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Vegetation shifts in desert environments: a multi-scale ecogeomorphic approach for the analysis of grassland-shrubland transitions

Final Report Summary - VEGDESERT (Vegetation shifts in desert environments: a multi-scale ecogeomorphic approach for the analysis of grassland-shrubland transitions)

Climate change and the massive alteration of natural habitats are major drivers of land degradation. Their effects may be especially significant in drylands, where ecosystems are particularly sensitive to degradation, usually involving largely irreversible landscape changes. A common form of land degradation in drylands involves the encroachment of shrub species into desert grasslands. An array of mechanisms are involved in shrub encroachment processes, including external triggering factors such as climate and land-use variations, and endogenous amplifying mechanisms brought about by soil erosion-vegetation feedbacks. Within this context, VEGDESERT (Vegetation shifts in desert environments: a multi-scale ecogeomorphic approach for the analysis of grassland-shrubland transitions, reference PIEF-GA-2012-329298) investigates grassland-shrubland transitions in desert environments. The project is hosted by Durham University (UK) and focus on grassland-shrubland transitions in the Chihuahuan desert, taking advantage of the information and facilities available at the Sevilleta Long Term Ecological Research (LTER) site (New Mexico, USA). The results of this project contribute to the understanding of the processes that regulate the dramatic changes that are taking place in arid and semi-arid landscapes worldwide, and provide practical tools for the management of dryland landscapes threatened by desertification.

The general aim of VEGDESERT is to develop an ecogeomorphic approach for understanding the processes that underlie land degradation in drylands, particularly in desert grasslands affected by shrub encroachment. The project is organized around three interrelated objectives: (i) to study the influence of precipitation variations and the activation of erosion-vegetation feedbacks in the progression of shrub encroachment in the northern margin of the Chihuahuan desert (Sevilleta LTER), (ii) to study the environmental conditions and limitations for plant establishment of key species for the shrub-encroachment phenomenon (black grama, Bouteloua eriopoda, a C4 grass species that broadly dominates Chihuahuan grasslands, and creosotebush, Larrea tridentata, a C3 encroaching shrub in the region), and (iii) to simulate early plant establishment and landscape dynamics at Chihuahuan grassland-shrubland transitions as a tool to determine the best land-management practices and design desertification-mitigation strategies. In the two years of duration of the project, VEGDESERT has applied a variety of methods, including remote sensing of vegetation (MODIS NDVI), soil and plant experimentation, and modelling of soil moisture and vegetation dynamics using a process-based modelling framework (DayCent model).

VEGDESERT has developed a novel and parsimonious methodology that can be applied to decompose and transform the NDVI into components of net primary production (NPP) for herbaceous and shrub vegetation, based on the analysis of NDVI-rainfall relationships [1]. Application of this methodology to the analysis of the Sevilleta LTER grassland-shrubland ecotone indicates that herbaceous production (primarily black grama) is synchronized with the summer monsoonal rainfall, while shrubs (primarily creosotebush) can sustain vegetation production using both summer and winter precipitation. Our results suggest that the Sevilleta ecotone has remained relatively stable within the last fifteen years, with no significant shrub expansion or grass recovery. However, future changes in the amount and temporal pattern of precipitation (i.e. reductions in monsoonal summer rainfall and/or increases in winter precipitation) may enhance the shrub-encroachment process, particularly in the face of expected upcoming increases in aridity for desert grasslands of the southwestern USA [1].

VEGDESERT has contributed (in collaboration with research partners of USA, UK and Australia) to generate an analytical framework that can be used to understand the influence of soil erosion-vegetation feedbacks on the degradation of arid and semiarid landscapes, by applying the hydrological concept of "connectivity" to dryland ecology [2]. This conceptual model has been applied in collaboration with partners of Spain and Australia to explore ecosystem stability and restoration potential of disturbed drylands [3, 4], and feeds into ongoing work within the EU-COST network CONNECTEUR.

Plant and soil experimentation within VEGDESERT has encompassed: (i) the collection and analysis of the soil seed bank across the Sevilleta LTER grassland-shrubland ecotone, (ii) the analysis of the environmental limitations (i.e. soil moisture and temperature) for the germination and early establishment of key species (black grama and creosotebush), and (iii) a deep analysis of the soil physico-chemical traits and characteristic soil-water retention curves in soil samples collected across the Sevilleta LTER grassland-shrubland ecotone. Results of the plant and soil experiments indicate that species composition in the soil seed bank is strongly affected by shrub encroachment, with seed densities of grass species sharply decreasing in shrub-dominated sites. Seed germination and seedling emergence for both black grama and creosotebush require exceptionally wet summers in the area [5].

Integration of VEGDESERT main experimental results with soil moisture simulations using the DayCent ecohydrological model [6] reveal that limited creosotebush seed dispersal and a narrow range of rainfall conditions for seedling establishment constrain shrub-recruitment pulses to localized and episodic decadal events. Black grama recruitment from seed is also episodic (5-8 years recurrence time). The lack of viable black grama seeds and decreased plant-available soil moisture strongly limit re-establishment of black grama in areas now dominated by creosotebush. Overall our results suggest that the early plant dynamics play a major role regulating species dominance and the rates of vegetation change at grassland-shrubland transition boundaries, and provide a biophysical explanation for the apparent stability of the Sevilleta LTER Chihuahuan ecotone, which is expected to remain stable in the absence of cattle overgrazing unless extensive grass suppression by an extreme and prolonged drought occurs in the future.

References:

[1] Moreno-de las Heras, M., Diaz-Sierra, R., Turnbull, L., and J. Wainwright. 2015. Assessing vegetation structure and ANPP dynamics in a grassland-shrubland Chihuahuan ecotone using NDVI-rainfall relationships. Biogeosciences, 12: 2907–2925.

[2] Okin, G.S. Moreno-de las Heras, M., Saco, P.M. Throop, H.L. Vivoni, E.R. Parsons, A.J. Wainwright, J., and D.P.C. Peters. 2015. Connectivity in dryland landscapes: shifting concepts of spatial interactions. Frontiers in Ecology and the Environment, 13: 20-27.

[3] Merino-Martín, L., Moreno-de las Heras, M., Espigares, T., and J.M. Nicolau. 2015. Overland flow directs soil moisture and ecosystem processes at patch scale in Mediterranean restored hillslopes. Catena, 133: 71-84.

[4] Saco, P.M. Azadi, S., Moreno de las Heras, M., and G. Willgoose. 2015. Ecosystem Stability Thresholds along a Precipitation Gradient in Australia. Poster presentation. 2015 EGU General Assembly (Vienna, Austria, April 2015), European Geosciences Union (EGU). Geophysical Research Abstracts Vol. 17, Abstract EGU2015-15020.

[5] Moreno-de las Heras, M., Turnbull, L., and J. Wainwright. 2015. Soil seed-bank structure and seed-germination dynamics contribute to pulsed shrub encroachment in a grassland-shrubland Chihuahuan ecotone. Poster presentation. 2015 EGU General Assembly (Vienna, Austria, April 2015), European Geosciences Union (EGU). Geophysical Research Abstracts Vol. 17, Abstract EGU2015-13806.

[6] Moreno-de las Heras, M., Turnbull, L., and J. Wainwright. Under review. Seed-bank structure and plant-recruitment conditions regulate the dynamics of a grassland-shrubland Chihuahuan ecotone. Submitted to Ecology on 12th July 2015.