PLANT MORPHOGENESIS FOR SCIENTIFIC MANAGEMENT OF RANGE RESOURCES 



179 



R N =LE4-Q+G (6) 



where LE is the energy used to evaporate water, 

 Q is that used in sensible heat transfer to the air, 

 and G is that used in heating the soil. 



The effect of stock on this partition of energy is 

 complicated. First, stock, by reducing vegetation 

 cover or inducing a change in dominant species, 

 alter the surface roughness, and thereby influence 

 the transfer process affecting the removal of heat 

 and water vapor from the vegetation and soil sur- 

 face. Thus, comparing a surface covered by well- 

 spaced shrubs with a bare soil surface, the trans- 

 fer of water vapor and heat is likely to be least 

 efficient of all for the shrub canopies, more effi- 

 cient for the bare soil surface, and most efficient 

 of all for the shrub canopies. Secondly, the parti- 

 tion of net radiation depends on the availability 

 of water for evaporation, an availability which 

 depends mainly in the present context on the 

 proportion of radiation intercepting plant sur- 

 face and soil surface. The latter is a less con- 

 trolled source of available water, unlike a plant 

 surface with its stomata. Also, the soil surface 

 usually shows an earlier decline in the avail- 

 ability of water because of the greater resistance 

 to evaporation provided by the soil surface layer 

 as it dries out. 



The complications outlined make quantitative 

 prediction of the differences in radiation micro- 

 climate difficult particularly when, in reality, the 

 altered water balance due to stock further com- 

 plicates the effects expected. 



The findings of Aase and Wight (2) on energy 

 balance components relative to percent plant 

 cover in Bouteloua-Carex-Stipa rangeland near 

 Sidney, Montana, are of interest as one of the few 

 examples in which such an evaluation has been 

 attempted in the field. They report a 40-percent 

 reduction in evapotranspiration (LE in equation 

 6) from bare ground in comparison with 100- 

 percent cover over a growing season of 113 days. 

 Ground covers of 25, 50, and 75 percent yielded 

 closely similar results, representing on average 

 a 21-percent reduction in evapotranspiration com- 

 pared with 100-percent cover. They also found 

 differences in daily energy partition between dif- 

 ferent vegetation covers and for periods of dif- 

 ferent water availability. Thus, the ratios of sen- 

 sible heat transfer (Q in Equation 6) to latent 

 heat transfer (LE in equation 6) were, for a wet 



period, 1.22, 0.49, and 0.18 and, for a dry period, 

 2.33, 1.95, and 1.88 for vegetation covers of per- 

 cent, mean of 25, 50, and 75 percent, and 100 per- 

 cent, respectively. Their results reflect, for both 

 periods, lower availability of water for evapora- 

 tion the smaller the proportion of vegetation 

 cover. What is not apparent from studies of this 

 type, however, is the effect on details of the mi- 

 eroenvironment between and within units of vege- 

 tation, that is on the spatial diversity of micro- 

 climate. 



Spatial Diversity Of Microclimate 



The most notable difference between areas which 

 have lost their perennial plants and unaffected 

 areas is the reduction of habitat diversity in the 

 former. This results from the reduction in area of 

 radiation intercepting surfaces above the ground 

 surface. Considerable microclimatic contrasts 

 exist as the air and soil are traversed from be- 

 neath vegetation units out into the open. At the 

 time of the albedo measurements reported in table 

 4, summer midday soil temperatures at 2 cm. 

 depth were found to differ by up to 16° C. be- 

 tween shrub shade and sun microhabitats. The 

 measured soil temperature at similar depth on a 

 denuded area was lower than the intershrub soil 

 temperature by 3° to 6°, largely attributable to 

 soil surface albedo difference. What appears more 

 important, however, was the spatial uniformity 

 of microclimate in this latter situation. Similar 

 shade and sun soil temperature differences were 

 noted in a natural community in a semihumid 

 environment (57). Many other examples of con- 

 trasting microenvironments exist in the general 

 literature on microclimatology (23). Further de- 

 scription of these is comparatively meaningless, 

 however, without reference to some specific eco- 

 logical problem. Instead, general aspects of mi- 

 croenvironmental diversity are discussed in the 

 next section. 



Discussion And Conclusions 



Interrelationships Of Stock-Microenvironment 

 Interactions 



Stock-microenvironment interactions have been 

 treated individually for the purpose of their defi- 

 nition and evaluation, as far as possible, of their 

 relative importance. However, in the field, no 



