The structure and function of ten Western North American grasslands: II. Intra-Seasonal dynamics in primary producer compartments

Abstract

(1) Intra-seasonal dynamics of the various above-ground and below-ground primary producer compartments for ten central and western North American grassland sites are presented. (2) The seasonal peak values of the primary producer compartments are examined, as indicative of the net accumulation of organic material, and the relationships of these peak values to various abiotic regimes at the sites are investigated. (3) Seasonal live biomass followed either a unimodal or a bimodal growth pattern. In general, grasslands with only cool-season or only warm-season plants showed a unimodal pattern, while grasslands dominated by both cool- and warm-season species had a bimodal seasonal growth pattern. There were no significant differences between grazed and ungrazed treatments in seasonal live biomass, although there was a significant site × treatment interaction. (4) Peak live biomass ranged from 84 to 336 g m^-2, and showed a linear increase with increasing amounts of growing-season precipitation up to 450 mm; at higher values of precipitation increases in live biomass tended to level out. (5) Maximum rates of accumulation of live biomass ranged from 0.4 to 6.5 g m^-2 day ^-1. Ungrazed grasslands had a peak rate of 4.2 g m^-2 day^-1 compared with 3.2 g m^-2 day ^-1 for grazed grasslands. (6) Generally the recent-dead compartment showed maximum values soon after the peak in the live compartment. Material in the old-dead compartment, however, was at a maximum early in the growing season, and a general decline in the standing crop of old dead material followed as material was transferred to the litter compartment. (7) Litter dynamics responded closely to precipitation events, and showed a rather erratic pattern. (8) Root biomass generally reached a maximum about midway through the growing season. On the cooler grasslands, grazed treatments typically had a larger peak in root biomass; in contrast, the warmer grasslands did not show a marked response in root biomass to grazing.