LB 6-56 - 23 years of plant functional group responses to CO₂ and N manipulation in the BioCON grassland experiment

Background/Question/Methods:

Plant functional group responses to global change factors can vary dramatically over time due to changing species composition and interactions, acclimation to environmental changes, or feedbacks that can modify the environment. Thus, temporal patterns in responses of functional groups may range from consistent responses over time, to decreasing or increasing sensitivity to global change or even switching responses over time. Yet, very few manipulative experiments have monitored long-term responses of different plant functional groups to changes in nitrogen (N) deposition and elevated carbon dioxide (CO₂) levels. Therefore, in this study, we explore temporal variation of biomass and cover for four functional groups (C3 grass, C4 grass, legumes and non N-fixing forbs) in response to N and CO₂ manipulation and their interactions with species richness, by leveraging data from a 23-year long grassland experiment at Cedar Creek, Minnesota. The BioCON experiment consists of 296 2 m X 2 m plots that spans two soil N availability treatments (ambient vs +N), two CO₂ treatments (ambient vs +CO₂) and four grassland plant diversity levels (1, 4, 9 and 16 species). Species-specific cover has been measured for each plot every year since 1998 which can be used to study temporal variation in functional groups.

Results/Conclusions:

We found considerable differences among the functional groups in their responses to the treatments. For instance, C3 grasses, legumes and forbs showed reduction in cover over time in mixtures, while C4 grasses showed a monotonic increase, irrespective of the experimental treatment. Furthermore, an increase in C3 cover under N addition was observed during the first few years of the experiment, but became nonsignificant during the second decade of the experiment. Similarly, an increase in legume cover under elevated CO₂ relative to ambient conditions shifted to a decrease in legume cover after the first few years of treatments. In contrast, an increase in C4 cover in response to resource addition treatments has remained consistent over time, although the magnitude of response depends on the planted diversity of the plots. We are now testing whether these patterns in functional groups are associated with changes in annual precipitation, soil nitrate or soil pH. Overall our results highlight the varying sensitivity of functional group responses over time and underscore the importance of long-term experiments to better understand plant responses to global change factors.