Grassland-shrubland transitions: Part 1

Previous LTER cycles (I-III) have focused largely of mechanisms and consequences of shrubland resilience

Lateral flux of water, sediment, nutrients Percolation

Schlesinger et al. (1990) “islands of fertility” or “Jornada” desertification model

Not clear how perennial grasses are lost from the ecosystem—a focus of long-term work since LTER IV

Feedback Feedback

Disruption of perennial grass cover, shrubs invade

Loss of dominant grasses can be highly abrupt and irreversible

Bestelmeyer et al., 2011, Ecosphere

Drought

Abruptness and hysteresis are hallmarks of a critical transition

Do grasslands cross critical thresholds at low, but positive, levels of grass cover?

Driver-control model: grass driven extinct directly by grazing/drought

Feedback-control model: critical threshold of grass cover below which soil erosion/hydrologicalfeedbacks drive remaining grass cover extinct

Driver

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Bestelmeyer et al, 2013, Ecology Letters

Experimental evaluation of threshold models

13-year pulse perturbation experiment, heavy grazing (initiated during LTER III in 1995 as the “Stressor Experiment”)

18 paddocks, 0.5 ha

Little support for feedback control model

Winter grazing resulted in biggest decline, but didnot affect rate of recovery

Mesquite presence exacerbated pulse effects, but did not constrain recovery

Recovery takes a long time

Might have approached a critical threshold in one plot

Could grass patch size mediate grassland resilience?

LTER VI Proposal, Hypothesis 1(a): As grass patches become smaller and more fragmented, grass resilience decreases such that plants and tillers within small grass patches will have lower fitness and greater water stress than those in large patches.

Scale-dependent feedback theory (after Rietkerk et al., 2004)

Leads to self organized patchiness, but also to abrupt transitions

Scale dependent feedback study

Short-term studies embedded within long-term study(Stressor)

144 focal plants classified to 4 patch classes(36 reps/class)

Patch type Code Patch Selection Rules Focal Plant Selection Rule

Large patch interior

L(I)Contain interior points >30cm from a patch edge

Plants located >30 cm from the edge of large patches

Large patch edge

L(E) Plants located within 30 cm of edge

Medium patch M >20cm wide in at least one dimension but do not contain any interior points >30cm from a patch edge

Plants located on the edge of medium patches

Small patch S ≤20cm from boundary to boundary in any direction

Plants located on the edge of small patches

Open ground OG Not in vegetation patch and >30cm from any BOER base

No focal plant

Least squares means for plant attributes (stolons, ramets and rooted ramets) of each patch class

(Small, Medium, Large (exterior), and Large (interior)) over all time periods.

Patch Classes

Attribute Small Medium Large (exterior) Large (interior)

Stolons 47.5 ± 5.6c 92.7 ± 5.6a 71.8 ± 5.6b 74.2 ± 5.6ab

Ramets 17.6 ± 4.3c 50.4 ± 4.3a 38.7 ± 4.3ab 34.6 ± 4.3b

Rooted ramets (RR) 0.8 ± 0.5b 3.8 ± 0.5a 2.5 ± 0.5ab 2.1 ± 0.5b

Plants in medium patches had higher rates of reproductive success (RR) and effort (stolons/ramets) than in large patch interiors or small patches.

Smallest patches were the worst environments, possible explanation for slow recovery

(Svejcar et al, 2015, Ecosystems)

Hypothesis 1(a): Scale-dependent feedbacks supported

Hypothesis 1(a): Scale-dependent feedbacks supported

But, reproductive success in small patches increased through a dry summer

Might be a stabilizing mechanism responsible for resilience in long-term data

Measurement Description Locations Frequency

Shallow water contents

Soil water content near surface (5 – 10 cm) using data loggers & 10HS

All Every 5 min after rain event. Every 8 hrs when dry. March 2011 to July 2014

Plant physiology

Predawn water potential, photosynthesis, and 0-12 cm soil water content (Hydrosense)

L(I), M, and S

Post summer rain event, 15 days in 2010 & 8 days in 2011 (no photosynthesis in 2011)

Soil profile water content

Soil water content measured with N-probe (10, 20, 30, 40, & 50 cm)

L(I), M, S, and OG

1x/month in winter & spring, every 2 wks in summer, & when plant physiology done (7/1/2010 – 5/14/2013)

Hypothesis 1(a): Do plants in smaller patches experience greater water stress?

(Duniway, Bestelmeyer, Svejcar, in prep)

Hypothesis 1(a): Ecohydrology and scale-dependent feedbacks

• Greater water stress and lower net carbon assimilation in medium and interior of large than in small patches.

• Greater infiltration under larger than smaller patches or open ground immediately following rain events.

• Mean water capture (difference between pre- and post-rain VWC) of 15 events show large patches have greater capture than small patches or open ground.

• Very few days with significant differences among patch classes, suggesting the effects of short-range facilitation on soil water balance are quickly erased by greater water use in large patches.

Hypothesis 1(a): Ecohydrology and scale-dependent feedbacks

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• Scale-dependent feedback-like patterns in plant reproduction seem to be poorly explained by ecohydrology

• May instead be related to how plants allocate resources in different patch contexts.

• Results point to the driver-control model of ecological thresholds in the Stressor case

• But, plant-soil erosion feedbacks may be important in other situations, such as when defoliation is sustained year after year (next).

Conclusions at this point

Future plans

Hypothesis 1(a):

1. Analysis of ultra-high resolution (UAV) imagery data from Stressor II to test new early warning signals (EWS) (Vishwesha Guttal, Indian Institute of Science)

2. Final record of grass recovery for current phase in fall 2016 (20 years)

3. New pulse perturbation to test for EWS along a smooth gradient using UAV imagery and soil nutrient/erosion measurements (possibly spring 2017)

4. Integrate interpretations with Nate Pierce’s work on grass responses to varying shrub density neighborhoods (tomorrow)