Reflections on vernal pool plant community restoration: Three lessons from a long-term field experiment

Background/Question/Methods

The goals of ecological restoration are to recover native populations, communities, and ecosystems degraded by human activities.  Successful restoration therefore requires establishment of both ecological structure and function to degraded systems. We study plant communities of California vernal pools, which are ephemeral wetlands that support many rare or endangered species. In this system, we consider plant species composition as a key component of ecological structure, and hydrological processes as key components of ecological function. Here we present results of a long-term field experiment involving 256 constructed vernal pools in which we compare hydrologic and vegetative characteristics of restored vernal pools to that of adjacent, naturally occurring reference pools. At the outset of the experiment (1999), three-fourths of the restored pools were seeded with three different combinations of five native species (“seeded”), and one-fourth of the pools were left unseeded (“controls”). We tracked pools from 2000 to 2009 and characterized effects of hydrology and seeding on restoration success, comparing seeded and control pools with naturally occurring reference pools. We ask whether appropriate hydrological conditions alone are sufficient to promote establishment of vernal pool plant communities in constructed pools, or if seeding enhances native species persistence.

Results/Conclusions

As expected, cover of seeded species was significantly higher in seeded than control pools in every year of the study.  The smallest observed difference in seeded species cover occurred in 2009. Furthermore, sampling plots in seeded and reference pools were similar through time in cover of seeded and other native species, as well as exotic species. Cover of seeded species in constructed pools was best modeled by seeding treatment and maximum depth of inundation. Pool depth and duration of inundation were similar among seeded, control, and reference pools through 2002, but by 2009 reference pools were significantly deeper than restored pools during the period of peak inundation. The depth of restored pools has decreased apparently as a result of climate-driven increases in the cover of exotic species that contribute to rapid thatch accumulation. These combined results suggest that vernal pool restoration can be facilitated by ensuring appropriate hydrological conditions and by seeding constructed pools with native species. However, our observed effects of climate on the outcome of competition between native and exotic species suggest impending effects of climate change, especially if exotic species facilitate their own dominance through thatch accumulation.