PS 43-159 - Restoration silviculture reduces risk of drought-induced hydraulic failure in temperate rainforest trees

Abstract: Global climate change has placed even coastal rainforests of the Pacific Northwest at risk of future mortality due to drought. Previous research, largely from dry forests, has suggested that restoration silvicultural treatments such as selective harvesting may increase the resistance and resilience of forests to drought. However, this remains largely untested in wet, temperate rainforests. Ellsworth Creek Preserve is a temperate rainforest preserve in SW Washington undergoing management by by The Nature Conservancy (TNC) to restore old-growth forest structure and function to second-growth forests and increase their resilience to climate change and, therefore, offers an ideal site to test the effects of restoration silviculture on tree drought resilience in relatively mesic forest types. Using a novel drone-based sampling method, we collected stem samples from the uppermost branches of mature individuals of the 3 most common conifer species in Ellsworth: western redcedar (Thuja plicata), western hemlock (Tsuga heterophylla), and Sitka spruce (Picea sitchensis) throughout the 2022 dry season from both restored and unmanaged (control) sites. We used these samples to compare in situ water status (predawn and midday leaf water potentials; Ψw),drought resistance traits from xylem vulnerability curves (P50, P88), and calculate hydraulic safety margins (HSM50, HSM88) – a known predictor of future drought resistance and mortality risk – between restored and unmanaged stands.



Analysis of hydraulic safety margins show two important patterns.First, the hydraulic trait-based metrics show that trees in restored stands are significantly more buffered against drought-induced stress (i.e. higher HSM50) and drought-induced mortality (i.e. higher HSM88) than trees in control stands across species. These patterns were strongest for Sitka spruce, intermediate for western redcedar, and weak for western hemlock. These results indicate that restoration silviculture involving ecological thinning treatments may increase the climate resilience of trees in treated stands, specifically through the mechanisms of reduced risk of drought-induced stress (cavitation) and mortality (hydraulic failure). Furthermore, these findings support the use of hydraulic traits as an important tool for evaluating the climate adaptation potential of silvicultural applications.