Professor University of Alberta Edmonton, Alberta, Canada
Abstract: Many theories try to explain how plants partition above- and below-ground resources, but experimental tests offer contrasting results. For instance, studies show nutrient addition reduces species richness, presumably via decreased light penetration to the understory because of increased standing biomass. However, fertilization may decrease richness independently of understory light levels. We observe that shade causes community-level traits to disperse, indicating plants respond to light passing through neighbours’ leaves. But how do responses to light, nutrients, and neighbours in turn affect whole communities? Here, we ask if resource manipulation and neighbour density havecomplementary or idiosyncratic effects on communities. To do so, in 2021 we established an experiment where we manipulated light, nutrients, and neighbour density in a native grassland. Plots were subject to one of twelve combinations of light (-, ambient, +), nutrient (ambient, +) and neighbour (-, ambient) treatments. Light availability was manipulated using vegetation tie-backs and shade cloth, fertilization was achieved using slow-release NPK, and neighbours were thinned by dripping herbicide randomly over plots. In August 2022 we measured plot-level light quantity (total light penetration) and quality (understory spectral characteristics), measured soil nitrogen (ammonia and total organic nitrogen), and recorded species’ percent abundance to calculate diversity metrics (species richness, evenness, and Shannon-Weiner diversity). We ordinated understory spectral absorbance values and extracted axes loadings to quantify light quality. We used these abiotic (light quantity, light quality, soil nitrogen) and biotic (richness, evenness, Shannon-Weiner diversity) descriptors as response variables in linear mixed-effect models where light, fertilization, and thinning treatments were fixed effects. Light manipulation significantly decreased understory light quantity in shaded treatments and fertilization significantly increased ammonia and total organic nitrogen in soil. In keeping with previous studies, Shannon-Weiner diversity and species evenness decreased in fertilized plots, and species richness increased in thinned plots. Surprisingly, we found total light penetration increased in fertilized plots, and light quality was affected by both light manipulation and thinning but not fertilization. These results suggest a more complex story than shade driving species loss in fertilized communities.
