Local species diversity is maintained in part by conspecific negative density dependence (CNDD). This pervasive mechanism occurs in a variety of forms and ecosystems, but research to date has been heavily skewed towards tree seedling survival in tropical forests. To address this research bias, we investigated how the growth rates of “saplings” (defined here as trees 1-5 cm in diameter at breast height) were affected by conspecific adult neighbors in a temperate deciduous forest (the 25.6 hectare Smithsonian Conservation Biology Institute plot in Front Royal, VA). We confronted this question by: 1) examining sapling growth rates as a function of their local adult tree neighborhood (via spatial autoregressive modeling), and 2) comparing the spatial positioning of fast-growing saplings and slow-growing saplings, with respect to adult conspecific trees (via bivariate point pattern analysis). In addition, to determine whether CNDD-driven variation in growth rates leaves a corresponding spatial signal (at a single point in time), we extended our point pattern analysis to a comparison of saplings and a larger size class (5-10 cm diameter stems). In all cases, we also considered the effects of heterospecific adult trees, and focused our analysis on common species capable of reaching canopy height.
Results/Conclusions
We found that negative conspecific effects dominated: most species (five of nine) exhibited CNDD, only one species showed evidence of a positive conspecific effect, and one to two species (depending on the analysis) displayed heterospecific effects. There was general agreement between the spatial autoregressive models and the point pattern analyses based on sapling growth rates (slow vs. fast), but point pattern analyses based on size classes (1-5 cm vs. 5-10 cm) yielded results that differed markedly from the other two approaches. As such, our results bolster concerns that spatial analyses based solely on size classes may fail to detect the process of interest (e.g. neighborhood-driven variation in growth rates), probably due to a multitude of confounding factors. Our work adds to the growing body of evidence that CNDD is a major force in temperate forests, and definitively extends the reach of this phenomenon to sapling growth rates.