Wetter Forest Trees More Drought-Sensitive, Study Finds

Annual tree-ring growth records from more than 122 species of trees show that trees growing in wetter forests are more sensitive to increasing drought. The findings – which tackle a research question that has yielded contradictory results in the past – suggest that land management and policy focused solely on drought effects in drier regions overestimates the resilience of forests in wetter regions. Forests cover roughly 30% of Earth's surface and, in addition to providing a host of valuable ecosystem services and harboring huge biodiversity, they play a crucial role in the planet's carbon cycle, absorbing more atmospheric carbon than all other terrestrial ecosystems. However, ongoing climate change is shifting the structure and function of forests worldwide, threatening tree growth and survival. What's more, research suggests that forests will continue to shift from carbon sinks to sources as the effects of climate change increase. Thus, to manage and respond to these changes, there is a critical need to predict which forests are most vulnerable to a hotter and drier future. Previous studies that have explored variation in drought sensitivity have produced contradictory results; while some have suggested that trees in drier areas of their ecological range are the most drought-sensitive, others have concluded that trees growing in wetter portions of their range are the most vulnerable as they lack adaptations that could make their counterparts in drier regions more resilient. To investigate this question, Robert Heilmayer and colleagues analyzed 6.6 million tree ring measurements from 122 species to evaluate trees' sensitivity to water and energy availability. Although drought-induced declines in tree growth frequently occur in dry regions, Heilmayer et al. found that trees growing in wet and warm regions of their range are the most vulnerable to drought and will likely experience the greatest declines in growth under climate change in the next century. The findings suggest that drought adaptations in trees from more arid regions could partially protect them from climate change-induced drought. "Policy-makers who seek to protect forests from climate change may need to expand the focus of conservation interventions beyond species' dry-range edges," write the authors. "By contrast, drought adaptations in populations from drier regions could be useful for management interventions, including assisted migration into wetter regions."