Increasing weight of evidence that thinning and burning treatments help restore understory plant communities in ponderosa pine forests

For more than a century ecosystems around the world have experienced an increase in the dominance of woody species. While the drivers of woody plant proliferation are complex, interactions between climate and land-use change are commonly invoked as primary contributing factors. In ponderosa pine forests of western North America, substantial increases in tree densities are impacting overall forest health and increasing the risk for severe wildfires and insect and disease outbreaks. Addressing this problem through the use of ecological restoration projects is widely advocated. Our objective was to quantify understory vegetation response five years after thinning and burning treatments in a southwestern ponderosa pine forest. We remeasured plant species richness and cover on thinned + burned, burned only, and untreated controls replicated four time in a BACI design. An untreated control (Untreated) was used for comparison of two alternative restoration treatments (1) thinning followed by prescribed fire (Thin + Burn) and (2) prescribed fire only (Burn-only). Understory species richness and total plant cover increased significantly in Thin + Burn compared to the Untreated and Burn-only treatments. Analysis of functional group composition revealed increases in species richness and cover was driven primarily by changes in annual–biennial forbs and graminoids. We then compared our results with those of three Long-term Ecological Restoration and Assessment Network (LEARN) sites. We found total and native plant cover showed evidence of a consistent response to Thin + Burn treatments across all sites. However, results suggest that understory response to restoration treatments is under strong environmental control. As a consequence the range of variability associated with understory responses should be expected to expand or contract depending on where treatments are implemented. Overall our findings add to a growing body of literature that restoration treatments combining mechanical thinning and prescribed fire are useful for increasing native understory abundance and diversity.