Annual trait variation in grassland plant species

George R. Wheeler, Chad E. Brassil and Johannes M. H. Knops, School of Biological Sciences, University of Nebraska, Lincoln, NE, Johannes M. H. Knops, Department of Health and Environmental Sciences, Xi’an Jiaotong Liverpool University, Suzhou, China

Background/Question/Methods
While plant functional traits are a valuable tool in addressing ecological questions, reliable application of these traits depends on a clear understanding of their variability, within as well as between species. In grassland systems, where most aboveground tissues are replaced annually, significant intraspecific variation may be occurring on a temporal basis. However, as many studies are limited to trait data collected in a single year, this potential source of variation remains poorly understood.
To measure whether, and to what degree, plant species’ functional traits vary from year to year, we conducted four years of trait measurements for twelve species common in the arid grasslands of Cedar Point Biological Station (Nebraska, USA). For each species, we measured four functional traits, height, specific leaf area, leaf dry matter content, and chlorophyll content. In order to compare annual variation to variation caused by nutrient availability, and to test for interactive effects of these two variables, we collected data in both control and nitrogen addition plots established as part of the Nutrient Network global collaboration.
Results/Conclusions
Based on linear mixed effects modeling, we found that three of the four analyzed traits showed significant intraspecific variation from year to year. The pattern of variation was consistent with a tendency towards more conservative trait values in drier years, though such comparisons could be made only on an observational basis. This annual trait variation was notable for its scale, representing effect sizes only slightly smaller than those of long-term nitrogen addition treatments. While we had predicted that trait values would respond more strongly to nitrogen in high rainfall years, our results did not show such a pattern. Instead, the effects of nutrient addition and sampling year were additive for most traits, with only chlorophyll content showing an interactive response. These results suggest that single year studies may fail to detect a substantial portion of intraspecific trait variation, though trait responses to other variables may nonetheless remain consistent across sampling years.