Experimental Protocol for Manipulating Plant-induced Soil Heterogeneity

Coexistence theory has often treated environmental heterogeneity as being independent of the community composition; however biotic feedbacks such as ...

Case Western Reserve University

Plant-soil feedbacks can affect plant community dynamics by influencing processes of coexistence or invasion, or by maintaining alternate stable states. Darwin's naturalization hypothesis suggests that phylogenetic relatedness should be a critical factor governing such feedbacks in invaded communities but is rarely considered in soil feedback studies. We investigated the effects of soil biota from experimentally established native and invaded California grassland communities on resource capture and allocation of three native and three exotic grass species, comprising three tribes, grown in the laboratory. Phylogeny was the single greatest determinant of grass biomass, root:shoot ratio, and growth rate, with presence of soil biota explaining the second greatest proportion of variance in total grass biomass. Similar trends were observed in soil collected from naturally occurring stands of native perennial and exotic annual grasses. Species of similar life history/provenance exhibited similar biomass responses to the same soil community, while more closely related species exhibited similar root:shoot ratio responses to the same soil community. Relationships between the plant community composition of a field plot and species responses to soil inoculum collected from that field plot were idiosyncratic, with many aspects of plant community structure potentially contributing to soil feedbacks. Thus, future studies should explicitly consider both phylogeny and provenance and evaluate soil feedbacks in a community setting.

Phylogeny and provenance affect plant-soil feedbacks in invaded California grasslands.

Coexistence through a variety of mechanisms is possible for species with differential responses to environmental conditions. Understanding the role of environmental heterogeneity in mediating coexistence of species of different provenance (i.e., native vs. exotic) has important implications for theory and management. We used two California grassland data sets, one spanning seven years at three reserves along a 500-km latitudinal gradient and one spanning 47 years at 11 sites within a single 1000-ha reserve, to determine how environmental heterogeneity in space and time contributes to variability in provenance group abundance and diversity, and whether native and exotic species respond similarly to spatial and temporal variability. We found that temporal environmental heterogeneity is the primary determinant of provenance group abundance, while spatial and temporal environmental heterogeneity both contribute to community diversity. Spatial and temporal heterogeneity must therefore be considered simultaneously when examining community dynamics and species coexistence. Provenance was a poor general predictor of species response; native and exotic species exhibit similar spatiotemporal patterns in some cases but not others. Plant persistence may depend more upon the abiotic environment than competition from the other provenance group as native and exotic diversity were generally positively correlated. Furthermore, mesoscale (10(2)-10(3) m) spatial heterogeneity may be a greater mediator of provenance group coexistence than temporal heterogeneity or spatial heterogeneity at other scales.

Regional and decadal patterns of native and exotic plant coexistence in California grasslands.

The effects of exotic species invasions on biodiversity vary with spatial scale, and documentation of local-scale changes in biodiversity following invasion is generally lacking. Coupling long-term observations of local community dynamics with experiments to determine the role played by exotic species in recruitment limitation of native species would inform both our understanding of exotic impacts on natives at local scales and regional-scale management efforts to promote native persistence. We used field experimentation to quantify propagule and establishment limitation in a suite of native annual forbs in a California reserve, and compared these findings to species abundance trends within the same sites over the past 48 years. Observations at 11 paired sites (inside and outside the reserve) indicated that exotic annual plants have continued to increase in abundance over the past 48 years. This trend suggests the system has not reached equilibrium > 250 years after exotic species began to spread, and 70 years after livestock grazing ceased within the reserve. Long-term monitoring observations also indicated that six native annual forb species went extinct from more local populations than were colonized. To determine the potential role of exotic species in these native plant declines, we added seed of these species into plots adjacent to monitoring sites where plant litter and live grass competition were removed. Experimental results suggest both propagule and establishment limitation have contributed to local declines observed for these native forbs. Recruitment was highest at sites that had current or historical occurrences of the seeded species, and in plots where litter was removed. Grazing history (i.e., location within or outside the reserve) interacted with exotic competition removal, such that removal of live grass competition increased recruitment in more recently grazed sites. Abundance of forbs was positively related to recruitment, while abundance of exotic forbs was negatively related. Thus, exotic competition is likely only one factor contributing to local declines of native species in invaded ecosystems, with a combination of propagule limitation, site quality, and land use history also playing important and interactive roles in native plant recruitment.

Angela J. Brandt

Department of Biology

Case Western Reserve University

Experimental Protocol for Manipulating Plant-induced Soil Heterogeneity

Angela J. Brandt

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Biology

Case Western Reserve University

Experimental Protocol for Manipulating Plant-induced Soil Heterogeneity

Department of Biology