Restoration of Old Forest Features in Coast Redwood Forests Using Early-stage Variable-density Thinning

Shade, light, and stream temperature responses to riparian thinning in second-growth redwood forests of northern California

Resource managers in the Pacific Northwest (USA) actively thin second-growth forests to accelerate the development of late-successional conditions and seek to expand these restoration thinning treatments into riparian zones. Riparian forest thinning, however, may impact stream temperatures–a key water quality parameter often regulated to protect stream habitat and aquatic organisms. To better understand the effects of riparian thinning on shade, light, and stream temperature, we employed a manipulative field experiment following a replicated Before-After-Control-Impact (BACI) design in three watersheds in the redwood forests of northern California, USA. Thinning treatments were intended to reduce canopy closure or basal area within the riparian zone by up to 50% on both sides of the stream channel along a 100–200 m stream reach. We found that responses to thinning ranged widely depending on the intensity of thinning treatments. In the watersheds with more intensive treatments, thinning reduced shade, increased light, and altered stream thermal regimes in thinned and downstream reaches. Thinning shifted thermal regimes by increasing maximum temperatures, thermal variability, and the frequency and duration of elevated temperatures. These thermal responses occurred primarily during summer but also extended into spring and fall. Longitudinal profiles indicated that increases in temperature associated with thinning frequently persisted downstream, but downstream effects depended on the magnitude of upstream temperature increases. Model selection analyses indicated that local changes in shade as well as upstream thermal conditions and proximity to upstream treatments explained variation in stream temperature responses to thinning. In contrast, in the study watershed with less intensive thinning, smaller changes in shade and light resulted in minimal stream temperature responses. Collectively, our data shed new light on the stream thermal responses to riparian thinning. These results provide relevant information for managers considering thinning as a viable restoration strategy for second-growth riparian forests.

Regeneration Dynamics of Coast Redwood, a Sprouting Conifer Species: A Review with Implications for Management and Restoration

Coast redwood (Sequoia sempervirens (Lamb. ex. D. Don) Endl.) is unique among conifer species because of its longevity, the great sizes of individual trees, and its propensity to reproduce through sprouts. Timber harvesting in the native redwood range along the coast of the western United States has necessitated restoration aimed to promote old forest structures to increase the total amount of old forest, the connectivity between old forests, and to enhance the resiliency of these ecosystems. After disturbance or harvest, healthy redwood stumps sprout vigorously, often producing dozens of sprouts within two years of disturbance. These sprouts form highly aggregated spatial patterns because they are clustered around stumps that may number less than 50 ha−1. Thinning of sprouts can accelerate individual tree growth, providing an effective restoration strategy to accelerate formation of large trees and old forest structures or increase stand growth for timber production. However, management, including restoration activities, is a contentious issue throughout the native range of redwood because of the history of overexploitation of this resource and perceptions that overexploitation is continuing. This paper reviews the science of early stand dynamics in coast redwood and their implications for restoration and other silvicultural strategies.

Management trade-off between aboveground carbon storage and understory plant species richness in temperate forests

Because forest ecosystems have the capacity to store large quantities of carbon (C), there is interest in managing forests to mitigate elevated CO2 concentrations and associated effects on the global climate. However, some mitigation techniques may contrast with management strategies for other goals, such as maintaining and restoring biodiversity. Forest thinning reduces C storage in the overstory and recruitment of detrital C. These C stores can affect environmental conditions and resource availability in the understory, driving patterns in the distribution of early and late-seral species. We examined the effects of replicated (N = 7) thinning experiments on aboveground C and understory vascular plant species richness, and we contrasted relationships between aboveground C and early- vs. late-seral species richness. Finally, we used structural equation modeling (SEM) to examine relationships among early- and late-seral species richness and live and detrital aboveground C stores. Six years following thinning, aboveground C was greater in the high-density treatment and untreated control than in moderate- (MD) and variable-density (VD) treatments as a result of reductions in live overstory C. In contrast, all thinning treatments increased species richness relative to controls. Between the growing seasons of years 6 and 11 following treatments, the live overstory C increment tended to increase with residual density, while richness decreased in MD and VD treatments. The richness of early-seral species was negatively related to aboveground C in MD and VD, while late-seral species richness was positively (albeit weakly) related to aboveground C. Structural equation modeling analysis revealed strong negative effects of live overstory C on early-seral species richness balanced against weaker positive effects on late-seral species richness, as well as positive effects of detrital C stocks. A trade-off between carbon and plant species richness thus emerges as a net result of these relationships among species traits, thinning treatments, and live and detrital C storage. Integrating C storage with traditional conservation objectives may require managing this trade-off within stands and landscapes (e.g., maintain early-seral habitat and species within dense, C-rich forests and, conversely, live and detrital C stores in early-seral habitats) or separating these goals across scales and species groupings.