Planted seedling regeneration using gap-based silviculture without herbicide in a wildfire-impacted forest of the Sierra Nevada

Gap-based silviculture, which we define as the creation and maintenance of multi-aged stands through the periodic harvesting of discrete canopy gaps, provides a potential mechanism for converting previously high-graded stands into more heterogeneous, multi-aged structures. An advantage of small canopy gaps, relative to even-aged regeneration methods, is their potential to suppress shrub competition while allowing seedling growth without the use of herbicides or other means of managing shrub competition. While this idea has been proposed in principle, it has not been tested. The objective of this study was to evaluate the use of small canopy gaps, which were 0.08 ha in size with a mean ratio of gap diameter to border tree height of 1.3, as a method to regenerate four mixed-conifer tree species. Gaps were randomly treated either with or without herbicide in a Sierra Nevada mixed-conifer forest. Our hypothesis was that, if this opening size provided sufficient edge effect to control shrub competition, there would be no or little difference in seedling growth between herbicide-treated and control gaps. For all species planted, height and basal diameter growth trends over time were very similar between gaps treated with and without herbicide. The similarity in seedling growth occurred despite a substantial difference in shrub cover between untreated (43%) and treated gaps (3%). We interpret this as evidence that a gap structure such as the one tested can allow a co-occurrence of mediated shrub development and seedling recruitment and that the primary limitation on seedling growth comes from overstory trees, not shrubs. We demonstrate an example of the difference in seedling growth that can be expected between small gaps and clearcuts in order to discuss tradeoffs between these silvicultural systems. Following repeated measurements of seedlings for 7 years following planting, a wildfire burned across the study area, allowing for an additional assessment of wildfire-related seedling damage and mortality in gaps with differing levels of shrub abundance. Substantial mortality of study trees occurred following wildfire (62% mortality 1-year post-fire), but mortality was similar between treated and control gaps. This suggests that shrubs do not have a negative impact with respect to fire related mortality in small canopy gaps despite higher shrub cover. Gap-based silviculture could be a valuable tool for developing multi-aged, multi-species stands without the use of herbicide.