Influence of deciduous tree representation on understory plant communities in Pacific Northwest forests

Dano  Holt

doi:10.7273/000005135

While broadleaf deciduous trees are historically a consistent component of forest canopy composition, the ecological importance of these trees has been understudied, in particular their impact on understory plant communities. Broadleaf deciduous trees are a unique source of functional diversity in the Pacific Northwest’s conifer dominated forests, and have been shown to alter soil nutrients, precipitation reaching the forest floor, epiphytic communities and provide critical habitat for a number of dependent wildlife species. However, their impact on understory plant communities has remained understudied. In particular, as one of the key functional differences in coniferous and broadleaf trees in the Pacific Northwest is a deciduous leaf habit which increases light transmission in the fall through winter, their impact on light availability to the understory has remained under investigated. It was hypothesized that understory plants growing beneath deciduous trees would be exposed to more light. This study investigated understory plant communities growing under overstory canopies of highly coniferous, low deciduous or highly deciduous compositions. Hypotheses were that 1) understory plant communities will differ significantly from communities under stands with differing overstory percent deciduousness, and that there would be changes to understory plant community diversity and 2) commonly occurring generalist species be acclimated to higher light availability in terms of functional traits, including specific leaf area (SLA), leaf area (LA), rachis angle (in Polystichum munitum), light compensation point (LCP), light response curves, saturation point (K) and intrinsic water use efficiency (iWUE). Understory community composition sampling indicated that canopy composition type had a significant effect on understory community, as indicated by a PERMANOVA test. Additionally, there were a number of species that only occurred under either highly coniferous or highly deciduous canopies. Results from functional trait sampling indicated that for the majority of traits generalist species including Acer circinatum, Berberis nervosa and Polystichum munitum exhibited changes to functional traits indicating that highly deciduous plots were higher light plots. Not only were understory communities growing under highly deciduous canopies significantly differ from those growing beneath low deciduous or highly coniferous canopies, but there were changes to morphology and physiology of plants under differing canopy types. These results suggest that broadleaf deciduous trees in the Pacific Northwest have a functional impact on heterogeneity of the understory environment and consequently the understory plant communities. These results imply that broadleaf deciduous trees play an important role in structuring forests in the Pacific Northwest, and their impact should be assessed in both forest management and conservation purposes.

Influence of deciduous tree representation on understory plant communities in Pacific Northwest forests

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