Changing boreal tree densities lead to higher wildfire risk and alter crucial forest functions

Boreal forests encircle the cold, northern regions of our planet. They host a diverse set of species, regulate global climate by storing huge amounts of carbon in the trees and soils, and provide for the livelihoods of millions of people. However, within decades, climate warming is now rapidly changing the conditions under which boreal forests have fulfilled their important roles for millennia. Understanding how boreal forests may change in the future is therefore key.

Because boreal trees grow slowly, it takes some time to see changes in the forest. “This time lag could keep the forests in a state that doesn’t match the changing climate”, explains Ronny Rotbarth, lead author of the new study, which he carried out as a PhD candidate at Wageningen University & Research. “Forests growing at the southern boreal distribution range may not be able to maintain the current amount of trees as climate becomes drier and warmer. Therefore, they may become less dense. Other forests, in the cold north, could support more trees as conditions warm up, and in fact become denser than the forests we currently find there.”

Past changes offer glimpse into the future

Future changes of ecosystems, such as boreal forests, are typically projected by complex simulation models. In the new study, researchers from Wageningen University & Research used an alternative innovative approach: “By examining two decades of tree density from space, we explored whether past changes could hint at the future state to which the forests may be shifting”, says Egbert van Nes, co-author of the study. The researchers used the changes in tree density during that time to project forest conditions until the year 2100.

Towards a single forest state

The results indicate that global boreal forests might change to a more open type, with trees covering about 30 to 50 percent of the area. “This is very different from what we see now, which includes both dense forests with more than 60 percent tree cover and sparse forests with around 5 to 15 percent tree cover”, says Marten Scheffer co-author of the study. “We were surprised by how consistent this projection is across the global distribution of boreal forests.” According to the model, tree cover in warmer boreal forests decrease, while cover in colder, northern boreal forests increase, moving the entire forest system to one single state.

Increased fire risk

The open forest type in a warmer world could lead to more forest fires, which would release a lot of carbon that is currently stored in the trees and soils. “The currently dominating sparse and dense forest types burn less frequently than the open type”, says Milena Holmgren, co-author of the study. “Sparse forests typically do not provide enough fuel for fires to burn and dense forests create micro-climates which prevent forest fires. The open type we project, however, meets the sweet spot and could lead to more fires than we observe at present. The crucial functions of boreal forests would in this case be at risk.”

Text: Wageningen University & Research
Photo: Jason Rossiter Flickr CC BY-NC 2.0 (lead photo: Alaska forest)
Figure: Rotbarth and colleagues