Drew W Purves, Jeremy W Lichstein, Nikolay Strigul, and Stephen W Pacala
29 October 2008
We have developed an analytically tractable demographic model of forest dynamics: the perfect plasticity approximation, PPA. We present the results of an application of the PPA to the eight most common tree species on each of four soils in the U.S. Lake States (Michigan, Wisconsin, and Minnesota). The PPA is defined in terms of parameters measureable at the scale of individual trees, including allometry, growth and mortality. We estimated these parameters using short-term data (= 15-year) from individual trees, then compared model predictions and observations for long-term (100-year) stand dynamics recorded in chronosequences. Predictions for the nature, timing and magnitude of the dynamics of basal area and ecological succession were accurate. Predictions for the diameter distribution of 100-year old stands were correct in form and slope. The PPA provides simple analytical metrics for early- and late-successional performance as a function of the species-specific, individual-level parameters. On each soil type, these metrics correctly classified the 8 species into their observed guilds (early or late), and they correctly predicted that Quaking Aspen (Populus tremuloides) should be early-successional on all soil types. Decomposing showed that: (1) succession in this region is driven both by the superior understory performance, and the superior canopy performance, of late-successional species; (2) this performance difference is primarily due to differences in mortality rather than growth. The predicted late-successional dominants matched the chronosequence data at the species level on xero-mesic soil (Quercus rubra) and on mesic soil (co-dominance by Acer rubrum and saccharum). On hydro-mesic and hydic soils, the literature reports that the current dominant species in old stands (Thuja occidentalis) is now failing to regenerate. Consistent with this finding, the PPA predicted that, on these soils, stands are now succeeding to dominance by other late-successional species (e.g. Fraxinus nigra, Larix laricina, Acer rubrum).
|Published in||Proceedings of the National Academy of Sciences USA|