Legacy effects of restoration disturbances on co-occurring native and exotic perennial grasses

Background/Question/Methods

The spread of exotic species has led to extensive research on how to restore native biodiversity in highly invaded ecosystems. Despite these efforts, minimal attention has focused on legacy effects caused by restoration disturbances and their influence on the reestablishment of native versus exotic taxa following restoration attempts. Some studies suggest disturbance is most effective as a restoration tool when exotic and native taxa respond differently and when such differences can be exploited to favor native taxa. While many studies have identified traits associated with invasiveness, few have assessed how responses of co-occurring natives and exotics vary across different post-restoration environments. Here, we summarize the findings of a fully-factorial field experiment designed to evaluate how legacy effects associated with season of mowing (late spring vs. fall) and herbicide application interact to influence post-disturbance responses of planted seedlings of two co-occurring species – Harding grass (Phalaris aquatica), an exotic bunchgrass currently spreading throughout California grasslands, and blue wildrye (Elymus glaucus), a native bunchgrass commonly used in restoration. In addition, we conducted a greenhouse germination study to evaluate the possibility that priority effects related to emergence timing and seedling growth have lasting effects on the success of individuals planted out in post-restoration environments.

Results/Conclusions

Our field experiment showed post-mowing environments had negative effects on the height of experimental seedlings for both species, with growth rates greatest in un-mowed environments. Similarly, post-herbicide environments increased stem number for both species, although this effect was greater for Harding grass. A mowing x herbicide interaction indicated that positive effects of herbicide on stem number decreased with the frequency of seasonal mowing. Post-herbicide environments also had a positive effect on inflorescence production, and this effect was greater for blue wildrye. Greenhouse studies showed earlier emergence and more rapid growth in blue wildrye relative to Harding grass, but this did not result in a greater advantage in growth rates after planting in the field. Collectively, our results demonstrate that legacy effects stemming from restoration disturbances had substantial effects on the growth and fecundity of native and exotic bunchgrass species. Although both species showed similar aboveground growth rates in each disturbance combination, a larger fraction of blue wildrye seedlings reproduced and produced more inflorescences than Harding grass. These results suggest tradeoffs between growth and fecundity may need to be considered when determining the most successful approach for using disturbances to simultaneously control invasive plants while promoting establishment of native species.