Effects of fire and invasive species legacy on plant communities in a Northwest prairie

Background/Question/Methods

In this study we used a matrix of restoration management practices in a glacial outwash prairie south of the Puget Sound in Washington to evaluate treatments of fire and herbicide as well as the legacy effects of the N-fixing non-native species Cytisus scoparius on prairie plant communities. We measured plant community diversity and abundance across five data sets spanning three years for both spring and fall seasons.  We compared native and non-native plant communities, community structure, composition, and diversity among 1) different aged burn treatments (2002, 2006, no burn) and pre/post burn treatments, 2) graminoid specific herbicide treatments over 1 and 4 years, and 3) we evaluated legacy effects of C. scoparious using historic aerial photos showing distribution and subsequent removal across all fire and herbicide treatments.  

Results/Conclusions

There are three major findings of this work: 1) Species richness was highest in the most recently burned site (2006; P < 0.05) compared to an older burn (2002) and control treatment (P < 0.05). These findings are also supported by data on 20 1m2 vegetation plots that were monitored pre, and post-burn in 2006/2007. However, both datasets suggest burn treatments increase richness through an increase in the diversity and abundance of non-native plants;  2) graminoid-specific herbicide showed promise as a restoration mechanism with significant reductions in invasive grass cover over 1-year and 4-years of consecutive treatment (P < 0.05);  and 3) using a combination of GIS modeling, non-metric multidimensional scaling ordination, and multiple response permutation analyses, we found relationships that suggest historical proximity to C. scoparious is a strong predictor of community differences through promotion of invasive plant abundance for all treatments (P < 0.05). This work suggests that while plant communities respond to fire and herbicide as restoration tools, fire may promote invasive species dominance over native species. Further,  key soil-modifying invasive species can have legacy effects that transcend the effects of even high-intensity restoration activities. Since these legacy effects and fire may both increase invasive success, targeted invasive species management in areas previously occupied by an N-fixing shrub may increase efficacy of restoration treatments over broad-landscape-level application of tools like fire and herbicide.