Abstract
Many fire-adapted forests across the United States where fire has been excluded have in-filled with shade-tolerant species, reducing the characteristic spatial heterogeneity. We used 1929 stem maps of historical forest conditions in the Sierra Nevada to develop a thinning prescription designed to restore a pattern of tree clumps, individual trees, and gaps. This "high variability" (HighV) thinning treatment was evaluated in a replicated experimental design along with a more standard regular “leave” tree spacing "low variability" (LowV) thinning treatment, and an unthinned control, all with or without prescribed fire. Both thinning treatments reduced tree density and basal area equally, but differed in spatial pattern, with the HighV thinning leading to greater variability in canopy closure and stem distribution. While creating small gaps with the HighV treatment required the removal of some larger trees, slightly more board foot (BF) volume was removed with the LowV treatment because larger trees often grow in groups and imposing a regular spacing resulted in the removal of some. Any difference in BF volume removed between treatments would likely be minimal on most Forest Service lands in the Sierra Nevada where a 30-inch diameter limit for cutting applies. Both thinning treatments improved tree survival in a severe drought compared with the untreated control. No difference was observed between thinning treatments indicating that leaving some trees in groups did not increase susceptibility to bark beetle attack at the stand scale. While more trees died in the prescribed burn treatments, secondary mortality in thinned units was relatively minor. The HighV with prescribed fire treatment not only produced a structure more closely approximating that of historical stands, but low surface fuel loads should make treated areas more resilient to future wildfires.
Parent Publication
Keywords
collaborative,
co-production,
stewardship,
implementation,
relationship building
Citation
Knapp, Eric E.; Carlson, Robert L.; North, Malcolm P.; Lydersen, Jamie L.; Collins, Brandon M. 2020. Restoring forest heterogeneity with thinning and prescribed fire: Initial results from the central Sierra Nevada, California. In: Pile, Lauren S.; Deal, Robert L.; Dey, Daniel C.; Gwaze, David; Kabrick, John M.; Palik, Brian J.; Schuler, Thomas M., comps. The 2019 National Silviculture Workshop: a focus on forest management-research partnerships. Gen. Tech. Rep. NRS-P-193. Madison, WI: U.S. Department of Agriculture, Forest Service, Northern Research Station: 216-226. https://doi.org/10.2737/NRS-GTR-P-193-paper28.
