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All About Discovery!New Mexico State Universityaces.nmsu.edu

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Patterns and Processes of Vegetation Change in Semi-arid

Ecosystems

Niall HananJornada Basin LTER & Plant and Environmental Sciences

Department (PES), New Mexico State University

Research Insights in Semi‐Arid Ecosystems Symposium (RISE)

October 21, 2017


™ Jornada Basin LTER &Plant and Environmental Sciences Department (PES)

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Patterns and Processes of Vegetation Change in Semi-arid Ecosystems

1. Should we rethink how we think about shrub encroachment?

Two provocative questions

2. Are competitive interactions (competition for water) as important in controlling shrub populations in semi‐arid

systems as we think they are?

Premise: Disturbance may be very common, and relationships noisy, but in large samples some sites provide information on the underlying resource limitationArid “stable” savannas (<650 mm): water limited; disturbance mediatedMesic “unstable” savannas (>650 mm): disturbance mediated

Sankaran et al. Nature, 2005.Using data from ~600 field sites

Inferring tropical savanna dynamics using field measurements (“process from pattern”)

0 200 400 600 800 1000 1200Mean Annual Rainfall (mm)

516 mm784 mm

Fire Cycle“Disturbance” =

& many other things

Explore broad scale (Western USA) patterns in tree and shrub populations (and change) relative to: Rainfall Aridity Soils Biome/vegetation types Land use, fire, grazing Etc.

Inferring temperate shrub process from pattern using aerial or satellite data for SW USA

Q: What drives patterns at

regional scale?

Landform Ranked by (e.g.) Water Availability

Peters et al., JAE 2010

Inferring temperate shrub process from pattern using NAIP airborne imagery for the Jornada Basin

Explore landscape scale (Jornada Basin) patterns relative to: Geomorphology Soils Landscape position …

Q: What drives landscape‐scale variability?

Playas

A priori hypothesis (with hypothetical data!). 95th%ile

Inferring tropical savanna dynamics using VHR satellite imagery

Axelsson & Hanan, Biogeosciences, 2017.Using data from ~800 VHR “plots”

Separate analyses for crown area (A), stem density (S) & cover (C)(note: C = A * S)

Increase in woody cover with rainfall driven by

crown size more than by

stem density

Inferring tropical savanna dynamics and shrub encroachment using VHR satellite imagery

Axelsson & Hanan, J. Biogeography, in review, using data from ~600 VHR “plots”

Woody cover change over 8‐10 year VHR satellite period

Potential – Initial Woody CoverAs (Potential‐Initial) ‐> 0, change ‐> 0.Thus “shrub encroachment” in African savannas

may simply reflect “recovery” of dynamic woody populations towards “potential” cover

Provocative Question 1Rethinking shrub encroachment



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Should we rethink how we think about shrub encroachment?

Shrub encroachment observations may often reflect dynamic population processes.Locally, management changes (e.g. fire suppression) modify dynamics But… perhaps no need to invoke climate change as an external driver…

Potential – Initial Woody Cover

Dohn et al., J. Ecology, 2013.Using field data from Africa and North America

Shrubs/trees often impact herbaceous growth, but the interactions is not always competitive

Shrubs facilitate herbaceous growth in drier systems (both tropical and temperate)

Shrubs compete with/suppress herbaceous growth in wetter systems

Temperate systems shifted left (competition “kicks in” at lower MAP: lower annual PET makes them less arid, than sites with equivalent MAP in tropical drylands)

Hunder > Hopen

Hunder < Hopen

“Facilitation”i.e. herbaceous NPP under a tree > herbaceous NPP away from trees

“Competition”

Are competitive interactions important in controlling shrub populations in semi‐arid systems?

1) Shrub effects on herbaceous growth


2) Grass effects on shrub growth

Figure 1. Relative growth rate (RGR) of shrubs in the presence and absence of grass on three soil types (with increasing clay content).

Figure 2. Percentage of total new growth of shrubs allocated to the woody component in the presence and absence of grasses on three soil types. ANCOVA suggests grass competition reduces allocation to above‐ground wood (but it’s a relatively weak effect)

Dohn and Hanan, (in prep), using field data from the Colorado Shortgrass Steppe

In most cases, grasses have little effect on growth or survival of adult shrubs… but grasses sometimes impact seedling growth and survival


3) Shrub competitive effects on shrub growth

Dohn et al., 2017, Ecology, 98, 478‐488, using data from East Africa

Shrub‐shrub competition may occur, but (for example) Sala’s rainout experiments suggest mesquite are not water limited

“Water limited”, but not necessarily a process of shrub‐grass or shrub‐shrub competition for water

Maximum woody cover correlates with rainfall. Initially we assumed: Patterns imply grass‐on‐tree or tree‐tree competition… Patterns are consistent with the Walter hypothesis… But competition observations are weak/inconsistent And in fact they don’t explain patterns very well… And may not be needed…!

Consider this simple 2‐stage population model:

Sapling population (S):

Adult tree population (T):

S − T

New samplings

“Loss” thru mortality and ageing up

New adult recruits “Loss” thru

mortality

“Water limited”, but not necessarily a process of shrub‐grass or shrub‐shrub competition for water

A simple 2‐stage shrub population model adult & seedling demographics

zero competition, just reproduction, growth, mortality ratesShould this be our null‐model for dryland shrub communities?

Water‐limited via seedling/sapling mortality, but not necessarily via competitive interactions…?

Maximum woody cover correlates with rainfall. Initially we assumed: Patterns imply grass‐on‐tree or tree‐tree competition… Patterns are consistent with the Walter hypothesis… But competition observations are weak/inconsistent And in fact they don’t explain patterns very well… And may not be needed…!



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Are competitive interactions (competition for water) as important in controlling shrub populations in semi‐arid systems as we think? Competitive mechanisms constraining shrub cover are inconsistent and/or weak

And… Perhaps no need to invoke competition if simple shrub demographics can explain patterns



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1. Rethinking shrub encroachment

Two provocative questions2. Are competitive interactions

important for shrubs?

Competitive mechanisms constraining shrub cover are inconsistent and/or weakPerhaps no need to invoke competition… if simple shrub demographics can explain patterns

Shrub encroachment observations may often reflect dynamic population processes.

Locally, management changes (e.g. fire suppression) modify dynamics

But… perhaps no need to invoke climate change as an external driver…


™ Plant and Environmental Sciences Department (PES)[email protected]

Thank you!




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Is shrub encroachment really a “thing”? Or, put another way: do we need to rethink how we think about shrub encroachment?There are many opportunities for sample bias in measuring shrub encroachment: spatial bias in site selection, and temporal bias in measuring slow growth and rapid declines. e.g. The temporal bias: If a shrub population is dynamically recovering from long‐term disturbance (e.g. population crashes at interval D years) then the probability (P↑) we observe an increase in shrub density/cover during a short observation interval (I years) is:

P↑ = 1 – I/DFor example, if major events that knock‐back the shrub population occur at approximately 100 year intervals (D=100 yr), and we measure change during an interval of 10 years (I=10 yr), the probability of observing an increasing shrub population is 0.9 (90%), even if at century time‐scales the population is dynamically stable.If management (e.g. fire suppression/grazing) increases D, then of course P↑ increases… and sites with larger D will tend towards the ‘potential’ shrub cover for those locations… If management goals favor shrub cover much less than ‘potential’ cover, the mechanism constraining maximum cover (demographic, competitive or both) is important

patterns and processes of vegetation change in semi-arid ... · vegetation change in semi-arid...

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