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

Throughout eastern North America, chronic white-tailed deer overbrowsing and widespread exotic plant invasions degrade native plant 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 predicted native plant diversity and abundance would be significantly higher during gap-phase regeneration where deer have been maintained at low densities (similar to 7 deer/km(2)) compared to where deer have been chronically overabundant. Simultaneously, we predicted 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. We studied 41 gaps throughout an 88 km(2) fenced military post where deer were maintained near pre-European-settlement densities for 67 years and in nearby forests where deer have been overabundant for decades. Native species richness, diversity, and cover were 37-65% higher, and exotic species cover was 80% lower in canopy gaps at low relative to high deer density. Stem 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. Deer were the primary filter underlying these different patterns in diversity, abundance, and plant height. Our findings demonstrate that gaps develop a vigorous and diverse native sapling layer, with far fewer exotics, in areas of low deer density without a history of chronic overbrowsing. We propose that military posts may be conservation and native plant regeneration hotspots wherever they regularly cull deer.

Automated summary

Chronic white-tailed deer overbrowsing and exotic plant invasions have negative impacts on native plant diversity and abundance in eastern North America. However, our study demonstrates that canopy gaps in areas with low deer density have higher native plant diversity and abundance, and lower exotic plant diversity and abundance compared to areas with chronically overabundant deer.