Effects of environmental factors on reproductive strategies: Resource heterogeneity and urban environments
Biological invasions by non-native plants have a massive cost to our societies, both economic and ecological. The capability for rapid spread through both seed and clonal propagation is a common trait in many invasive plants, and poses a difficult challenge in designing control methods. We investigate this reproductive versatility by examining environmental factors that impact resource allocation between clonal and sexual reproductive methods. Our design is focused on the heterogeneity of resource distribution and the effects of human development. Urban environments are highly heterogeneous, and this high variability should select for a different suite of traits than more homogenous environments. We utilized Trifolium repens as a model system to test if we could experimentally manipulate the reproductive strategy of plants through alteration of spacial heterogeneity for resources. Fertilizer was applied in homogenous and heterogeneous distributions at two intensity levels, we then measured resource allocation through biomass of new tissue growth. Additionally, we conducted a germination study on seeds of a major invasive tree, Ailanthus altissima, to determine if urban and exurban environments impact seed viability. Although not directly related to resource allocation, lower seed viability would result in a lower fitness and selection against sexual reproduction at the population level.
Under controlled greenhouse conditions we demonstrate a link between resource heterogeneity and greater emphasis on traits leading to horizontal spread across the surface of soil, allowing for greater clonal reproduction. The proportion of total biomass dedicated to horizontal stem growth increased in response to uneven resource distribution (p=0.025), as did the frequency of adventitious root growth (p=0.05). These two shifts act to favor asexual reproduction through fragmentation. Flower production showed no response, but this was likely due to the overwhelming impact of total nutrient load. We also demonstrate a lower rate of germination for seeds of Ailanthus altissima taken from urban sites (p=0.044). The large resource investment required to produce seeds means that lowered viability acts as a selective pressure against sexual reproduction. By necessity this means a selection pressure for Ailanthus altissima in urban environments that will favor clonal reproduction, either through fragmentation or root suckers. These findings are biologically relevant as reproductive strategy effects both the range of propagule dispersal and genetic diversity of the resulting populations, which impacts resistance to both control efforts and disease. Further work being conducted by our group examines this genetic diversity directly through STR (short tandem repeat) analysis.