Understory species have often been proposed as indicators of forest community type and environmental conditions. Reasons for this include their sensitivity to local conditions, rapid recovery from disturbance relative to overstory species, and their richness – representing on average 80% of species in forests of eastern North America. However, there has been limited research regarding the niche relations of understory species, impeding their effective use as environmental indicators. Niche modeling has received increased attention recently, as it is well-recognized as a limitation to the accuracy of species distribution models. The primary goal of our study was to improve niche modeling for understory species to determine their ability to indicate environmental conditions. To accomplish this, we sampled 200 plots across the White Mountain National Forest, New Hampshire. Species presence-absence data were collected for 214 vascular plant species and soils were analyzed by genetic horizon. We used a lidar-derived digital elevation model to calculate predictor variables related to topography, including topographic wetness index as well as variables such as elevation and aspect. The indicator value of species was determined using species response curves from a variety of modeling techniques, including generalized linear models (GLM), generalized additive models (GAM), artificial neural networks (ANN), and recursive partitioning (RP).
Results/Conclusions
Preliminary results suggest that soil chemistry accounts for a considerable amount of variation not explain by topographic predictors. Our results from NMDS ordination show a strong influence of soil fertility and elevation on understory communities. NMDS axes were most strongly correlated with elevation, B horizon C:N, O/A horizon C:N, and O/A horizon pH. Lidar-derived topographic metrics had a statistically significant, but less predicative relationship with NMDS axes. Niche modeling techniques suggest that B horizon chemistry is more predictive of individual species’ distributions than O/A or C horizons. B horizon acid-base cations such as aluminum and calcium were important to numerous species’ niche models, providing evidence of a fertility gradient driving the distribution of many understory species. Our results have important implications for the use of understory species as environmental indicators and the development of species distribution models.