Common generalizations concerning the ecologic significance of C photosynthesis were tested in a study of plant gas exchange, productivity, carbon balance, and water use in monospecific communities of C and C salt desert shrubs. Contrary to expectations, few of the hypotheses concerning the performance of C species were supported. Like the C species, Ceratoides lanata, the C shrub, Atriplex confertifolia, initiated growth and photosynthetic activity in the cool spring months and also exhibited maximum photosynthetic rates at this time of year. To compete successfully with C species, Atriplex may have been forced to evolve the capacity for photosynthesis at low temperatures prevalent during the spring when moisture is most abundant. Maximum photosynthetic rates of Atriplex were lower than those of the C species. This was compensated by a prolonged period of low photosynthetic activity in the dry late summer months while Ceratoides became largely inactive. However, the annual photosynthetic carbon fixation per ground area was about the same in these two communities composed of C and C shrubs. The C species did not exhibit greater leaf diffusion resistance than the C species. The photosynthesis/transpiration ratios of the two species were about the same during the period of maximum photosynthetic rates in the spring. During the warm summer months the C species did have superior photosynthesis/transpiration ratios. Yet, since Ceratoides completed a somewhat greater proportion of its annual carbon fixation earlier in the season, the ratio of annual carbon fixation/transpiratory water loss in the two communities was about the same. Atriplex did incorporate a greater percentage of the annual carbon fixation into biomass production than did Ceratoides. However, this is considered to be a reflection of properties apart from the C photosynthetic pathway. Both species displayed a heavy commitment of carbon to the belowground system, and only about half of the annual moisture resource was utilized in both communities.
Prescribed burning was conducted in the fall and spring to evaluate the effects of fire on productivity of 3 forage species. Yield measurements were obtained throughout the growing season at biweekly intervals on western wheatgrass, blue grama, and tbreadleaf sedge. Supplementary measurements were made on vegetation cover and soil moisture. Herbage yield depended upon individual species, sampling date, and treatment. Spring burning of western wheatgrass and blue grama stimulated production by mid-and late-June, whereas fall burning also stimulated productivity but to a lesser degree. Production of threadleaf sedge was relatively unaffected by spring burning and reduced by fall burning. Fire can be used as a management practice to increase forage yield in the Northern Great Plains, but timing of utilization by livestock must receive careful consideration to assure maximum benefit. Fire has played an important historical role in the development of grassland communities (Daubenmire 1968; Komarek 1964, 1965). Nevertheless, negative attitudes toward burning have frequently limited application of fire as a management tool. Factors such as the threat of fire escaping the boundaries of a prescribed burn, temporary elimination of potentially usable forage, and the destructive effects of wildfires have all contributed to such attitudes. Only limited research has been conducted on the effects of burning on forage species in the Northern Great Plains. Lodge (1960) examined the effects of burning on crested wheatgrass (Agropyron desertorum), while Dix (1960) and Kirsch and Kruse (1972) reported on the effects of wildfire on mixed grass prairies in North Dakota. Prescribed burning has also been conducted in western wheatgrass (Agropyron smithii) communities in South Dakota (Gartner et al. 1978) and in the fescue grasslands of Canada (Bailey and Anderson 1978; Anderson and Bailey 1980). In general, these studies demonstrated that fire has management potential although additional research is needed to determine how individual species may respond (Bailey 1978). Our study was designed to evaluate the effects of prescribed burning on important forage species in the mixed grass prairie of eastern Montana. The primary objectives of the study were: (1) to examine post-burn herbage production of 3 species throughout the growing season, (2) to compare the effects of both fall and spring burning and (3) to determine if burning conditions could be simulated by a clipping treatment. Methods Three species were selected for investigation: western wheatgrass, blue grama (Bouteloua gracilis), and threadleaf sedge (Carex Authors are research plant physiologist and range scientist, respectively, USDA-ARS, Livestock and Range Research Station, Route I, Box 2021, Miles City, Montana 5930 I.
Changes in basal cover of vegetation were predicted in response to variation in precipitation and grazing intensity. Multiple regression analysis was used with basal cover as a dependent variable and precipitation parameters as independent variables to develop predictive equations. Predicted cover values were used to develop three dimensional response surfaces which describe individual species responses to fluctuating precipitation and different grazing intensities. Results indicate that each species reacts to precipitation regimes and grazing pressure in a unique manner. Continual changes in basal cover can be expected in the plant community as the precipitation regime changes. Moderate grazing intensity, approximately 0.92 ha (2.3 acres) per AUM, appears to be most conducive for maintaining vegetative cover that is desirable for livestock production. However, stocking rate changes need to be anticipated and planned to coincide with available forage because of large fluctuations in cover due to varying precipitation. Climate appears to be the major factor controlling plant growth in the Great Plains. Whitman et al. (1943) stated that drought is the primary influence of vegetational change. Reed and Peterson (1961) Hurtt (1951) and Clark et al. (1943) stated that major trends in vegetation are primarily determined by fluctuations in climatic conditions, and that changes within these major trends are influenced by grazing intensity.
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