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The objective of this study was to develop empirical regression equations for describing forest overstory-understory relationships in Arizona mixed conifer forests, and for predicting responses of potential mule deer (Odocoileus hemionus crooki) and elk (Cervus canadensis nelsoni) forage resources following thinning and clearcutting. Data were collected in the White Mountains from unlogged stands, a 2-year-old thinned stand, a 4-year-old thinned stand, an 8-year-old clearcut, and a 16-year-old clearcut. A combination of high variability in understory production (due presumably to inherent structurally-complex overstory conditions) and substantial intercorrelation between independent variables yielded multiple regression equations of questionable value. However, average understory production levels can be predicted using stand basal area alone. Relationships between basal area and understory production components were generally best described using log or log-log functions. Despite considerable diversity in slope and aspect conditions, potential insolation was generally not correlated with understory production; nor was potential insolation per unit of basal area often a better predictor than basal area alone. On unlogged sites production of potential deer forage was positively correlated with presence of spruce and fir, but negatively correlated with combined pine and Douglas-fir composition, and not significantly correlated with aspen composition. Potential elk forage was positively correlated with spruce and combined pine and Douglas-fir composition, but inversely correlated with aspen and fir composition. Understory production was generally inversely related to periodic annual forest growth and thickness of the forest floor. Browse production was generally less related to overstory conditions than herbage components, but was correlated with slope gradient and soil rockiness. Although total browse production could be estimated from basal area data, production of browse potentially useful to deer and elk could not be. Total understory production under virgin mixed conifer stands ranged from about 150 pounds per acre at 50 square feet of basal area (BA) to about 40 pounds at BA 400. The portion of this biomass potentially valuable as deer forage declined from 30% at BA 50 to 8% at BA 400, while elk forage declined from 49 to 11%. Forbs were the dominant understory component for all stand densities higher than about BA75, but were out-produced by graminoids below this level. Browse was the least productive understory component and increased only slightly with decreasing stand density. Total understory production on the 4-year-old thinned site was higher, but not statistically different than on unlogged sites; however, from 65 to 74% of the biomass consisted of forage potentially valuable to deer and elk. Depending on stand density, understory production under the 2-year-old thinning was comparable to or less than that of unlogged sites. Overstory-understory regression curves were often statistically different between thinned and unlogged stands. Diversity of principal deer and elk forage species increased as stand density declined under both unlogged and thinned stands. Unlogged stands produced slightly more potential elk than deer forage at lower stand densities, but production of deer and elk forage after thinning was nearly identical over the entire range of stand densities sampled on both thinned sites. The 8- and 16-year-old clearcuts produced about 750 and 1160 pounds per acre of biomass, respectively, with about 34% consisting of potential deer and elk forage on the younger site, versus 56% in the older clearcut. The 8-year-old clearcut produced 30% more elk than deer forage, and the 16-year-old site 7% more. The duration of beneficial deer and elk forage responses following clearcutting appears substantially longer than 16 years where tree regeneration is unsuccessful.

A large proportion of the world's land surface is covered by semi-arid grasslands, and they provide an important source of income as a grazing resource. A more comprehensive understanding of these complex ecosystems is vital for the effective management of rangelands, as it will lead to an increased and more sustainable economic output. Herbivores modify the spatial pattern of vegetation distribution and their response to spatially heterogeneous forage resources affects their performance. The spatial aspect of herbivory is often ignored although it is a necessary component of understanding grazing dynamics and the factors affecting herbivore condition. A spatial model is developed which incorporates vegetation and animal dynamics and the interactions between these two components. The effect of different spatial foraging strategies on animal performance and vegetation was investigated. Model results were compared with the output of a non-spatial model to assess the importance of spatially explicit modelling in the context of monitoring animal performance. The relative significance of a number of aspects relating to spatial grazing and animal condition was explored. The results from this research show that significant differences in output are obtained from spatial versus non-spatial models. While the purpose of a model will determine its nature, the results imply that in certain contexts, a spatial model is essential for accurate results and insight. The results also indicated that foraging strategies have a large affect on herbivore condition and that spatially explicit models are necessary in the context of investigating the effect of foraging strategies on animal performance. Various aspects that significantly affected animal condition were highlighted and are useful in directing future investigations into grazing dynamics. It is difficult to conduct field studies under spatially and temporally variable conditions where the interactions between vegetation and herbivores are so complex. In the light of this, modelling was found to be an effective tool that can be used in investigating and revealing important dynamics of semi-arid grazing systems. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1999.