Base-line Simulations
In these simulations, the forest landscape was a square, 300 m on a side (area, 9 ha), typically containing several thousand trees of varying sizes on the landscape at any given time. As in other SORTIE models (see Pacala et al. 1993; Pacala et al. 1996), these runs used a 5-year time step and reported the total basal area (stem area in square meters per hectare) of each species throughout the 1000-year run. Additionally, a complete map of the forest was output at years 500 and 1000 for the visualizations. Both "undisturbed" and "disturbed" forests were simulated. In the former, no multitree gaps were created on the landscape, although single-tree gaps formed naturally as the result of adult mortality. In the latter, circular disturbances in which all trees were destroyed were generated.
In the undisturbed-forest simulations, fast-growing species like black cherry, white pine, and red oak reach the canopy first, attaining high initial basal areas. As the light reaching the forest floor declines, the seedlings of shade-intolerant species fail to recruit into the canopy, and the more shade-tolerant species (mainly beech, hemlock, and, to a lesser extent, yellow birch) increase in relative abundance. By the end of the simulation, beech is the dominant species in the forest. In the disturbed-forest simulations, the same suite of species (white pine, red oak, and black cherry) dominate the early growth. However, the continued opening of the canopy by disturbance leads to the much enhanced performance of yellow birch (for more details see Deutschman 1996).
Disturbed and undisturbed forests at two points during the simulations.
Visualization of individual trees from these simulations allows a more complete understanding and description of the forest dynamics. The early forest is a diverse mixture of all species with little spatial pattern. As competition among trees intensifies, patches begin to form on the landscape. Patches of beech exclude all other species, whereas patches of hemlock permit occasional adult yellow birch trees. Competition between species becomes localized at the boundaries between patches, resulting in the relatively slow dynamics. The competitive advantage of beech under undisturbed conditions is evidenced by both its more exclusive patches and its ability to advance into the areas occupied by other species. Multitree disturbance provides an exogenous source of patch formation on the landscape. Yellow birch colonizes the open areas, leading to dense patches. Beech and hemlock thrive on areas that, by chance, remain undisturbed for long periods. Although beech and hemlock can slowly displace individuals of yellow birch, the relatively high rate of disturbance guarantees that yellow birch will dominate the landscape.
Detailed description of base-line behavior
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Copyright © 1997 by the American Association for the Advancement of Science.
