Efficacy of forest restoration treatments across a 40-year chronosequence at Redwood National Park

Kevin R Soland, Forestry & Wildland Resources, Humboldt State University, Arcata, CA, Lucy Penn Kerhoulas, Forestry and Wildland Resources, Humboldt State University, Arcata, CA, Nicholas John Kerhoulas, Wildlife, Humboldt State University, Arcata, CA and Jason R Teraoka, Redwood National Park, Orick, CA

Background/Question/Methods
Due to past logging practices, much of the redwood (Sequoia sempervirens) range consists of dense, second-growth stands with slow tree growth and low biodiversity. Thinning treatments are widely used in these stands to increase tree growth rates, ecosystem biodiversity, and old-growth forest characteristics. To better understand forest responses to restoration treatments, we used unthinned control plots and a 40-year chronosequence of plots treated with a 40% basal area reduction. We asked: Does treatment affect redwood physiology, redwood growth, and redwood forest biodiversity, and if so, how persistent are these responses? To assess physiology, during the 2018 and 2019 growing seasons we measured predawn and midday water potential using a pressure chamber and stem psychrometers and we measured stomatal conductance using a leaf porometer. To assess tree growth, we used dendrochronological analyses to calculate annual basal area increment across time since before thinning treatments to the present. To assess biodiversity, in 2018 and 2019 we surveyed understory plants, conducted point county surveys to inventory birds, and set camera traps to inventory mammals.

Results/Conclusions
Treatments did not influence redwood water potential in the short- or long-term; all trees maintained a relatively hydrated water status during all measurement periods. This finding suggests that post-treatment increases in evapotranspiration are unlikely to induce water stress in residual trees of this temperate rainforest. Stomatal conductance was significantly higher two-years post-treatment in thinned compared to unthinned stands, suggesting that increased light following treatment can increase gas exchange in residual trees. In the long-term, treatments increased radial growth rates in residual trees compared to trees in unthinned stands. Time between treatment and increased growth ranged from 4 to 10 years, with the increased growth invariably persisting to the present. Assessments of plant, bird, and mammal diversity collectively indicate that diversity increases post-treatment and that this increase slowly reduces over time as the overstory recloses. Our study demonstrates that thinning second-growth redwood forests has the potential to accelerate the development of old-growth forest characteristics; this is an important verification of efficacy, as landscape-scale applications of these treatments are currently underway. Further, our measurements of tree physiology, tree growth rates, and forest biodiversity provide useful baseline data to aid future assessments of long-term forest responses to contemporary restoration efforts.