Of the original riparian habitats in the western U.S., The Bureau of Land Management and the U.S. Forest Service estimate that only 20 percent still exist and what remains continue to decline largely due to cattle. However, some recent work shows that other ungulates have significant effects on riparian health. In developing a Healthy Watershed Protection Plan, we studied the effects of white tail deer and exotic axis deer foraging on riparian structure and function along the South Llano River in Junction, Texas. The whitetail deer population density is 4 deer/acre. The axis deer population has not been quantified, but are visually as abundant as white tail and move in herds of 100 or more. We used coordinant sampling to assess riparian tree composition and age class, one large exclosure (15x15m) and 10 smaller (5x5m) riparian exclosures to assess deer browsing and herbivory effects. We conducted replicated quadrat (.25m2) samples and identified plants and forbs, including plant diversity, height, biomass and litter formation inside and outside of exclosures.
Plant species richness was higher in exclosures than open riparian areas, especially among grasses, forbs and saplings. Very few seedlings/saplings were quantified along riverbank transects, whereas within exclosures, pecan, basswood, hackberry and elm were common with few china berry and mesquite. Standing crop above ground plant biomass was 3-25 times greater within exclosures and litter layer biomass was 2-5 times higher than outside. Highly preferred browse species (pecan, hackberry, elm, wild four o’clock) were taller and denser inside the exclosures.
Clearly, ungulate herbivory pressures are impacting riparian zones of the South Llano River, resulting in streambank erosion, periodic bacterial exceedances and lack of streamside forest canopy regeneration. Implementation of the Upper Llano Watershed Protection Plan calls for a coordinated effort to managing white-tailed deer and non-native, exotic populations (primarily axis deer) through increased landowner participation in wildlife management plans and the establishment of exclosures along critically degraded riparian zones.
Moving forward, there is immediate urgency for research on herbivore impact, exclosure effectiveness and demonstration value of restored riparian plant communities to further watershed protection efforts.