Climate Change May Have Altered U.S. Forests: Study

A recent analysis of U.S. Forest Service data conducted by the University of Florida Biology researchers J. Aaron Hogan and Jeremy W. Lichstein exposed the disparate impacts of climate change on forests across the United States. The study, published in the Proceedings of the National Academy of Sciences, outlines the regional imbalances in forest productivity, emphasising the toll that rising temperatures, escalating droughts, wildfires, and disease outbreaks are taking on the American West's ecosystems.

As the Western U.S. grapples with severe climate change effects, the study suggests a notable slowdown in forest productivity, while the Eastern U.S., experiencing milder climate effects, witnesses slightly accelerated growth.

Regional Imbalances

Forests, vital in regulating the Earth's climate by acting as carbon sinks, are experiencing significant regional variations in productivity. The study, utilising national-scale forest inventory data, tracked trends from 1999 to 2020, analysing 113,806 measurements in non-plantation forests. The Western U.S., faced with more severe climate change impacts, exhibits a conspicuous slowdown in productivity. In contrast, the Eastern U.S. shows slightly accelerated growth, emphasising the dichotomy in climate effects across the nation.

Climate change acts as a dynamic force with region-specific influences, pushing some forests past tipping points. While some forests are approaching or surpassing thresholds, becoming sources of carbon, others remain sinks that remove carbon from the atmosphere. This dynamic underscores the importance of understanding climate change's nuanced impact on diverse ecosystems.

Fertilization Hypothesis

The study delves into the complex interplay of various drivers affecting forest productivity. Some drivers, such as droughts and forest pathogens, have negative impacts, while others, like carbon-dioxide fertilization, are predicted to have positive effects. The latter suggests that increased carbon dioxide levels enhance plant growth through increased photosynthesis. However, the study reveals that in the Western U.S., extreme climate effects overshadow any positive growth trends, challenging assumptions about forests' continued ability to store carbon.

J. Aaron Hogan emphasises, "We are witnessing changes in forest functioning as forest ecosystems respond to global change drivers, such as carbon dioxide fertilization and climate change." The delicate balance between the positive and negative effects of climate change will determine forests' functioning in the years to come.

Carbon Sequestration at Risk

The decline in productivity in the Western U.S. cannot be solely attributed to increased rates of tree mortality caused by wildfires. Jeremy W. Lichstein notes, "Our study shows that additional ecosystem carbon loss in Western forests is occurring due to declining tree growth rates." With trees growing slower due to adverse climate change effects, including decreased precipitation, the carbon sink in Western forests weakens, even without intensifying wildfires.

Urgent Action

The transformations observed in U.S. forests underscore the urgency for coordinated efforts to address climate change. Researchers advocate for immediate action to reduce human greenhouse gas emissions, emphasising the essential connection between healthy forests and emissions reduction to restore the global carbon balance and limit climate change.

The study, developed in collaboration with experts from the U.S. Forest Service and the University of Michigan, highlights the need for governments and industries to work together. 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 contributed to the study, emphasising the interdisciplinary approach required to tackle the multifaceted challenges posed by climate change in U.S. forests.