What causes effects of early summer harvesting on restored grassland productivity: An investigation of interactions between functional groups
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 (C4) 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.
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.