Impacts of California grassland species on multiple ecosystem services

Background/Question/Methods

In wildland and human-dominated landscapes, there is increasing interest in managing for multiple ecosystem services. However, we are still lacking in our understanding of how to simultaneously enhance multiple services, or whether distinct tradeoffs exist between production, conserving native species, enhancing water quality and storage, increasing soil carbon and nitrogen, and minimizing weeds, surface runoff and erosion. The objective of this study was to determine the impacts of California grassland species on multiple ecosystem processes and properties and establish how these impacts change over space and time.  Measurements were taken in experimental plots, and in vegetation patches across Yolo County, California. Experimental plots were planted as monocultures and mixtures of 14 species, including natives commonly used in restoration, naturalized exotics, and newly invasive exotics. Some of these vegetation treatments were exposed to N addition, and to different timing of clipping.  Observational studies were also conducted across the landscape, evaluating how ecosystem service indicators differed in restored vs. unrestored areas, and in patches dominated by invasive weeds vs. naturalized annual exotics. We measured plant biomass and quality, infiltration rates, water holding capacity, soil organic matter and soil C/N by depth, soil cohesion (an indicator of surface erosion control) and soil compaction.

Results/Conclusions

Species and species mixtures significantly differed in their impacts on ecosystem processes and properties, and the impact of a given vegetation type varied across sites and treatments.  As anticipated, tradeoffs exist so that there is no vegetation group or species that optimizes all ecosystem processes simultaneously.  Surface soil cohesion is significantly higher in soils dominated by annual exotics, compared with soils associated with native grasses, and is highest in soils dominated by weeds. Infiltration rates varied across species, however we did not observe any significant differences across broad groups (weeds vs. annuals vs. natives). Across clipping treatments, infiltration rates were significantly higher in the spring clipped treatment compared to the fall clipped and unclipped treatments.  Species and species mixtures had little impact on soil compaction by depth, but did impact depth of soil organic matter.