Abstract
Compositional stability in forests has traditionally been evaluated by comparing understory with overstory composition. Such comparisons have generally been qualitative. Transition probability analysis has recently allowed quantitative prediction of future community states. We used transition probability analysis of an undisturbed spruce-fir forest to evaluate the sensitivity of the overstory/understory comparison to underlying assumptions. The predictions of future composition differed widely depending on understory size class used, ecological situation (gap versus forest plots), and stand descriptor (density versus frequency). Species longevities and interactions between understory and overstory species also affected the predictions. Understory data generally led to a predicted increase in importance for the most tolerant species (Abies fraseri) and the conclusion that a previous disturbance allowed the least tolerant species (Betula lutea) to become established. Inventory of stems in gaps led to a predicted increase in importance for the least tolerant species (Betula) and the conclusion that disturbance frequency was increasing in the stand. Data incorporating more detailed observations of the gap capture process led to the inference that this old growth stand was in compositional equilibrium. In this community, the species that was densest in the understory species (Abies) had the shortest lifespan and thus, the fastest canopy turnover rate. This lead to counter-intuitive behavior in the models; in some cases Abies had a 40% higher relative density in the understory than in the overstory at equilibrium.
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White, P.S., MacKenzie, M.D. & Busing, R.T. A critique on overstory/understory comparisons based on transition probability analysis of an old growth spruce-fir stand in the Appalachians. Vegetatio 64, 37–45 (1985). https://doi.org/10.1007/BF00033452
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DOI: https://doi.org/10.1007/BF00033452