Herbivores influence nutrient cycling and plant nutrient uptake:

Insights from tundra ecosystems

Hélène Barthelemy

Ecology and Environemntal Sciences

901 87 Umeå

Umeå 2016

This work is protected by the Swedish Copyright Legislation (Act 1960:729)

Copyright©Hélène Barthelemy

ISBN: 978-91-7601-456-1

Cover: Reindeer in the moutains, photo by Hélène Barthelemy

Electronic version available at http://umu.diva-portal.org/ Printed by: Service Center KBC

Umeå, Sweden 2016

To Marthe and Adrien Barthelemy

my so loved and wonderful grandparents

Contents

List of papers i

Authors contribution ii

Abstract iii

1. Introduction 1

1. 1 The role of large herbivores in ecosystem nutrient cycling 1

1. 2 Study system: the arctic tundra 2

1. 3 Reindeer grazing and nutrient cycling in the Arctic 6

1. 4 Objectives of the thesis 8

2. Materials and Methods 9

2. 1 Study sites 9

2. 2 Experimental designs 10

2. 3 Field sampling and laboratory procedures 13

2. 4 Statistical analysis 17

3. Results and Discussion 18

3. 1 The role of reindeer grazing for ecosystem N cycling 18

3. 2 The role of dung and urine deposition 19

3. 3 How does herbivore grazing influence plant nutrient uptake? 22

3. 4 The complex role of mosses 23

3. 5 Conclusion and perceptive 25

References 27

Acknowledgments 34

i

List of papers

This thesis is a summary of the following four studies, which will be referred

to by their roman numerals

I. Barthelemy, H., S. Stark, and J. Olofsson. 2014. Strong Responses

of Subarctic Plant Communities to Long-Term Reindeer Feces

Manipulation. Ecosystems 18: 740-751

II. Barthelemy, H., S. Stark, A. Michelsen, and J. Olofsson. 2016.

Effect of herbivory on the fate of added 15N-urea in a grazed Arctic

tundra.

Manuscript

III. Barthelemy, H., E. Dorrepaal, and J. Olofsson. 2016.

Defoliation, soil grazing legacy, dung and moss cover influence

growth and nutrient uptake of the common grass species, Festuca

ovina

Manuscript

IV. Barthelemy, H., S. Stark, M. M. Kytöviita, and J. Olofsson. 2016.

Grazing decreases N partitioning among coexisting plant species.

Manuscript

Papper I is reproduced with the kind permission from the publisher

ii

Authors contribution

Paper I:

JO established and maintained the field sites and collected initial data

together with SS. HB collected field data, performed chemical analysis,

analyzed the data and wrote the first version of the manuscript with

supervision from JO and SS.

Paper II:

HB designed the experiment with supervision from JO. HB performed field

work, collected data, and prepared samples for chemical and isotopic analysis.

HB analyzed the data and wrote the manuscript with comments from JO, SS

and AM.

Paper III:

HB designed the experiment with supervision from JO and ED. HB performed

greenhouse work, field work and chemical analyses. HB analyzed the data and

wrote the first version of the manuscript with comments from JO and ED.

Paper IV:

HB designed the experiment with supervision from JO. HB performed field

work, chemical analysis and mycorrhizal analysis with help from JO, SS and

MMK. HB analysed the data and wrote the first version of the manuscript with

comments from JO, SS and MMK.

Authors:

HB: Hélène Barthelemy, JO: Johan Olofsson, SS: Sari Stark, AM: Anders

Michelsen, ED: Ellen Dorrepaal, MMK: Minna-Maarit Kytöviita

iii

Abstract

Reindeer appear to have strong positive effects on plant productivity and

nutrient cycling in strongly nutrient-limited ecosystems. While the direct

effects of grazing on vegetation composition have been intensively studied,

much less is known about the indirect effect of grazing on plant-soil

interactions. This thesis investigated the indirect effects of ungulate grazing

on arctic plant communities via soil nutrient availability and plant nutrient

uptake.

At high density, the deposition of dung alone increased plant productivity

both in nutrient rich and nutrient poor tundra habitats without causing major

changes in soil possesses. Plant community responses to dung addition was

slow, with a delay of at least some years. By contrast, a 15N-urea tracer study

revealed that nutrients from reindeer urine could be rapidly incorporated into

arctic plant tissues. Soil and microbial N pools only sequestered small

proportions of the tracer. This thesis therefore suggests a strong effect of dung

and urine on plant productivity by directly providing nutrient-rich resources,

rather than by stimulating soil microbial activities, N mineralization and

ultimately increasing soil nutrient availability. Further, defoliation alone did

not induce compensatory growth, but resulted in plants with higher nutrient

contents. This grazing-induced increase in plant quality could drive the high

N cycling in arctic secondary grasslands by providing litter of a better quality

to the belowground system and thus increase organic matter decomposition

and enhance soil nutrient availability. Finally, a 15N natural abundance study

revealed that intense reindeer grazing influences how plants are taking up

their nutrients and thus decreased plant N partitioning among coexisting

plant species.

Taken together these results demonstrate the central role of dung and

urine and grazing-induced changes in plant quality for plant productivity. Soil

nutrient concentrations alone do not reveal nutrient availability for plants

since reindeer have a strong influence on how plants are taking up their

nutrients. This thesis highlights that both direct and indirect effects of

reindeer grazing are strong determinants of tundra ecosystem functioning.

Therefore, their complex influence on the aboveground and belowground

linkages should be integrated in future work on tundra ecosystem N dynamic.

1

1. Introduction

Large herbivores are ambassadors of wildlife and of vast and pristine

landscapes. Their migration provide one of the most astonishing and sublime

sight on earth. Large herbivores are vital to healthy and balanced ecosystems

and are of a great importance to human welfare, especially for many

indigenous communities.

The question of how herbivores control the structure and the functioning of

natural ecosystems has a long history in modern ecology. During decades, the

study of the effects of herbivory on nutrient cycling focused mainly on

invertebrate herbivory in particular on phytophagous insects in terrestrial

ecosystems and on phytoplanktivorous zooplankton in aquatic ecosystems.

The role of large herbivore grazing on the regulation of nutrient cycles was

largely neglected. Large herbivores consume only a relatively small proportion

of the ecosystem net primary production, but their effects on plant

productivity and nutrient cycling are intensified through positive and negative

soil-plant feedbacks (Wardle et al. 2004). Among wild large herbivores, the

effects of ungulate communities of African savannas and the ungulate

populations in the Arctic are probably the most widely studied. Still, the

mechanisms behind their control over plant and soil interactions are not fully

understood.

1. 1 The role of large herbivores in ecosystem nutrient cycling

Effects of grazing on ecosystem functioning

Aboveground herbivory strongly influence plant community structure and

function and soil processes via three main mechanisms acting simultaneously

over different spatial and temporal scales: (1) Defoliation directly affects

individual plants by removing and consuming plant tissues; (2) By converting

plant biomass into dung and urine, herbivores redistribute highly

decomposable resources rich in labile nutrients within the ecosystem (Hobbs

1996). Dung and urine deposition have been reported to stimulate soil

microbial activities which in turn increase nitrogen (N) mineralization and

promote plant nutrient acquisition and plant productivity (McNaughton et al.

1997; Bardgett et al. 1998a; Hamilton and Frank 2001); (3) Finally, trampling

by large herbivores has pronounced effects on soil microclimate (i.e.

2

temperatures and water balance) and soil structure (i.e. infiltration rate and

soil pore size) by decreasing vegetation cover and compacting the soil

(Veldhuis et al. 2014). In the long term, by feeding selectively on plant species

based on their nutritional status and their digestibility, herbivores can induce

large changes in species composition and can either decelerate or accelerate

ecosystem nutrient cycling (Bardgett et al. 1998b; Wardle et al. 2004; Bardgett

and Wardle 2003; Ritchie et al. 1998). While, the effect of herbivory on plant

species composition have been widely studied, much less is known about the

indirect effects of grazing on ecosystem nutrient dynamic and plant nutrient

uptake.

Plant tolerance to grazing

Plant tolerance to herbivory has been defined as the capacity of a plant to

maintain growth and reproduction following a defoliation event (Strauss and

Agrawal 1999). Compensatory growth occurs when plant growth increase after

defoliation resulting in smaller losses for the plant than expected and has been

intensively studied in African grasslands where intense ungulate grazing

optimizes plant primary production (McNaughton 1979; McNaughton 1984;

McNaughton et al. 1997).

Aboveground herbivory often enhances shoot N concentration (Hamilton

and Frank 2001) either directly through the reallocation of nutrients from the

plant belowground parts into shoot (Bardgett and Wardle 2003) or indirectly

through the grazing-induced increase in soil nutrient availability as discussed

above. Foliar herbivory can also induce a pulse of root exudates which

stimulate soil microbial activities and thus N mineralization and ultimately

plant nutrient uptake (Hamilton and Frank 2001; Guitian and Bardgett

2000). These defoliation induced changes in the quality and quantity of plant

resources entering the soil will affect plant litter decomposition rate (Diaz et

al. 2007) and have further consequences for ecosystem nutrient cycling.

Exploring plant tolerance to grazing is thus highly important to understand

the relationship between grazing and nutrient cycling.

1. 2 Study system: the arctic tundra

Nutrient availability for plants

3

Arctic tundra ecosystems are strongly N-limited environments. Litter

decomposition and nutrient mineralization is slow in cold arctic soils (Van

Cleve and Alexander 1981; Stark 2007; Weintraub and Schimel 2003) and the

slow release of nutrients to plant uptake controls primary production (Aerts

and Chapin 2000; Schimel and Bennett 2004). Due to variation in topography

over very short distances affecting soil microclimate, snow accumulation and

wind exposure, tundra vegetation commonly consists of a mosaic of vegetation

types. These range from nutrient poor tundra heaths dominated by mosses

and dwarf shrubs to nutrient rich meadows, dominated by graminoids and

forbs (Bjork et al. 2007) (Fig 1). N is commonly seen as the main limiting

nutrient for plant productivity in the tundra. However, there is an increasing

recognition that co-limitation by N and phosphorus (P) could be operating in

more fertile tundra habitats (Giesler et al. 2012; Sundqvist et al. 2011) (Fig. 1).

Figure 1 Characteristics of the two most common vegetation types in the tundra. The heath

vegetation is dominated by slow-growing and long-lived dwarf shrubs and a thick moss layer

covering the ground surface almost entirely. Typical species are Betula nana, Vaccinium

myrtillus, Vaccinium vitis-ideae and Empetrum hermaphroditum. The meadow

vegetation, commonly found in shallow depression, is dominated by the fast-growing

graminoids such as Festuca ovina and Carex bigelowii and forbs such as Saussurea alpina,

Viola biflora and Bistorta vivipara. Most of the graminoid species found in arctic secondary

grasslands have a clonal growth.

4

In arctic soils, dissolved organic N (DON) dominates N cycling where most

of the N is bound in complex organic compounds and thus not directly

available for plant uptake (Neff et al. 2003). In addition, a considerable

amount of N can be immobilized by soil microbes acting as strong competitors

with plants for available N (Jonasson et al. 1996; Jonasson et al. 1999;

Michelsen et al. 1999). However, recent evidence suggest that arctic plants can

compete more efficiently with soil microbes than expected (Schmidt et al.

2002). Indeed, in the long term, a larger proportion of soil nutrients will be

immobilized within plants, since nutrients are retained for longer periods in

plant tissues compared to soil microbial biomass (Hodge et al. 2000).

Plant nutrient uptake and mycorrhiza symbiosis

Resource partitioning of available nutrients in space (i.e. root depth), in time

(i.e. differential uptake during the growing season) and on their chemical

forms (i.e. organic and inorganic forms) is an important mechanism

facilitating plant coexistence in nutrient poor ecosystems (McKane et al.

2002) (Fig. 2). Most plants commonly bypass N mineralization processes by

using organic N as a large part of their nutrition. In the Arctic, plant nutrient

uptake then occurs primarily through the symbiosis between mycorrhizal

fungi and plant fine roots (Read and Perez-Moreno 2003). Complex forms of

organic N and P are passed from the fungus to the plants, while the fungal

partner receives labile carbohydrates in exchange (Hobbie and Hobbie 2008).

Ericoid mycorrhizae (ERI) and ectomycorrhizae (ECM) dominate in

nutrient-poor heath (Fig. 2). Although both ERI and ECM fungi have

considerable proteolytics enabling them to depolymerise complex organic N

compounds (Read and Perez-Moreno 2003; Schimel and Bennett 2004), ERI

fungi have higher saprotrophic capacities than ECM fungi (Bending and Read

1996). Arbuscular mycorrhizae (AM) are found in more fertile habitats (Fig.

2). They access mainly organic form of P and lack of the enzymatic capacities

to degrade recalcitrant form of organic N (Read and Perez-Moreno 2003).

However, recent studies reveal that AM could be more active in the uptake of

DON than previously thought (Leigh et al. 2009; Whiteside et al. 2012).

Although non-mycorrhizal tundra plants (NON) access mainly inorganic

forms of N in the soil solution, they can also directly take up low molecular

weight organic N (i.e. amino acids) (Chapin et al. 1993; Kielland 1994;

McKane et al. 2002) (Fig.2).

5

Figure 2 Plant mycorrhizal associations and plant N partitioning in the Arctic. Arctic plant

species are commonly well differentiated in chemical forms, timing and depth of available

N uptake.

Stable N isotopes and arctic plant ecology

The relative abundance of the stable isotopes 15N and 14N provides an effective

tool for analysing tundra ecosystem N dynamics, since changes in plant and

soil 15N natural abundance integrates the net effect of different mechanisms

in plant nutrient uptake and N cycling (Nadelhoffer and Fry 1994; Högberg

1997; Robinson 2001) (See Box 1, Materials and Methods). Foliar δ15N

depends on plant N sources (proportion of proteins, amino acids and

ammonium taking up) and on nutrient uptake mechanisms (direct or

mycorrhizal-mediated nutrient uptake) (Hobbie and Hogberg 2012).

Differentiation in foliar δ15N is strong in nutrient poor tundra ecosystems

as a result of a high plant N partitioning (Nadelhoffer et al. 1996; Michelsen

et al. 1996; Michelsen et al. 1998; Hobbie et al. 2000). In arctic soils, inorganic

N pools are 15N enriched relative to organic N pools (Yano et al. 2010) and ERI

plants have commonly the lowest foliar δ15N followed by ECM plants and NON

plants have the highest foliar δ15N while AM are intermediate (Fig. 2)

(Michelsen et al. 1996; Michelsen et al. 1998). Any changes in tundra

ecosystem N cycling is likely to be reflected in foliar and δ15N signatures.

6

1. 3 Reindeer grazing and nutrient cycling in the Arctic

In the strongly N-limited tundra ecosystem, any processes that could modify

plant-soil interactions can have dramatic consequences on the whole

ecosystem. Grazing by the large and semi-domesticated reindeer populations

is one of them. While negative effects of grazing on nutrient cycling have been

sometimes observed in reindeer winter range (i.e. nutrient poor heath in the

boreal forest) (Stark et al. 2000), reindeer grazing in the summer ranges

appears to have strong positive effects on ecosystem nutrient cycling and plant

community (Fig. 3).

Figure 3 Effects of summer reindeer grazing on plant species composition and nutrient

cycling in tundra ecosystems.

Although the deposition of dung and urine provides highly decomposable

resources to the arctic belowground communities (Fig. 3) (van der Wal et al.

2004; Olofsson et al. 2004), their specific role for plant productivity and soil

nutrient availably is still poorly understood and experimental evidence is

scarce. In addition, the effect of dung seems also to differ between ecosystems

of contrasting fertility (Bardgett and Wardle 2003). Since tundra ecosystems

are composed of a mosaic of vegetation types, it is thus highly important to

explore the strength and the direction of dung deposition for tundra

ecosystem functioning. The return of nutrients through urine deposition has

attracted even less attention probably because tracing and quantifying urine

7

in natural ecosystems is highly complex. However, exploring the effect of urine

deposition on nutrient dynamic is highly important, since urine is more

readily available to plants than most N compounds in dung which have to

undergo the slow process of organic matter decomposition (Hobbs 1996).

Mosses are a key component of tundra plant communities and with their

large insulating capacities they have strong effects on arctic ecosystem

functioning by keeping soils cold and wet (Gornall et al. 2007). Large

herbivores play a central role in regulating the extent of the moss cover in

tundra ecosystems. Reindeer have been shown to decrease the extensive moss

layer directly by tramping over the vegetation (Fig. 3) (Olofsson et al. 2004)

(van der Wal and Brooker 2004; van der Wal et al. 2001). A dung

manipulation study in the High Arctic has also demonstrated that reindeer

can also reduce the moss cover by the deposition of dung alone which

stimulate organic matter decomposition and thus increase moss

decomposition rate (Fig. 3) (van der Wal et al. 2004). The decrease in moss

cover increases soil summer temperatures and the maximum depth thaw of

the permafrost, and thus positively affects N mineralization (Olofsson et al.

2004; van der Wal et al. 2001; van der Wal and Brooker 2004). Exploring the

effects of reindeer grazing on soil microclimate is thus important for a better

understanding of tundra ecosystem nutrient cycling.

With their large effects on N mineralization processes and soil microbial

communities, reindeer have a strong potential to affect the proportion and the

type chemical forms of N in the soil and the strength of the symbiosis between

plant root and mycorrhizal fungi. Reindeer can thus to influence how plant

acquire their nutrients from the soil solution and affect plant resource

partitioning of available nutrients. However, it is still unclear to which extend

herbivores can influence plant nutrient uptake.

On their summer pasture, reindeer feed preferentially on the most palatable

plants such as forbs and graminoids (Fig. 3). Under some circumstances, such

as intense summer grazing, reindeer can induce a shift in vegetation from a

moss and dwarf shrub vegetation to graminoids dominance (van der Wal

2006) and there is increasing evidence of grazing induced promotion of

graminoids in the Arctic (Olofsson et al. 2001; van der Wal and Brooker 2004;

van der Wal et al. 2004). Although it has been suggested that the positive

effect of aboveground herbivory occurs when graminoids respond to

defoliation by compensatory growth (McNaughton 1983), the mechanisms

behind plant tolerance to herbivory are still poorly understood.

8

1. 4 Objectives of the thesis

The overall objective of this thesis is to explore the indirect effect of reindeer

grazing on arctic plant community via soil nutrient availability and plant

nutrient uptake. More precisely I examine the following questions:

(Paper I) What are the effects of dung deposition alone on arctic plant

productivity and structure? Do these effects differ depending on soil fertility?

(Paper II) Which ecosystem N pools will sequester reindeer urines?

(Paper III) Which mechanisms regulate plant tolerance to grazing?

(Paper IV) How does reindeer grazing influence plant N partitioning

among coexisting plant species?

9

2. Materials and Methods

2. 1 Study sites

Kärkevagge valley (Paper I)

Kärkevagge valley is located in northern Sweden (Fig 4). The landscape is

dominated by intermingling patches of tundra heath and tundra meadow

vegetation (Fig. 5). The study site is an important summer grazing area for the

Gabna reindeer herding district and reindeer are mainly grazing the area in

July and August.

Reisa valley (Paper, II, III and IV)

Reisa valley is located in northern Norway (Fig. 4). At the research site, a 50

years old reindeer fence divide the landscape over several kilometres and

separate the summer grazing range from the spring and autumn range where

reindeer only pass the area during their migration (Fig. 4). The spring and

autumn range is dominated by tundra heath vegetation. It only experiences

little grazing and thus will be referred to as lightly grazed sites. By contrast,

the summer range experiences intense grazing in August and the dwarf shrub

and moss rich vegetation has been replaced by a graminoid vegetation

(Olofsson et al. 2001). These sites will be referred as heavily grazed sites.

Figure 4 Map of northern Fennoscandia showing the locations of the research sites. On the

right, the reindeer pasture fence in Reisa.

10

2. 2 Experimental designs

Paper I: Reindeer dung manipulation (2004-2010)

A 7 years old dung manipulation experiment was performed in a nutrient rich

tundra meadow and in a nutrient poor tundra heath. The effects on plant

community composition and nutrient cycling were analysed. 10 sites were

selected in the two habitats types by a random block design (Fig. 5). The

following treatments were allocated and repeated every year in early August:

(1) Control, i.e. natural reindeer dung deposition; (2) Removal of pellets and

(3) Double, i.e. all reindeer pellets in the removed plots were transferred to

the Double plots; (4) High, addition of about 10 times the natural dung density

(pellets were collected in the surrounding area).

Figure 5 Experimental design of the reindeer dung manipulation (in total 20 blocks). As

reindeer preferentially grazed in the meadow (forage of a better quality), reindeer density

was 2-3 times higher in these control plots and thus also in the double treatments.

Paper II: 15N tracer study (2011-2012)

A 15N tracer study (Box 1) using labelled urea was performed in Reisa. 10

blocks were selected along the fence and in each block, a control plot and one 15N-urea addition plot were randomly allocated at the lightly grazed and at the

heavily grazed sites (in total 40 plots).

11

Box 1: N stable isotope in plant-soil ecology

N stable isotopes: N has two stabile isotopes: 15N is the rare (0.36% of natural N) and

heavy (8 neutrons) variant, and 14N the abundant (99.63% of natural N) and light (7

neutrons) one. δ15N is the relative natural abundance of 15 N and 14N in samples expresses

as

δ15N (‰) = 1000 × (R𝑠𝑎𝑚𝑝𝑙𝑒 − R𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑

R𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑)

with Rsample the stabile isotope ratio 15N/14N in a sample compared to atmospheric N2.

Isotopic fractionation and N cycling: Fractionation is the changes in the partitioning

of 15N and 14N between a source and a product. Biologically mediated isotope fractionation

is called discrimination, the light and less stable 14N (forming bonds that are more easily

broken) will react faster and is likely to accumulate more rapidly in the product than the

heavier 15N (Robinson 2001; Dawson et al. 2002). As a consequence, the major soil N

transformations such as N mineralization and nitrification lead to a product depleted in 15N

compared to the residual N pool.

Higher soil nutrient availability is often associated with a higher rate of ecosystem N cycling.

This increase in N cycling will result in an 15N enrichment of soil N pools, since the lighter

14N isotopes are preferentially lost during N leaching and denitrifications (Amundson et al.

2003). If herbivores enhance soil N cycling, they are thus also expected to increase the δ15N

of the system.

Isotopic fractionation and mycorrhizal-mediated N uptake: In the arctic,

mycorrhizal arctic plants have been reported to have lower δ15N signatures than non-

mycorrhizal plants (Michelsen et al. 1996): (1) They access a higher proportion of organic N

sources (with lower δ15N than inorganic N sources) and (2) The large 15N fractionation

associated with N assimilation with fungal hyphae (Macko et al. 1986) and N transfer

processes in fugal symbionts (Hobbie and Hobbie 2006) leave the plants depleted in 15N.

15N enriched (tracer) study: This method involves applying trace amount of N

compounds highly enriched in 15N (labelled N compound). Tracing and quantifying the N

compounds derived from urine is methodologically challenging. In paper II, the molecule

of urea was enriched in 15N by 98%. Since its abundance will be much higher than any

natural occurring level, it is thus possible to follow its transportation and partitioning in a

system without altering its natural behaviour (Robinson 2001; Dawson et al. 2002).

12

All the plots were selected within a distance of 5 m from the fence. The 15N

urea, diluted in 2 litres of stream water, was sprayed uniformly over the

vegetation at a rate of 0.2 g m-2 at the peak of the growing season. Following

each tracer addition, the control plots received the equivalent amount of non-

enriched water. The distribution of 15N-urea into the different N pools of the

two grazing sites was examined after 2 weeks and 1 year after tracer addition.

Paper III: Common garden with Festuca ovina (2011-2012)

To test the effects of soil origin (heavily or lightly grazed soil) and simulated

herbivory on plant growth and soil N bioavailability, a full factorial common

garden pot experiment was established at the Scientific Research Station in

Abisko (Fig. 4). Festuca ovina was used as model species. It is a grazing-

tolerant graminoid very common in arctic secondary grasslands. Soil samples

were collected at 5 location along the reindeer fence in Reisa (Fig. 4) and half

of the plants were grown in soils from the heavily grazed sites and the other

half in soils from the lightly grazed sites. The following treatments were

allocated in 5 blocks corresponding to each soil sampling site (Fig. 6): (1)

Shoot defoliation, plants were cut down to 3 cm two times in 2011 and one

time in 2012. This clipping treatment mimics reindeer defoliation; (2)

Reindeer dung fertilization (volume 1 dl) with pellets collected close to the

reindeer herd in Reisa and (3) Presence or absence of a thick layer of feather

mosses to manipulate soil microclimate (soil temperature and moisture).

13

Figure 6 Experimental design of the common garden with F. ovina with C = Control; F =

reindeer dung fertilization; M = presence of a moss layer; and D = defoliation. On the right,

the photos illustrate one experimental block and one pot unit of the common garden in

Abisko. In each block, each treatment had two replicates (in total 32 experimental plots in

each block). The plant root simulators are the orange anion probes and the purple cation

probes inserted in some of the experimental pots.

Paper IV: 15N natural abundance study in the heavily grazed and lightly

grazed tundra

The long term effect of reindeer grazing on plant N uptake of coexisting tundra

plants was examined by exploring differences in 15N natural abundance

signatures in plants, microbes and soil (Box 1) and in mycorrhizal colonization

rate. 10 sites along the reindeer fence in Reisa were selected with similar

abiotic conditions. The studied plant species included two ERI dwarf shrubs

Vaccinium myrtillus and Empetrum hermaphroditum, an ECM dwarf shrub

Betula nana, an AM grass Dechampsia Flexuosa and a NON sedge Carex

bigelowii. These were the only plant species that were common enough to get

a representable sample from both sites of the reindeer fence.

2. 3 Field sampling and laboratory procedures

Plant sampling

In paper I and II, all aboveground vegetation (including litter) was harvested

in a subplot (25 cm x 25 cm) located in the middle of each plot (Fig. 7a). Root

biomass was sampled from soil cores (depth 5 cm, area 44 cm2) in the middle

of each subplot. In paper III, the common garden was ended shortly after the

peak of the second growing season. The entire aboveground plant biomass was

clipped and the belowground plant biomass was collected. In paper IV, foliar

tissues of the 5 studied plant species were collected from 5 to 10 individuals

for each plant species. Root samples were also harvested by tracing the fine

root from the main root down into the soil. Fresh reindeer dung was also

collected at close location of the reindeer herd for nutrient and isotope

analysis (paper I and IV).

14

Aboveground plant biomass was sorted at the species level except for the

graminoids and cryptogram groups. In paper I, to estimate aboveground

primary production, the current year shoots of dwarf shrubs were separated

from the previous years´ growth. All graminoids and forbs were considered to

represent new biomass. All root samples were thoroughly washed to remove

any soil and other organic materials. In paper IV, roots were stored in ethanol

until mycorrhizal analysis. All other plant and dung samples were oven dried,

weighted for biomass estimation and finely grinded and sent for analysis.

Mycorrhizal analysis

In paper IV, root samples were cleared and acidified. Darkly pigmented roots

were bleached to remove any phenolic compounds left in the cleared roots

prior acidification. The fungal structure for ERI and AM fungi was stained and

mycorrhiza colonization frequency was estimated as the proportion of coils or

arbuscules intercepted in 1 cm long root segment (diameter < 1 mm) by a

modification of the line intersection procedure (McGonigle et al. 1990). ECM

colonization rate was measured by the line intersection procedure (Fig. 7b)

(Brundett et al. 1996).

Figure 7 a) Aboveground and belowground plant biomass sampling b) Root mycorrhizal

preparation and analysis c) Soil samples extraction and filtration

15

Soil sampling and soil measurements

A set of three soil cores (volume 500 cm3) was collected at each experimental

plot (paper I and II) or sampling location (paper IV). Only the thin humus

layer was sampled while the mineral layer was discarded. Soil samples were

kept frozen until analysis. In paper III, plant root simulators, PRSTM probes

(Western Ag Innovations, Saskatoon, Canada) were buried in the soil of

selected experimental pots for the whole growing season 2012. These ion

exchange membranes (Fig. 6) are effective surrogate for biomimicking

nutrient absorption by plant roots and provide a dynamic measure in situ of

N available in the soil. Soil temperature and moisture were also measured in

each experimental unit (Paper I, III and IV).

Soil preparation

Soil samples for soil extractable and microbial N (Paper I, II and IV), soil and

microbial δ 15N signatures (Paper II and IV) and soil extractable and microbial

P (Paper I) were thawed and sieved to remove plant material and stones.

Water content was measured gravimetrically and organic matter content was

determined by loss on ignition. Soil microbial N and P was determined as the

chloroform labile fraction using the chloroform-fumigation-extraction

technique describes by Brookes et al. (1985). Non-fumigated soil extracts were

used to determine soil extractable N. Soil extracts were filtered and total

extractable N and P were determined by oxidizing the entire extractable N and

P to NO3- and PO3

- (Williams et al. 1995) (Fig. 7c). To determine soil

extractable and microbial δ15N signatures, fumigated and non-fumigated

extracts were prepared following a modification of the acid trap diffusion

technique (Holmes et al. 1998). Glass microfiber filter disk were incubated for

7 days to trap all the volatilized NH3 onto the acidified disks.

Chemical analysis and calculation

Samples for elemental C and N and δ15N signatures were encapsulated in pre-

weighted tin capsules and were analysed on a continuous flow CHN analyser

interfaced to an isotope ration mass spectrometer by the stable isotope facility

of the University of Davis and the University of Copenhagen (Paper II, III and

IV). Kjeldahl N and P content of dung and a selected forage species were

analysed on a continuous flow colorimetric analyser by the Landcare Research

16

laboratory (Paper I). The PRSTM probes were analysed colorimetrically using

an automated flow injection analysis system by western Ag Innovation (Paper

III). Soil extracts for N and P availability were analysed by flow injection

analysis and by spectrophotometry respectively (Paper I, II and IV).

The microbial N and P content was calculated as

𝑁𝑚𝑖𝑐 = 𝑁𝑓 − 𝑁𝑒

𝑃𝑚𝑖𝑐 = 𝑃𝑓 − 𝑃𝑒

In paper II and IV, the isotope signature of the microbial biomass N pool was

determined using isotope mass balance as

δ 15𝑁𝑚𝑖𝑐 (‰) =(δ15𝑁𝑓 × 𝑁𝑓 − δ15𝑁𝑒 × 𝑁𝑒)

𝑁𝑚𝑖𝑐

In paper II, The 15N enrichment in each ecosystem N pools was calculated as

the 15N atomic frequency in excess as

Atom15𝑁 % excess = Atom % 𝑡𝑟𝑎𝑐𝑒𝑟 − Atom % 𝑏𝑎𝑐𝑘𝑔𝑟𝑜𝑢𝑛𝑑

The percentage tracer recovery in each ecosystem N pool was estimated as

% 𝑡𝑟𝑎𝑐𝑒𝑟 𝑟𝑒𝑐𝑜𝑣𝑒𝑟𝑦 15𝑁 = (𝐴𝑡𝑜𝑚15𝑁 𝑒𝑥𝑐𝑒𝑠𝑠 × 𝑁 × 𝑀𝑎𝑠𝑠)/104

𝑇𝑜𝑡𝑎𝑙 𝑎𝑚𝑜𝑢𝑛𝑡 𝑜𝑓 𝑡𝑟𝑎𝑐𝑒𝑟 𝑎𝑑𝑑𝑒𝑑 ∗ 100

Box 2: Notation

Nmic and Pmic: soil microbial N and P

Ne and Pe: soil extractable N and P (from the non-fumigated extracts)

Nf and Pf: N and P availability in the fumigated extracts

δ 15Ne and δ15Nmic: δ 15N signature in soil solution and in the microbial N pool

Atom % tracer: 15N atomic frequency of a sample collected in the 15N urea addition plot

Atom % background: 15N atomic frequency of the corresponding sample collected in the

non-labelled plot

N: N content (%) and Mass: total mass of each pool (biomass or soil g m-2)

17

2. 4 Statistical analysis

For statistical analysis, analysis of variance (ANOVA) without repeated factors

(Paper IV) and with repeated factors (Paper I, II and III)) was mainly used.

The use of linear mixed effect models allow the incorporation of random

factors (i.e. time, sampling location or both) to account for variation between

sampling locations and to avoid pseudo-replication, for example by taking

into account the different recording of the same plots at a different sampling

time. For all ANOVAs, when the analysis showed a significant effects between

categorical variables, differences among means were further explored using

post hoc tests. Akaike´s information criteria and residual plots were used to

assess the model fit. Other statistical methods were also used, for example

student t-test (Paper I). Data were log or arcsine-transformed when necessary

to fulfil the assumptions of normality and homoscedasticity. All statistical

analysis were conducted using the R statistical package (R development Core

Team 2016).

18

3. Results and Discussion

This thesis aimed at getting a better understanding of the indirect effects of

grazing by large herbivores on ecosystem N dynamic, soil nutrient availability

and plant N uptake in the nutrient limited tundra ecosystems. The key

findings of each paper are presented and briefly discussed together.

3. 1 The role of reindeer grazing for ecosystem N cycling

Changes in arctic plant community and soil processes

Intense reindeer grazing transformed the plant community from a moss and

dwarf shrubs dominated vegetation to a vegetation dominated by graminoids

and forbs, creating an arctic secondary grassland (Paper II and IV). The shift

in plant community composition was also accompanied by a shift in

mycorrhizal community composition, from the dominance of ERI and ECM

fungi at the lightly grazed sites to the dominance of AM fungi and NON plant

species at the heavily grazed sites (Paper IV). 50 years of reindeer grazing have

also increased soil nitrogen availability, evident both from a higher extractable

N in the soil and higher N concentrations in all plant species investigated.

These results are in line with previous work reporting a reindeer-induced

vegetation shift and increase N cycling in tundra ecosystems (Olofsson et al.

2001; van der Wal et al. 2004; van der Wal and Brooker 2004; Olofsson et al.

2004; Stark and Väisänen 2014) and highlight the strong effect of herbivory

on ecosystem N cycling.

Further, Paper IV reveals that intense reindeer grazing resulted in a 15N-

enrichment of both soil N and microbial N pool δ15N signatures. This 15N-

enrichment is likely due to the grazing-induced faster N turnover, since

mineralization processes discriminate against the 15N isotope (Högberg 1997;

Bedard-Haughn et al. 2003) and the deposition of dung and urine providing

nutrient-rich resources with relatively high δ15N compared to other soil N

sources. This finding demonstrates the powerful used of 15N natural

abundance in capturing the positive effect of reindeer grazing in soil N

dynamics.

Plant responses to aboveground defoliation and soil grazing legacy

19

Aboveground herbivory severely reduced the growth of F. ovina (Paper III).

This result demonstrates that even a grazing tolerant graminoid species which

increases largely in abundance under heavy reindeer grazing (Olofsson et al.

2001) was damaged following defoliation. Both final aboveground and

belowground biomass decreased more than expected from the proportion of

biomass removed. Plants did not tolerate defoliation better at higher nutrient

availability, i.e. when grown in soil from the heavily grazed or after dung

fertilization. This indicates that an increase in tolerance to aboveground

herbivory at more fertile conditions might be restricted to when plants in

natural communities experience light N-limitation in the absence of

defoliation (Johnson and Gough 2013).

By contrast, defoliation increased N concentrations and decreased C to N

ratio both in shoots and in roots, likely due to the production of new biomass

with higher N concentration than older plant parts. This finding indicates that

the grazing-induced increase in plant quality could drive the high N cycling in

arctic secondary grasslands by providing plant litter of a better quality to the

soil, resulting in a faster decomposition rate (Olofsson and Oksanen 2002).

Further, plants grown in previously heavily grazed soils produced more

biomass, had higher nutrient uptake and higher N concentration than plants

grown in previously lightly grazed soils. This finding highlights the strong

effect of soil grazing legacy on plant productivity and indicates that herbivores

could have long lasting effects on plant growth and ecosystem N dynamics

even after the herbivory has ceased.

3. 2 The role of dung and urine deposition

The effect of dung deposition for tundra plants

Paper I and Paper III revealed that herbivores can increase plant productivity

by the deposition of dung alone. Dung addition increased both shoot biomass

and N concentrations of the grazing tolerant plant F. ovina (Paper III).

Further, dung addition at high density increased plant productivity in the

meadow and heath vegetation types (Paper I). Doubling and removing the

natural dung deposition had only weak effects, mainly in the heath vegetation.

Graminoids and deciduous shrubs responded the strongest to dung additions

and were thus the plant functional groups the most sensible to changes in

resource availability. Dung addition had mixed effects on plant nutrient

20

status, with an increase in N concentration for F. ovina (Paper III), while no

changes were observed for plants in natural communities (Paper I), where

added nutrients were diluted in plant tissues.

Although reindeer preferentially used the more fertile meadow vegetation,

the effect of long term dung manipulation was stronger in the nutrient poor

heath vegetation (Paper I). This finding does not support the prediction that

dung is more important in nutrient rich habitat, as a larger proportion of plant

biomass should be converted into dung and urine (Bardgett and Wardle

2003). Further, since dung addition did not decrease the proportion of slow

growing and nutrient poor plant species, Paper I suggest that dung deposition

alone cannot drive the dramatic vegetation shifts reported in Paper II and IV.

Aboveground defoliation, trampling and urine deposition may also be needed

in order to induce a large vegetation transition (van der Wal 2006).

Urea partitioning in plant ecosystem N pools

Most of the added 15N-urea was incorporated into aboveground part of plants.

Vascular plants in each of the grazing regime appeared to sequester 15N-urea

in proportion to their abundance in the vegetation (Paper II). Dwarf shrubs

were as efficient as graminoids and forbs in taking up the added 15N-urea. This

result is in contrast with studies were 15N was injected in the soil (Oulehlea et

al. 2016) and with Paper I, where the opportunistic and fast-growing plants

took the most advantages of enhanced nutrient availability from dung

addition. A possible explanation for this finding could be that, in addition to

root uptake, a large proportion of urea from reindeer urine might have been

absorbed directly by the plant canopy (Baur et al. 1997; Schreiber 2005). Litter

was also a large sink for the added 15N-urea, highlighting the biological

importance of this layer for tundra ecosystems functioning. These findings

provide further evidence of a key role of urine for arctic plant productivity.

Dung and urine availability to plants

Although both dung and urine appeared to be important for arctic plant

nutrition, their effects were decoupled in the short and long term. Plants

responded to dung addition after two years in the meadow and after 7 years in

the heath (Paper I). Similarly, dung addition effects on the growth and

productivity of F. ovina were still weak two years after dung fertilization

21

(Paper III). This slow effect of dung is consistent with previous studies in

arctic tundra, indicating a delay of some years in the response of plant

communities to dung fertilization (van der Wal et al. 2004). Previous work

indicates that the decomposition rate of reindeer dung is strongly positively

correlated to soil temperatures and soil moisture (van der Wal et al. 2004;

Skarin 2008). The faster response to dung addition in the meadow probably

originated from more favourable soil microclimate conditions for dung

decomposition with higher soil temperature and moisture content than in the

heath. By contrast, urine assimilation by plants can be very rapid since 15N-

urea recovery in plants was high only after 2 weeks after tracer addition (Paper

II). This demonstrates that nutrients from urine are used much faster than N

added through dung (Paper I and Paper III) or plant litter (Olofsson and

Oksanen 2002), as for both it can take years before nutrients are released into

plant available forms.

Strong competition between plants and microbes for the incoming nutrients?

Dung fertilization, even at high density, had only weak effects on soil nutrient

availability and no effect on microbial N and P immobilization in both

vegetation types (Paper I) and did not affect soil N bioavailability to the

graminoid F. ovina (Paper III). Similarly, although soil extractable N and

microbial N constituted a considerable ecosystem N pool, they sequestered

only a marginal proportion of the N added in the form of 15N urea, irrespective

of the grazing intensity (Paper II). Taken together, this indicates that arctic

plant communities are well adapted to take advantage of the pulse of incoming

N compounds derived from dung and urine and that arctic plants are efficient

competitors to soil microbes for the soil available N (Jonasson et al. 1999;

Schmidt et al. 2002; Stark and Kytoviita 2006). These findings suggest a direct

effect of dung and urine for plant productivity by providing nutrient-rich

resources for plant uptake, rather than an indirect effect by stimulating

microbial activities, N mineralization and ultimately plant nutrient uptake.

Further, dung addition did not affect moss layer decomposition (Paper I).

These findings are in contrast with previous work demonstrating that the

deposition of dung alone can induce cascading effects in high arctic tundra

ecosystems (van der Wal et al. 2004).

22

3. 3 How does herbivore grazing influence plant nutrient

uptake?

Intensive reindeer grazing induced a large shift in plant 15N natural abundance

(Paper II and Paper IV). The large variation in δ15N among coexisting plant

species in the lightly grazed sites was dramatically reduced in the heavily

grazed sites. This indicates a reduced N partitioning on the basis of chemical

N forms. All dwarf shrubs, forbs, mosses and lichens had higher and

graminoids lower δ15N at the heavily grazed sites.

First, all plants could access more easily available mineral N sources, with

higher δ15N compared to most complex organic N compounds (Yano et al.

2010), at conditions of high mineral nutrient availability that predominate at

the heavily grazed sites (Stark and Väisänen 2014) (Fig 8). Plants at the

heavily grazed sites were also more defoliated and the more similar δ15N

signatures might thus result from a reduced capacity of defoliated plants to

access organic N sources. Paper II and III supported this thesis. Defoliation

caused a strong decrease in plant N uptake for F. ovina (inducing an increase

in soil N bioavailability), likely due to the observed decrease in root biomass

(Paper III). Further, heavily grazed shrubs were also less efficient in taking up

the added 15N urea than lightly grazed shrubs (Paper II).

Figure 8 Key findings for paper IV: intense reindeer grazing decreases N partitioning

among coexisting plant species. Plain arrows indicate direct nutrient uptake and dashed

23

arrows mycorrhizal-mediated nutrient uptake. The thickness of each arrow indicate size

effect. DON = dissolved organic nitrogen and DIN = dissolved inorganic nitrogen.

Secondly, with increasing soil N availability, the importance of mycorrhizal-

mediated N uptake could be reduced and plants could shift toward a more

direct uptake of N sources. Since N assimilation within fungal hyphae (Macko

et al. 1986; Jin et al. 2005) and N transfer processes (Hobbie and Hobbie

2006) highly discriminate against the 15N isotope, a decrease in mycorrhizal-

mediated N uptake could be at the origin of the higher δ15N signatures for all

dwarf shrubs. Thus is supported by the strong decline in mycorrhizal

colonization for the ECM B. nana and the ERI E. hermaphroditum.

Finally, the uptake of labile nutrients from dung and urine could also be a

central mechanism for the observed shift in plant δ15N, since all investigated

plants had δ15N values converging toward the δ15N signature in dung and

urine. Evidence comes from the δ15N signature of mosses (Paper II). If direct

uptake of soil N by diverse bryophyte species have been reported (Ayres et al.

2006), wet deposition is the main pathway of N acquisition. The large increase

in moss δ15N at the highly grazed sites could thus only result from a change in

incoming N over the vegetation, through the deposition of dung and urine

with higher δ15N than those recorded in moss tissues.

Paper II suggest thus that the combined effect of a reduced mycorrhiza-

mediated nitrogen uptake and a shift towards a more mineral nutrition of

labile nitrogen from dung and urine was at the origin of the observed shift in

plant δ15N in heavily grazed areas. These findings indicate that herbivores do

not only influence the soil nutrient availability, but also how plants acquire

nutrients. This may have potential consequences for the effects of herbivores

on plant coexistence, as it reveals that nutrient availability in the soil in

different grazing regimes does not fully depict the nutrient availability for

plants.

3. 4 The complex role of mosses

Mosses and lichens were the largest 15N sink both at the lightly and heavily

grazed tundra (Paper II). This findings is consistent with previous tracer

studies in arctic tundra, demonstrating the high capacity of cryptogams for

24

nutrient uptake (Gundale et al. 2014; Tye et al. 2005). Bryophytes possess very

absorptive surfaces and can capture large proportions of mineral and organic

N compounds for several years (Jonsdottir et al. 1995; Weber and Van Cleve

1981; Krab et al. 2008; Street et al. 2015) and thus could negatively affect plant

nutrient uptake (Jonsdottir et al. 1995). Since intense reindeer grazing caused

a large decrease in moss biomass, reindeer have the potential to increase N

cycling and the growth of vascular plants by enabling more nutrients from

urine to enter the soil. Although urea may rapidly contribute to a higher

primary production in tundra ecosystems, it does not automatically result in

a higher abundance of high quality forage, since large amounts of N

compounds derived from urine could be trapped for extended periods plants

not preferred as forage by reindeer (Thomas and Kroeger 1981). Paper III also

revealed that the presence of a thick moss layer had a negative effect on growth

of F. ovina, reducing in particular belowground plant productivity. Further

mosses reduced plant compensatory growth after a defoliation event.

By contrast, Paper III also demonstrated that the presence of a moss layer

increased plant N concentrations and enhanced total nutrient uptake. Since

precipitation is low at the common garden site in Abisko (Kohler et al. 2006)

and mosses increased soil moisture without affecting soil temperature, it is

likely that the positive effect of mosses on plant nutrient status and plant

nutrient uptake resulted from an improvement in soil hydrology. This is in

line with previous work reporting that mosses, especially in harsh and dry

environments, reduce evaporation and retain moisture from snow melt and

precipitation (Sohlberg and Bliss 1987; Gornall et al. 2007; Sand-Jensen and

Hammer 2012). N leached from the mosses could have been an additional

facilitative effect on plant growth since feather mosses have N fixing

symbionts (Stuiver et al. 2015). Mosses also did not seem to prevent nutrients

from the added reindeer dung to enter the soil. Since moisture is an important

determinant of the growth of mosses, it is likely that the dry environment that

mosses were experiencing in the common garden in Abisko reduced their

absorbing capacity.

Taken together these findings indicate that mosses can retard or reinforce

the positive effect of reindeer on plant productivity and ecosystem N cycling.

Soil microclimate appears to be of a particular high important in controlling

the direction of the effect of mosses on tundra ecosystem functioning.

25

3. 5 Conclusion and perceptive

The results presented in this thesis provide a more detailed framework about

some of the key mechanisms through which grazing by large herbivores can

influence plant productivity, soil nutrient availability and N cycling in tundra

ecosystems. This thesis brings evidence for a central role of herbivore dung

and urine in regulating plant community composition and ecosystem N

dynamics in both nutrient-poor and nutrient-rich tundra ecosystems where

herbivore density is high (Fig. 9). This work reveals that, although both dung

and urine provide highly decomposable resources to the system, these

resources differ in their availability to plants. It can take years before plants

can take advantages of the nutrients contained in dung which have to undergo

a long decomposition pathway, while nutrients in urine are readily available

to plant. Further, this thesis suggests a strong effect of dung and urine for

plant productivity by providing directly nutrient-rich resources for the system

rather than by stimulating soil microbial activities, N mineralization and

ultimately increasing soil nutrient availability. This more detailed

understanding of how nutrients derived from dung and urine are sequestered

by different ecosystem pools improve our understanding of nutrient cycling in

the Arctic. Still, there is a crucial need for further investigations on how

nutrients from dung and urine are entering the system, are transformed and

cycled by plant and soil microorganisms.

Figure 9 Key findings for the Paper I, II and III. Central role of herbivore dung and urine

and defoliation-induced changes in plant quality in regulating plant community

composition and ecosystem N dynamics.

26

Importantly, this work indicates that the increase in plant N contents

following defoliation reported for the graminoid F. ovina could drive the high

N cycling in arctic secondary grasslands. The mechanism is a provision of

litter of better quality, which enhances organic matter decomposition (Fig. 9).

Defoliation commonly not only changes plant nutrient status but also plant

chemical and structural mechanisms, such as the increase in the proportion

of C-based secondary metabolic (Coley et al. 1985) or the increase in silicon

uptake to increase leaf abrasiveness for graminoids (McNaughton and

Tarrants 1983). After leaf senescence, the concentration of such defence

compounds control litter decomposition rate. Exploring how different plant

functional groups respond to and tolerate aboveground herbivory is thus

highly important, if we want to better understand how herbivores control N

cycling by changing litter quality.

The chapters of this thesis reinforce a growing literature of a strong

positive effect of summer reindeer grazing on tundra ecosystem nutrient

cycling. For the first time, they reveal that reindeer do not only influence soil

nutrient availability, but also by which mechanisms plants acquire their

nutrients (Fig. 8). Intense reindeer grazing strongly reduced δ15N variation

between coexisting plant species. This indicates a reduced N partitioning on

the basis of chemical N forms, and might have strong implication for plant

coexistence and tundra ecosystem functioning. Further investigations are

needed to disentangle the driving mechanisms behind this reported decrease

in plant N portioning following defoliation. Particular knowledge gaps exist

regarding the specific importance of mycorrhizal mediated N uptake, the shift

toward a more mineral nutrition for plants and the effect of aboveground

herbivory on plant nutrient uptake capacity. The application of different

organic and inorganic labelled N compounds into tundra habitats of

contrasting grazing intensity could bring direct evidence of grazing induced

shift in plant N nutrition.

Finally, this thesis highlights that the effects of mosses can range from

negative to positive, depending on how they influence soil microclimate and

nutrient cycling. Arctic climate is currently changing at an unprecedented rate

(Serreze and Barry 2011) with major changes in temperatures and

precipitation. Consequences for plant species composition including a large

decrease in moss cover are therefore expected (Sorensen et al. 2012).

Understanding the effect of herbivore grazing in a changing arctic involves

integrating the effect of soil microclimate (temperature and water balance)

and considering the specific effect of mosses.

27

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Acknowledgments

I would like to first thank my supervisor, Johan Olofsson, for trusting me and

giving me the great opportunity to do my PhD studies on a so interesting

research area. Thanks for taking me to so many beautiful places far, far north.

I am grateful for all your valuable comments on my manuscripts and for your

help with statistical analysis. I am also very grateful to your supportive

attitude to new ideas and projects. Working with you has been sometimes

challenging but it also taught me to be imaginative, to develop good

organisation skills and, more importantly, to work independently.

I am also very grateful to Sari Stark. When I started my PhD, I was a student

passionate about arctic plant ecology, but little interest in the "things"

happening in the soil. Thank you so much for introducing me to the world of

soil processes and nutrient cycling and teaching me the importance of plant-

soil interactions! You were truly inspirational to me! Thanks for your patience

in the laboratory in Rovaniemi when teaching me soil extraction and diffusion

techniques. I learned so much from you!

I would also like to address many thanks to my two co-supervisors Ann Milbau

and Ellen Dorrepaal. Thanks for taking part in some of my PhD projects, for

your valuable comments on my ideas and all your practical advices. I only wish

we could have had much more interactions! Minna-Maarit Kytöviita, it was

fantastic to finally meeting you in Umeå after so many years of email

correspondences! It was a real pleasure to work with you and I am so grateful

for all your help with mycorrhizal analysis. I hope to be able to visit you

at Jyväskylä one day! Anders Michelsen, it has been great working with you in

Greenland and thanks for all your valuable comments on the manuscript.

This thesis would not have been possible without the fantastic help from all

the field and laboratory assistants. I am very grateful to Carine Le Piouf,

Andreas Hagenbo, Katharina Brinck, Nicki Leblans, Diana Santiago, Anna

Swärd, Jonas Forsberg, Aline Schneider and Jonas Gustafsson. Thanks to all

of you for the good company in the field, your great work under any kind of

weather and surviving with me all the mosquito attacks! I am grateful to all

the staff at the Abisko Scientific Research Station with special thanks to

Linnéa Wanhatalo and Noella Jonsson for always making me feel so welcome.

Carin Olofsson, what would I have done without your help for the preparation

of hundreds and hundreds of samples? I just don´t know! You precision and

35

your dedication in the work was amazing. Many thanks for overcoming many

methodological challenges with me! Niklas Nord, I am so grateful for your

excellent work with the root sorting, I think I will have become insane without

your help and good company in the laboratory!

I would like to thank the Department of Ecology and Environmental Science,

EMG. I have always felt welcome and everyone has been very helpful. A special

thanks to Ingrid Forsmark with all the help with the bureaucracy aspect of my

PhD, many thanks for your patience and your understanding! Thanks to

Mariska Te Beest and Dagmar Egelkraut for the nice company in the reindeer

research group.

I would like to address a special thanks to my two office mates and friends.

Elina Kaarlejärvi, thanks for sharing with me so many lunches, tea breaks and

interesting conversations. Thanks for having been there to listen and to

support me. Judith Sitters! Thanks for bringing so many positive and

energetic waves into the office! Thank you for introducing me the vegan world!

We definitely changed the way we eat at home after meeting you!

Joanna Paczkowska, Alessia Uboni, Judith Sarneel and Junwen Guo, what

would I be without you? You are my dear friends and my strongest support!

Thanks for walking up and down the hill with me and all your encouragement.

I am so grateful for all the wonderful time we had together in Umeå. Soon, we

will probably disperse over the world but you will always have a special place

in my heart! Maria Väisänen, thanks for making me feel so welcome during all

my visits to Rovaniemi. I am so grateful I could stay at your home and thanks

for all the delicious dinners you prepared. Thanks for the countless

conversions we had about science but also about gardening, sewing and

cooking. Thanks for being so supportive! I wish I could take you one day to the

French Alps where I spend so many of my summers... Tim Horskotte, I truly

appreciated your sweet and peaceful company during your time with us at

EMG. Many thanks for your helpful comments on this thesis summary.

I would also to thank all the nice colleagues/friends I had the chance to meet

at EMG, in particular Mehdi Cherif, Jon Moen, Tom Korsman, Reiner Giesler,

Jonatan Klaminder, Ulla Carlsson-Granér, Stig-Olof Holm, Svetlana Serikova,

Fidele Bognounou, Anne Deininger, Wojciech Uszko, Marcus Klaus, Christine

Gallampois, Gesche Blume-Werry, Toni Liess and Carolina Olid Gracia.

Thanks for always being very friendly with me and making my time rich and

36

memorable in Umeå. Valerie Hudon, thanks for all the lunches in the

mathematic department! I truly enjoyed a lot all our French session!

I would like to address a special thanks to all my non-academic friends.

Evelina Nilsson and Christer Andersson, I am very grateful for the great

friendship and min underbar Evelina, thanks for your great support and for

being you! Linnea is saying so often Edvin! Edvin! when she is home from

daycare! Soon the summer is coming and we can spend so much time again

all together! Jonas Forsberg and Susanne Persson thanks all your kindness

and your encouraging correspondences! Many thanks also to Johannes

Hedtjärn, Sara Forsell, Jenny Runesson, Zsuzsanna Holmgren, Marie Sion,

Carine Le Piouf, Fauhn Minvielle, Hanna Johansson and Arvid Lundberg,

Hadrien Lalagüe and Brigitte Santoni. I am so glad I have you all in my life!

Mom, dad and Cécile thanks for your tremendous support. Thanks for

believing in me and all your encouragement! I am so grateful for all the letters

and small packages you sent from Marseille, they helped to make the distance

smaller between us! Merci d'être venus si souvent nous aider avec Linnea. Je

vous aime tant! Simon, thanks for taking so good care of Linnea during the

last months of this thesis, to give me so many extra hours of sleep and time for

writing during the evenings and weekends, du är min hjälte! Thanks for

listening and all your patience with me. Linnea, thanks for giving me so much

love! I am so grateful I have you in my life! Simon, Linnea … you are my true

motivation and energy! Jag älskar er!