< 



._ 



NITRATE NITROGEN 



Figure 1.--The hypothesized relation between herbage production potential, 

 percent. 



nitrate-nitrogen level, and crown cover 



Within a region of limited APR-range and soil parent 

 material (limestone versus other), site productivity can be 

 expected to vary by reason of local differences in such 

 environmental factors as topography and microclimate. 

 The long-term integrated effects of these factors are 

 reflected in the amount of tree cover developed for climax 

 stands; so cover constitutes an excellent within-region 

 index to site quality. Cable (1975), Clary and others 

 (1966), and Pechanec and others (1954), have documented 

 the positive relation between the amount of original 

 woody-plant cover and, subsequent to removal of this 

 cover, the amount of herbaceous plant growth in semiarid 

 ecosystems. The hypothesized herbage production/ 

 cover relation is pictured at the right edge of figure 1. When 

 climax cover is scant, the site is expected to be poor and 

 potential herbage production low. As cover increases, both 

 site and potential herbage production improve, the latter 

 possibly reaching the asymptote shown over the upper 

 range of cover. 



When cutting, catastrophic fires, and other recent 

 disturbances have decimated the climax cover to a greater 

 or lesser extent, cover is no longer an uncompromised 

 measure of site productivity. Under these circumstances, 

 a supplementary index to productivity is needed. Studies 

 conducted in both grassland and forest ecosystems 

 suggest that ecosystems at the climax stage inhibit 

 nitrification (Rice and Pancholy 1972, 1973). Release of 



a site, through disturbance of the climax overstory, could 

 be expected to result in conditions again favorable to the 

 accumulation of nitrates (model acronym N03) in the soil 

 to the extent permitted by residual climax trees (Vitousek 

 and Melillo 1979). 



With no residual trees, the opportunity exists for the 

 accumulation of N03 to a level determined by the char- 

 acteristics of the site (Jenny 1941). The level of accumula- 

 tion will of course differ among sites because each is 

 unique in its exact combination of characteristics. Accum- 

 ulation could vary as indicated by the full range of N03 at 

 zero cover in figure 1. The sigmoidal form for herbage pro- 

 duction over N03 was assumed to be approximately cor- 

 rect because experience in the agronomic field has shown 

 diminishing returns from higher nutrient levels (Black 

 1957) and results in semiarid natural ecosystems have 

 shown that only very modest nutrient levels can be effec- 

 tively utilized by such systems (Hyder and others 1975). 



It is assumed that when climax cover reaches a maximum 

 virtually no nitrification takes place. Thus, the surface 

 shown in figure 1 is truncated on the diagonal at the rear be- 

 cause maximum soil nitrates and maximum tree cover are 

 not expected to occur simultaneously. The interactive 

 nature of N03 and climax cover is apparent in that herbage 

 production potential varies differently over N03, depend- 

 ing on the level of climax overstory. 



2 



