Native tree species prosper while exotics falter during gap-phase regeneration, but only where deer densities are near historical levels

Abstract

Abstract Throughout eastern North America, chronic white-tailed deer overbrowsing and widespread exotic plant invasions degrade native plant species diversity and abundance. However, because deer densities are high across entire regions, it is challenging to find large areas where deer have been at low density for long periods. We tested the hypothesis that native plant diversity and abundance would be significantly higher during gap-phase regeneration where deer have been maintained at low densities (~ 7 deer/km²) compared to where deer have been chronically overabundant. Simultaneously, we tested the hypothesis that exotic plant diversity and abundance would be lower in gaps where deer have been maintained at low versus high densities. We also evaluated whether deer density interacts with gap age, gap size, and gaps in dry versus more mesic forest patches. To test these hypotheses, we studied 41 gaps throughout an 88 km2 fenced military post where deer were maintained near historical densities for 67 years compared to nearby forests where deer have been overabundant for decades. Deer were the primary filter. Within canopy gaps at low deer density, native species richness, diversity, and cover were 30–65% higher, and exotic species cover was 80% lower. Sapling height and density of multiple native canopy tree species (e.g., two Quercus spp.) were 5-20-fold higher within canopy gaps at low deer density. Moreover, 10 native woody species were indicators of low deer density, including three Quercus species, whereas no native woody species were indicators of high deer density. Our findings demonstrate that gaps develop a vigorous and diverse native sapling layer that is not plagued by exotics at low deer density. We propose that military posts may be conservation and regeneration hotspots whenever they regularly cull deer.