Temporal variability in hydrology modifies the influence of geomorphology on wetland distribution along a desert stream
Both geomorphic setting and dynamic environmental variables influence riverine wetland vegetation distributions. Most studies of species distributions in riverine systems emphasize either hydrological variability or geomorphic controls, but rarely consider the interaction between the two. It is unknown whether and to what extent the relationship between the geomorphic template and species distribution is modified by fluctuating environmental conditions.
This study examines how spatial patterns of riverine wetlands in a desert stream change in response to environmental shifts brought about by interannual variability in the hydrologic regime. We surveyed wetland spatial distribution and measured its abundance every June over 5 years (2009–2013) by recording patch size and presence/absence of five wetland plant species along the 12-km main stem of Sycamore Creek, Arizona, U.S.A. The study period encompassed a very large flood in January 2010, a wet year (2010), two average years (2009 and 2013) and two extremely dry years (2011 and 2012). We used a Bayesian statistical approach to analyse the relationship between geomorphic variables and wetland distribution under different hydrological conditions.
The geomorphic variables provided much greater explanatory power in dry years than in average to wet years. Hydrological conditions modified the interactions between geomorphic template and species distribution. Annual hydrological conditions affected the direction (i.e. positive or negative effect) and magnitude (i.e. the size and significance level of an effect) of these interactions, both of which gave rise to spatial patterns of wetlands. Ecosystem temporal variability, such as inter-annual and multi-year hydrological variability and longer-term ecosystem state changes, triggered complex species responses.
Synthesis. The effect of geomorphic setting on stream wetland plant distribution in this desert system is conditioned on the temporal variability in hydrology among years. Temporal transferability of the relationship between geomorphology and species distributions is therefore questionable.