Effects of drought and non-native grass invasion on plant diversity and composition in longleaf pine communities
Invasive species threaten vulnerable natural ecosystems and climate change has the potential to increase the success and exacerbate the effects of invasions. However, few studies have evaluated the interaction between climate change and plant invasions. Imperata cylindrica (cogongrass) is one of the most problematic invasive plants in the southeastern US. Under future climate scenarios, the southeastern US is expected to experience increased drought conditions, which might interact with invasions to suppress native species and alter ecosystem functions. Our goal was to investigate the individual and interactive effects of drought and I. cylindrica invasion on regeneration of a highly threatened ecosystem, longleaf pine forests. We expected that drought would enhance the success of I. cylindrica and its effects on native plant communities. We established 40 4 m x 4 m plots planted with longleaf pine and native plant species, applied a factorial combination of I. cylindrica invasion and rainout shelters, and measured plant community diversity and composition and root characteristics. This design allows us to evaluate the direct effects of cogongrass and drought on native plant communities, as well as the indirect effect of drought on cogongrass invasion.
After the first year of data collection, we found that I. cylindrica invasion had a significant effect on the native plant community based on PERMANOVA of Bray-Curtis dissimilarity matrix, but did not result in a change in the species diversity in the plots. Drought has not affected native plant community composition or diversity. During a single growing season, percent cover of I. cylindrica increased from 18% in June to 50% in October 2014. I. cylindrica maintained 50% cover in February 2015 and light availability at the ground was 12.6% compared with native plots at 38%. I. cylindrica had a strong interactive effect with drought on soil moisture, where drought treated plots with the invasion had higher soil moisture than uninvaded drought plots. Root depth distributions were similar among treatments in the top 30cm but I. cylindrica plots had higher root length below 30cm. Our data demonstrates that I. cylindrica can alter plant community composition and suppress longleaf pine performance possibly due to changes in light and water availability. Further work over the next several years will elucidate the longer-term effects of invasion and drought on longleaf community diversity and ecosystem function.