Overstory Removal and Biological Legacies Influence Long-Term Forest Management Outcomes on Introduced Species and Native Shrubs

Forests in the western United States are actively managed to increase resilience to fire and extract timber resources. Disturbance from these management activities, however, may reduce forest resilience to invasion and promote shrub expansion, which could compromise treatment efficacy in the long-term. To assess mid- and long-term impacts of forest management practices, we coupled a field experiment with a chronosequence of patch cuts (similar to 0.5 ha cuts) located at Blodgett Forest Research Station in the north-central Sierra Nevada. The field experiment, which was part of the national Fire and Fire Surrogate (FFS) study, compared three fuel treatments to control plots 12-13 years post-treatment. Fuel treatments included mechanical thinning, prescribed fire (that burned twice), and mechanical thinning plus fire. In contrast, the chronosequence assessed the impacts of patch cuts on vegetation across 33 years. The FFS fuel treatments led to increases in introduced species richness initially and in the mid-term. Introduced species richness and cover in the fuel treatments, however, were considerably lower than in patch cuts, which reached over 100% cover six years post-harvesting. In addition, while fuel treatments initially reduced shrub cover and richness, shrubs increased significantly in all fuel treatments in the mid-term. Understory vegetation dynamics in the fuel treatments were influenced by pretreatment flora richness and cover. In contrast, overstory removal was likely the strongest driver of high invasion in patch cuts within the first ten years, suggesting that severe disturbance (e.g., 100% removal of the overstory) may overwhelm biological legacies. Finally, overstory canopy was significantly and negatively associated with introduced species in the fuel treatments and the patch cuts, highlighting that canopy gap dynamics likely influenced invasion in this system. Our study underscored that while pretreatment species composition was a strong determinant of invasion in fuel treatments, management strategies that sustain overstory canopy cover may reduce growth and recruitment of introduced species and native shrubs.