Allocation and the Transient Dynamics of Succession on Poor Soils

Biomass and nitrogen allocation to leaf, root, stem, and reproduction was determined in a 35—field chronosequence that spans the first 60 yr of secondary succession on a Minnesota sand plain. Biomass (grams per square metre) in leaf and root increased during succession, but reproductive biomass declined, and that in stem remained constant. Because root biomass increased twice as rapidly as leaf biomass, the proportion of total biomass in root increased during succession, whereas that in leaf, reproduction, and stem declined. In an additional study, biomass allocation was determined on a species—by—species basis for 46 species common at different times during succession. This study showed a similar pattern of increasing proportional root allocation and declining proportional reproductive and stem allocation during succession. These changes were accompanied by an increase in total soil nitrogen and a decrease in light penetration to the soil surface during succession. Increasing root allocation and decreasing reproductive allocation suggest that succession on these nutrient—poor soils is the transient dynamics of colonization and competitive displacement, with later successional species being superior nitrogen competitors because of higher root allocation. Allocation trade—offs root, stem, leaf, and seed can lead to initial dominance by species with high seed and leaf allocation, presumably because of greater colonization and/or maximal growth rates. Thus, this succession differs markedly from successions on rich soils, for which stem allocation is increasingly important. These results contradict the resource ratio hypothesis as an explanation for the pattern of early succession on impoverished soils.