An ecotone effect on tree community composition: decoupling from the soil environment and increased spatial structure consistent with mass effects

Background/Question/Methods

Areas of transition between adjacent ecosystems are referred to as ecotones.  Here we propose the new hypothesis that ecotones decouple community-environment relationships because transport of abiotic material and dispersal of organism propagules across ecotones are independent processes.  From the perspective of metacommunity organization, this suggests that mass effects gain importance near ecotones, where there should be an increase in the importance of spatial structure that is independent of the environment.  We tested this hypothesis by examining the effects of ecotones on relationships between trees and soil properties in a temperate deciduous forest.  The study area included different landforms defined by topography, hydrology, and geomorphology, which we designated upland, bottomland, and riparian forests.  The site also included a mowed herbaceous corridor.   

Results/Conclusions

We found that soil properties and tree community composition significantly differed among landforms, and thus they could be treated as differing ecosystem types.  However, inclusion of plots near ecotones significantly reduced the variance explained by landform.  In agreement with our hypothesis, we found that ecotones introduced significant unexplained variation into correlations between tree community composition and soil properties.  In addition, spatial variation in tree community composition that was independent of soil properties was only detected when ecotones were included in the analysis.  Together, these results indicate that dispersal limitation and mass effects in the tree community take on increased importance near ecotones.  This decoupling of community-environment relationships near ecotones has important implications for accuracy of models predicting community distributions from abiotic information.