RESULTS AND ANALYSIS 



Aerial Production 



Tables 1 through 12 and figures 2 through 5 summarize productivity and 

 litter accumulation for the 1977-1980 study period. Means are expressed in 

 kilograms per hectare with 90% confidence limits. (For pounds per acre, 

 multiply kilogram per hectare values by 0.892.) When data from both halves of 

 each exclosure were compared, all exclosures were found to be internally 

 homogeneous at the .05 probability level, which further supports the hypothesis 

 (DNRC 1979a) that high precipitation increases heterogeneity for these 

 communities. During the four-year study period, aerial production has varied, 

 for the most part, with precipitation. Aside from the great variability in 

 productivity, plant community types with similar production in some years can 

 have quite different production in other years, and the ranking of community 

 types by production can change dramatically. 



Species have evolved different strategies to cope with the factors affecting 

 survival and reproduction, so changes in species composition as well as total 

 production are not remarkable. Six communities were selected to portray some 

 effects of climatic conditions on species composition (figures 6 through 12). 

 The response of mid-grasses is far more variable than the response of short- 

 grasses. However, this phenomenon may be explained better in terms of root 

 structure or phenology than plant height. (In reviewing figures 6 through 17, 

 the reader should note that the number of samples taken each year was based on 

 statistical requirements concerning total production, not statistical 

 requirements for each major species.) 



Not only does aerial production vary among species, it also varies within 

 certain species in different communities and on different sites. Effectiveness 

 of precipitation may be modified by slope position, horizontal configuration, 

 edaphic factors, interception, and precipitation intensity and duration. When 

 moisture is available, competition affects production of each species 

 differently, depending upon the species composition of the community. Figures 

 13 through 17 portray the variable responses of some major species in different 

 communities. Production of Bouteloua gracilis is amazingly variable; this high 

 variability is corroborated by canopy-coverage data. 



Frequency and Canopy-Coverage 



Table 13 shows canopy-coverage data for 1980. Tables 14 through 25 

 summarize canopy-coverage and frequency of major species for the study period. 



Plant canopy-coverage has varied with precipitation, but not synchronously 

 with production due to the obvious differences in measurement techniques. 

 Because the same plots were sampled each year, these data may clarify changes 

 suggested by the production data, if it is not clear whether the suggested 

 changes are real or an artifact of sampling. 



The following discussion compares data from 1978, a very wet year, and 

 1980, a very dry year, to demonstrate the contrast in species composition based 

 upon canopy -cove rage. Canopy-coverage estimation is not a precise 

 measurement , and sampling error may be + 10% or more for single-stem grasses. 



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