Rates of Succession and Soil Changes on Southern Lake Michigan Sand Dunes

1. Geological dating of the Great Lakes shore lines and other methods now provide approximate absolute age estimates for many dune systems in Indiana. 2. Plots on these dunes provide estimates of rates of change in Cowles's classic succession from dune grasses (Ammophila, Calamovilfa, Andropogon) to pine (Pinus banksiana or P. strobus) and black oak (Quercus velutina). 3. Soil analyses of carbon, nitrogen, moisture equivalent, carbonates, acidity, and cation exchange relations show how most soil improvement of the original barren dune sand occurs within about a thousand years after stabilization. The pattern of change on older dunes promises little further improvement and perhaps even deterioration of fertility. 4. Low fertility favors vegetation with low nutrient requirements. But such vegetation probably is relatively ineffective in returning nutrient to the dune surface in its litter and thus aggravates low fertility. Leaching of nutrients out of the sand-dune ecosystem and the low moisture reserve of most dune surfaces help account for the poor prospects for successional replacement of the black oak-blueberry community by the more exacting species of the mesophytic forests. 5. Fire history probably (a) helped keep black oak in some areas that could otherwise have supported a richer forest; (b) aggravated the normal loss of nutrients from the black oak-blueberry community; and (c) helped favor persistence or return of grass undercover in prairie-margin areas and probably slowed soil acidification here. 6. A succession of basswood → red oak → sugar maple (Tilia americana, Quercus rubra, Acer saccharum) develops in moister sites, on lower lee slopes and dune pockets that are protected from drying and burning, usually without passing through stages like those leading to black oak-blueberry communities. These and associated species seem to have higher nutrient requirements; the higher nutrient content of their litter apparently helps maintain the fertility of soils in these local areas at a higher level than in the surrounding pine and oak dunes. 7. These conclusions fit into a mathematical framework for analyzing ecological succession and climax. Vegetation, soil, and other properties of an ecosystem usually change rapidly at first and more slowly later on. If they approach some limit asymptotically or fluctuate around it, this limit should describe the climax community or mature soil. In some cases like the soil nitrogen or cation exchange relations discussed here, this limit might be computed theoretically from rates of gains and losses. 8. The limit itself may vary with time and place. Ideally it describes a gradational "climax pattern" of communities or ecosystems in any region-generally not a uniform "climatic climax." 9. These points follow simply from assuming a quantitative meaning for Cowles's own statement of the fundamental principle of ecological succession: that it represents a "variable approaching a variable rather than a constant."