A new study has found that forests in the eastern United States are becoming more vulnerable to drought due to a combination of climate change and human land-use practices such as logging and fire suppression, which may have major implications for forest management in Indiana, the region, and beyond.
Justin Maxwell, Professor and Chair of the Department of Geography within the College of Arts and Sciences at Indiana University Bloomington, is the lead author of the study, published in the journal Global Change Biology.
In the study Professor Maxwell and his co-authors note that forest management activities, development, and climate change have altered the mix of tree species in the region. This is the result of a shift from forests in the region historically dominated by species of trees such as oaks and hickories, which thrive in dryer, more arid environments known as “xeric” ecosystems, to forests today increasingly made up of species that require wetter conditions for growth, like tulip poplars and sugar maples, which thrive in moisture-rich “mesic” ecosystems.
The researchers used tree-ring data from approximately 1,300 trees across 100 sites across the eastern U.S. to measure how different types of trees respond to periods of both wet and dry conditions. The study focused on how mesic tree species like tulip poplars and sugar maples respond to drought, due to their preference for moist environments.
The study’s findings highlight that during periods of drought, tulip poplars and sugar maples exhibit significantly reduced growth compared to periods of increased moisture. The research suggests that while severe droughts cause noticeable declines in growth, frequent mild droughts have a more substantial long-term impact.
Meanwhile, climate models indicate that by 2100, droughts could become three to nine times more frequent than they are today. This anticipated increase in drought could lead to a decline in the ability of forests in the eastern U.S. to absorb carbon from the atmosphere.
“Today we see a lot of forests comprised of trees that flourish in wetter conditions,” Maxwell said. “This got me thinking about how vulnerable our forests are to climate in general, and in the future. As global climate models predict more frequent and severe droughts, it’s crucial to understand how these species will cope.”
Maxwell’s research bridges climate science and ecology using dendrochronology—the study of tree rings, which he analyzes to understand how climatic extremes impact forest ecosystems over time, and to determine how forests will respond to climate change.
Prior to Europeans’ arrival in North America centuries ago, the region’s forests were dominated by oak and hickory trees, explained Maxwell. “They’re more resilient to drought and grow in these dry, xeric environments. They’re like rock stars, in that it’s a very risky strategy to be photosynthesizing and growing during really dry conditions, so they’re much more likely to die from that lifestyle, but it has historically worked.”Due to fire suppression since the 20th century, species of trees such as maples and beech are taking over forest canopies in the eastern U.S.
However, mesic species of trees such as tulip poplars and sugar maples, which require wetter conditions to grow the thrive, are particularly sensitive to drought, and are expected to absorb less carbon if their growth is stunted by more frequent dry periods.
While it remains unclear to experts how this shift will affect overall tree mortality, the potential decrease in carbon sequestration is a significant concern for climate change mitigation efforts. Forests play a critical role in absorbing carbon dioxide, and reduced growth during droughts means less carbon capture.
The study underscores the importance of maintaining a diverse mix of tree species in forest management strategies, Maxwell said.
“The big takeaway of the study is that having a mix of species is crucial for forest resilience,” Maxwell explained. “Focusing solely on drought-tolerant species like oaks and hickories might not be the best approach.”
But because oaks and hickories, which historically dominated these forests, are more resilient to drought conditions but also face higher mortality risks due to their intense growth strategies during dry periods, this complexity makes forest management a challenging task.
Forest managers are already working to address these issues through various strategies, including controlled burns and selective thinning to promote oak and hickory regeneration. However, there have been mixed results, and Maxwell cautioned against relying too heavily on a narrow array of tree species to promote forests’ viability.
“The goal should be to encourage a diverse forest composition that can better withstand future climate changes,” noted Maxwell.
The study highlights the need for continued research and adaptive management practices to ensure that forests in the eastern U.S. remain resilient in the face of changing climate conditions. Maintaining a balance of tree species will be crucial in supporting forest health and carbon storage capabilities in the future.
