Attention Trillion Tree planters. There are some lessons to learn.
J. Barquin et al in Science 29 September 2022 (paywall):
This summer, Europe broke high temperature records across the continent (1) and experienced the largest widespread drought in 500 years (2). The extreme climatic conditions contributed to an increased number and extent of forest fires across Europe (3). Large forest fires are boosted not only by warmer and dryer conditions but also by the type and state of the vegetation (i.e., fuel models) (4). Many recent forest fires worldwide have been fueled by large areas of evergreen monoculture plantations, such as Eucalyptus spp. in Chile and Portugal in 2017 (5) and Pinus spp. in France this past summer (6). Given increasing temperatures and fire risks, the traditional practices for evergreen tree plantations are no longer a responsible forest management strategy.
Evergreen trees have been shown to serve as efficient fuel for fires. Specific plant traits, such as essential oils or enhanced terpene content in Eucalyptus spp. and Pinus spp. leaves, respectively, increase the trees’ flammability (canopy ignition and the subsequent resprout or seed release help the species to outcompete others) (7). The higher evapotranspiration rates of evergreens compared with native forests translate to lower soil humidity and dryer conditions (8). In addition, monoculture plantations often suffer from poor or nonexistent design and forest management, which translates into high tree densities, species homogeneity, and increased continuity of fuel loads (9).
Evergreen monoculture plantations have increased worldwide over the past two centuries (10). In many cases, they are the selected species for large-scale reforestation for erosion control, carbon sequestration, or restoration of burned areas. Current environmental policies continue to favor these strategic responses, so additional monoculture plantations are likely [e.g., (11)]. However, given the fire risks they pose and the increasing probabilities of heatwaves, the lack of innovative design and management strategies for these evergreen monocultures is creating the perfect conditions for larger and more devastating forest fires worldwide.
Leverkus et al M 5 May 2022 Science (paywall):
Grassroots movements such as the Trillion Tree Campaign (1) and international policies such as the EU Biodiversity Strategy for 2030 (2) aim to mitigate climate change through ambitious tree planting objectives. However, tree planting targets could produce counterproductive side effects (3), including an increase in the amount and continuity of fuels, a key driver of large fires in a warming world (4, 5). Drier weather, coupled with afforestation dominated by extensive, dense, even-aged, monospecific conifer or eucalypt plantations, has already promoted megafires in places like Chile and Portugal (6), and burnt areas are more likely to reburn when postfire management includes extensive reforestation instead of natural regrowth (7). Reforestation programs should prioritize the mitigation of fire risks.
Scientists, public administrations, and the forestry industry must reconsider tree planting design, strategies, and management actions when dealing with evergreen monoculture plantations. Otherwise, we might be just casting more wood on the bonfire.
- Strict anti-fire policy was implemented in the 1940s aimed at stomping out wildfires and preventing them from starting.
- But many scientists and wildlife experts say fire suppression can be harmful to ecosystems.
- A new study finds fire suppression has contributed to a big compositional change in eastern U.S. forests, with less resilient tree species supplanting drought- and fire-resistant species.
According to a new study, strict wildfire control has changed the forests in the eastern U.S. over the past century, leading to less drought resistance. Researchers are warning this could make them more vulnerable to the effects of climate change.
Wildfire was a common visitor to many eastern U.S. forests for millennia, encouraging the growth of vegetation species that evolved to survive periodic burning events. But then heavy logging began 140 years ago, spurred by the industrialization of the U.S. and followed by a spate of fires that destroyed much of the region’s remaining forest. As a result, strict anti-fire policy was implemented in the 1940s aimed at stomping out wildfires and preventing them from starting.
The iconic Smokey Bear became the mascot of movement, and “Only you can prevent forest fires” became his call to arms. But many scientists and wildlife experts decry the friendly-looking character and the policies he represents, saying fire suppression can be harmful to ecosystems.
A study published earlier this month in the journal Tree Physiology adds its voice to the dissent, with its authors finding 70 years of fire absence has fundamentally changed the forests of the eastern U.S.
“Our forests are in a state of flux from these two very contrasting land-use history events,” said Marc Abrams, professor of forest ecology and physiology at Pennsylvania State University, and co-author of the study. “We had a lot of fire on the landscape during the time of Native American occupation and also during early European settlement and the associated clear-cut era — so we went from a moderate to large amount of fire to an era of overprotection. With Smokey Bear, we have lost fire, and we need to get it back. This issue is on the radar screen of foresters.”
Specifically, Abrams and co-author Gregory Nowacki of the USDA Forest Service found that tree species more sensitive to drought and fire like maple and birch have replaced species that are less sensitive, such as oak and hickory. This, they say, may have serious implications as climates shift due to global warming.
“Eastern forests are changing in a way that we haven’t seen for thousands of years, and this is basically because they have gone through major changes in disturbance regimes and land-use history,” Abrams said. “The change to less drought resistance — part of a process known as mesophication — has serious implications in a warming climate, which portends more frequent and more severe droughts.”