SPECIES DIVERSITY 



The community types cover a ten-fold range of richness. Somewhat sur- 

 prisingly, the tall shrub types are more diverse than the tree dominated types, 

 The Scam, Dist, and Bogr c.t.'s stand apart from the other types with respect 

 to dominance and equitability due to intensive selective pressure caused by 

 soil salts and grazing. 



Figure 4 shows the relationships among the community types with respect 

 to species diversity. 



The community types are ordered according to richness. Dominance and 

 equitability generally follow this order, with a few interesting deviations. 

 The Calo and Ansc c.t.'s have higher dominance and lower equitability than 

 their richness might suggest. This is due to the fact that these types have 

 a single dominant. 



A similar deviation might be expected for the Yugl c.t., but this is not 

 the case. A few grasses are usually codominant with Yucca glauca , and field 

 observations suggest that this type might better be called the Yugl/Calo-Ansc 

 c.t. and Muhlenbergia cuspidata was co-dominent in some stands. More samples 

 would be necessary to determine if this is true generally. 



An opposite deviation is found in the Pode/Rosa-Syoc c.t. The structure 



graphs show that the coverage of a nanophanerophytes exceeds that of the trees, 



The result is lower dominance and higher equitability than the richness would 

 suggest. 



PRODUCTIVITY 



Productivity data, expresses as means (x) and standard deviations (s), 

 are given in kilograms per hectare. Coefficients of variation (CV) are pre- 

 sented as decimals. The two replicates of each exclosure were compared for 

 homogeneity using total productivity data, not biomass. The .05 probability 

 level was used for this comparison. The number of samples (N) necessary to 

 give a sampling accuracy of + 20% at the 0.2 probability level have been 

 calculated. This represents a minimal requirement. 



The effects of drought and grasshoppers, which are discussed below, 

 should be considered before evaluating the productivity data. 



Drought 



Reed and Peterson (1961) noted that the major trends in mixed prairie 

 vegetation are set by major weather cycles, while the range of change within 

 trends are set by grazing intensity. Clark et al . (1943) also found that 

 grazing had less than climate in modifying plant cover. Coupland (1959) 

 found that during a period of 12 years characterized by moist, cool weather, 

 the yield capacity of the same stands sampled earlier increased by an average 

 of one hundred and thirty-seven percent. 



63 



