Divergent successional pathways of stand development following fire in a California closed-cone pine forest

Questions What are the major trends in vegetation community structure and forest stand structure over a 14-yr post-fire period in a California closed-cone pine forest? Which biotic and abiotic factors best explain variation in stand structure at different stages of post-fire succession, and does the relative importance of these factors remain constant? Is there evidence of multiple successional pathways of forest stand development? Location Post-fire Pinus muricata (bishop pine) forests at Point Reyes National Seashore, CA, USA. Methods We quantified post-fire vegetation change from field data collected 1, 2, 6 and 14yr following stand-replacing wildfire that occurred in 1995. General linear models were used to assess trends in composition (plant functional groups and species diversity) and generalized linear models were used to assess trends in stand structure (post-fire P.muricata density) and determine the relative importance of abiotic and biotic factors on stand structure in different early-successional post-fire years. Results Species richness and diversity peaked in the first 2yr following fire, and then declined through year 14. Initial post-fire P.muricata tree regeneration was high (mean 249750stemsha(-1) in year 1) and remained well above pre-fire stand density levels by year 14 (mean 15179stemsha(-1)). Post-fire P.muricata seedling density was associated with topographic factors in years 1 and 2, negatively associated with cover of a non-native herb in year 2, and negatively associated with cover of an early/mid-successional shrub and positively associated with slope in years 6 and 14. Two alternative pathways of post-fire stand development have emerged by year 14. A high-density, closed-canopy pathway (mean 40875stemsha(-1)) with early intra-specific thinning resulted on steep slopes and ridges with low shrub cover. In contrast, a low-density, open-canopy pathway (mean 1250stemsha(-1)) resulted on gentle slopes and where shrub cover was high. Conclusions This study provides evidence of divergent successional pathways and illustrates the importance of early-successional species interactions and topography on longer-term stand development trajectories in serotinous conifer forests. Early heterogeneity in vegetation establishment set the course for variability in stand structure in mid-seral stages and may persist into later stages.