Local testing and calibration of species-specific competition indices in Sierran mixed-conifer forests: application transfer to evolving objectives

Forest growth processes are driven by site productivity and species functional traits, which are ultimately constrained by cumulative resource demand, resulting in competitive dynamics across successional forest communities. Historical efforts to quantify competition used density metrics or neighborhood crowding indices for yield modeling and reforestation surveys. These methods have expanded to include dendroclimatology and restoration applications that commonly assume similar competitive response across species of various functional types. We assessed the competitive indices of two focal species, Pinus lambertiana (Dougl.) and Pinus ponderosa (Dougl. ex P. Laws. & C. Laws.) in mixed-conifer forests of the Sierra Nevada to estimate stem radial growth under current stand structure. We ranked correlations of basal area increment of the last 10 years (BAI(10)) separately across 20 competition indices (CIs). The best-ranked CIs were used to test the relative influence of competition, tree size, and site variables on BAI(10) with linear mixed models. While crown overlap was a common variable in CIs among both species, BAI(10) of P. lambertiana was less impacted by intraspecific competition, and P. ponderosa appeared sensitive to all competing stems. The results suggest that local calibration of CIs with crown parameters may aid in interpreting Pinus species growth patterns and that the relative impact of competition on growth is species specific.