Understory light, regeneration, and browsing effects in irregular structures created by partial harvesting in coast redwood stands

Regeneration of commercial species is central to long-term success of multiaged management for wood production. We used a replicated uneven-aged silviculture experiment to study regeneration by stump sprouting (Chapter 1) and planted seedlings (Chapter 2). In Chapter 1, we present relationships between understory light, varying overstory tree retention, and growth of coast redwood (Sequoia sempervirens) and tanoak (Notholithocarpus densiflorus) stump sprouts initiated by group selection (GS) and single-tree selection harvesting. First, we quantified understory light throughout this 20 ha experiment comparing four different silvicultural treatments repeated at four sites. Then, we related understory light to post-treatment stand density and treatment type (i.e., complete harvest in 1 ha (2.5 acre) GS opening, low density dispersed retention (LD), and either aggregated (HA) or dispersed high-density retention (HD)). Finally, we quantified height increment of stump sprouts in response to understory light, treatment type, and other candidate variables influencing growth of stump sprout regeneration after partial harvesting. Mean and maximum understory light did not differ significantly between high density treatments. However, the HD treatment had lower minimum light levels when compared to the HA treatment. At all light levels, the dominant sprout within clumps of redwood stump sprouts generally grew faster than dominant tanoak sprouts within tanoak sprout clumps. Differences in sprout height growth between high density aggregated and dispersed treatments were minimal. In the LD treatments, redwood stump sprouts outperformed tanoak sprouts by the greatest margin. Regeneration of redwood and tanoak was most rapid in high light within GS openings. In Chapter 2, we studied how incidences of animal browsing or mortality of planted seedlings related to multiaged treatment type, stand, and site variables. Deer browsing of planted seedlings was a pervasive problem. Incidence of browsing differed among seedling species, treatment type, and position on the landscape (elevation or distance to watercourse). Coast Douglas-fir (Pseudotsuga menziesii var menziesii) seedlings were preferred by browsers over redwood seedlings in this study. The most instances of browsing were recorded in GS treatments, followed by LD, HA, and HD treatments. In treatments with higher densities, browsing was less likely. As distance to watercourse and elevation increased, the probability of browsing diminished for both species. Like browsing, survival of planted seedlings was largely dependent on their position on the landscape. Seedlings planted on a southwest aspect had the lowest survival rates, while seedlings planted on a northeast aspect had nearly complete survival, regardless of species. Overall, Douglas-fir seedlings had higher mortality rates than redwood. Mortality was highest in GS, followed by HA and HD treatments, and was lowest in LD treatments. Seedling survival exhibited a rise-peak-fall pattern with increasing stand density. This pattern was the most distinct on southwest facing slopes. In general, dispersed treatments gave better results than aggregated and GS treatments when trying to maximize survival and minimize the occurrence of browsing. These results inform forest managers implementing a conversion towards multiaged management in coast redwood stands receiving partial harvesting without site preparation or herbicide treatment of re-sprouting hardwoods. Presumably, a reduction in below ground competition from hardwood control would enhance survival of planted seedlings. However, any enhancement of seedling growth and vigor may result in elevated browsing activity. Specific recommendations for management include planting extra seedlings on southern slopes and in stands of lower densities such as group selection openings (in anticipation of elevated mortality), and implementing seedling protection measures (e.g., shelters, repellant, fencing) near watercourses where browsing occurs most often