Dead tree removal after drought mortality increases understory plant diversity in a mixed conifer forest

Altered disturbance regimes during a time of rapid global change present challenges for decision-making concerning ecosystem recovery. In the Sierra Nevada of California, USA, 129 million trees died due to severe drought from 2012-2017, raising concerns over forest ecosystem health and dead fuel loads available for future wildfires. One approach to managing forests after tree mortality is to cut and remove standing dead trees, yet the ecological effects of this management practice are not well understood, particularly in western U.S. montane coniferous forests. We compared the management practice of dead tree removal versus no removal following severe drought and insect-induced tree mortality in the Sierra Nevada and evaluated the initial effects on understory plants in terms of diversity, vegetative cover, and community composition. Understory plants were sampled in 122 paired plots (treated and untreated; 22 m diameter) spanning 300 km of the Sierra Nevada one to five years after the removal of recently dead trees. We found that plant species diversity was higher where dead trees were removed across all plant subgroups (annuals, perennials, shrubs, natives, and non-natives). Higher total plant species richness and annual species richness were associated with lower live tree cover, but shrub and perennial richness were associated with higher estimated soil moisture and not with live tree cover. Understory vegetative cover was modestly lower with treatment, driven largely by decreases in tree sapling cover, but annual and non-native species cover were higher with treatment. Plant community composition was significantly different in treated and untreated plots (perMANOVA; p = 0.006), but communities were highly overlapping. Our results align with research in other western U.S. forests that found a near-term increase in understory diversity following drought-killed dead tree harvest, but future research is needed to identify longer-term patterns. Our project provides essential information to assess whether recovery goals related to understory vegetation are likely to be met under different post-disturbance management scenarios.

Methods
Field data were collected in 2020 and 2021 at six sites in the Sierra Nevada, California, USA on the Stanislaus, Sierra, and Sequoia National Forests, the Sequoia National Monument, on private lands owned and managed by Southern California Edison, and at Mountain Home Demonstration State Forest. 

Soil lab data were processed at the University of California, Davis, Analytical Laboratory. 

Treatment consisted of felling and removal of dead trees via single-tree selection. 

Data are observational. Treatments were not implemented experimentally but rather as part of separately planned management actions.