An assessment of the contribution of Canadian forest ecosystems and forestry activities to the global carbon budget has been undertaken. The first phase of this study consisted of the development of a computer modeling framework and the use of pub lished information to establish the sector's current role as a net source or a net sink of atmospheric carbon. The framework includes age-dependent carbon sequestration by living forest biomass, net detrital litter fall of carbon to the forest floor, subsequent accumulation and decomposition release in three soil compartments, retention of carbon in manufactured products derived from harvested forest biomass, and burning of forest biomass for energy. There is explicit representation of the role of ecosystem disturbances, such as fire, insect-induced stand mortality, and harvesting (clear-cutting, clear- cutting and slash burning, and partial cutting), as they affect carbon releases and transfers to the forest floor and to the forest product sector. Regrowth of biomass and changes in soil decomposition processes following disturbance are also simulated within the model. In the first phase of the work, national and provin cial data bases were used to provide the first compre hensive estimates of the net carbon exchange between Canadian forest ecosystems and the atmosphere for the reference year 1986.

Summary Using four detailed and complex simulation models we derive a framework for predicting behavior of any defoliating insect/forest system. The framework uses simple and easily gathered biological information on four sets of state variables, each with a characteristic temporal scale, to predict presence, absence or form of key ecological processes acting on or between the variables. The combination of these key processes enables prediction of system equilibrium structure and this structure can be used to derive the temporal behavior of the system. Four qualitatively different classes of system behavior arise from the equilibrium structures. The framework is tested against twelve other systems and field invalidation experiments are outlined. Forest defoliator research and management implications are discussed.

Can. Enr. I I 1: 55-66(1979) A theory for the population behavior of the eastern blackheaded budwom is presented. The qualitative properties and key processes of the system are identified and a description of the theory as a simulation model is given. Two domains of stability exist for the insect; one is caused by bird predation, the second by the effects of food limitation. The model predicts that the budworm's cyclical population fluctuations are produced by the disappearance and reappearance of these stable domains and consequent movement of densities to new levels. Parasitism is identified as the principal mechanism causing this behavior. A qualitative comparison of model behavior with historical behavior is made and a critical field experiment to test the validity of the theory is proposed. Finally it is suggested that the population behavior of many forest defoliators may be explained using this theory.Takahashi (1964) proposed a model which suggested a mechanism by which a discrete-generation insect species could exhibit rapid changes in population density. Using stock-recruitment curves (Ricker 1954), he postulated the existence of two stable equiIibrium densities for the insect, one endemic, the other epidemic, with an unstable equilibrium at some intermediate density (Fig. 1). Onset and collapse of outbreaks were viewed as changes in the population level from one stable density across the unstable equilibrium to the other stable density. HolIing (1973) extended this concept and defined "domains of stability," regions of stable population density separated by unstable equilibria. Therefore, in Fig. 1, there are two stable domains: d l and d2. Peterman et al. (in press) described the dynamics of multi-stabIe systems; as a consequence of processes in the system, stability domains can disappear and reappear and result in rapid shifts in density.This paper presents a model that has been developed for the eastern blackheaded budworm, Acleris variana (Fern.). The model was constructed using the identified key processes of the system and then used to investigate its real-world behavior. The theory proposes that the system has two domains of attraction and the population fluctuations of the insect can be characterized by the collapse and reappearance of these stable regions.

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