Population persistence and reproductive ecology of a forest herb: Aster acuminatus

Aster acuminatus Michaux, a patchily distributed herb of eastem forests, is commonly represented in all stages of forest succession. To determine its mechanisms of persistence, we carefully examined the distribution pattems ofthis species as affected by its reproductive ecology and demography. Using A. acuminatus as a model, we hypothesized that 1) availability of propagules and location of adequate recruitment sites impose first-order control over plant distributions in stands of all ages by determining where plants might be found and 2) light and other resources exert a second-order control over herb distributions by determining where established plants cannot persist. Results from our study at the Hubbard Brook Experimental Forest in New Hampshire demonstrated this species' ability to exploit resource-rich disturbances through immediate and profuse ramet and seed production. This ensured rapid colonization of new sites and provided genetic exchange for longer-term survival. Relict colonies which survived the disturbance served as the sole propagule-producing loci for the first year of colonization. After several years, resources became more limiting and the aster shifted to a more conservative asexual reproduction strategy which provided population stability until the next disturbance. The clumped distribution of A. acuminatus in mature forests is explained by strict adherence to asexual ramet replacement in which there is no year-to-year change in aster importance. Location of persistent colonies probably is dictated by location of suitable seedbeds during the first few years of recovery after disturbance. Whereas numerous sites may be colonized initially, timing of colonization and quality of site impose constraints on which colonies persist. IN THE UNDERSTORY of temperate forests, herbs generally are distributed in sparse clumps and patches (Watt, 1947), and species often are represented in all stages of forest succession (Grime, 1979). Herbs growing in mature forests exhibit life history traits and resource allocation patterns that are quite different from those of herbs inhabiting less stable environments (Struik, 1965; Kawano and Nagai, 1975). Most notably, forest herbs usually are smaller in size, rarely establish from seed, and generally make little investment in sexual reproduction (Tamm, 1956). These distinctive life history traits may favor their persistence in the resource-poor understory of a mature forest. Although most studies of plant distributions in the forest understory have focussed on the role of specific environmental variables in regulating patterns (e.g., light and water, Anderson, Loucks, and Swain, 1969; microtopog' Received for publication 26 February 1987; revision accepted 7 October 1987. We thank the following individuals for field assistance: Steve Compton, Tom Kowalski, Mou Pu, Scott Shipley, and Zhihua Xu. Drs. James P. Lassoie, Peter L. Marks, and two anonymous reviewers provided helpful comments on earlier versions of the manuscript. This research was supported by a grant from the National Science Foundation (BSR 83-16953). raphy, Bratton, 1976), we suspected that the reproductive ecology of forest herbs may be equally critical in determining herb distribution, abundance and long-term persistence. Through careful examination of the distribution patterns of a forest herb, as affected by its reproductive ecology and demography, we sought a better understanding of the factors which 1) cause clumped distributions of forest herbs, 2) limit their use of unoccupied sites, and 3) permit their persistence in many stages of forest succession. We used as a test species Aster acuminatus Michaux, the whorled wood aster, because it has been much discussed in the literature (e.g., Pitelka, Stanton, and Peckenham, 1980; Pitelka and Curtis, 1986) and has distribution patterns characteristic of other forest herbs (Siccama, Bormann, and Likens, 1970). In this paper we show how life history traits and population dynamics ofA. acuminatus are directed by the interaction of resource and propagule availability and how distribution patterns ultimately are determined by the predisturbance patterns of distribution within the forest. We hypothesize that light and other resources are of second-order importance in that they do not determine the location of patches, but rather where they cannot be located. This