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.

The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or sen/ice. Intermountain Research Station 324 25th Street Ogden, UT 84401 User's Manual for Western Root Disease Model Tree root diseases pervade forested lands of the United States. Timber losses caused by root diseases have been estimated to average nearly 240 million cubic feet per year (Smith 1984). Root diseases typically originate from infected stumps of previous stands and spread to root systems of other trees in the immediate vicinity (Thies 1984;Wargo and Shaw 1985). The ability of these diseases to survive for many years in roots and stumps and to spread in stands throughout a rotation (possibly without early detection) imposes constraints on the management prescriptions available for regenerating stands (Bloomberg and others 1980). Management practices such as transplanting of trees, deep planting, partial cutting, fire prevention, or excessive removal of imderstory biomass may initiate or enhance the spread of one or more root diseases (Wargo 1980). Other practices, including use of diseaseresistant tree species, trenching around infection centers, and careful cutting and commercial thinning methods, are known to reduce root disease impacts (Shaw and Roth 1978).In response to managers' concerns for lack of information about the future development of diseased stands, a model was designed to predict the spread and impact of pathogenic Armillaria spp. or Phellinus weirii (Murr. [Gilbn.]) in mixed species, multiaged stands in the Western United States. The model was developed through a series of workshops in which the knowledge of many experts on the biology and management of root diseases was captured (Brookes 1985 and appendix I). With this information, and after several rounds of refinement, the model was produced; it currently operates in conjunction with the Prognosis Model for Stand Development. Details on the process of model development appear in Shaw and others (1985), Eav and Shaw (1987), and McNamee and others (1985). The combined Prognosis/Root Disease Model can be used with existing forest inventories to evaluate possible outcomes of silvicultural prescriptions and root disease control activities for sites where root disease may influence stand development. Its uses include long-term planning for management of stands affected by root disease and for highlighting potential uncertainties and research needs for better understanding of root disease dynamics and effects. To model effects of silviculture on root-disease organisms, and their effects on stand dynamics, requires a model of stand development that can represent stands of mixed species and ages. Furthermore, the stand model should represent the species and sizes of regeneration that are expected to fill openings created by tree mortality. The Stand Prognosis Model described by Stage (1973) and Wykoff and others (1982) has these capabilities. In addition, it is extensively used f...

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