Modelling stand dynamics in whitebark pine ( Pinus albicaulis) forests

Abstract

The ecological process model firesum (a fire succession model) has been adapted to whitebark pine ( Pinus albicaulis) forests of the inland Northwest and Rocky Mountains, U.S.A. firesum simulates tree establishment, growth, and mortality on a 400-m 2 plot. Also modelled are live and dead fuel accumulations, fire behavior, fuel reduction, and insect and disease mortality. The following influences on tree establishment and growth are simulated in the model: temperature, water stress, site quality, and light conditions. An additional submodel in firesum simulates the mutualistic relationship between the Clark's nutcracker ( Nucifraga columbiana) and the whitebark pine. Seed caches made by the Clark's nutcracker are evidently responsible for most whitebark pine regeneration. Whitebark pine seeds are also an important food for red squirrels ( Tamiasciurus hudsonicus), black bear ( Ursus americana), and especially grizzly bears ( Ursus arctos horribilis). Infestations of white pine blister rust ( Cronartium ribicola) and mountain pine beetle ( Dendroctonus ponderosae), and successional replacement by shade-tolerant conifers can reduce whitebark pine populations, which can adversely impact dependent wildlife species. The model firesum was used to investigate the effects of fire, insects, and disease on whitebark pine regeneration and growth. Model predictions of basal area by tree species are presented for contrasting disturbance scenarios. Simulation predictions indicate severe reduction in some whitebark pine populations if current trends are not altered. Model predictions from a test were compared with inventory data from actual postfire stands.