Wetlands without Water? A systematic review of drought effects on wetland plant communities

Abstract

Wetlands in arid regions, like the western United States, regularly experience water shortages which likely will be exacerbated by climate change and increased human impacts on water supplies. Unfortunately, little consensus exists on the effects of drought on wetlands. Nor is it clear how wetland research can best inform management of wetlands in the face of declining water supplies. To address these limitations, we conducted a systematic literature review of the impacts of drought on wetland vegetation. We approached this analysis with four broad questions. First, where and why are studies about wetlands and drought being conducted? Second, how is drought defined and evaluated in the literature? Third, what are the known effects of drought on wetland plant communities, and have threshold effects been identified? Finally, how can we best manage wetlands in the face of drought and climate change? The results of the systematic review of 157 peer-reviewed studies show that most research is conducted in temperate-humid biomes, particularly in North America, primarily by academic researchers. The vast majority of studies are conducted in palustrine (non-tidal, emergent marshes) and freshwater systems. Most research analyzed one short-term drought event and the definitions of drought and measurements of its impact varied greatly. Drought was not defined at all in 30% of the studies; terms like drawdown and drought are often used interchangeably. Thresholds for drought impacts are rarely mentioned, but when they are, the focus is on thresholds with regard to the depth of a drought, the speed of a drought, or the life stage of vegetation experiencing drought. Very few studies sought to improve wetland management strategies; instead most wetland research tested general ecological theories. These findings suggest the need for more consistent research methods and definitions that cross ecology, hydrology, engineering, science policy, and other disciplines to make comparing or aggregating results of wetland studies possible and useful. There is also a need for more collaboration between researchers and managers who may have long-term data sets and insights into wetland response to various on-going management manipulations. In this way, research efforts can be made more relevant to management needs and to the types of drought that naturally occur, which are generally longer-term or more severe than what are evaluated in the literature. Finally, more research is needed to identify the impacts of drought on many different wetland types, particularly in regions that are not as well studied but are likely to experience water scarcity