Recent droughts have precipitated large-scale woody plant die-offs. These woody plants dominate the systems in which they inhabit; thus, their mortality has widespread implications for how these systems function. Often co-occurring species are differentially susceptible to drought such that some persist while others exhibit high mortality. Many studies have sought to distinguish the traits of species that resist drought from those that succumb. Fewer studies have focused on aspects of drought that could affect drought resistance of species.
Drought is commonly defined by annual metrics such as precipitation totals or drought severity indices that integrate temperature and precipitation relative to long-term mean values. While these are useful metrics, they have limitations because how species respond to drought may be affected by aspects of droughts that occur on shorter than annual time scales and because droughts can interact with several other variables.
In this study, we examined drought in the context of ecological disturbance models to define a new concept referred to as drought regime. We synthesized existing approaches and used them to define the ecologically relevant aspects of drought. Our objective was to create a framework for analyzing and communicating the important aspects of drought as a stressor.
Drought regime for a region, and on an annual basis, can be defined by intensity, frequency, and predictability of precipitation. On an intra-annual basis, important factors include consecutive months without rainfall, and size and seasonality of rainfall events. These intra-annual factors are important because they can affect soil-moisture availability to vegetation and affect how droughts might interact with temperature extremes. It is also important to consider pre-disposition of a region to drought, which will depend on its disturbance history and successional status.
Recent studies of multi-year droughts have identified drought duration as being the key factor triggering widespread woody plant die-offs. While this may be true in some cases, an alternative hypothesis is that intra-annual factors have been important drivers of die-offs. Long-term monitoring of mortality of various systems dominated by woody species, coupled with available high resolution weather data, provides opportunities to assess which aspects of drought regime contribute most to plant mortality.
In the context of climate change, as droughts become more common and precipitation more erratic, a more refined understanding of drought characteristics and how they influence species and systems will increase the predictive power of vegetation models and aid natural resource management.