Effects of Ecological Restoration Alternative Treatments on Nonnative Plant Species Establishment

Abstract—Disturbances generated by forest restoration treatments have the potential for enhancing the establishment of nonnative species thereby impeding long-term native plant recovery. In a ponderosa pine forest next to the Fort Valley Experimental Forest, Arizona, we examined the establishment of nonnative species after three alter-native treatments with different intensities of tree thinning, coupled with prescribed burning and an untreated control, in relation to total species abundance and rich-ness. Pretreatment data were collected in 1998 and postreatment responses were measured from 2001 through 2006. Total herbaceous cover and richness were signif-icantly higher in the two more intensely thinned areas compared to the control over the entire post-treatment period. Native species were the most prevalent in terms of cover (92%) and richness (90%) across all treated units, though greater understory plant responses were linked to heavier amounts of tree thinning. Nonnative species abundance and richness also increased significantly in response to restoration treat-ments, particularly in the two more intense treatments. The proportion of nonnative abundance to the total abundance within the two heavily treated areas decreased through time and began to converge back towards the undisturbed control unit. One year following treatments, 15 % of the total cover (27%) was composed of nonna-tive species in the heaviest treated unit. This proportion dropped almost 50 % by the fifth year following treatment. Our results suggest that disturbances associated with restoration treatments can facilitate establishment of nonnative plants, however the post-treatment plant community was increasingly dominated by native species