Effect sizes and standardization in neighborhood models of forest stands

Abstract

Effects of conspecific neighbors on growth and survival of trees have been found to be related to species abundance. Both positive and negative relationships may explain observed abundance patterns. Surprisingly, it is rarely tested whether such relationships could be biased or even spurious due to influences of spatial aggregation, distance decay of neighbor effects and standardization of effect sizes. To investigate potential biases, we simulated communities of 20 identical species with log-series abundances but without species-specific interactions. We expected no relationship of conspecific neighbor effects with species abundance. Growth of individual trees was simulated in random and aggregated spatial patterns using no, linear, or squared distance decay. Regression coefficients of statistical neighborhood models were unbiased and unrelated to species abundance. However, variation in the number of conspecific neighbors was positively or negatively related to species abundance depending on spatial pattern and type of distance decay. Consequently, effect sizes and standardized regression coefficients were also positively or negatively related to species abundance depending on spatial pattern and distance decay. We argue that tests using randomized tree positions and identities provide the best bench marks by which to critically evaluate relationships of effect sizes or standardized regression coefficients with tree species abundance.