The terrestrial biosphere functions as a sink for a large percentage of human greenhouse gas emissions, and forests represent the majority of this function. Accurately understanding forest ecosystems critically impacts our ability to understand, predict, and mitigate the trajectory of climate change. Much research has focused on the functional differences between tropical, boreal, and temperate forests and the relative sizes of their carbon fluxes and pools. More than 23% of northeastern forests are within 30 meters of a non-forested land cover, and yet our understanding of forests and the role they play in carbon cycling is still predominantly based on intact, interior forests. As anthropogenic forest fragmentation continues to spread, these intact forests grow increasingly rare. Recent studies suggest that trees grow differently near the edge of forest fragments, and the resulting carbon uptake may vary substantially from our current projections based on intact forests. Our study extends earlier plot-based studies by examining fragmentation at a continental scale. We used the USFS’s Forest Inventory and Analysis database to identify more than 17,000 forest edge plots across 20 states in the northeastern US. Using a causal inference framework, we are able to quantify differences in biomass and growth between forest edges and the interior.
Results/Conclusions
Our results confirm preliminary findings that trees along forest edges have a 17% higher basal area increment than their counterparts in the interior, with large differences as a function of forest type. This pattern is most pronounced along anthropogenically-formed edges, which show an average 30% increase in basal area increment compared to the forest interior. Conversely, naturally-occurring edges show no significant difference in tree growth between the edge and interior. Using a causal framework, we are able to separate the effects of forest fragmentation from other variables that have been shown to control growth, including light, water, and temperature limitations, as well as nitrogen deposition. Overall, our results demonstrate that forest fragmentation leads to significant differences in tree growth across the region. Our current understanding of forest dynamics, particularly as expressed in carbon cycle models, does not fully account for the unique ecosystems that are fragmented forests. To accurately predict how forests will respond to pressure from many other global change drivers, we must update our current framework of forest ecosystems to reflect the reality of an increasingly fragmented landscape.