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Climatic and landscape influences on soil moisture areprimary determinants of soil carbon fluxes in seasonallysnow-covered forest ecosystems

Clare M. Stielstra • Kathleen A. Lohse • Jon Chorover •

Jennifer C. McIntosh • Greg A. Barron-Gafford •

Julia N. Perdrial • Marcy Litvak • Holly R. Barnard •

Paul D. Brooks

Received: 19 May 2014 / Accepted: 8 February 2015 / Published online: 20 February 2015

� Springer International Publishing Switzerland 2015

Abstract A changing climate has the potential to

mobilize soil carbon, shifting seasonally snow-covered,

forested ecosystems from carbon sinks to sources. To

determine the sensitivity of soil carbon fluxes to

changes in temperature and moisture, we quantified

seasonal and spatial variability of soil carbon dioxide

(CO2) fluxes (N = 746) and dissolved organic carbon

(DOC) in leachate (N = 260) in high-elevation, mixed

conifer forests in Arizona and New Mexico. All sites

have cold winters, warm summers, and bimodal soil

moisture patterns associated with snowmelt and

summer monsoon rainfall. We employed a state factor

approach, quantifying how distal controls (parent

material, regional climate, topography) interacted with

proximal variability in soil temperature (-3 to 26 �C)

and moisture (2–76 %) to influence carbon effluxes.

Carbon loss was dominated by CO2 flux (250–1220 g

C m-2 year-1) rather than leached DOC (7.0–9.4 g C

m-2 year-1). Significant differences in mean growing

season CO2 flux were associated with parent material

and aspect; differences appear to be mediated by how

these distal controls influence primarily moisture and

secondarily temperature. Across all sites, a multiple

linear regression model (MLR) relying on moisture and

temperature best described growing season CO2 fluxes

(r2 = 0.63, p \ 0.001). During winter, the MLR

Responsible Editor: Melany Fisk.

C. M. Stielstra � J. C. McIntosh

Department of Hydrology and Water Resources,

University of Arizona, Tucson, AZ, USA

K. A. Lohse

Department of Biological Sciences, Idaho State

University, Pocatello, ID, USA

J. Chorover

Department of Soil, Water and Environmental Science,

University of Arizona, Tucson, AZ, USA

G. A. Barron-Gafford

School of Geography & Development, University of

Arizona, Tucson, AZ, USA

G. A. Barron-Gafford

B2 Earthscience, Biosphere 2, University of Arizona,

Tucson, AZ, USA

J. N. Perdrial

Department of Geology, University of Vermont,

Burlington, VT, USA

M. Litvak

Department of Biological Science, University of New

Mexico, Albuquerque, NM, USA

H. R. Barnard

Department of Geography and INSTAAR, University of

Colorado, Boulder, CO, USA

P. D. Brooks (&)

Department of Geology/Geophysics, University of Utah,

115 So. 1460 East, Salt Lake City, UT 84112-0102, USA

e-mail: [email protected]

123

Biogeochemistry (2015) 123:447–465

DOI 10.1007/s10533-015-0078-3

describing soil CO2 flux (r2 = 0.98, p \ 0.001) relied

on distal factors including snow cover, clay content, and

bulk carbon, all factors that influence liquid water

content. Our findings highlight the importance of state

factors in controlling soil respiration primarily through

influencing spatial and temporal heterogeneity in soil

moisture.

Keywords Carbon cycle � Climate � Forests � Soil

moisture � Soil respiration

Introduction

Soils are the largest terrestrial carbon pool, represent-

ing a long-term repository of organic carbon input

from vegetation (Davidson and Janssens 2006).

Seasonally snow-covered, mid-latitude soils contain

20–30 % of global organic carbon stores (Jobbagy and

Jackson 2000; Post et al. 1982; Schimel et al. 2001)

and are currently a significant sink for atmospheric

CO2 (Schimel et al. 2002). It is unclear, however, how

soil carbon in these systems will respond to changes in

climate that simultaneously alter temperature and

water availability.

Soil organic carbon (SOC) is stabilized via

chemical (e.g., inherent recalcitrance, bonding inter-

actions with surfaces and metals) and physical

(occlusion within aggregates) mechanisms, both of

which limit biological degradation so that residence

time of SOC may vary from days to 1000s of years

(Conant et al. 2011; Gaudinski et al. 2000; Torn et al.

1997; Trumbore 1993). Stabilization mechanisms are

dependent on soil mineral composition and texture

(Von Lutzow et al. 2008), making parent rock type a

potentially key control on subsequent carbon losses for

a given climatic forcing (Heckman et al. 2009). In the

absence of erosion and mass wasting, soil carbon can

be lost via two primary pathways: gaseous fluxes and

leaching of dissolved carbon. Reported dissolved

carbon (DOC) fluxes from surface soils vary in

magnitude from 5 to 84 g C m-2 year-1 (Jones and

Mulholland 1998; Kindler et al. 2011; Neff and Asner

2001) and are related to soil carbon content, hydro-

logic flowpaths, microbial activity, and soil distur-

bance (e.g. Bishop et al. 2004; Hope et al. 1994, 1997;

Brooks et al. 1999; Haei et al. 2010). Gaseous soil C

losses, predominately as CO2, typically are much

larger than dissolved C fluxes, ranging from 60 to

1260 g C m-2 year-1 (Raich and Schlesinger 1992),

and account for up to 80 % of total ecosystem

respiration (Barron-Gafford et al. 2011; Janssens

et al. 2001; Law et al. 1999; Milyukova et al. 2002).

Growing season soil respiration is strongly related to

direct controls of temperature, moisture, and substrate

concentration (Raich and Schlesinger 1992) with

perhaps moisture being the least well quantified. A

growing body of recent work, however, has focused on

how precipitation, soil characteristics, and landscape

position interact to influence soil moisture and thus

soil respiration (e.g., Cable et al. 2008; Davidson and

Janssens 2006; Harrysson Drotz et al. 2010; Huxman

et al. 2004b; Kang et al. 2003; 2005; Kieft et al. 1987,

1998; Kurc and Small 2007; Lee et al. 2002; McCulley

et al. 2007; Pacific et al. 2011; Potts et al. 2006; Raich

and Potter 1995; Riveros-Iregui and McGlynn 2009;

Van Gestel 1991; Wood et al. 2013).

Seasonally snow-covered ecosystems are particu-

larly vulnerable to changing climate, as small varia-

tions in climate can produce large changes in snow

cover, soil temperature, soil moisture, and soil frost

(Hamlet et al. 2005, 2007; Harpold et al. 2012; Lemke

et al. 2007; Regonda et al. 2005). A recent review and

meta-analysis (Blankinship and Hart 2012) suggests

that winter and summer biogeochemistry are inter-

twined with experimentally manipulated changes in

winter snowpack associated with both the timing and

magnitude of summer CO2 efflux. Furthermore, the

response of summer CO2 fluxes early in the growing

season to snow cover manipulation the previous winter

varied due to local climate, with warmer sites more

likely to exhibit an increase in efflux. These observa-

tions highlight the need for consistent observations of

CO2 flux that span the range of temperature and soil

moisture associated with a given site (Molotch et al.

2009; Brooks et al. 2011).

Recent observations suggest that both growing

season and winter soil respiration will be strongly

related to liquid water availability, which is closely

linked to precipitation. Some regions in the western

US already have experienced decreases in snow cover

(Mote et al. 2005), higher ablation rates, and decreased

duration of winter snow cover (Harpold et al. 2012).

Shorter snow-covered seasons and transitions from

continuous to intermittent snow cover at lower

latitudes and elevations lead to greater ecosystem

reliance of soil moisture on summer rainfall (Molotch

448 Biogeochemistry (2015) 123:447–465

123

et al. 2009; Hayhoe et al. 2006, 2007; Huntington et al.

2009; Harpold et al. 2012; Lundquist et al. 2008).

Although a shorter snow-covered season results in a

longer growing season, it may result in reduced net

carbon uptake during the growing season (Hu et al.

2010; Monson et al. 2005). Conversely, reduced snow

cover in the previous winter may increase CO2 flux the

following spring, primarily at warmer sites (Blankin-

ship and Hart 2012), because summer rain wets

surface soils without recharging deeper water stores

(Molotch et al. 2009). Hence, a greater proportion of

precipitation falling in summer may increase soil

respiration without alleviating plant water stress

(Curiel-Yuste et al. 2003; Fierer and Schimel 2002;

Huxman et al. 2004a; Inglima et al. 2009; Miller et al.

2005). This pattern has been observed in semiarid

ecosystems, in which increased summer rains stimu-

late net primary production (NPP) (Scott et al. 2009),

but also significantly increase soil respiratory loss of

CO2 to the atmosphere (Barron-Gafford et al. 2011).

Importantly, the seasonality of precipitation controls

surface soil moisture and is essential in driving these

inter-annual patterns of carbon flux. In contrast, soil

CO2 fluxes in more mesic systems may be limited by

high soil moisture that not only limits the gaseous

transport of CO2 from soil to atmosphere, but also

diminishes O2 availability for heterotrophic oxidation

(Wood et al. 2013).

Winter soil respiration, which releases back to the

atmosphere up to 50 % of C fixed during the previous

growing season, is similarly sensitive to climate-

driven changes in available soil moisture (Brooks et al.

2005; Monson et al. 2005). Shallower snow cover and

a shorter snow-covered season result in increased soil

frost that effectively reduces liquid water availability

(Campbell et al. 2010; Decker et al. 2003; Dyer and

Mote 2006; Groffman et al. 2001a; Hardy et al. 2001;

Henry 2007; Isard and Schaetzl 1998; Oquist and

Laudon 2008; Venalainen et al. 2001). The subsequent

effects of soil frost on CO2 flux are complex. For

example, frequent soil frost can increase winter CO2

flux if followed by consistent snow cover (Brooks

et al. 1997, 1999, 2011). However, if the subsequent

snow pack is ephemeral and soils remain frozen, both

CO2 and DOC fluxes are reduced (Brooks et al. 1997,

1999, 2011; Groffman et al. 2001a). Projected

increases in soil frost (Hayhoe et al. 2007), decreases

in winter precipitation, and snowmelt-derived soil

moisture (Collins et al. 2013) highlight the need to

quantify the interactions between climate and land-

scape that influence soil moisture, carbon storage, and

soil carbon flux.

These climate and landscape mediated interactions

can be framed and understood within the context of the

state factor model (Jenny 1941). The major factors

determining the spatiotemporal variation in soil

carbon fluxes in ecosystems range from proximate

controls that determine seasonal variation in carbon

flux to more distal controls, or state factors, that result

in ecosystem variation in carbon fluxes (Chapin and

Matson 2012). In particular, climate, biota, and parent

material are thought to be the primary determinants of

carbon fluxes mainly through interacting controls on

carbon quantity and quality inputs and secondarily

through proximal controls on soil water and tempera-

ture (Chapin and Matson 2012). However, there is an

emerging consensus that these other factors need

further consideration to understand and predict carbon

fluxes and storage (Conant et al. 2011; Schmidt et al.

2011). Indeed, it is currently thought that landscape

mediated environmental variables such as soil moist-

ure and temperature and other physical processes

(physical protection, freeze–thaw, wetting–drying)

may be as important or more important in determining

soil carbon fluxes and storage than the distal landscape

and climate characteristics that have resulted in recent

carbon inputs and losses. Thus, despite advances in

understanding the controls on soil fluxes, the relative

importance of changes in soil moisture versus

temperature in mobilizing C from near-surface soils

remains unclear.

To address this knowledge gap, we quantified soil

CO2 and DOC fluxes in four mixed conifer forests in

Arizona and New Mexico. Specifically, we asked two

questions: How do soil carbon fluxes vary across

physical landscape characteristics in broadly similar

forested ecosystems? What climatic and landscape

characteristics have predictive capacity to identify the

dominant carbon loss pathways? We used a modified

state factor approach and contrasted montane forest

ecosystems that varied in climate. Nested within this

framework, we examined the relative importance of

parent material and topographic controls such as

aspect in driving variation in carbon fluxes. The

variability in local and regional climate, soils, and

landscape associated with the montane study sites

provided observations that spanned soil temperatures

from -3 to 26 �C, and gravimetric soil moisture

Biogeochemistry (2015) 123:447–465 449

123

contents from 2 to 76 %, allowing a cross-site

comparison of how temperature and moisture influ-

enced soil carbon fluxes in seasonally snow-covered

forest ecosystems.

Study sites

We conducted our study in four subalpine forests that

are part of the Catalina-Jemez Critical Zone Observa-

tory (JRB-SCM-CZO, http://criticalzone.org/catalina-

jemez/, Chorover et al. 2011), located in the Santa

Catalina Mountains in southern Arizona (AZ) and

within the Valles Caldera National Preserve in north-

ern New Mexico (NM, Fig. 1). All study areas have

distinctly bimodal precipitation and soil moisture

regimes, with winter snowfall (November to April), a

dry early summer with minimal precipitation (May

and June) and summer rainfall associated with the

North American Monsoon (NAM) (July–September).

Annual precipitation at the AZ sites ranges from 650 to

940 mm and air temperature ranges from -5 �C in

winter to 32 �C in summer, with an annual mean of

11 �C (Brown-Mitic et al. 2007; Pelletier and Ras-

mussen 2009). Annual precipitation at the NM sites

ranges from 600 to 1021 mm (Liu et al. 2008; Zapata-

Rios et al. 2012), and air temperature ranges from

-15 �C in winter to 25 �C in summer (annual mean

4 �C) (Broxton et al. 2009). Although the seasonal

precipitation patterns are similar, the NM sites typi-

cally have continual snow cover during winter months

(Gustafson et al. 2010), whereas the AZ sites typically

have ephemeral snow cover that may accumulate and

ablate several times over the winter.

The two AZ sites are located in the Marshall Gulch

catchment, a 1.6 km2 basin located near the crest of

the Santa Catalina Mountains, and comprise similar

vegetation, but contrasting parent materials and soil

types. One site is underlain by Precambrian and

Tertiary aged granite and granodiorite that weathers to

produce a coarse-grained soil, while the other is

underlain by Paleozoic aged metamorphic rocks of

schist and quartzite that weather to yield a finer-

grained soil (Pelletier and Rasmussen 2009, Table 1).

Fig. 1 JRB-SCM CZO study sites in Arizona and New Mexico with a sites of contrasting parent material (granite and schist) at

Marshall Gulch, AZ and b sites of differing aspect as part of the zero-order basin in the Valles Caldera, NM


123

http://criticalzone.org/catalina-jemez/

http://criticalzone.org/catalina-jemez/

The weathering profile is deeper in the schist soils than

the granitic soils, with lower hydraulic conductivity,

higher moisture retention capacity for a given matric

potential, and longer water residence times (Heidbu-

chel et al. 2012). Both sites are located between 2340

and 2400 m above sea level (a.s.l), with northeast

aspects and slopes ranging from 15� to 25�. The

vegetation is a mixed conifer forest consisting of

Douglas-fir (Pseudotsuga menziesii), white fir (Abies

concolor), ponderosa pine (Pinus ponderosa), white

pine (Pinus strobiforma), scrub oak (Quercus gambe-

lii) and bigtooth maple (Acer grandidentatum) (Pel-

letier and Rasmussen 2009).

The two NM sites are within the Valles Caldera

National Preserve and situated, respectively, on

northeast (NE) and southwest (SW) aspects of a

catchment draining Redondo Peak, a 3,431 m resur-

gent dome within the caldera. The caldera collapsed

ca. 1.2 million years ago and subsequently filled with

ash and fragments of megabreccia from the crater

walls (Goff and Gardner 1994; Heiken et al. 1986).

The parent material is rhyolitic tuff with occasional

sandstone megabreccia, which weathers to produce

deep, fine-grained, well-drained soils. Compared to

the AZ sites, the NM sites are at higher elevation

(3020–3040 m a.s.l.), with slopes ranging from 3� to

12� (Table 1). Study sites are characterized by mixed

conifer forest consisting of Douglas fir (Pseudotsuga

menziesii), white fir (Abies concolor), blue spruce

(Picea pungens), corkbark fir (Abies lasiocarpa var.

arizonica), Englemann’s spruce (Picea englemannii)

and poplar (Populus tremuloides) (Parmenter et al.

2007). All four sites are located on hillslopes away

from convergent zones and riparian areas.

Methods

We established three sites during the winter of

2009–2010, two in Arizona and one in New Mexico,

and measured soil carbon fluxes for 2 years. A fourth

site was added in New Mexico during the winter of

2010–2011 and fluxes were quantified for 1 year from

this location. At each of these four sites, we

established either three (NM) or five (AZ) 1 m2

experimental plots to measure seasonal CO2 fluxes and

environmental controls on these fluxes (soil moisture

and temperature). The smaller number of plots in NM

was due to logistical concerns associated with access

and sampling all sites within a single day. On each

sampling date we collected three CO2 flux measure-

ments per plot yielding nine to fifteen observations per

date. CO2 fluxes were calculated only when concen-

trations at the ground surface were significantly

different than in free air at 2 m (Brooks et al. 1999;

2005). At each plot, three soil cores 7–10 cm deep and

4 cm in diameter were collected before and after each

season (before snowmelt, after snowmelt, before onset

of NAM, after NAM) and stored cool (\4 �C) until

arrival in the lab. Within 48 h of collection, field-moist

soil samples were sieved to 2 mm and a subset of soil

samples were weighed before and after drying at

105 �C to determine gravimetric soil moisture.

Continuous observations of soil temperature were

obtained from a met station at each site. Additional

observations were made manually at individual plots

during sample collection using a thermometer inserted

approximately 5 cm under the soil surface. Manual

observations were strongly related to continuous

observations from met stations (R2 [ 0.9, p \ 0.01).

Table 1 Physical characteristics of the O/A horizon (0–10 cm depth) by site: bedrock and soil characteristics (bedrock, % bulk C, %

sand, % silt, and % clay) and topographic characteristics [elevation, slope, aspect and topographic contribution index (TCI)]

Site Geology and soil characteristics Topographic characteristics

Bedrock Bulk C (%) Sand (%) Silt (%) Clay (%) Elev. (m) Slope (�) Aspect TCI

Santa Catalina Mountains (AZ)

Granite Granite 6.8 ± 0.3 56.7 36.4 6.9 2396 18.2 32.9 7.15

Schist Schist 8.1 ± 0.4 51.6 39.8 8.6 2349 24.2 30.3 7.5

Jemez River Basin (NM)

SW-aspect Rhyolite 11.4 ± 1.4 45.6 37.4 17 3027 4.9 222.8 15.46

NE-aspect Rhyolite 7.9 ± 1.2 39.2 35.8 25 3038 9.2 110.3 8.95

Uncertainties represent the SE of the mean


123

At the AZ sites, precipitation and air temperature data

were collected using a RAINEW 111 Tipping Bucket

Wired Rain Gauges and a HOBO U20-001-01 probe

and datalogger. Snow depth was measured manually

during each site visit by inserting a graduated snow

probe vertically to the ground to assess spatial

heterogeneity across; continuous records (at 5-min

intervals) of snow depth and duration of snow cover

were measured with a Judd Ultrasonic Depth Sensor

(Judd Communications LLC, Salt Lake City, UT)

mounted 1.5 m off the ground at a meteorological

station located approximately 3 km from Marshall

Gulch at similar elevation. For the NM sites, pre-

cipitation (tipping buckets and Geonor all weather

precipitation gage), air temperature (HMP45C Vai-

sala, Helsinki, Finland), and continuous snow depth

and duration data (Judd ultrasonic snow depth sensor)

were monitored at a meteorological station adjacent to

the NE site.

Soil CO2 fluxes were measured in situ using a

portable infrared gas analyzer, PP systems soil respi-

ration system, with an environmental gas monitor

(EGM-4, PP Systems, Hertfordshire, UK). During

snow-free periods, the EGM-4 was attached to an

SRC-1 Soil Respiration Chamber (Riveros-Iregui and

McGlynn 2009). During snow-covered periods, CO2

fluxes were estimated by measuring the vertical

concentration gradient of CO2 through the snowpack

using a probe attachment with the portable gas

monitor. The gradient was then used to calculate flux

through the snowpack based on a steady state diffusion

model (Brooks et al. 1999):

JCO2¼ DCO2

d CCO2½ �

dz

� �f ; ðð1ÞÞ

where JCO2is the flux, DCO2

is a diffusion coefficient, z

is the depth of the snowpack, and f is the porosity of the

snowpack. The diffusion coefficient DCO2was as-

sumed to be 0.139 for CO2 gas, consistent with the

literature (e.g., Brooks et al. 1996, 1999; Perry et al.

1963; Sommerfeld et al. 1993). Porosity was estimated

as the inverse of snowpack density. Molar gas volumes

were corrected for local temperature and pressure

conditions. Although advective transport due to

pressure pumping may increase fluxes 10–20 % over

diffusion alone (Bowling et al. 2001, 2002; Massman

and Lee 2002; Massman et al. 1997), the steady state

model allows a consistent, conservative estimate of

winter gas flux across a wide concentration range and

wide number of sites.

Edaphic controls on CO2 fluxes, including DOC, total

soil carbon (%) and bulk density were measured

seasonally. Soil exchangeable DOC was measured using

ion exchange resin traps following protocols described

by Brooks et al. (1999). Specifically, ion exchange resin

traps (three per plot, 9–15 per site) were installed at

10 cm depth prior to the onset of each precipitation

season to capture DOC leached from surface soils (O/A

soil horizon). The traps consisted of *20 cm3 of mixed

anion and cation exchange resins (J.T. Baker, IONAC

NM-60 H?/OH- Form, 16–50 Mesh) sewn into acid-

washed nylon bags and secured within 1.5 cm-diameter

PVC pipe open at the top and bottom to collect leachate

from overlying soils. Areal DOC fluxes were calculated

using the open area of the 1.5 cm collection pipe. Traps

were recovered at the end of each season and processed

in the laboratory where they were air dried and extracted

with 2 M KCl (1:5, weight:volume). Extracts were

filtered through combusted Whatman GF/F 0.7 lm glass

fiber filters and stored at 4 �C until analyzed for non-

purgable organic carbon (NPOC) by high temperature

combustion followed by CO2 quantification by non-

dispersive infrared analysis (Shimadzu TOC-Vcsh/

TNM-1, Columbia, MD). A subset of soil was dried

(60 �C), ground, packed in tins, and sent for total carbon

(C) analysis on a PDZ Europa ANCA-GSL elemental

analyzer interfaced to a PDZ Europa 20-20 isotope ratio

mass spectrometer (Sercon Ltd., Cheshire, UK) at the

UC Davis Stable Isotope Facility. Finally, one of the

replicated cores was used to determine soil bulk density

following a modified excavation method (Grossman and

Reinsch 2002). In brief, soils of known volume were

collected from the field, dried to determine the dry total

mass, and then sieved to 2 mm to determine the mass of

the fine and coarse fraction.

Statistical analyses were performed using JMP 9

software (SAS System, 2007). There were no consistent

differences between CO2 flux observations at individual

plots within sites, so all observations from a site were

grouped. To avoid potential artifacts arising from

uneven sample sizes and the recognized difficulties in

objectively identifying fixed versus random effects

(Bennington and Thayne 1994), we used the non-

parametric Wilcoxan/Kruskal–Wallis (rank sums) test

to evaluate mean seasonal differences between sites.

The growing season was defined as beginning with the


123

onset of snowmelt—since plants respond rapidly as

soon as liquid water becomes available (Monson et al.

2005)—and ending with the accumulation of the next

winter’s snowpack. For multiple linear regression

analyses, CO2 flux data were log transformed to meet

assumptions of normality and heteroscedasticity. For

the transformation, we added one to all flux values to

retain zero flux values. We used a step-wise multiple

linear regression model to predict gaseous flux from

measured parameters (soil temperature, soil moisture,

leached DOC, bulk soil C, and soil clay content), their

interactions, and snow cover (ephemeral or continu-

ous). We accepted models with the highest adjusted r2

and lowest Akaike information criterion (AIC).

Results

We obtained 746 individual observations of soil CO2

flux and 260 observations of DOC leachate across our

sites in AZ and NM. CO2 fluxes were large and

variable in both space and time relative to those for

DOC leachate. Daily CO2 fluxes ranged from below

detection limits to 12 g C m-2 day-1, spanning a

range of soil temperatures from -3 to 26 �C, and

gravimetric soil moisture from 2 to 76 % (Figs. 2, 3).

Climatic and landscape characteristics

Both sites were subject to a relatively wet year in 2010

and a dry year in 2011, with NM sites receiving

approximately 100 mm more precipitation each year

than AZ sites (Table 2). Total precipitation at the AZ

sites was 813 mm in 2010 and 461 mm in 2011, and a

greater proportion of the annual precipitation fell as

snow in 2010 (39 %) than in 2011 (11 %). Continuous

snow cover in 2010 lasted 139 d (1 Dec 2009–18 April

2010). In contrast, the snow season lasted only 65 days

(29 Dec 2010–3 March 2011) during 2011 with an

ephemeral snow cover developing and ablating

throughout the season. Growing season rainfall was

slightly higher in 2010 than 2011, with the monsoon

arriving later in 2011, resulting in a longer early-

summer dry season. The NM sites also received more

total annual precipitation during 2010 than 2011 (928

compared to 546 mm). Snow cover lasted 143 days (7

Dec 2009–28 April 2010) in 2010, and 103 d (16 Dec

2010–28 March 2011) in 2011, and both winters had

continuous snow cover. In contrast to AZ, more rain

fell during the growing season in NM during 2010

(635 mm) than during 2011 (320 mm).

Soil temperatures concurrent with CO2 flux mea-

surements ranged from -3 to 26 �C in AZ and -3 to

12 �C in NM. In AZ, no significant difference in soil

temperature was observed between growing season

2010 and 2011 (F = 1.37, p = 0.24), and probe

failures during winter 2010 resulted in inadequate

data for comparison between years. In NM, soil

temperatures were significantly warmer during grow-

ing season 2011 than 2010 (F = 13.52, p \ 0.001) but

no significant difference between winter 2010 and

2011 was found (F = 3.42, p = 0.066).

Soil moisture during the growing season in AZ was

significantly higher in 2010 compared to 2011

(F = 39.62, p \ 0.001) (Table 3). Furthermore, soil

moisture was significantly higher in schist soils than

granitic soils during both the growing season

(F = 6.12, p = 0.016) and winter (F = 10.84,

p = 0.004) of the wet year (2010). In contrast, during

the drier year (2011), there was no significant

difference between granite and schist soils during the

growing season (F = 1.04, p = 0.312) or during

winter (F = 2.76, p = 0.116). Both granite and schist

soils were significantly wetter during winter 2011 than

winter 2010 (F = 18.34, p \ 0.001, and F = 10.76,

p = 0.004, respectively). In NM, there was no

significant difference in growing season soil moisture

between 2010 and 2011 (F = 0.87, p = 0.35). How-

ever, soils on northeast aspects had significantly

higher soil moisture than soils on southwest aspects

during the growing season 2011 (F = 29.18,

p \ 0.001). No significant effect of aspect on soil

moisture in NM sites was observed for winter 2011

(F = 0.46, p = 0.511).

Total organic C measured in soil core samples was

significantly lower at the AZ sites (7.5 % ± 0.27)

compared to the NM sites (9.7 % ± 1.0, F = 4.60,

p = 0.039). In AZ, schist soils had significantly higher

soil organic C than granite soils (Table 1, F = 8.54,

p = 0.01). At the NM sites there was no significant

difference in total soil organic C between aspects

(Table 1, F = 3.69, p = 0.073).

Seasonal and landscape patterns of carbon flux

The primary determinant of winter CO2 flux at all sites

was the presence or absence of consistent snow cover.

With relatively deep and early snow cover during


123

winter 2010, CO2 fluxes were not significantly

different (F = 0.50, p = 0.48) between AZ sites

(0.57 ± 0.02 g C m-2 day-1) and NM sites

(0.50 ± 0.01 g C m-2 day-1) (Fig. 2; Table 3). There

were also no significant differences in fluxes between

granite and schist soils during winter 2010 (F = 0.002,

p = 0.96) or 2011 (F = 0.27, p = 0.61). During

winter 2011, CO2 flux under inconsistent snow cover

in AZ dropped to less than 0.1 g C m-2 day-1 and was

significantly lower than fluxes from consistently snow-

covered sites in NM (F = 10.28, p = 0.002) as well as

under continuous snow cover in AZ during winter 2010

from both granite (F = 27.13, p \ 0.001) and schist

soils (F = 36.09, p \ 0.001). In contrast to the lower

CO2 fluxes observed in AZ during the low snow winter

of 2011, fluxes from NM sites were significantly higher

during winter 2011 than winter 2010 (F = 53.34,

p \ 0.001). Expanded NM sampling in winter 2011

identified slightly higher (F = 4.75, p = 0.036) fluxes

on SW aspects (mean 1.8 ± 0.16 g C m-2 day-1) than

NE aspects (1.6 ± 0.33 g C m-2 day-1).

Fig. 2 Precipitation, air temperature, snow cover, and 746 individual CO2 flux for AZ and NM sites for water years 2010 and 2011

Fig. 3 Measured CO2 fluxes (N = 746) from sites in AZ and

NM show no clear relationship between with soil temperature

either in winter or during the growing season


123

Growing season CO2 fluxes remained low at all

sites until monsoon rains began and then increased

markedly (Fig. 2). There was no significant difference

in growing season CO2 fluxes (F = 0.007, p = 0.93)

between the AZ (2.4 ± 0.18 g C m-2 day-1) and NM

(2.2 ± 0.31 g C m-2 day-1) sites in 2010. Within the

AZ sites, however, schist-derived soils had signifi-

cantly higher fluxes (3.3 ± 0.30 g C m-2 day-1) than

granite soils (1.6 ± 0.18 g C m-2 day-1, F = 10.58,

p = 0.001) (Table 3). During 2011, mean growing

season fluxes in AZ sites (1.0 ± 0.10 g C m-2 day-1)

were significantly (F = 16.21, p B 0.001) lower than

the previous year (2.4 ± 0.18 g C m-2 day-1). Unlike

the previous year, there was no significant difference

in fluxes between soil types (F = 0.08, p = 0.783),

presumably due to similar soil moisture conditions

(Table 3). Similar to AZ, the drier 2011 growing

season in NM resulted in lower CO2 fluxes (F = 7.75,

p = 0.006) on the SW aspect in 2011 (1.60 ± 0.24 g

C m-2 day-1) than in 2010 (2.25 ± 0.31 g C

m-2 day-1). Fluxes measured in 2011 at the wetter

sites on NE aspect (2.8 ± 0.30 g C m-2 day-1) were

significantly higher (F = 22.40, p \ 0.001) than on

the SW aspect (1.6 ± 0.24 g C m-2 day-1) and

similar to fluxes observed at all sites in 2010 (Table 3).

Relative to the large and variable CO2 fluxes, soil

DOC leaching losses were over an order of magnitude

smaller at all sites during both years (Table 4). DOC

fluxes at the AZ sites were significantly higher during

the growing season (4.30 ± 0.27 g C m-2) than in the

winter/spring (3.4 ± 0.11 g C m-2; F = 7.70,

p = 0.007). Average seasonal DOC fluxes ranged

from 3.2 ± 0.21 to 4.6 ± 0.68 g C m-2 season-1,

and there was no significant difference between mean

DOC fluxes for 2010 and 2011 despite dramatic inter-

annual variability in the amount of precipitation

(F = 1.33, p = 0.251). There was also no significant

difference in DOC fluxes between soil types in AZ

(F = 0.03, p = 0.866). Similar to AZ, mean DOC flux

values at the NM sites were significantly higher during

the growing season (4.6 ± 0.28 g C m-2) compared

to winter (2.9 ± 0.23 g C m-2; F = 20.47,

p \ 0.001), but there were no significant differences

in DOC with aspect (F = 0.77, p = 0.386). Summer

seasonal DOC fluxes, but not winter or annual DOC

fluxes, were correlated (albeit weakly) to summer CO2

fluxes (r2 = 0.17, p = 0.010).

Proximal versus distal predictors of carbon flux

There was no clear relationship between CO2 flux and

temperature for either soil type in AZ during the

growing season (r2 B 0.04, p C 0.41) (Fig. 4a).

However, a significant, positive relationship between

CO2 flux and gravimetric soil moisture was observed

for both soil types (Fig. 4b), explaining 42 % of the

variance in CO2 flux from granite soils (p = 0.003)

and 88 % of the variance in CO2 flux from schist soils

(p \ 0.001). In contrast, however, the relationship

between CO2 flux and soil temperature was significant

(r2 0.52–0.60, p \ 0.001) at the cooler NM sites. Also

in contrast to the AZ sites, there was no significant

relationship between CO2 flux and gravimetric soil

moisture in the NM sites (r2 B 0.10, p C 0.40)

(Fig. 4d). Although growing season respiration

responded more strongly to temperature at the NM

sites and moisture at the AZ sites, a multiple linear

regression (MLR) model including soil moisture, soil

temperature, and leached DOC explained 63 % of the

variability in CO2 flux for the summer fluxes across

sites and years (Fig. 6a; Table 5).

Large differences in winter CO2 fluxes were observed

between ephemerally snow-covered sites (AZ, 2011)

and continuously snow-covered sites (AZ, 2010; NM,

both years) (Fig. 5). Winter CO2 fluxes did not increase

with temperature at any site (Fig. 5a), but were

significantly related to soil moisture at continuously

snow-covered sites (r2 = 0.61, p = 0.005, Fig. 5b).

Table 2 Precipitation totals (in mm) and proportions of seasonal precipitation relative to total annual precipitation

Marshall Gulch, AZ Valles Caldera, NM

2010 2011 2010 2011

Winter 315 (39 %) 52 (11 %) 293 (32 %) 226 (41 %)

Summer 498 (61 %) 409 (89 %) 635 (68 %) 320 (59 %)

Total 813 461 928 546


123

Across all sites and years, a single MLR model for winter

fluxes that included snow cover (ephemeral or contin-

uous), gravimetric water content, soil clay content, the

interaction between gravimetric water content and soil

clay content, and bulk soil carbon explained 98 % of the

variation in winter CO2 fluxes (Fig. 6b; Table 5).

Discussion

The combination of inter-annual climatic variability

between 2010 and 2011, regional climatic differences

between the NM and AZ sites, local aspect, and soil

characteristics provided a wide range of state factors

for evaluating the interactions between soil moisture

and temperature on seasonal soil DOC and CO2 fluxes

(Figs. 3, 4). DOC fluxes were relatively small and

within the published range of values for DOC leached

from surface soils (Neff and Asner 2001, Brooks et al.

1999) and so we focus our discussion on CO2 flux.

Across all observations, soil moisture exhibited the

greatest predictive in our MLR model suggesting that

quantifying the spatial and temporal dynamics of

water availability will be critical for predicting the

response of soil carbon to climate change.

Seasonal and landscape patterns: growing season

carbon fluxes

Growing season soil CO2 fluxes were within the range of

values reported in other recent studies (e.g., Barron-

Gafford et al. 2011; Liptzin et al. 2009; Riveros-Iregui

and McGlynn 2009). Similar to other studies of soil

respiration under seasonally dry conditions, fluxes

during the growing season but prior to summer rains

were low suggesting water limitation (e.g., Curiel-Yuste

et al. 2003; Inglima et al. 2009). This was especially

apparent during the summer of 2011 when soil moisture

at both sites was lowest following the dry winter

(Table 3). The large increases in CO2 flux following

monsoon rains in both AZ and NM (Fig. 2) were

consistent with pulsed respiration response observed in

lower elevation, drier, and warmer ecosystems (e.g.,

Huxman et al. 2004b; Lee et al. 2002; Potts et al. 2006).

These data support the expectation that an increase in

precipitation to warm soils results in increased soil

respiration even in high elevation, seasonally snow-

covered forests where primary productivity is tempera-

ture limited (Anderson-Teixeira et al. 2011). ThisTa

ble

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123

overriding importance of soil moisture in controlling

soil respiration is in contrast to observations in more

mesic sites. In the current study, CO2 flux exhibits a

quasi-parabolic relationship to growing season soil

temperature (Fig. 3), similar to the relationship

observed between CO2 flux and soil moisture at warmer,

wetter locations (e.g. Wood et al. 2013).

Soil physical characteristics also exerted control

over CO2 flux, primarily through their capacity to

modulate water availability. For example, we observed

higher soil moisture and higher CO2 fluxes from fine,

schist soils relative to coarse granitic soils (Table 3),

consistent with other studies (e.g., Gupta and Larson

1979; Vereecken et al. 1989). The absence of a

relationship between soil temperature and CO2 flux at

these sites suggests that the higher CO2 fluxes from the

schist soils are not due to a temperature response of

plant roots or microbes, but rather the result of higher

soil moisture influencing both autotrophic and hetero-

trophic activity. Soil textural controls on soil moisture

and CO2 fluxes are consistent with numerous studies

showing higher respiration rates from fine-textured soil

compared to coarse textured ones following rainfall

pulses (Cable et al. 2008; Kieft et al. 1987, 1998;

McCulley et al. 2007; Raich and Potter 1995; Van

Gestel 1991). Collectively, these observations support

Table 4 Estimated seasonal losses as soil CO2 and DOC (g m-2 season-1)

Site Location Soil CO2 fluxes (g m-2 season-1) DOC leached (g m-2 season-1)

Winter

2010

Summer

2010

Winter

2011

Summer

2011

Winter

2010

Summer

2010

Winter

2011

Summer

2011

AZ Granite 79.5 ± 4.2 414.0 ± 45.7 3.3 ± 1.3 224.1 ± 35.9 3.8 ± 0.4 4.6 ± 0.7 3.2 ± 0.2 3.8 ± 0.4

Schist 79.2 ± 4.2 835.7 ± 76.2 5.9 ± 2.0 298.8 ± 37.4 4.3 ± 0.4 4.2 ± 0.4 3.4 ± 0.3 4.3 ± 0.4

NM SW 71.5 ± 1.4 519.8 ± 71.6 185.4 ± 16.5 337.6 ± 50.6 5.5 ± 0.5 3.9 ± 0.3 2.9 ± 0.4 5.5 ± 0.5

NE N/A N/A 164.8 ± 34.0 590.8 ± 63.3 N/A N/A 2.9 ± 0.3 4.5 ± 0.3

Soil CO2 fluxes were estimated from the product of the mean season flux rate and 139 and 143 days for winter 2010 in AZ and NM,

respectively, compared to 65 and 103 days for AZ and NM in 2011. Total growing season (summer) was estimated by difference.

Uncertainties represent the SE of the mean

Fig. 4 Relationships of

a soil temperature and

b moisture to summer CO2

fluxes at granite and schist

sites in Arizona compared to

relationships of c soil

temperature and d moisture

to summer CO2 fluxes in

east and west facing aspects

in New Mexico. Values are

mean ± SD


123

Table 5 Multiple linear regression models for summer growing season and winter CO2 fluxes

Season Term Estimate SE t ratio Prob [ t

Summer Intercept -0.2986 0.1078 -2.77 0.009*

Gravimetric water content (%) 0.0073 0.0014 5.21 <0.001*

Soil temperature (�C) 0.0320 0.0093 3.43 0.002*

Leached DOC (g/m2) 0.0632 0.0220 2.87 0.007*

Winter Intercept -0.1759 0.0442 -3.98 0.002*

Snow cover 0.0945 0.0137 6.90 <0.001*

Soil clay content (%) 0.0170 0.0023 7.52 <0.001*

Gravimetric water content (%) 9 Soil Clay content (%) 0.0008 0.0002 4.11 0.002*

Bulk soil C (%) 0.0124 0.0044 2.83 0.016*

Gravimetric water content (%) 0.0012 0.0009 1.28 0.227

Soil temperature, soil moisture, and their interaction explained summer CO2 fluxes (r2 = 0.63, r2adj = 0.60, F = 18.82, p \ 0.001);

snow cover (continuous or ephemeral), soil clay content, soil moisture, the interaction between soil clay content and soil moisture,

and bulk soil carbon explained winter CO2 fluxes (r2 = 0.98, r2adj = 0.97, F = 122.07, p \ 0.001)

Bold text indicates individual loadings that were significant to the MLR

Fig. 5 Relationships of a soil temperature and b moisture to winter CO2 fluxes for continuous versus ephemeral snowpack covers for

all data from NM and AZ. Values are mean ± SD

Fig. 6 Observed

(measured) CO2 fluxes

versus results predicted by

the multiple linear

regression model for

a summer fluxes and

b winter fluxes for all data

from NM and AZ sites


123

the ‘‘inverse–inverse texture’’ hypothesis introduced by

Austin et al. (2004), that suggests that fine- textured

soils should have higher soil microbial activity due to

increased water holding capacity and higher nutrient

availability during the growing season (Cable et al.

2008; Fierer and Schimel 2002).

Our observation that both soil moisture and CO2

fluxes were higher on NE-facing aspects than paired

SW aspects in NM (Table 3) highlights the role that

topography and landscape position play in mediating

water availability (Broxton et al. 2009; Hanna et al.

1982; Kang et al. 2005; Wang et al. 2002; Western

et al. 1999). Northeast-facing sites had higher CO2 flux

than southwest-facing sites, consistent with observa-

tions by Kang et al. (2003, 2005). These observations

suggest that landscape position has the potential to

strongly modulate the magnitude of soil C flux by

differentially affecting how precipitation is partitioned

into the amount and duration of available soil

moisture. Recent studies of soil respiration in the

northern Rocky Mountains found a positive correla-

tion between soil respiration and catchment drainage

patterns (Riveros-Iregui and McGlynn 2009).

Although the mechanisms driving these patterns are

unclear, the authors speculate that observed diel

fluctuations in soil respiration result from greater

photosynthetic activity and labile carbon availability

in areas where local drainage increases plant available

water. Other recent work has suggested that growing

season CO2 fluxes are related to snow cover the

previous winter, especially in warmer site (Blankin-

ship and Hart 2012). The relative contribution of

autotrophic versus heterotrophic respiration to our

observed spatial patterns in soil respiration is

unknown. Several studies have identified coupling

between assimilation and belowground processes

using stable isotope techniques (Bahn et al. 2009;

Carbone and Trumbore 2007, Carbone et al. 2007;

Hogberg et al. 2008; Kayler 2008; Plain et al. 2009;

Ruhr et al. 2009), wavelet analysis of the synchroni-

city of photosynthesis and soil CO2 flux (Vargas et al.

2011), and Bayesian modeling (Barron-Gafford et al.

2014).

These previous studies highlight the need to

quantify both the contemporaneous, direct effects of

soil temperature and moisture on heterotrophic activity

as well as longer-term feedback mediated through

terrain, soils, vegetation structure, composition, bio-

mass, and activity (Chorover et al. 2011; Perdrial et al.

2014). In the interim however, our results suggest that

warming temperatures during the growing season may

not result in large increases in soil respiration unless

accompanied by an increase in soil moisture.

Seasonal and landscape patterns: winter carbon

fluxes

Similar to growing season values, observed winter

CO2 fluxes were within the range of values typically

reported for seasonally snow-covered ecosystems

(Brooks et al. 1997, 2005; Brooks and Williams

1999b; Fahnestock et al. 1998). Winter fluxes from

snow-covered soil are primarily heterotrophic in

origin (Brooks et al. 1996, 1999, 2005; Sommerfeld

et al. 1993) and have been shown to respond to both

moisture and labile carbon availability (Brooks et al.

2005; Harrysson Drotz et al. 2009; Oquist and Laudon

2008; Panikov et al. 2006). Although winter fluxes

varied over an order of magnitude, this variability was

not related to temperature or landscape position. The

overall pattern in winter respiration is consistent with a

conceptual model that correlates winter soil respira-

tion positively to snow cover at low snow sites (e.g.,

AZ) and negatively to snow cover at higher snow sites

(e.g., NM) (Brooks et al. 2011; Brooks and Williams

1999a; Mellander et al. 2007).

In this conceptual model, the primary control on

winter CO2 fluxes was the presence of a consistent

snowpack (Fig. 2; Table 5) that insulates soils and

regulates soil temperature and, more importantly,

liquid water content (Brooks et al. 1996; Groffman

et al. 2001a; Mellander et al. 2007; Oquist and Laudon

2008; Stieglitz et al. 2003). Thin or intermittent snow

cover in combination with cold temperatures leads to

increased soil frost, reducing the availability of liquid

water necessary for microbial activity (Brooks et al.

2011; Brooks and Williams 1999b; Edwards and

Cresser 1992; Grogan et al. 2004; Nobrega and

Grogan 2007; Oquist and Laudon 2008; Sturm et al.

2001). The fraction of moisture in liquid form is a

function both of temperature (Stahli and Stadler 1997)

and soil characteristics (Anderson and Banin 1974;

Harrysson Drotz et al. 2009), with higher organic

matter content associated with higher fractional liquid

water content at a given temperature (Harrysson Drotz

et al. 2010; Oquist and Laudon 2008; Sparrman et al.

2004). This may partially explain the high CO2 fluxes

from the NM sites, which are higher in organic matter,


123

even though soils were well below 0 �C in winter

2011. Alternatively, shallow and later accumulating

snow cover results in cellular lysis, increasing labile

carbon content resulting in higher microbial activity

and CO2 flux (Brooks et al. 2005; Mellander et al.

2007; Monson et al. 2006; Schimel and Mikan 2005).

A warming climate is likely to decrease the amount

of snow, delay the establishment of seasonal snow

cover, and increase the frequency and severity of soil

frost (Groffman et al. 2001a, b, 2012; Venalainen

et al. 2001). Observations from this study are

consistent with predictions (Brooks et al. 2011;

Brooks and Williams 1999b) that this delay in snow

cover will result in higher winter respiration rates and

greater carbon loss in many forested ecosystems.

Exceptions will be in areas where soils either remain

frozen and respiration is reduced, or where soils never

freeze and winter precipitation falls as rain. The latter

case likely is restricted to coastal areas where high

humidity buffers temperature. However, in drier,

inland forests, similar to our AZ site, soil temperature

variability and soil frost are likely to increase as

snowfall decreases.

Proximal versus distal predictors of carbon flux

Our results provide insight into how state factors,

specifically topography and parent material, influence

proximal controls of moisture and temperature on soil

respiration directly, rather than indirectly through

primary productivity and carbon availability. During

the growing season, soil moisture contributes most

strongly to the MLR model followed by temperature and

then DOC leachate, an index of available, labile organic

matter. These proximal controls are related to how state

factors, regional climatic, landscape (specifically aspect

but not slope or TCI), and edaphic (clay and carbon

content) influence primarily moisture but also tempera-

ture across all sites. For example, fine textured schist-

derived soils and NE aspects both have higher soil

moisture and higher CO2 fluxes than paired sites in

granite or SW facing slopes, respectively (Table 3). The

observation that neither leached nor bulk carbon were

significantly related to soil respiration suggest that CO2

flux is a direct effect of moisture and temperature on soil

biology. Consequently, spatially explicit models that

use terrain attributes to predict microclimate will

provide mechanisms for scaling soil CO2 flux across

landscapes characterized by similar ecosystem struc-

ture, but complex geological structure.

In contrast, the three characteristics that contribute

to the MLR for winter CO2 flux (consistent snow

cover, clay, and carbon content) are distal controls

associated with state factors of climate and parent

material. The strongest control was the presence or

absence of consistent snow cover that exerted a

threshold-type response in winter soil respiration. Soil

clay and carbon content also contributing to the

predictive power of the MLR model; each of these

characteristics are strongly associated with the sever-

ity of soil frost and soil liquid water content in winter

(Harrysson Drotz et al. 2009, 2010: Brooks et al.

2011), and support the proposition that the amount and

severity of soil frost is the primary control on winter

CO2 flux (Clein and Schimel 1995; Brooks et al.

2011). These observations highlight how state factors

differentially influence growing season and winter

biogeochemical processes, highlighting the need for

research linking these two seasons.

Collectively, these findings indicate that climate,

parent material, and topography are primary determi-

nants of carbon fluxes but mainly through their

interacting controls on soil moisture and temperature

and secondarily through carbon availability. The MLR

for growing season CO2 flux supports the emerging

views that environmental variables such as moisture

and temperature may be more important than the

quality of the carbon in determining carbon fluxes

(Conant et al. 2011; Schmidt et al. 2011). These

proximal controls vary predictably with topographic

and soil characteristics providing a framework for

spatially distributing CO2 fluxes. Our findings also

highlight the need for more contemporaneous studies

evaluating these controls in snow-dominated environ-

ments during winter. Winter fluxes are potentially

large and variable, yet we do not have the ability to

develop spatially distributed proximal drivers during

winter and thus rely on proxies associated with climate

and landscape.

Acknowledgments This research was funded by the National

Science Foundation (NSF) Critical Zone Observatory (CZO)

awards to Jemez-Catalina (EAR 0724958 and 1331408), Reynolds

Creek (EAR 1331872), Boulder Creek (EAR 0724960); the

Department of Energy Award # DE-SC0006968; the National

Science Foundation Award # EPS-0814387; NASA Award #

NNX11AG91G, and the University of Arizona Water,

Environmental, and Energy Solutions program through the


123

Technology and Research Initiative Fund. We extend our gratitude

to Tim Corley, Mary Kay Amistadi and Allison Peterson for their

assistance and expertise in the laboratory, and to Dr. Tyson

Swetnam, Dr. Ingo Heidbuchel, Dr. Erika Gallo, Dr. Adrian

Harpold, and all who assisted with field work, data collection, and

analyses. We also thank two anonymous reviewers and Dr.

Melanie Fisk, Associate Editor, for valuable feedback and

comments on the manuscript.

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http://dx.doi.org/10.1371/journal.pone.0080965

climatic and landscape inﬂuences on soil moisture are ... · 2 ﬂux that span the range of...

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