Production Ecology of Grassland Plant Communities in Western North Dakota

Net primary productivity and distribution of mixed grassland plant communities in western North Dakota were studied. The semiarid continental climate supports several grassland types, delimited on the bases of topography and substratum. The dominant species and herbage (shoot) production (mean of 2 yr) of stands located on what were classified as silty range sites included, on rolling upland, (1) Stipa viridula (128 g m— ²), Stipa comata (128 g m— ²), and (3) Bouteloua gracilis (110 g m— ²); on ravine slopes, (4) Stipa spartea—forbs (135 g m— ²) and (5) Andropogon spp. (126 g m— ²). On sandy range sites were found, in shallower ravine bottoms and at bases of slopes, (6) Sporobolus heterolepis—Stipa spartea (160 g m— ²) and (7) Sporobolus heterolepis (185 g m— ²); on slopes with very sandy soils, (8) A. scoparius—Stipa spartea (132 g m— ²), (9) A. scoparius—forbs (107 g m— ²), and (10) A. scoparius (119 g m— ²). The highest annual herbage production was measured in Stand 7 (200 g m— ²) and the lowest in Stand 3 (77 g m— ²). Root mass ranged from 6230 g m— ² (Stand 7) to 3030 g m— ² (Stand 9). Annual root production could not be measured but was estimated to be equivalent to herbage production. Variation in herbage production between sites and between years was attributed to differences in soil moisture and fertility (especially the latter in sites with sandy soils). Fresh mulch was highest in the Stipa—dominated sites (288—320 g m— ²) and lowest in Andropogon— and Bouteloua—dominated stands (212—234 g m— ²). The highest fresh mulch decay constant and shortest equilibrium time were determined to occur in sites dominated by Sporobolus heterolepis, and the converse for Stipa—dominated sites. Chlorophyll values ranged from 78 mg m— ² (Stand 3) to 542 mg m— ² (Stand 7); variation depended on plant morphology and phenology as well as biomass. Maximum efficiency in herbage production occurred later in the year for stands with a large warm—season component (Sporobolus heterolepis an Andropogon spp.) than in sites dominated by cool—season grasses (Stipa spp.). On an annual basis, Stand 7 was most efficient (0.12%) and Stand 3 least (0.06%). The interrelationships between drought tolerance, phenology (cool— vs. warm—season development), physiology, and production are discussed.