Forecasting semi‐arid biome shifts in the Anthropocene

Abstract

Summary Shrub encroachment, forest decline and wildfires have caused large‐scale changes in semi‐arid vegetation over the past 50 years. Climate is a primary determinant of plant growth in semi‐arid ecosystems, yet it remains difficult to forecast large‐scale vegetation shifts (i.e. biome shifts) in response to climate change. We highlight recent advances from four conceptual perspectives that are improving forecasts of semi‐arid biome shifts. Moving from small to large scales, first, tree‐level models that simulate the carbon costs of drought‐induced plant hydraulic failure are improving predictions of delayed‐mortality responses to drought. Second, tracer‐informed water flow models are improving predictions of species coexistence as a function of climate. Third, new applications of ecohydrological models are beginning to simulate small‐scale water movement processes at large scales. Fourth, remotely‐sensed measurements of plant traits such as relative canopy moisture are providing early‐warning signals that predict forest mortality more than a year in advance. We suggest that a community of researchers using modeling approaches (e.g. machine learning) that can integrate these perspectives will rapidly improve forecasts of semi‐arid biome shifts. Better forecasts can be expected to help prevent catastrophic changes in vegetation states by identifying improved monitoring approaches and by prioritizing high‐risk areas for management.

Toward an extreme world: The hyper‐arid ecosystem as a natural model

“…Climate change models project a future biosphere dominated by hydro‐climatic extremes characterized by high variation in precipitation regime, namely severe droughts and pulses of floods. In many regions, the effects of hydro‐climatic extremes on ecosystem are projected to induce state change and even biome shifts (Kulmatiski et al, 2020). Understanding the direction and magnitude of ecosystem changes under these predictions is critical for coping with the ecosystem dynamics of an extreme world.…”

“…hydro-climatic extremes on ecosystem are projected to induce state change and even biome shifts (Kulmatiski et al, 2020). Understanding the direction and magnitude of ecosystem changes under these predictions is critical for coping with the ecosystem dynamics of an extreme world.…”

Desertification vulnerability under accelerated dryland expansion

“…To avoid such a phenomenon, trees reduce their water loss through stomatal closure, thus leading to a decline in carbon sequestration (N. G. McDowell & Sevanto, 2010). With this process, coupled with carbon consumption for xylem restoration (Choat et al, 2018; Kulmatiski et al, 2020), trees suffer from a constant negative carbon balance, thus resulting in the onset of tree dieback. This explains why accelerated EVI and woodland proportion declines follow the SOC threshold (Figure 5).…”

“…Because the roots of herbaceous plants are relatively short (Canadell et al, 1996), once the SAC exceeds a certain level, precipitation will quickly penetrate deep soil beyond what the herbaceous plant roots can absorb. Conversely, woody plants can use water from deeper soil because of their longer roots, thus leading to shrub encroachment in grasslands (Kulmatiski et al, 2020). Shrub encroachment is considered one of the typical manifestations of land degradation (although the association is not universal) because it can increase the bare soil fraction by altering the spatial distribution of soil resources (D'Odorico et al, 2012), particularly in areas with high soil pH and sand content (Ochoa‐Hueso et al, 2018).…”