Forb Standing Crop Response to Grazing and Precipitation

Reducing grass standing crop by grazing may increase forbs and benefit wildlife that depend on forbs. However, precipitation and soil texture also strongly influence forb standing crop. We determined if standing crop of forbs selected by white-tailed deer (Odocoileus virginianus Zimm.) is more strongly influenced by grazing or precipitation. Ungulates typically graze in patches with greater standing crop than the surrounding vegetation. Our second objective was to determine if predicted relationships of forb standing crop with grazing, precipitation, and percent sand were similar with less productive sites included or excluded from models. We estimated standing crop of grasses and forbs in 50 paired grazing exclosures and grazed plots on each of six 2 500-ha study sites. Standing crop of forbs selected by white-tailed deer (selected forbs) was strongly related to precipitation and percent sand but not estimated percent use of grasses. For our second objective, we examined grazing effects on forbs by removing pairs of exclosures and grazed plots from the data where grass standing crop in nongrazed exclosures exceeded the average standing crop of grass after grazing. Percent use of grasses did not influence selected forb standing crop when we included only productive patches. For overall forb standing crop in productive patches, percent use of grasses and percent sand interacted. Forbs declined with increasing percent use of grasses in less sandy soils and increased with percent use of grasses in sandy soils. Grazing is not useful to increase forbs selected by white-tailed deer in our study sites because standing crop of selected forbs is more strongly dependent on precipitation and soil texture than on grazing. Grazing did influence forb standing crop in productive areas, which suggests accounting for grazing effects in productive versus less productive areas of the landscape provides insight into herbivore-vegetation relationships.