Examining the influences of tree-to-tree competition and climate on size-growth relationships in hydric, multi-aged Fraxinus nigra stands

Abstract

Most research on tree-tree competition and size-growth relationship (SGR – a stand-level metric that infers the relative efficiency with which different sized trees utilize available resources) has focused on upland systems. It is unclear if inferences from these studies extend to wetland forests. Moreover, no study to date has thoroughly investigated the relationship between individual tree-tree competition and SGR. To fill these research gaps, we conducted a dendrochronological study examining the relationship of tree-tree competition, SGR, and climate in late-successional Fraxinus nigra (black ash) wetland forests in northern Minnesota, USA. We took advantage of a detailed, stem-mapped dataset of 1670 trees in five late-successional, multi-aged stands to explore the following research questions: (1) how do competitive interactions, particularly size symmetry, influence individual-tree growth; (2) do late-successional F. nigra stands display inverse asymmetric SGR; and (3) do short-term variations in drought influence SGR in F. nigra wetland forests? Using neighborhood competition indices, which characterize the growth of individual trees based on the size, number, and distance of competitors, we examined the nature and strength of individual tree-tree interactions. Additionally, we used SGR to determine how tree size and individual tree contributions to stand productivity relate to changes in stand growth and competitive interactions during stand development. At the individual tree level, we found evidence of size-asymmetric competition, with larger trees disproportionately suppressing the growth of smaller trees. However, tree size was a stronger predictor of growth than competition at all sites. At the stand level, our multi-aged F. nigra sites showed consistent patterns of inverse size-asymmetric SGR (i.e., smaller individual trees growing at disproportionately higher rates relative to larger trees), which is generally consistent with previous observations of mature upland forests and supports the hypothesis that large trees decline in relative growth as stands age. While seemingly counter-intuitive, the simultaneous presence of size-asymmetric individual tree-level competition and stand-level inverse asymmetric SGR suggests declines in large tree production efficiency. Drought effects on SGR, as expressed by PDSI, while sometimes evident, appeared weak on both relatively mesic and extremely wet sites. Our findings, which are consistent with previous studies of both F. nigra wetlands and upland forests, demonstrate that the combined results of individual-tree competition models and stand-level SGR can provide deeper insights into growth and competition in F. nigra and other forest types.