The Relative Importance of Biotic and Abiotic Controls on Young Conifer Growth after Fire in the Klamath-Siskiyou Region

In the Klamath-Siskiyou region of northern California and southwest Oregon, the mixed-severity fire regime and short fire return interval has created diverse vegetative landscape with frequent regenerating patches of forest vegetation. Considerable evidence from this region suggests that intense competition from neighborhood vegetation can greatly reduce the survival and growth of young conifers following severe site disturbance, including fire. However, relatively little information exists regarding the growing conditions one to three decades following disturbance, particularly in unmanaged stands. We measured the effects of neighborhood vegetation on regenerating Douglas-fir (Pseudotsuga menziesii) growth in northern California, 10-20 years following severe wildfires. Steep climatic gradients owing to complex topography in the region can cause significant variation in local temperatures, levels of precipitation, and solar radiation; thus our measurements were dispersed across two abiotic environmental gradients, elevation and heat load. We used a nested sampling design to elicit the tree-level effects of neighborhood vegetation, the plot-level effects of elevation and heat load, and the interactions among tree-level and plot-level variables on Douglas-fir growth. We used a combination of of hierarchical regression models and non-parametric, multiplicative regression modeling for 3-dimensional graphical data presentation. Our results indicate that conifers are exposed to both positive (facilitative) and negative (competitive) neighborhood interactions depending on location in the abiotic environmental landscape. However, growth appears to be much more sensitive to abiotic environmental conditions than to the effects of neighborhood interactions. Considering the spatially explicit patterns in our data, we suggest that site-specific conditions play a significant role in the development of post-fire forest management strategies in the Klamath-Siskiyou region.