The Effects of Thinning and Ungulate Exclusion on Conifer Understory Vegetation and Key Mule Deer Forage Species in Northeastern New Mexico, USA

Background/Question/Methods

Over the past 150 years, conifer species have been expanding into southwestern grasslands displacing resident plant communities. The expansion of juniper (Juniperus spp.) and ponderosa pine (Pinus ponderosa) has led to reductions of grass and forb communities, and altered wildlife habitat. This has created potential problems for wildlife managers and ranchers who often desire to remove or reduce juniper or ponderosa stands in an effort to improve wildlife or livestock habitat. The goal of this study was to determine the effects of juniper and ponderosa pine removal by selective logging and hydraulic mulching on mule deer habitat in northeastern New Mexico. The primary objectives of this study were to: (1) examine differences in plant community richness, species occurrence, and cover between areas with and without removal of juniper and ponderosa; and (2) determine if removal of juniper and ponderosa improves biomass production, abundance and cover of key forage species available to mule deer (Odocoileus hemionus). Plant composition, biomass production, and plant density were assessed during June and July in 2011 and 2012. Analysis via Proc General Linear Models and Tukey Honestly Significant Difference was used to evaluate the treated and untreated plant communities and their differences.

Results/Conclusions

An ongoing regional drought probably reduced vegetation response, but key forage species, grass and forb biomass, species richness and herbaceous cover responded to conifer thinning or ungulate exclusion during one or both years. In 2011, areas from which ungulates were excluded had twice the forb biomass of unexcluded areas.  By 2012, forb production was similar between excluded and unexcluded areas, but grass biomass production was almost two times higher in areas of ungulate exclusion. Key forage species responded to conifer thinning in 2011 with thinned areas having 1.3 times more key forage species than unthinned areas. In 2012, there was a slight increase in species richness (~20%) in areas where ungulates were excluded. The results of this study suggest that even during drought, thinning conifers can have positive effects on biomass and abundance of key forage species, especially when ungulates are excluded.