Climate change‐induced vegetation shifts lead to more ecological droughts despite projected rainfall increases in many global temperate drylands

Abstract

Drylands occur worldwide and are particularly vulnerable to climate change because dryland ecosystems depend directly on soil water availability that may become increasingly limited as temperatures rise. Climate change will both directly impact soil water availability and change plant biomass, with resulting indirect feedbacks on soil moisture. Thus, the net impact of direct and indirect climate change effects on soil moisture requires better understanding. We used the ecohydrological simulation model SOILWAT at sites from temperate dryland ecosystems around the globe to disentangle the contributions of direct climate change effects and of additional indirect, climate change-induced changes in vegetation on soil water availability. We simulated current and future climate conditions projected by 16 GCMs under RCP 4.5 and RCP 8.5 for the end of the century. We determined shifts in water availability due to climate change alone and due to combined changes of climate and the growth form and biomass of vegetation. Vegetation change will mostly exacerbate low soil water availability in regions already expected to suffer from negative direct impacts of climate change (with the two RCP scenarios giving us qualitatively similar effects). By contrast, in regions that will likely experience increased water availability due to climate change alone, vegetation changes will counteract these increases due to increased water losses by interception. In only a small minority of locations, climate change-induced vegetation changes may lead to a net increase in water availability. These results suggest that changes in vegetation in response to climate change may exacerbate drought conditions and may dampen the effects of increased precipitation, that is, leading to more ecological droughts despite higher precipitation in some regions. Our results underscore the value of considering indirect effects of climate change on vegetation when assessing future soil moisture conditions in water-limited ecosystems.

Functional Group, Biomass, and Climate Change Effects on Ecological Drought in Semiarid Grasslands

“…We report results for a future climate near the 50th percentile of the 16 GCMs based on RCP8.5, leading to ~1,370 ppm CO 2 (Van Vuuren et al, ). We present results only for RCP8.5 because results for RCP4.5 are qualitatively similar (Tietjen et al, ) and because RCP8.5 simulates a plausible scenario of heavy dependence on fossil fuels until at least 2100.…”

“…There are important limitations to the use of simulation models like SOILWAT in studying the effects of plant functional groups on ecological drought. First, our analysis did not incorporate feedback (Turnbull et al, ), and vegetation changes that follow climate change further exacerbate drought in SOILWAT (Tietjen et al, ). Similarly, biomass increased by factors such as fertilization, grazing respite, or invasion by identical functional groups with greater productivity should all increase ecological drought, but grassland aboveground biomass in turn varies strongly with water availability independently of other drivers (Lauenroth & Sala, ).…”

“…Grassland hydrology can also be profoundly affected by the dominant plant functional group (Pysek et al, ; Wilcox et al, ). Whether changes in plant functional groups exacerbate or mitigate climate‐related drought has received little attention (Cavaleri & Sack, ; Prevéy & Seastedt, ), even though the role of plant communities in modulating ecosystem responses to global change has recently received increased attention (Suding et al, ; Tietjen et al, ).…”

Heat Response of Global Vegetation Biomes to Ongoing Climate Warming Based on Remote Sensing

“…Earth greening implies that the capacity of the biospheric carbon pool is being boosted because of enhanced vegetation photosynthetic activity [7], which might be expected to mitigate the magnitude of climate warming and its associated negative effects on the earth system [1,3]. On the other hand, climate warming directly impacts the dynamics of the terrestrial vegetation biomes by increasing the frequency, duration and severity of drought and heat stress [8][9][10][11], while other studies have suggested that climate warming tends to relax temperature constraints for plant growth [12,13]. Many studies assume that plants have an optimal threshold for growth and reproduction [14], suffering cold stress when temperatures are below this threshold, and heat stress when above [15].…”

“…Many studies assume that plants have an optimal threshold for growth and reproduction [14], suffering cold stress when temperatures are below this threshold, and heat stress when above [15]. Accordingly, we can deduce that when the in-situ temperature is suboptimal, climate warming may relax the temperature constraint for plant metabolism and promote its growth, while supraoptimal temperature conditions and the accompanying rise in the vapor pressure deficit and evaporation may damage the plant's cellular tissues [8,16,17]. Here, we ask: what have been the responses in diverse vegetation biomes to ongoing recent climate warming?…”