Effects of photosynthesis capacity on community structure in tallgrass prairie

Background/Question/Methods

Mixtures of native grassland perennials are characterized by high productivity, large carbon sequestration capacity and have been considered as potential biofuel feedstock. However, the provision of these services is affected by community structure and photosynthetic capacity of species in the community. In order to test photosynthetic capacity of different species and its effects on community structure, we measured photosynthesis rate, chlorophyll content, specific leaf area (SLA), leaf carbon(C), leaf nitrogen (N), leaf C: N ratio, species cover and abundance of 13 most common species in a tall-grass prairie from June to October 2010. The 13 species belong to 4 plant functional types (PFT): C4 grass, C3 grass, forb and legume. The C3 Farquhar, von Caemmerer and Berry (FvCB) model (Farquhar et al. 1980) and C4 coupled photosynthesis-stomatal conductance model (Collatz et al. 1992) were parameterized using Bayesian Markov Chain-Monte Carlo (MCMC) methods to examine the effects of photosynthesis parameters on community composition. The C3 photosynthesis parameters [including maximum Rubisco carboxylation rate (V_cmax), potential light-saturated electron transport rate (J_max), and dark respiration (R_d )] and C4 photosynthesis parameters (V_cmax, R_d) were determined by fitting the model to field photosynthesis data sets.

Results/Conclusions

The net carbon assimilation rates (A_net) of the 13 prairie species examined ranged from 1.47 µmol m^-2 s^-1 to 20.55 µmol m^-2 s^-1. The growth rates of grasses peaked in June and July, while the peak growth of forbs was in July and August. Seasonal variety of A_net was very high among all species. A_net was positively correlated with species abundance, cover and chlorophyll-A content across the growing season. However, the positive correlation was more significant within each PFT, demonstrating that physiological photosynthesis traits played a more important role in competition within PFT. The positive correlation between A_net and SLA found in broad-scale GLOPNET analysis was not observed in our study. Our results suggest that growth rates play an important role in species competition and structuring the community. Tallgrass prairie tends to maintain high carbon assimilation rate because of the dominant position of species with high photosynthesis capacity. Our study presents a new approach to parameterize photosynthesis models, which overcomes the data limitation problem of single-curve parameterization. In addition, the parameter values can be updated with new data. Our research also provides a methodology for estimating the productivity of tallgrass prairie.