Measuring the sustainability and community responses to ten-years of exotic control in Southern California grasslands

Background/Question/Methods

Re-establishment of native-based communities and assemblages to replace exotics infested lands is the primary goal of any restoration project. However, uncertainties in invader response to control efforts challenge this approach. Distinguishing among different scenarios of response during control and re-organization following control is critically important. We present results of a long-term herbicide control program conducted in the Nature Reserve of Orange County (NROC) to eradicate Cynara cardunculus. We specifically asked: (i) is passive control program reducing Cynara? (ii) If so, does Cynara removal result in greater native cover, or increase in other problematic exotics cover? (iii) Does success vary depending on environment, initial site conditions or disturbance level? (iv) Post long-term control, are sites on a sustainable trajectory or will Cynara re-invade? Answers to these questions are important to management decisions faced by the NROC, and also provide relevance to other restoration programs worldwide. In 2008, we resurveyed over 1000 acres initially surveyed in 1998 to assess change in Cynara and other species cover. We used path analysis to determine how multiple variables directly and indirectly influenced native restoration. Then, in a subset of smaller plots, we stopped herbicide treatments to assess vegetation responses of treated areas post long-term control.

Results/Conclusions

We found that control efforts have substantially reduced Cynara cover. However, between 1998 and 2008, Brassica nigra, another equally undesirable species increased in abundance and averaged 15% of the study area. Such secondary invasion resulted in overall decline of native cover and native richness in 2008. Our results suggest that overgrazing and erosion at the NROC could have shifted microbial communities, nutrient cycling and other soil processes advantageous to Cynara. Such legacy effects may last even after Cynara is controlled and provide an environment that facilitates the establishment of another invasive species such as Brassica as shown by path analysis. Our results shed new light on how legacy effects between invasive species and soil environment might contribute to cases where restoration gets “stuck” in a degraditive cycle or take a trajectory that leads to unplanned outcomes. After two years of excluding herbicide treatments, we found Cynara re-invasion in 61% of the plots. These results emphasize the importance of long-term large-scale invasive plant management efforts. Overall, our results point to the need for more careful consideration of unplanned and undesirable consequences of control efforts.