The structure and function of ten Western North American grasslands: III. Net primary production, turnover and efficiencies of energy capture and water use

Abstract

(1) Levels of net primary production and the efficiencies of energy capture and water use were investigated in six grassland types encompassing ten western North American grasslands. Emphasis was placed on between-site analysis to show the importance of abiotic variables in the functioning of the ecosystem at the producer level. (2) Above-ground primary production (ANP) ranged from 54 to 523 g m^-2 yr^-1. Average ANP for grazed grasslands was 212 compared to 236 g m^-2 yr^-1 for ungrazed grasslands (P ≤ 0.05). (3) There was an apparent linear increase in ANP with increasing precipitation up to approximately 500 mm yr-1. Likewise, ANP increased linearly with increases in both growing-season and annual actual evapotranspiration. (4) Net root production (RP) ranged from 148 to 641 g m^-2 yr^-1. The RP was significantly higher on grazed treatments compared to ungrazed grasslands. In general, RP increased with decreasing levels of long-term mean annual temperature. (5) Total net primary productivity (TNP) ranged from 225 to 1425 g m^-2 yr^-1. Approximately 46% and 58% of the variability in TNP were explained by annual precipitation in ungrazed and grazed grasslands, respectively. (6) Generally, the warmer grasslands had higher rates of turnover of crown material than did cooler grasslands. As annual usable incident solar radiation and annual actual evapotranspiration increased, so the rate of crown-turnover increased. (7) The average rate of turnover of root material was 0.18, 0.30 and 0.49 for the mixed-grass, tallgrass and shortgrass prairies, respectively. There was a positive curvilinear relationship between root turnover and total annual usable incident solar radiation. (8) Efficiency of energy capture in TNP ranged from 0.12% to more than 1.4% for both ungrazed and grazed grasslands. It appears that plant communities dominated by cool-season species were comparable to or more efficient in energy capture than the communities dominated by warm-season plants. Grasslands having higher efficiencies of water use for TNP also had greater efficiency of energy capture. Consequently, these two important functional properties of the producer system are positively related.