Effects of climate change on the coupled dynamics of water and vegetation in drylands

Abstract

Drylands worldwide are exposed to a highly variable environment and face a high risk of degradation. The effects of global climate change such as altered precipitation patterns and increased temperature leading to reduced water availability will likely increase this risk. At the same time, an elevated atmospheric CO 2 level could mitigate the effects of reduced water availability by increasing the water use efficiency of plants. To prevent degradation of drylands, it is essential to understand the underlying processes that affect water availability and vegetation cover. Since water and vegetation are strongly interdependent in waterlimited ecosystems, changes can lead to highly non-linear effects. We assess these effects by developing an ecohydrological model of soil moisture and vegetation cover. The water component of the model simulates the daily dynamics of surface water and water contents in two soil layers. Vegetation is represented by two functional types: shrubs and grasses. These compete for soil water and strongly influence hydrological processes. We apply the model to a Namibian thornbush savanna and evaluate the separate and combined effects of decreased annual precipitation, increased temperature, more variable precipitation and elevated atmospheric CO 2 on soil moisture and on vegetation cover. The results show that two main factors control the response of plant types towards climate change, namely a change in water availability and a change in water allocation to a specific plant type. Especially, reduced competitiveness of grasses can lead to a higher risk of shrub encroachment in these systems.

It is getting hotter in here: determining and projecting the impacts of global environmental change on drylands

“…Although this phenomenon has not been found to significantly affect ecosystem attributes such as biodiversity at the global scale [100], it may either enhance or reduce plant species richness at local and regional scales [97,101]. Some studies have suggested that different global change drivers, such as increases in [CO 2 ] and in the frequency of large precipitation events, may favour shrubs at the expenses of grasses in drylands [49,102,103]. Thus, it is likely that shrub encroachment will be augmented in the future [35], even if other factors known to promote this land-cover change (e.g.…”

Threshold behaviour in hydrological systems as (human) geo-ecosystems: manifestations, controls, implications

“…6), in which the dense biomass patches form bands, stripes or arcs (Saco et al, 2007;Ludwig et al, 1999) aligned along contour lines. These patterns exert a strong influence on the re-distribution of water and nutrients within the system (Saco et al, 2007;Tietjen et al, 2009a). The underlying reason is that infiltra- tion in bare soil patches is impeded due to surface soil crusting but very effective in vegetated patches due to the presence of inter-connected root channels that act as preferential pathways (Thiery et al, 1995;Ludwig et al, 2005;Saco et al, 2007).…”

“…These ecosystems thus provide a service that is most crucial for global food production, especially in developing countries; the provision of ecosystem service depends crucially on the fragile equilibrium between woody and herbaceous vegetation. By applying a recently developed eco-hydrological model to a Namibian thornbush savannah, Tietjen et al (2009a) evaluated the separate and combined effects of decreased annual precipitation, increased temperature, more variable precipitation, and elevated atmospheric CO 2 on soil moisture and on vegetation cover. They suggest that expected climate change tends to promote shrub growth in Namibian thornbush savannah.…”

“…They suggest that expected climate change tends to promote shrub growth in Namibian thornbush savannah. As a consequence infiltration into the deeper subsoil and transpiration losses would increase and redistribution of water resources due to runoff would be impeded (Tietjen et al, 2009a, b). The drier surface soil moisture regime implies worse conditions for grass growth.…”

“…These authors compared 41 different models to simulate coupled vegetation and water dynamics of savannah ecosystems and found a series of deficiencies concerning the representation of water dynamics and feedbacks of vegetation on key hydrological processes such as the lateral redistribution due to Hortonian overland flow and infiltration. Tietjen et al (2009a, b) developed their own coupled eco-hydrological model that accounts for the complete water cycle in a simplified form, sound vegetation dynamics of shrubs and grasses and key feedbacks such as the effects of shrub cover on infiltration, runoff redistribution and transpiration. With their model they were able to reproduce observed soil water dynamics at several different sites in Israel and observed vegetation patterns at several locations.…”