Abstract A growing interest in the restoration of young second-growth forests by managers of reserves in the redwood region has led to a need to evaluate restoration-based silvicultural strategies. This case study assessed the effectiveness of low thinning as a forest restoration tool via analysis of stand structure at Redwood National Park's Whiskey Forty Forest Restoration Study. The second-growth stand had more than 5,500 trees ha−1 and 57.0 m2 ha−1 basal area and consisted chiefly of three species: Douglas-fir (the dominant species), redwood, and tanoak. Low thinning reduced stand density but also reduced species richness by eliminating scarce species. Seven years after thinning, growth was enhanced (33.6% gain in basal area), and mortality was minor (3% of all stems); however, Douglas-fir remained competitive in the upper canopy. Its average basal area increment was less than redwood's, but its radial growth was equal and its rate of basal area growth was greater in the years following thinning. We conclude that the thinning improved stand conditions but did not fully satisfy restoration goals and that other thinning methods, such as variable-density thinning, are likely to be more effective at promoting redwood dominance.
For more than 30 years, Redwood National Park has been working to establish a Forest Restoration Program to rehabilitate its impaired, second-growth forests. This case study outlines the Park’s history of using silviculture as a restoration tool, which began in 1978 after the Park's expansion. The most recent effort was the 1,700 acre South Fork of Lost Man Creek Forest Restoration Project where two silvicultural prescriptions were used. Low thinning on ridge-top sites reduced basal area by 40-percent, and wood generated was sold as forest products. Crown thinning on steep mid-slope sites reduced basal area by 25-percent, and the wood was lopped-and-scattered. Permanent plots were established before thinning and were re-assessed after thinning. Data were analyzed to determine whether the silvicultural prescriptions altered stand structure and species composition to promote redwood dominance. Before thinning, Douglas-fir dominated stand density. Both prescriptions shifted composition in favor of redwood. The ridge-top prescription resulted in 191.3 percent more redwood trees/acre than Douglas-fir, and 80.2 percent more redwood basal area, making redwood the dominant species. The mid-slope prescription resulted in 4.3 percent more redwood trees/acre than Douglas-fir, but Douglas-fir had 5.9 percent more basal area. This project is the Park's first successful attempt at large-scale forest restoration.
The growing impacts of climate change and uncertainty about forest vulnerability to these changes make understanding forest response to drought increasingly important. Many of the studies investigating forest response to drought focus on inland forest types rather than coastal forests. This study examined tree growth and growth response to drought in coastal forests at restoration thinning sites, evaluating responses to local climate, tree-level competition, and site-level factors. Tree cores were extracted at three restoration sites in Redwood National Park, California, from both thinned and unthinned stands. Douglas-fir (Pseudotsuga menziesii) and coast redwood (Sequoia sempervirens) trees > 20 cm diameter at breast height were sampled (n = 268). Thinned stands had lower tree density, individual level competition index, and higher stand basal area than control stands. We modeled the influence of competition, size, site, and species identity on tree growth as measured by basal area increment (BAI). Prior to a period of drought, Douglas-fir had greater BAI relative to redwood, and competition was negatively associated with tree growth for both tree species. There was variability among sites, with the stand closest to the coast and with the most recent thinning treatment showing the fastest growing trees. Relative to the pre-drought period (2010–2012), average BAI declined slightly during the drought period (2013–2015) for Douglas-fir and coast redwood; however, we found less evidence for this effect in coast redwood. Notably, the relationships between growth and competition did not change during the drought period for either species. These results suggest that reducing competition via restoration thinning treatment improves tree growth, and this advantage is maintained even during short (3-year) periods of drought.
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.
Following 20th century logging, much of the natural coast redwood (Sequoia sempervirens [D.Don.] Endl.) range consists of dense second-growth stands with slow tree growth and low biodiversity. There is a landscape-scale effort in much of coastal northern California to increase tree growth rates and ecosystem biodiversity via thinning treatments, thereby hopefully accelerating the development of old-growth forest characteristics. Redwood National Park has been experimenting with thinning in these forest types since the 1970s. This study investigated short- (1 year post-thinning) and long-term (40 years post-thinning) responses of redwood physiology (water potential and stomatal conductance), redwood growth (via tree-rings), and forest biodiversity (understory plants, birds, and mammals) to restoration thinning treatments. We found that thinning second-growth redwood forests with 40% basal area reduction treatments 1) did not meaningfully influence tree water potential, 2) increased tree stomatal conductance in the short-term, 3) increased tree growth in the long-term, 4) increased understory plant diversity, and 5) did not affect bird or mammal diversity. Collectively, these findings demonstrate that thinning second-growth redwood forests has the potential to accelerate the development of old-growth characteristics. This verification of the efficacy of restoration treatments is important information for land managers, as plans are currently underway to apply these treatments at the landscape-scale. This study can provide useful baseline data to aid future assessments of long-term forest responses to contemporary restoration efforts.
