A century of fire suppression, active management, and altered disturbance regimes has changed the structure and composition of western forests. These changes influence seedling establishment and survival through alterations to the abiotic and biotic regeneration environment, including the herbaceous and shrub community, which can modify the suitability of seedling establishment sites. Although historic datasets are frequently used to identify such changes, few studies have used these datasets to examine and compare the effects of establishment conditions on seedling abundances under historic versus current forest conditions. We used a historic dataset from the mixed conifer forest of the western Sierra Nevada to examine the effect of the regeneration environment and co-establishing shrub community on the abundance and success of Pinus, Abies, and Calocedrus spp. seedlings. We re-measured four USFS Methods of Cutting plots originally established in 1929 to examine the effects of different harvest practices on forest composition. Overstory, regeneration, and understory conditions were mapped prior to logging in 1929 and periodically until 1947. We resampled the plots in 2016, following original methodologies, to track changes over the last 85+ years and used a negative binomial generalized linear model to identify significant predictors of seedling abundance under historic versus current conditions.
Results/Conclusions
Density of incense cedar and white fir trees increased from 1931 to 2016, especially for smaller diameter trees, while shrub cover decreased from 5.1% to 0.3% (p<0.001). In 1929, burned patches were negatively associated with white fir seedling abundance, while overstory biomass inhibited abundance in 2016 (p<0.01). Burned area and overstory biomass were negatively associated with incense cedar density in 1929, while biomass alone was significant in limiting cedar regeneration in 2016 (p<0.01). Pine seedling abundance was negatively correlated with biomass in 1929 but did not inhibit 2016 abundances, which were low across all plots. A century of fire suppression altered forest conditions through increases in tree density and reduced light availability, which led to a decrease in pine and fir seedlings and increase in cedar abundance. High forest density and associated surface fuels increases the likelihood of a future high severity fire, which could produce favorable light and understory conditions for pine and fir establishment, but high post-fire tree mortality might also make seed sources scarce. Ongoing climate change, bark beetle outbreaks, and altered fire return intervals are likely to further influence forest character, seedling success, and control future forest community composition.