Vegetation constitutes a renewable resource that can be harvested through its consumption by herbivores. However, in seasonal environments the production of edible plant material does not take place continuously. Plants regenerate much of their above-ground biomass at the start of a growing season (early summer or rainy season), partly by reallocation from below-ground reserves. Later when conditions become adverse (too dry or too cold), they cease growth, and progressively shed the senescent parts. Superimposed on this seasonal cycle of growth and attrition are changes in plant populations, either through vegetative growth of new ramets, or via the recruitment of new individuals (genets) from seeds (Harper 1977).

Vegetation growth is not only phased seasonally, but also fluctuates in response to variability in weather during seasonal periods. Plants are a renewing resource for only a portion of the year, and a depleting resource for the remainder. Hence, no balanced equilibrium between production and consumption is maintained, except perhaps transiently. Because senescent parts are less nutritious than actively metabolizing leaves, herbivores must respond to seasonal changes in food quality as well as quantity. Different plant types and parts have distinctive patterns of growth and attrition in biomass and nutritional value.

In this chapter, an appropriate production function for the annual dynamics of the vegetation resource, accommodating this temporal and nutritional variability, is developed. The basis for this production, in terms of fluxes in sunlight, available moisture and mineral nutrients in soils, will not be considered explicitly.