Article: Increasing Rates of Subalpine Tree Mortality Linked to Warmer and Drier Summers by Andrus and colleagues (2021)
Background: Severe events such as fires, insect outbreaks, and intense droughts are becoming more frequent with climate change. These events are capable of killing thousands of trees and destroying entire forests; however, severe events are not the only cause of tree death. Trees die from other factors apart from severe events such as competition with other trees for resources, endemic insects (as opposed to insect outbreaks), and unfavorable weather conditions. This ongoing, natural tree death, termed “background mortality”, is typically responsible for the deaths of 0.5-2% of trees in a forest each year. Background mortality has no single cause. It is instead due to multiple causes; each one making the tree more vulnerable to the next stressor. Warming climate and other effects of climate change have essentially applied a blanket of stress to all trees, making them more vulnerable to natural causes of death and increasing the background mortality rate. Even increases as small as 0.5% to background mortality rates can, over many years, cause even greater losses of trees and carbon storage than extreme events (e.g., fires, insect outbreaks, severe drought). Understanding how the background mortality rate is changing is essential to predicting future climate change, water cycles, and forest habitat health.
Methods: Andrus and colleagues examined subalpine forests in Colorado to understand how background tree mortality is changing over time. They tracked over 5,000 trees from 1982 to 2019. Each tree was surveyed every 3 years, and the cause of every tree death was investigated. The trees studied varied with respect to elevation, soil moisture, and forest age to understand how these factors affect background mortality. Because of the long length of the study, researchers were also able to draw conclusions on the effects of weather trends on background mortality.
Findings: After 37 years, 908 of the initial 5,043 trees had died. More than half of these mortalities occurred in the last 12 years. Across all tree populations, mortality increased over time. Mortality was also generally higher than recruitment--this means there were more deaths than new seedlings each year, resulting in an overall decrease in the number of trees. Across most tree populations studied, the highest rates of mortality occurred in the hottest and driest years, and the lowest rates of mortality occurred in the coolest and wettest years. Across the 37-year period, over 80% of trees that died of bark beetles (i.e., endemic insect) died in the last 12 years, suggesting that insect infestations are becoming worse with climate change. Results also showed that older trees are more susceptible to background mortality than younger trees.
Conclusions: Forests in drier soils are especially susceptible to increased mortality due to warming conditions. Climate change will continue to increase the rates of background tree mortality. Increasing mortality due to climate change might lead to a slow decline of subalpine forests--decreasing the landscape's ability to store excess carbon. Furthermore, the larger and older trees, which store the most carbon, are most susceptible to increased mortality. The loss of subalpine forests can serve as an early warning sign of habitat degradation and loss of ecosystem services (e.g., storing carbon, filtering water, controlling floods, limiting erosion, sustaining biodiversity, etc.), which makes the conservation of forested lands even more important and stresses the need for active conservation management techniques.
Figure: (a) Map of study area. (b, c, d) Before and after photographs of different tree species studied from 1982 to 2019.
Reference:
Andrus, Robert A., et al. "Increasing rates of subalpine tree mortality linked to warmer and drier summers." Journal of Ecology (2021).
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