Forest restoration treatments increase native plant diversity but open the door to invasion in the Colorado Front Range

Forest thinning treatments are often done with objectives to re-establish historical forest structure and increase the system’s resilience against future wildfires. But little is known about their long-term efficacy, and effects on understory plant composition. This is especially true in the Colorado Front Range (CFR). We used a before/after control/impact study design to assess the effects of forest restoration treatments on forest structure and understory plant composition one, five, and ten years after treatment. Five and ten years after treatments, treated areas had lower basal area and tree density, and higher quadratic mean diameter, consistent with treatment objectives, and had higher native cover and native richness, and graminoid cover. Species accumulation curves showed region-wide increases in native richness at five years, and further increases at 10 years, with a net increase of 47 native species in treated areas compared to 11 in untreated controls. However, treatments also had higher relative numbers of non-native plant species, and increased probability of non-native plant invasion. Native richness, native cover and graminoid cover were associated with stand structure and topography, while non-native invasions were more strongly associated with moisture deficit. This suggests that climate played a stronger role in constraining the introduction of species than treatment alone. In the CFR, forest restoration treatments can benefit the native understory, but also provide an opening for invasion in uninvaded sites. However, the overall impact of invasions was low after ten years.