Direct, indirect, and interacting effects of overabundant deer and invasive autumn olive on native vegetation.

Abstract

Temperate plant communities are being simultaneously affected by plant invasions, including the invasive shrub, autumn olive (<italic>Elaeagnus umbellata </italic>), and by native but overabundant white-tailed deer (<italic>Odocoileus virginianus</italic>). Previous studies have shown that both invasive plants and deer, acting individually, may reduce vegetation diversity and floristic quality. Acting together, they have unknown effects. In this dissertation, I examined how deer and autumn olive affect vegetation diversity and tree regeneration, and whether their individual effects are additive or whether the interaction counteracts or modifies them. My initial study found that dense deer populations in two southeastern Michigan forest plots were correlated with diminished vegetation diversity, floristic quality, and tree seedling survival. I then examined how deer, interacting with autumn olive, affected two old-field plant communities. Autumn olive significantly reduced vegetation biomass, species density, and floristic quality. Deer had no significant effects or interactions with autumn olive. However, analysis of cumulative rather than average plot species richness (beta vs. alpha diversity), showed that deer reduced richness and floristic quality. Both deer and autumn olive increased numbers of invasive and native weedy species, which further reduced floristic quality. Because tree regeneration is of concern, I conducted a field experiment to examine how deer and autumn olive interact with additional animal and plant species, including small-mammal herbivores and herbaceous plant competitors, to affect old-field recolonization by black oak, <italic>Quercus velutina </italic>, and pignut hickory, <italic>Carya glabra</italic>. Seed predators removed or consumed 99% of seeds. Deer did not affect seed removal rates, but autumn olive increased on-site seed consumption. When seeds were protected from predation, however, autumn olive facilitated germination. Once seedlings were established, autumn olive reduced biomass or survival, depending on site. Deer did little damage to tree seedlings until the fourth year, because small mammals repeatedly browsed seedlings before deer could find them. Small-mammal damage was more frequent under autumn olive, and it was linked to higher mortality. Therefore, despite serving as a nurse tree to facilitate germination, autumn olive's net effect was to inhibit tree survival or biomass. This effect was largely indirect, through protection of small mammals, rather than direct, through resource competition.