Difference in regeneration conditions in Pinus ponderosa dominated forests in northern California, USA, over an 83 year period

Link To Share This Poster: https://cdmcd.co/WkzRdp
Live Discussion Link: https://cdmcd.co/xvxBpk

Sushil Nepal and Bianca N.I. Eskelson, Forest Resource Management (Biometrics), The University of British Columbia, Vancouver, BC, Canada, Bianca N.I. Eskelson, Forest Resource Management, University of British Columbia, Vancouver, BC, Canada, Martin W. Ritchie, USDA, Forest Service Pacific Southwest Research Station, Redding, CA

Background/Question/Methods
Understanding regeneration density and stocking in dry forests is a key element that supports silvicultural decision-making processes for sustainable forest management. Understanding how forest regeneration in stands that were minimally impacted by Euro-American settlement is related to ecological factors can help us mimic historic stocking in contemporary management decisions. This study was conducted in a ponderosa pine (Pinus ponderosae Dougl. ex P. and C. Law) dominated forest located within the Blacks Mountain Experimental Forest (BMEF) in northeastern California, United States. Field survey regeneration data were available for 1935 (pre-harvest), collected prior to any timber harvest, and for 2018 (contemporary), collected after six management scenarios were implemented. Our objectives were to: (1) provide a quantitative description of pre-harvest regeneration by species and height class, (2) identify the association of regeneration with overstory, soil, and topography variables, (3) describe contemporary regeneration across various management treatments, and (4) compare differences in regeneration between historical and contemporary forests. Survey data were tallied for three species—ponderosa pine, incense-cedar, and white fir—by four height classes: class 1: 0–0.31 m, class 2: 0.31–0.91 m, class 3: 0.91–1.83 m, and class 4: >1.83 m and <8.9 cm diameter at breast height. Stocking was used as a proxy for regeneration density in this study.
Results/Conclusions
BMEF in 1935 pre-harvest condition was dominated by shade-intolerant ponderosa pine in height classes 2 and 3. Using a beta regression model with pseudo-R2 of 0.49, we found that overstory basal area per hectare (m2 ha-1) and the amount of water (cm) available for plants up to a depth of 150 cm were significantly associated with pre-harvest stocking. We found significant differences (p-value<0.05) in the 2018 stocking among six management scenarios using a Kruskal–Wallis non-parametric one-way ANOVA. Control compartments had the highest stocking followed by burned compartments. In contemporary forest stands, recent treatments involving a combination of burning and thinning resulted in high stocking in height classes 2 and 3. Overall, the stocking in pre-harvest BMEF stands was higher than in contemporary stands and was dominated by ponderosa pine. Future forest stand structures in ponderosa pine forests of northern California depend on the regeneration response to management and environmental factors. Our study results on pre-harvest and contemporary regeneration allow managers to understand the relationship between regeneration and ecological factors and management conditions, which in turn shape the overall stand structure.