A recent analysis of U.S. Forest Service data reveals distinct regional shifts in forest productivity across the United States due to the impacts of climate change. The study, led by UF Biology researchers J. Aaron Hogan and Jeremy W. Lichstein and published in the Proceedings of the National Academy of Sciences, emphasizes the disparate effects of rising temperatures, droughts, wildfires, and disease outbreaks on Western and Eastern U.S. forests.
Over the last two decades, the Western U.S., grappling with more severe climate change impacts, has experienced a significant slowdown in forest productivity, while the Eastern U.S., facing milder effects, has witnessed slightly accelerated growth. This regional imbalance in productivity, a crucial indicator of forest health measuring tree growth and biomass accumulation, underscores the vulnerability of Western forests to climate-related stressors.
Forests play a pivotal role in climate regulation, acting as carbon sinks that absorb approximately 25% of human carbon emissions annually. However, the study warns that the delicate balance between positive and negative climate change effects on forests is shifting, impacting their ability to store carbon effectively.
Using national-scale forest inventory data, the researchers analyzed trends from 1999 to 2020, examining 113,806 measurements in non-plantation forests. The findings challenge assumptions about the continued increase in forests’ carbon-storing ability. While some factors like carbon-dioxide fertilization are predicted to have positive effects on tree growth, adverse impacts such as droughts and forest pathogens are shown to outweigh these benefits, particularly in the Western region.
The study questions the prevailing notion that higher carbon-dioxide levels lead to increased photosynthesis and subsequent tree growth. The data from the Western U.S. suggests that extreme climate effects are overshadowing any positive growth trends, challenging the assumption of forests continuing to serve as effective carbon sinks.
The researchers caution that future projections of climate change impacts may be overly optimistic, emphasizing the need to acknowledge regional-specific influences. The study also highlights that climate change can push forests past tipping points, with some already transitioning from carbon sinks to carbon sources.
The decline in productivity in the Western U.S. is not solely attributed to increased tree mortality from events like wildfires but is primarily linked to declining tree growth rates. Even without intensifying wildfires, the weakened carbon sink in Western forests implies a continued decrease in their ability to sequester carbon unless urgent action is taken to reduce human greenhouse gas emissions.
The study underscores the crucial connection between healthy forests and emissions reduction to restore the global carbon balance and limit climate change. The researchers emphasize the need for collaborative efforts by governments and industries to reduce greenhouse gas emissions and achieve net-zero emissions to ensure the future resilience and sustainability of U.S. forests.
Collaborating on this study were Grant Domke from the U.S. Forest Service Northern Research Station, Kai Zhu from the University of Michigan, and Dan Johnson from UF’s School of Forest, Fisheries, and Geomatics Sciences.