COS 107-4 - Deciduous canopy tree representation in a mixed forest: impacts on understory plant communities in the Pacific Northwest

Abstract: Deciduous broadleaf trees in the coastal and western Cascade forests of the Pacific Northwest represent a unique source of functional diversity in conifer-dominated landscapes. While these trees can support alternative successional development and ecosystem functions, their ecological importance has not been thoroughly investigated. While the most common upland deciduous tree, Acer macrophyllum, has been shown to impact soil chemistry, hydrodynamics of precipitation through the canopy, epiphytic and mycorrhizal communities, their impact on understory plant communities has not been investigated. As forest understories often represent the majority of plant diversity in these forested systems, and these can impact important functions such as nutrient cycling and soil processes, the drivers of understory composition and function are key to understanding forest community composition and dynamics.

We first investigated understory plant community composition across a spectrum of overstory tree diversity ranging from conifer to deciduous-dominated. Across three study sites, each containing six plots, we observed large disparities in understory community composition. Species richness was highest in highly deciduous sites, and these were the only sites where graminoids and annual forbs were present. Based on visual interpretation of a non-metric multi-dimension scaling plot of species composition, there was strong separation between the highly deciduous plots and the coniferous and lightly deciduous plots, as well as strong visual discrimination between sites in the ordination space, apparently reflecting a productivity and moisture availability gradient between sites.

To establish a physiological basis for these trends in functional diversity, we investigated the role of light availability differences in these plots in driving the morphological variability of generalist species. We examined leaf area, specific leaf area (area per mass), light compensation point and water use efficiency in the three most common understory species, Polystichum munitum, Acer circinatum and Berberis nervosa. Preliminary results suggest that there was a significant difference in at least the specific leaf area of Acer circinatum and Berberis nervosa, indicating that these species are responding to differences in the environment created by the deciduous canopy.

Our preliminary results suggest that certain attributes of understory plant communities in the coastal temperate Northwest are uniquely associated with strongly deciduous overstories, and that overstory differences correlate with morphological differences in generalist species responses to light availability. Our research helps illuminate the compositional and functional mechanisms behind the ecosystem function in forests comprised of both deciduous broadleaf and evergreen conifer forest canopies.