Drought as an emergent driver of ecological transformation in the twenty-first century

Abstract

Under climate change, ecosystems are experiencing novel drought regimes, often in combination with stressors that reduce resilience and amplify drought's impacts. Consequently, drought appears increasingly likely to push systems beyond important physiological and ecological thresholds, resulting in substantial changes in ecosystem characteristics persisting long after drought ends (i.e., ecological transformation). In the present article, we clarify how drought can lead to transformation across a wide variety of ecosystems including forests, woodlands, and grasslands. Specifically, we describe how climate change alters drought regimes and how this translates to impacts on plant population growth, either directly or through drought's interactions with factors such as land management, biotic interactions, and other disturbances. We emphasize how interactions among mechanisms can inhibit postdrought recovery and can shift trajectories toward alternate states. Providing a holistic picture of how drought initiates long-term change supports the development of risk assessments, predictive models, and management strategies, enhancing preparedness for a complex and growing challenge.

Keywords: climate change; disturbance; drought; ecological transformation; vegetation shift.

Figure 1.

Changes in future precipitation and drought regimes. (a) Projected changes in annual precipitation by mid-century (2036–2065), indicating increased precipitation across much of the globe. (b) Projected changes in the frequency of extreme atmospheric drought events defined as monthly potential evapotranspiration values exceeding the 99th percentile threshold during a historical (1971–2000) period. (c) Projected changes in the frequency of extreme soil moisture drought events by mid-century, defined as monthly soil moisture deficits exceeding historical 99th percentile thresholds. All projection estimates are obtained from 40 simulations of the Community Earth System Model version 1 Large Ensemble (Kay et al. 2015) under the RCP8.5 emissions scenario.

Figure 2.

Recent examples of in-progress (solid outlines) or emerging (dotted lines) transformations that were triggered or strongly mediated by drought, compiled from literature review and previous syntheses (Martínez-Vilalta and Lloret , Cobb et al. , Batllori et al. , Lloret and Batllori 2021). The mechanisms involved with each transformation are included where evidence exists, although the absence of a particular mechanism may reflect a lack of information. While this is not an exhaustive list, it reflects the diverse mechanisms by which drought contributes to the transformation of ecosystems across an aridity gradient. The aridity gradient was defined by the average monthly water balance (precipitation – potential evapotranspiration) over the reference period 1980–2010 using the TerraClimate data set (Abatzoglou et al. 2018). More detailed descriptions of examples are found in the supplemental material.

Figure 3.

The interrelated mechanisms involved when drought triggers an ecological transformation. The solid lines represent key pathways, whereas the dotted lines are involved in some but not necessarily all transformations. The impact of drought on organisms is mediated by heterogeneity in water availability, which can be influenced by land management, human water use, and landscape features. Water deficits can alter recruitment and mortality rates, leading to shifts in community composition and structure. The effects of drought on demographic rates can be direct (i.e., because of physiological damage) or can result from drought's effects on other linked stressors (e.g., fire, insect outbreaks). Additional, compounding stressors (e.g., land-use change, invasions) may exacerbate drought's impacts or mediate postdrought dynamics in ways that promote transformation. Interactions between mechanisms (a subset of which are shown) can amplify or dampen these pathways, determining whether communities follow a trajectory that leads to recovery or to any number of trajectories that lead to ecological transformation. Novel climate conditions affect nearly all of these processes.

Figure 4.

Example pathways by which drought can initiate or promote ecological transformations. These hypothetical examples are drawn from contemporary case studies but are simplified for illustrative purposes. Many pathways are possible, and we have depicted a subset to represent how interacting mechanisms can lead to transformations.