Reference: Asaro, C., Koch, F. H., & Potter, K. M. (2023). Denser forests across the USA experience more damage from insects and pathogens. Scientific Reports, 13(1), 3666. https://doi.org/10.1038/s41598-023-30675-z
Outbreaks Take Root
Forests are under attack. They are receiving blows from drought brought on by climate change, unsustainable logging practices, and the introduction of damaging insects and pathogens. When these stressors join forces, global forest health is severely threatened.
Scientists have joined the battle by trying to understand what makes a forest happy and healthy. Identifying when a forest is unhealthy and what it needs to regain health relies on a consistent and measurable definition of a healthy forest. For example, adjectives like resilient, stable, and vigorous get sprinkled into discussions of what makes for a productive forest, but quantifying these poses a challenge.
Why do we need healthy forests? It’s probably no surprise that forests house diverse animals, plants, and fungi, and provide us with food, shelter, and a peaceful escape from our stressful lives. Importantly, trees help buffer against climate change by moderating temperatures and using up carbon dioxide, a leading greenhouse gas. Further, trees safeguard against floods by soaking up water and protecting against erosion.
Climate change and human activity are also increasing the prevalence of insect and pathogen outbreaks in forests. Identifying at-risk forests could help prevent or intervene in these outbreaks before the forest undergoes significant damage. Scientists have recently developed a tool to determine areas that should be monitored for potential outbreaks.
Scientists Branch Out to Find Solutions
Trees need water, nutrients, and light to survive. Trees constantly compete for these resources in forests, especially when they are limited. Not getting enough resources can lead to stress. Like humans, stress weakens a tree’s immune system, increasing vulnerability to diseases. With the increased frequency of droughts, water has become severely limited in many forests, making trees more susceptible to outbreaks of insects and disease.
Diseases spread rapidly between people living in cities because dense populations give more opportunities for a disease to pass from person to person. Similarly, dense forests of highly vulnerable trees are likely hotspots for detrimental epidemics. With a team of researchers, Christopher Asaro at the USDA Forest Service tested whether forest density could be used to predict areas of outbreak concern.
The researchers used a measure called tree basal area to estimate forest density. This measure calculates how much area is occupied by tree trunks; it is an indicator of both biomass and forest structure. Using 20 years of data, they looked at tree basal area for forests across the United States and looked for overlap with recorded tree damage from insects and pathogens. They found forests with evidence of damage from outbreaks were two to three times denser than undamaged forests. The researchers suggest that tree basal area is a beneficial tool for identifying forests across the world that should be monitored closely for damaging insects and pathogens.
‘Leaf’-ing the Pests Behind?
The patterns of northern forests often deviated from the other regions of the study; tree basal area was similar between damaged and undamaged areas. The northern United States is the (unwelcome) home of many invasive attackers, like the emerald ash borer, spongy moth, and hemlock wooly adelgid. Non-native insects commonly attack the understory and slowly kill off their host, regardless of forest density, making their detrimental impacts challenging to pick up from satellite images like those used in the study. Plus, invasive species can often thrive because their new environments lack their natural enemies.
While native species are still harmful to trees, their hosts have sometimes had millions of years to adjust to their damaging effects. Still, climate change puts environmental stress on forests, making them vulnerable to even these long-time enemies. Tree basal area seems to be a valuable tool for identifying forests at risk of native outbreaks. However, the researchers caution that it may be less effective at detecting the more looming threat of invasive attacks.
The researchers suggest that this new tool can be a helpful first filter for identifying forests that need more fine-scaled monitoring programs. Insect and pathogen outbreaks are a global challenge and threat, meaning trees are not out of the woods yet. For more information, check out these articles about the Hemlock Woolly Adelgid and Bark Beetles.