Insect and pathogen outbreaks, fires, hurricanes, severe drought and other disturbances alter the structure, composition and function of forests. Results from silvicultural experiments suggest disturbances in lower density forests lead to better mitigation of drought stress for surviving trees than those in higher density stands. As drought and insect and pathogen outbreaks are expected to become more severe and frequent, understanding how surviving trees’ climatic response is affected by disturbance provides valuable insight into how future compound events alter tree growth.
Hemlock woolly adelgid (HWA) is an exotic insect pest introduced to the eastern U.S. in the 1950s. At our old-growth study site in New Jersey, HWA began killing Tsuga canadensis in the mid-1990s. We hypothesized HWA-related mortality would avail resources to surrounding trees, reducing sensitivity to summer drought post-disturbance, especially for drought-sensitive species. We used bootstrapping methods and a state-space model analysis to examine climate-growth correlations before and after disturbance.
Our results reveal climate-growth relationships clearly differ before and after HWA in all species except Pinus strobus and Acer rubrum. Surviving Tsuga canadensis, Betula lenta, Quercus montana, and Quercus rubra trees followed the expected pattern and have become less sensitive to June-July water stress over time. This suggests disturbances alter forest response to changing climate by affecting the resources and climatic limitations of surviving trees. These responses will likely vary depending on species and disturbance type. Our findings emphasize the need to consider the role of disturbances to understand forest vulnerability to climate change.