Polyvariant ontogeny in woodreeds: Novel models and new discoveries

详细

Polyvariant ontogeny (PVO) gets a visual expression in the life cycle graphs (LCGs) for Calamagrostis woodreeds as a variety of pathways for individual plants to develop through many of their states, which are distinguishable by the ontogenetic stage and chronological age (in years). PVO is recognized as the basic mechanism of adaptation in local populations of grasses to their environments, while a quantitative measure of adaptation is found by constructing a matrix model for the double-structured population, calibrating its matrix of vital rates from empirical data, and calculating the dominant eigenvalue λ₁. This approach encounters an obstacle typical for grasses: while the rates of aging and ontogenetic transitions can be determined from field data mainly by the morphology of aboveground parts of the plant, the rates of vegetative propagation can be reliably determined only from digging up the belowground rhizome system, i.e., by destroying the sample plot (“reproductive uncertainty”). Therefore, the former (nondestructive) calibrations of matrix models were, to an extent, subjective, resulting in correspondingly subjective estimations. A novel method to overcome the reproductive uncertainty makes use of the maximization hypothesis: the uncertain rates are such that λ₁ attains its maximal possible value under the given conditions. To test the hypothesis, we have conducted a field study by a new technique with a model species, the woodreed Calamagrostis epigeios (L.) Roth, which vegetatively reproduces in a meadow phytocenosis and a spruce forest clearance. Excavating the whole system of ramets with their rhizomes and analyzing the parent–offspring links in laboratory, we have gained (in addition to the former data on the local population structures and ontogenetic transitions) a new kind of data to calculate the status-specific rates of reproduction. The novel calibration method has enabled us to find an exact range of λ₁ values, i.e., the true quantitative bounds of adaptation for a given local population. Obtained under the reproductive uncertainty and maximality hypothesis, the values of λ₁ have turned out to be close to the upper bounds of the ranges, thus verifying the hypothesis. The study has discovered some generative subsidiary plants that sprout from the rhizomes of maternal ramets without entering the virginal stage. As a result, the LCG is enriched with new reproductive pathways, and there are new (not yet published) situations, in which λ₁ fails in its accuracy as a measuring tool of comparative plant demography. We propose a general method to adjust the adaptation measure in this kind of situation.