What causes effects of early summer harvesting on restored grassland productivity: An investigation of interactions between functional groups

Background/Question/Methods

We investigated the effects of harvesting biomass from a restored grassland on interspecific interactions within this community and how those interactions may influence total productivity. Grasslands are potential sources of biofuel feedstocks and haying in both the summer and fall has been proposed as a way to increase their annual net primary productivity (ANPP). The efficacy of such a harvest may depend on interspecific interactions between members of these communities as a result of variations in their responses to this disturbance. Cool season (C₃) species grow primarily in the spring and fall while warm season (C_4­­) grasses grow during hotter and dryer summer months. Harvesting in the early summer may take advantage of this niche partitioning by removing biomass produced in spring which may competitively reduce C₄ grass productivity later in the season. Alternatively, early summer harvesting may disturb C₄ grasses in early growth stages, reducing their ANPP. We harvested either C₃ or C₄species individually or both functional groups and included an un-harvested control in a diverse restored grassland. All functional groups were harvested in the fall and we compared the productivity of each functional group and community wide ANPP among treatments. 

Results/Conclusions

C₃ species growth response to harvest depends on the concomitant removal of C₄ biomass and C₃ ANPP increased only when both species were harvested (C₃ removal x C₄ removal interaction; p = 0.039). The removal of C₄ species early in the summer resulted in a decrease of C₄ ANPP regardless of C₃ removal (p = 0.021). The result of these different responses of C₃ and C₄ species to harvest is that total ANPP was not significantly decreased when harvesting both C₃ and C₄ species in the early summer because C₃ regrowth offset reduced regrowth of C₄ species. These results suggest that multiple harvests can occur in a year, which may prove useful in agricultural situations where biomass for forage is especially valuable in early summer and biomass for biofuel is more valuable in the fall. The positive effects of early summer harvesting on C₃ species productivity may also result in the modification of community composition and an increase in pollinator resources.