Temperate forest regeneration dynamics: a comparison of three mapped forests

Background/Question/Methods

Forests are dynamic systems and their composition can have a great impact on the sustainability of natural and anthropogenic ecosystems.  Regeneration of tree seedlings determines the successional trajectory of forests.  We examined the role of density dependence in shaping the tree seedling communities in three large forest plots in Indiana, Virginia and Wisconsin where all stems larger than 1 cm d.b.h. have been mapped, measured and identified.  We use generalized linear models with a spatially lagged weighting to ask whether the local neighborhood density of conspecifics trees affected seedling density.  We tested five species at each site that were drawn from across the species abundance spectrum at each site.  We examined the strength of density dependence on species abundance and diversity in both intra- and inter-forest comparisons.

Results/Conclusions

Almost half of the species examined showed a significant negative density dependence of seedling establishment in relation to the density or basal area of conspecific trees.  Additionally, the strength of density dependence was positively correlated with the relative abundance of the species at each forest where rare species had a stronger signal of negative density dependence than common species.  Significant differences were found between the average heights of seedlings at the different sites indicating possible differences in herbivore pressure. As seedlings survive and grow to maturity, they must cope with multiple stressors both biotic and abiotic.  Although density dependence has been frequently investigated in tropical forests, our results indicate that temperate forest dynamics are influenced by negative density dependence and suggest that it is a general mechanism that influences the diversity of forests.