Linking photosynthesis to community dynamics in a tallgrass prairie

Background/Question/Methods

Maintenance of species diversity is an important ecological concern in restored prairies given that plant community composition is a major driver of prairie ecosystem function. In addition, plant community composition is a good indicator of prairie ecological restoration values associated with soil conservation, disease resistance and wildlife benefits. Carbon assimilation rate affects relative growth rate and thus may play an important role in shaping community composition. However, the linkage between photosynthetic rate and community dynamics has rarely been studied, but it has been hypothesized that high assimilation rate leads to more biomass production and light capture, which in turn results in higher photosynthetic rate. This positive feedback benefits species with high photosynthetic rates and contributes to high competitive ability leading to species dominance. In order to test this hypothesis, we conducted field experiments in a newly established tallgrass prairie restoration and an older mature prairie (Trelease prairie, established in 1940s). Species-level photosynthetic rate, percent cover and abundance were measured monthly from June to October in the two prairies. The species were assigned to four plant functional types (PFT): C3 grass, C4 grass, forb and legume.

Results/Conclusions

The photosynthetic capacity (A_max, photosynthetic rate at saturating light and ambient CO₂) ranged from 1.15 µmol m^-2 s^-1 to 40.56 µmol m^-2 s^-1. Within-species seasonal variety of A_max was very high for all species. In the newly established prairie, A_max was positively correlated with percent cover within species for most forbs through the growing season. However, no within-species relationship was observed between photosynthetic capacity and percent cover for grass species. For the most abundant species, a consistent across-species positive relationship between photosynthesis and percent cover was found, but the relationship was only significant in mid-growing season. These results suggest that photosynthesis may influence community composition by affecting dominance of relatively common species in the prairie. In the mature prairie, two forb species revealed a positive within-species relationship between A_max and percent cover. However, no across-species relationship was found. This suggests that the positive correlation between cover and photosynthetic capacity may be a phenomenon affected by the development stage of the prairie.