A three-dimensional model of forest development and competition

Abstract

Recent developments in ecological modelling (Mauchamp et al., 1994, Ecol. Model., 71: 107–130) and in the study of ecosystem function (Pickett and White, 1985, The Ecology of Natural Disturbances and Patch Dynamics, Academic Press) have emphasised the importance of spatial effects on vegetation dynamics. The mosaic of patches which are regarded as comprising an ecosystem, such as a closed canopy forest, can be defined by the morphology and distribution of component organisms in space. Specifically, the size and distribution of trees is an important determinant of gap phase dynamics, and the nature of plant competition depends upon the spatial location of the plants, and the plant morphology that defines the pattern of spatial occupancy by an individual. Arcadia is an individual-based forest stand model based on these hypotheses, detailing the establishment, growth and mortality of hardwood and coniferous species over century timescales. The model comprises a fine-scaled three-dimensional framework within which spatial relationships, including three-dimensional leaf area distributions, are explicitly simulated to define competitive interactions. Productivity is modelled through a carbon-budget approach, linked to the unique light environment within each of the 300 000 cells which comprise the framework. Diurnal and seasonal variations in solar azimuth angles, zenith angles and insolation are simulated. Arcadia was used to model the development of old-growth forest in the north-eastern US, and satisfactorily simulated stand structure and productivity relationships derived from tree rings. Experiments with the model revealed that the more productive conifers were outcompeted by hardwoods due to the flexible crown morphology and greater shade tolerance of the latter. Conifers were also rendered uncompetitive if variation in solar azimuth and zenith angles was not simulated.