Species composition in California’s grasslands impacts the magnitude and timing of ecosystem services

Background/Question/Methods

A comprehensive understanding of vegetation impacts on multiple ecosystem services is critical for ecosystem management. Documenting the multiple effects of plant composition on soils enhances our ability to predict the ecosystem consequences of vegetation shifts in response to changes in environmental conditions and management. It also provides an “ecological toolbox”, allowing managers to selectively manage for species based on the multiple services they provide.

 In the fall of 2007, we planted monocultures and mixtures of prevalent species in California grasslands:  (1) Naturalized annuals that have dominated California grasslands for the past two to three centuries (Avena fatua, Bromus hordeaceus, Festuca perennis, Trifolium subterraneum); (2) Native species that are prevalent in restoration, including short-lived species (Festuca microstachys, Bromus carinatus, Lupinus bicolor), and long-lived perennial bunchgrasses ( Stipa pulchra, Elymus glaucus, Elymus triticoides); and (3) more recently introduced annual noxious weeds that are displacing the naturalized annuals in many areas (Aegilops triuncialis, Elymus caput-medusae). Measurements included: seasonality of plant biomass and nutrient content, seasonality of soil nitrogen supply (resin bags), water infiltration, water holding capacity, soil cohesion, and soil carbon storage.

Results/Conclusions

Vegetation composition impacted all measured processes. Compared to naturalized communities, natives and weeds had higher aboveground production, which occurred later in the growing season. In all communities, soil N supply was highest in the winter, but this winter peak was much lower in weed communities, compared with native and naturalized communities. Water infiltration rates and soil cohesion were highest in the naturalized communities.

While the community types differed from one another, variation in ecosystem processes across communities was smaller than the variation between monocultures within a community type.  For example, within the naturalized community type, Avena had much higher winter N availability than other annuals. Within the natives, winter N supply was highest in plots containing the legume and two of the bunchgrass species. All mixture and monoculture treatments had a winter peak in soil N availability, except for monocultures of the weed Aegilops, which had the lowest N availability at this time.

Ecosystem processes in mixed-species plots were not predictable based on process measures in the component monoculture plots. Similarly, ecosystem processes measured in each community type (natives, naturalized, weeds) could not be used to predict ecosystem processes in mixtures of these communities.