We studied biophysical (biotic and abiotic) factors correlating with dominant height and basal area (BA) development within 110 ha of evergreen mixed conifer forest dominated by coast redwood (Sequoia sempervirens (D. Don) Endl.) in north coastal California. Various descriptors of species composition, topography, soil properties and moisture, light and harvest disturbance were assessed in a grid of 234 permanent sample plots. Within the study area characterized by heterogeneous topography and variable species composition, site descriptors alone were poor predictors of stand volume production in multiple linear regressions. Multivariate canonical correlation analysis revealed that redwood height growth was related to a different suite of biophysical variables than BA growth. Greater stand volume production was measured in areas with less hardwood, more Douglas-fir (Pseudotsuga menziesii var. menziesii (Mirbel) Franco), higher soil pH and at lower slope positions. Sheltered lower slopes were more amenable to redwood height growth. Exposed upper slopes and ridges receiving more light favoured BA over height growth in redwood. Our analysis provides a framework for refining estimates of forest growth, yield and carbon stocks in natural forests in accordance with divergent dimensions of productivity and allied biophysical gradients.

Research Highlights: Bark thickness (BT) in coastal redwood (Sequoia sempervirens (D.Don) Endl.) varies in accordance with tree size, crown ratio, position within the canopy, height along the tree stem, genetic identity and latitude. However, current BT predictive equations do not account for such variability, leading to inaccurate BT estimations. We present improved BT models to increase the accuracy of BT estimates for coastal redwood in northern California. Background and Objectives: BT is an important metric that has many practical applications in forest management. However, BT varies substantially across species and environments, as well as across individuals and populations. Our objectives were to investigate BT along various gradients of change, with factors accounting for genetics, tapering of BT along the tree bole, differences in BT according to tree crown position within the stand, and the latitude. Materials and Methods: We collected BT data throughout most of redwood’s natural range along a north–south latitudinal gradient. Subsets of these data were used to examine the influence of particular variables on BT while holding the other variables constant. Results: Regionally, the bark was thicker among more xeric southern redwoods and thinner among more mesic northern redwoods. We found that the BT of codominant, intermediate and suppressed trees was around 8%, 14%, and 18% thicker, respectively, than bark of the same size dominant tree. Redwood trees growing in the partial shade of an overstory had thicker bark than trees growing in even-aged stands and incorporating genetic identity yielded major improvements in the BT model estimates, suggesting that BT is under genetic control. Bark thickness decreased with increasing height along the tree stem, with notable differences in the BT above and below breast height. Conclusions: We recommend utilizing the best available BT equations (over standard ‘bark factors’) in forest science, modeling and management applications. We also recommend the adoption of our drilling method for BT measurement on larger trees due to the potential for error associated with traditional bark gauge measurements.

Forest restoration thinning has the potential to enhance the structural complexity and accelerate the development of large trees important to wildlife, aesthetics, and wildfire resistance. These are key objectives for the restoration of even-aged secondary forests within Redwood National Park in Humboldt County, CA, USA. We evaluated the tree growth and stand structure 10 years after two thinning methods were applied at two intensities in a 40-year-old mixed redwood ( Sequoia sempervirens (Lamb. ex D. Don) Endl.)/Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco var. menziesii) stand. Heavy thinning enhanced the diameter growth of redwood and Douglas-fir trees more than light thinning. Crown thinning generally enhanced the structural diversity more than low thinning, and structural diversity increased progressively over the 10 years following thinning. Understory plant richness fluctuated between measurement years. Heavy thinning enhanced the understory shrub cover. The fastest-growing trees in heavily thinned stands were much more likely to sustain bear damage, especially redwood trees. Overall, different thinning methods and intensities induced a different suite of outcomes, yet none restored redwood dominance, but all treatments enhanced some other ecosystem values important for old-growth restoration such as large overstory trees, understory plant and shrubs, and elements of structural complexity, including tree-size variability, snags, down logs, and trees exhibiting stem or top damage.

Multi-year drought and climate change can impact tree growth, especially in California's Mediterranean climate where growing season rainfall is limited or absent. Active forest restoration has the potential to mitigate climate impacts by reducing stand density and conversion towards more resilient species' composition. We used dendrochronology methods to examine climate–growth relationships for coast Douglas-fir ( Pseudotsuga menziesii var. menziesii) trees in mixed multiaged stands near the species’ natural southern range margin. We found positive correlations of ring width with spring–early summer and prior October precipitation and an evapotranspiration index. Additionally, cooler spring temperature was negatively correlated with growth. We also studied tree resistance, resilience, and recovery from two multi-year drought events. Restoration treatments enhanced resistance and resilience to drought relative to trees growing in untreated plots. We did not detect differences in drought resistance and resilience between two common restoration methods, giving managers options for restoration to lessen drought impacts on tree growth.

We sought to answer the question: How do we restore characteristics of old-growth evergreen mixed-conifer forests in young even-aged stands on upland terrain at Headwaters Forest Reserve (HFR)? We described the old-growth reference condition for three stands at HFR. In each old-growth stand, trees within a 1-ha plot were inventoried. We found coast Douglas-fir (Pseudotsuga menziesii var. menziesii) and tanoak (Notholithocarpus densiflorus) well represented while coast redwood (Sequoia sempervirens) dominated in terms of size. Numbers of understory and overstory trees ha⁻¹ and average tree sizes were similar among sites, suggesting these were useful goals for restoration. Adjacent to each old-growth stand, we measured recent growth rates of second-growth trees and remnant old trees across a range of tree sizes, stand structures, and densities. The resultant growth models of redwood and Douglas-fir enabled us to project the development of precommercially thinned young stands at HFR forward in time under two silvicultural prescriptions: (i) no further management; and (ii) partial harvesting simulated before trees attained 30 cm dbh. The partial-harvesting prescription reduced stand density and set the young stand on a more rapid trajectory towards the reference condition found at HFR.