Ripe old age: Do large, old trees contribute disproportionately to forest recruitment?

Background/Question/Methods

Large, old trees serve as keystone structures in many forest ecosystems, acting as important providers of resources and habitat. Due to a greater proportional reproductive allocation than smaller, younger trees, large, old trees may also contribute disproportionately to forest recruitment. In this way, large, old trees may have the potential to strongly influence the trajectory of forests exposed to environmental changes. Yet, due to their rarity, investigation into the demography of large, old trees is limited, and their contribution to recruitment is poorly understood. The objective of this study is to investigate the relative contribution of large, old trees to seed production and seedling recruitment in Pacific Northwest conifer forests. To address this objective, we monitored seed production, seedling germination, and seedling establishment of Douglas-fir over three years and across four mature and old-growth stands of varying ages, including one particularly old stand dominated by large trees. We evaluated the effects of large, old trees at the stand level by applying a Bayesian regression model to quantify the effects of stand age and large tree basal area on recruitment after accounting for the effects of climate and other stand effects. At the individual tree level, we used a Bayesian neighborhood modeling approach to quantify the effects of individual tree size and species identity on seed and seedling densities.

Results/Conclusions

Seed and seedling counts were significantly higher in the two older stands than in the two younger stands in all three years. Regressions considering the independent effects of stand age, stand basal area, large tree basal area, climate, and other stand effects showed a significant and positive effect of stand age, basal area, and large tree basal area on seed and seedling counts. Stand age explained a greater proportion of the variance in seed and seedling counts than stand or large tree basal area. At the individual tree level, the neighborhood analysis revealed a positive yet moderate effect of tree size on seed and seedling densities. Together, our results support the hypothesis that large, old trees have a greater contribution to recruitment than smaller, younger, trees. Yet, this effect was not as pronounced as expected and appears proportional to tree size and age. Nevertheless, these findings do indeed underscore the important role that large, old trees play in providing opportunities for successful forest regeneration; such opportunities will continue to be important for forest persistence and migration in a changing environment.