Plant traits that influence ecosystem processes vary independently among species

Most predictions of plant species effects on ecosystems are based on single traits (e.g., litter chemistry) or suites of related traits (functional groups). However, recent studies demonstrate that predictions of species effects on ecosystems are improved by considering multiple traits. In order to develop this multiple trait approach, it is critical to understand how these multiple traits vary in relation to one another among species. The ecosystem effects of traits that strongly covary can likely be summarized by one of these traits. In contrast, it will be necessary to determine the ecosystem effects of specific trait combinations for those traits that vary independently across species. In the field, I established monocultures of eight herbaceous species common in California annual grasslands. Plant species significantly differed in their litter quantity and quality, live biomass, and effects on soil labile C, soil temperature, and soil moisture. Species effects on soil moisture and temperature were only significant at the times of the growing season when each of these limited plant and microbial activity. Some of these traits correlated with one another, such as litter biomass and species effects on soil temperature during the winter. However, for the most part, plant species exhibited unique combinations of these traits. For example, species with similar litter chemistry had the largest differences in plant biomass, soil moisture, and soil labile C. Species rankings for many traits changed over the growing season (e.g., biomass), so that the relationship among traits varied seasonally. The independent variation of these fraits suggests that predictions of plant species effects on ecosystems will likely be enhanced by an, understanding of how the ecosystem effects of plant traits may vary depending on the combination of traits.