873 Ecology, 80(3), 1999, pp. 873–881  1999 by the Ecological Society of America INTERACTIONS BETWEEN PLANT SPECIES AND EARTHWORM CASTS IN A CALCAREOUS GRASSLAND UNDER ELEVATED CO 2 JOHANN G. ZALLER 1 AND JOHN A. ARNONE III 2 Botanisches Institut, Universita ¨t Basel, Scho ¨nbeinstrasse 6, CH-4056 Basel, Switzerland Abstract. We tested the hypothesis that the spatial proximity of a plant species to nutrient-rich earthworm casts (e.g., 100% more ammonium and 30% more phosphate than in adjacent soil) is an important determinant of a plant’s responsiveness to elevated at- mospheric CO 2 . In 1995 we mapped the location of both earthworm surface casts and plants in each of 16 1.2-m 2 plots in a species-rich calcareous grassland in northwestern Switzerland. Eight plots have been maintained under current ambient CO 2 concentrations (350 L CO 2 / L), and eight have been maintained at elevated CO 2 (600 L CO 2 /L) since March 1994. In addition, total ramet production of each species, as a measure of performance, and cu- mulative cast production at each location (cell) were recorded at peak community biomass in 1995. Plant species within functional groups (graminoids, non-legume forbs, and leg- umes) differed markedly in their degree of association with casts; however, after two growing seasons elevated CO 2 had no effect on plant species or functional group associations with casts. No statistically significant relationship could be demonstrated between plant- species response (i.e., ramet production) to elevated CO 2 and the degree of association with casts within any of the functional groups. However, a positive relationship was observed between the mean response of graminoid species to elevated CO 2 (measured as the per- centage change in mean total ramet production of graminoid species, relative to mean total ramet production at ambient CO 2 ) and their mean degree of association (%) with surface casts at ambient CO 2 . Thus, graminoid species more frequently associated with casts (e.g., Anthoxanthum odoratum and Carex caryophyllea) produced more ramets per square meter at elevated CO 2 than those less frequently associated with casts (e.g., Agrostis tenuis and Danthonia decumbens). These results, along with the strong and significant positive cor- relations observed between ramet production and associated cumulative cast mass across CO 2 treatments for most plant species in all functional groups demonstrate: (1) that plant species differ significantly in their degree of association with nutrient-rich earthworm sur- face casts, regardless of the relative abundance of plant species in the community; (2) that graminoid species that are more highly associated with casts may respond more strongly to rising CO 2 than those less highly associated with casts; and (3) that nutrient-rich earth- worm casts stimulate the growth (ramet production) of most plant species in these grassland communities, even at current levels of atmospheric CO 2 . The data further suggest that these species-specific relationships between plants and casts have helped define the current struc- ture of these highly diverse grassland communities and will likely influence their future structure as global CO 2 levels continue to rise. Key words: CO 2 enrichment; earthworm activity; earthworm surface casts; elevated CO 2 , plant species responses; fertile soil microsites, role of earthworm casts; global warming, plant species responses; Lumbricidae; plant–animal interactions; plant–cast spatial distribution; soil ecology; spa- tial distribution of plants, earthworm effects. INTRODUCTION Numerous studies suggest that rising atmospheric CO 2 will alter the competitive interactions among plant species and thus lead to changes in the structure and composition of plant communities (e.g., Bazzaz 1990, Manuscript received 10 November 1997; revised 6 April 1998; accepted 13 April 1998; final version received 18 May 1998. 1 Present address: Department of Rangeland Resources and The Ecology Center, Utah State University, Logan, Utah 84322-5230 USA. E-mail: jgz@cc.usu.edu 2 Present address: Biological Sciences Center, Desert Re- search Institute, P.O. Box 60220, Reno, Nevada 89506 USA. Ko ¨rner and Bazzaz 1996). These interactions are in- fluenced strongly by soil fertility, with species in low- fertility ecosystems responding more slowly than those in high-fertility systems (Ko ¨rner 1996, Reynolds 1996). However, low nutrient supply may not neces- sarily preclude pronounced effects on plant community structure in the long term (Leadley et al. 1998). In the grassland communities we studied, causes for changing species abundance are numerous but may ultimately derive from species-specific gas-exchange responses to elevated CO 2 (Lauber and Ko ¨rner 1997). However, in- direct effects of elevated CO 2 , such as increases in soil moisture (Niklaus et al. 1998) and greater inputs of carbon to the rhizosphere (e.g., Hungate et al. 1997,