Size-growth relationship, tree spatial patterns, and tree-tree competition influence tree growth and stand complexity in a 160-year red pine chronosequence

Extended rotations have been suggested as a strategy for balancing timber production and ecological objectives. By lengthening the period of stand development, extended rotations may increase tree size inequality and other elements of structural complexity, thus reducing the disparity between managed and old-growth stands. A potential limitation of extended rotations is the tradeoff between reduced stand-level productivity and greater large-tree growth that typically occurs with stand age. The mechanisms driving this tradeoff have not been fully explored. To fill this knowledge gap, we investigated the size-growth relationship (SGR), tree spatial patterns, and tree-tree competition along an established 160-yr chronosequence of 19 single-cohort, unthinned red pine (Pinus resinosa) stands in northern Minnesota, USA. We analyzed SGR, a stand-level metric used to estimate the relative efficiency with which different sized trees utilize available resources, to assess how the relationship between tree size and growth changed over an extended period of stand development. We performed spatial analysis to examine whether tree spatial clustering, a criterion of structural complexity, increased with stand age. We modeled individual-tree biomass increment to test whether competition along the chronosequence was size-symmetric (access of individual trees to resources is directly proportional to size) or size-asymmetric (larger trees suppress the growth of smaller individuals by preempting resources), and how SGR, tree spatial patterns, and competition together influenced individual-tree growth. We found low SGR (i.e., disproportionately slow growth of larger trees compared to smaller trees) across the chronosequence, a finding that contrasts with hypothesized models of SGR during stand development but is consistent with previous research on pine-dominated systems. Tree spatial patterns trended towards clustering with stand age, indicating higher structural complexity over time. In agreement with our SGR findings, competition across the chronosequence was size-symmetric, suggesting that competition reduced individual-tree growth while maintaining relative size equality. Individual-tree biomass increment was strongly dependent on tree size, with the growth of small trees appearing relatively less affected by competition. Differences in SGR did not translate into individual-tree growth, and tree spatial clustering was associated with reduced growth, especially in larger trees. Our results indicate that disproportionately slow large-tree growth and size-symmetric competition throughout stand development may delay the emergence of stand structural complexity in extended rotation red pine stands. Silvicultural treatments may be required to promote stand structural complexity and increase large crop-tree growth.