How do ecological restoration treatments affect understory plant communities in dry conifer forests of the Colorado Front Range?

Includes bibliographical references.2022 Fall.Ecological restoration efforts are progressing in dry conifer forests across the western United States to increase resilience to fire and other disturbances. While such treatments primarily aim to create overstory change, impacts beyond the canopy should also be considered – such as effects on understory plants. Several studies have investigated outcomes of ecological restoration thinning treatments for understory plants, but few of these have examined effects across a landscape and at a time interval long enough for plants to potentially adjust to the disturbance. Additionally, none have investigated how specific aspects of treatment and local climate might interact to modify understory responses. In this study, we investigated the effects of ecological restoration thinning treatments on understory plant communities in dry conifer forests of the Colorado Front Range using a Before/After/Control/Impact study design. We collected data at 1-2 years pre-treatment, 1-2 years post-treatment, and 4-6 years post-treatment in 156 plots distributed across 8 sites, encompassing 15 treatment units and 15 nearby untreated areas. We found 1.6 times higher native understory plant cover and 1.1 times higher richness in treated compared untreated plots at 4-6 years after treatment. Heightened cover and richness values in treated plots were not driven by a single native plant functional group, but by a large portion of the community. Short- and long-lived, forb and graminoid, and vegetatively spreading and non-vegetatively spreading native plants all grew in cover. Both lifespans, forb, and non-vegetatively spreading native plants had heightened richness. Introduced plants showed 2.3 times higher cover and 3.9 times higher richness in treated plots compared to untreated, but were still present at very low levels. Greater native plant cover and richness were associated with lower basal areas that more closely resemble historical norms for the landscape. Thirty year average climatic water deficit (CWD) was not as strong of a predictor of native cover or richness as was a short-term relative measure, final spring CWD z-score, which describes how different the spring climatic conditions of the sampling year were from average conditions. Overall, the broad longer-term benefits to the native understory plant community that were found for numerous sites across the Colorado Front Range suggest that these results may generalizable to elsewhere on this and similar landscapes