Influence of competition and age on tree growth in structurally complex old-growth forests in northern Minnesota, USA
Factors influencing tree growth in structurally complex forests remain poorly understood. Here we assessed the influence of competition on Pinus resinosa (n = 224) and Pinus strobus (n = 90) growth in four old-growth stands in Minnesota, using mixed effects models. A subset of trees, with accurate age estimates, was used to further test the influence of tree age. Our analyses included the weighted Voronoi diagram (WVD) as a novel competition index, representing a detailed description of the spatial structure of a tree's neighborhood. Competition was variably expressed depending on stand developmental history and tree species. For P. resinosa in single-cohort stands, and P. strobus in multi-cohort stands, tree size relative to the population mean size best predicted tree growth. In contrast, for P. resinosa in multi-cohort stands, the spatial configuration of competitors became important, as shown by the superior performance of the WVD index. Surprisingly, while tree age had a negative influence on growth, it did not influence the intensity of competition. Our results highlight the importance of considering stand developmental history and tree age in analyses of tree growth and competition, and the potential for improving assessments of competition in complex stands, using detailed quantification of neighborhood structure.