A first look at subcontinental-scale spatial patterns of herbivory on forest landscapes of the Midwest and Northeast US
It is well known that over-browsing of woody plants has pervasive negative effects on forest ecosystems that include biotic homogenization of understory flora, invasion by native and exotic plants, and elimination of palatable tree seedlings. Herbivory stress threatens natural regeneration following stand replacement disturbances. Advance regeneration is often absent in areas with high browsing and compensatory seedling recruitment does not develop until well after herbivory is reduced. Studies that quantify browse for large landscapes are very rare. Existing studies are typically local in scale, use unique sampling protocols, or are not repeated. In addition, public data archives are nearly nonexistent, making follow-up investigation difficult. This has left important questions about geographic distribution and hot spots left unanswered for Midwest and Northeast US forests. To address this, new measurements of browse impact were installed on the US national forest inventory sample grid across 24 states of the Midwest and Northeast in 2012. About 30 percent of the baseline samples (1,700) are currently available and the survey should be complete in 2018. Mapped results provide a first look at herbivory stress across ecoregions, cover types, and states.
The results indicate that about two-thirds of the samples had at least medium browsing of understory plants. One out of ten acres of forest, or about 22 million acres, had high impacts that will likely alter compositional and structural trajectories for understory vegetation and tree reproduction. Rate of occurrence of high-impact samples was highest in the Laurentian Mixed Forest, Eastern Broadleaf, and Central-New England Mixed Forest-Coniferous Forest-Alpine Meadow provinces and the Adirondack Highlands section. Deciduous forests were characterized by higher prevalence of high-impact samples than coniferous forests. Quercus/Carya was the group with the highest frequency of high-impact samples (18 percent) representing over 11 million acres of forest land. High-impact samples were most frequent in Pennsylvania, Wisconsin, and the Upper Peninsula of Michigan; and for sub-regions of New York, Ohio, and West Virginia. Roughly half the samples in Delaware, New Jersey, and Maryland had high impact; however, this needs to be clarified by further sampling. The findings suggest that treatments to control competing vegetation, available light, and browse will be required to replace native forests under herbivory stress for large geographic areas within the study region.