Regional changes to forest understories since the mid-Twentieth Century: Effects of overabundant deer and other factors in northern New Jersey

Abstract

Increased densities of white-tailed deer (Odocoileus virginianus) in eastern North America cause multiple impacts to forest understories, but few studies have documented their effects at large spatial and temporal scales. I studied long-term, regional changes in forest understories in northern New Jersey, comparing data from 62 stands in 2014–2018 to historical data gathered from the same stands in 1948–1973, when statewide deer densities were substantially lower. Significant declines occurred in nearly all forest layers, including densities of large seedlings (80%), saplings (75%) and trees (18%), with parallel declines in the cover of native herbs (76%) and shrubs (72%). In contrast, the cover of exotic shrubs, lianas, and herbs increased by 5×–40×, resulting in a major shift in species composition away from mostly native species cover (≥95%) to mostly exotic shrub and liana cover (54%). Significant changes in tree species composition were also observed in relation to deer browse preferences, especially in the large seedling and sapling layers, which exhibited the greatest declines. Sites with higher estimated deer populations show conspicuously lower sapling densities (R2 = 0.47). Experimental data from nine deer exclosures of different ages showed consistent increases in large seedling density and heights over time, and full recovery of large seeding abundance and species composition to historic levels after 11–20 yrs. Native lianas and herbs and exotic shrubs each increased or maintained their dominance in exclosures. Species afflicted by introduced pests and diseases (e.g., Cornus florida and Tsuga canadensis) also declined greatly, but their declines were not sufficient to explain overall forest understory declines. Declines in tree recruitment could not be explained by changes in disturbance regimes and/or light levels over time. Because deer strongly reduce tree recruitment, shift species composition, and reduce understory cover across large spatial scales, they represent a significant concern for forest managers and an issue that should be effectively addressed.