Effect of tree density on seed production and dispersal of birch (Betula pendula roth and betula pubescens ehrhs)

Silver and downy birch (Betula pendula Roth and B. pubescens Ehrhs) are pioneer species which play an important role in forest regeneration in disturbed areas. Knowledge of birch seed production and dispersal is key to making good predictions of the persistence and colonization of birch. Both processes can be affected by the density of trees in the neighbourhood. In this study, we studied the seed production and dispersal of birch trees in two plots in Wytham Woods, UK, in 2015, and investigated the potential effect of neighbourhood tree density. We applied inverse modelling to seed trap data, incorporating tree density around the source tree and on the seed path to estimate birch fecundity and the dispersal kernel of the seeds. We show that the pattern of dispersed seeds was best explained by a model that included an effect of tree density on seed dispersal. There was no strong evidence that conspecific or heterospecific tree density had an effect on birch fecundity in Wytham Woods. A birch with diameter at breast height (DBH) of 20 cm is estimated to have produced ~137,000 seeds in 2015. Mean dispersal distance in an open area is estimated to be 65 m but would be reduced to 38 m in a closed stand. Both the mean dispersal distance and the probability of long-distance dispersal of birch decreases in dense environments. Areas with higher tree density also would intercept more seeds. These results highlight the importance of considering tree density in the neighbourhood and in the overall landscape when predicting the colonization and recruitment of birch.