It has been estimated that one third of global forest cover has disappeared and 20% of the world’s remaining forest is within 100 m of an edge. It’s no surprise, then, that edge effects are a key component of understanding how forests may respond to future environmental conditions. Throw in the heat stress of climate change, and predicting that response may become even more complicated.
When trees are found on the border of a fragmented forest, edge effects may create a unique microclimate and more exposure to disturbance. Previous research has shown that edge effects negatively affect tropical rainforests, temperate rainforests, and boreal forests, but does this hold true for all types of forests?
The answer seems to be no. New research by Reinmann and Hutyra in PNAS looks at trees on the edge of temperate broadleaf forests in the northeastern U. S., where much of the remaining forest exists as patches in a matrix of human development. They measured individual trees on a gradient from the edge of the forest to the interior, and asked how position affected growth, carbon storage, and response to unusually high temperatures.
Two big results emerge. First, in contrast with other studies, overall forest growth and aboveground carbon storage consistently increased with proximity to the edge. Second, forest growth was more impacted by heat stress at the edge than at the interior, implying that rising temperatures due to climate change are likely to cause problems.
So what is the net result of edge effects on trees in temperate forests? Although being near the edge seems to have a positive effect on growth and carbon storage, the negative effects of higher temperatures and fragmentation in general ultimately outweigh these benefits. This is especially true given that heat stress could nearly double by the end of the 21st century, potentially reducing regional forest growth by over one-third. There seems to be no long-term benefit for trees on the edge, although understanding how edge effects influence forest growth and carbon storage will lead to a more accurate picture of future forest dynamics and better estimates of the global terrestrial carbon sink.
Reinmann, A. and L. R. Hutyra. 2017. Edge effects enhance carbon uptake and its vulnerability to climate change in temperate broadleaf forests. Proceedings of the National Academy of Sciences 114(1): 107-112.