Intraspecific variation matters: the forces that explain it and its impact on community weighted means in the Sandhills region of North Carolina
The use of functional traits to understand the structure and functioning of communities has gained traction in recent years, and has been used to explain community composition, production, and function in a variety of systems. Specifically, community weighted mean (CWM) trait values are now commonly used as a metric to quantify community response to environmental forcing. Studies generally take for granted that interspecific variability in traits is sufficiently large compared to intraspecific variation to justify using species’ mean trait values to calculate the CWM. To assess the factors responsible for trait variation, and to test the effect of intraspecific trait variation on the CWM, we examined data collected for a multi-year biodiversity study on the Fort Bragg near Fayetteville, NC. In 2011 we set up 142 vegetation plots at 32 sites spanning a large range of soil moisture conditions and fire return intervals. In each plot we collected environmental covariates, the aerial cover and biomass production of over 150 herbaceous species, along with a suite of functional traits related to nutrient acquisition/retention and fire tolerance/promotion.
We show that both inter- and intraspecific variation in functional traits such as specific leaf area are largely explained by a small number of key environmental drivers such as time since the last fire and plot soil moisture. Because of this variation between and among species along the environmental gradients, the use of species’ global average trait values to calculate CWM traits results in estimates that are off by 14-21% when compared to CWM trait measurements calculated using only local specimens. Model selection based on these global CWM trait estimates results in radically different inference about trait-environment relationships than inference based only on local data. These results highlight the importance of intraspecific trait variability in determining community function and suggest caution in interpreting results of community analyses that are based on species’ average traits. Further, intraspecific variability in functional traits that drive key ecosystem processes has the potential to change ecosystem functioning, independent of changes in community composition.