The Structure and Function of Ten Western North American Grasslands: IV. Compartmental Transfers and Energy Flow Within the Ecosystem

Abstract

1) Seasonal and daily rates of inputs to and outputs from the various primary producer compartments of ten North American grassland sites are presented. (2) Net accumulation rates of total net primary production (TNP) ranged from c. 1 g m-2 day-1 in the desert grassland to over 11 g m-2 day-1 in the mountain grassland. The average average net accumulation rate of TNP was 4.4 g m-2 day -1 for ungrazed grasslands, and slightly more for the grazed grasslands. A significant amount of the variability in mean daily accumulation rates of TNP for ungrazed and grazed grasslands was related by regression analysis to variations in long-term mean annual temperature and long-term mean annual precipitation. (3) Above-ground net primary production (ANP) accumulated at rates of c. 1.4 and 1.2 g m-2 day-1 for ungrazed and grazed grasslands respectively. Variations in annual precipitation explained 60% and 76% of the variability in ANP rates for ungrazed and grazed grasslands respectively. (4) The amount of live material transferred during the growing season to the standing dead compartment averaged 182 g m-2 for the ungrazed treatment and 162 g m-2 for the grazed treatment. The desert grassland had the smallest amount of live material transferred to the standing dead compartment, while the tallgrass prairie had the greatest amount. (5) Transfers of standing dead material into the litter compartment during the growing season averaged 165 g m-2 for ungrazed grasslands and 134 gm-2 for grazed grasslands. Net disappearance of litter during the growing season averaged 147 g m-2 for both the grazed and ungrazed treatments, giving an average rate of litter disappearance of c. 0.9 g m-2 day-1 across these North American grasslands. (6) All grasslands, except the desert grassland, had higher mean rates of net accumulation of dry matter below-ground in the grazed treatment as compared with the ungrazed treatment. Generally, the rate of net accumulation of dry matter below-ground increased with decreasing mean annual temperature, and, within a particular temperature regime, with increasing aridity of habitat. (7) The percentage of incident solar energy captured in the ungrazed and grazed grasslands was 0.64 and 0.69% respectively. On the ungrazed grasslands there was three times more energy trapped in the below-ground compartment than in the above-ground compartment; for grazed grasslands, six times more energy was trapped below-ground compared to the above-ground vegetation compartments.