Ecological Relationships of Co-Occurring Goldenrods (Solidago: Compositae)

Abstract

Comparisons are made of propagule characteristics and niche relationships among co-occurring species of goldenrods (Solidago) in a 23-yr-old field and a virgin tall-grass prairie. Five of the six goldenrod species from the prairie site are the same species as those found in the old field, allowing an empirical study of differences between "open" and "closed" communities postulated by earlier theoretical treatments. In general, each species produced fewer and heavier propagules with a reduced dispersal capacity in the prairie compared with the old-field populations. Under increasing competitive pressure, selection favors larger propagules. Assuming a species does not increase the absolute amount of resources allocated to propagule production, then heavier propagules should result in fewer propagules. Both changes result in decreased dispersal capabilities. The relationship between the annual number of propagules (N) per basal stem to the mean weight of an individual propagule (W) is N = kwb, where k and b are constants. The equation indicates a trade-off between numbers and weight of propagules within the resources allocated to sexual reproduction by the plants. In the case of goldenrods, b is near - 1; hence NW = k. The more general form ∑t 0 Nt = kWb, where t equals the number of reproductive years, allows comparisons of species with greatly varying life spans or varying propagule production with age. Tests of the generality of the three equations are in order. In both habitats, species in the dry end of the moisture gradient produce heavier propagules with lower dispersal ability. A possible countercompetitive advantage to the larger seed of the dry-area species is the increased foliage height of wet-area species. While the species of goldenrods are distributed similarly (i.e., peak frequencies in the same order) along a soil-moisture gradient on each site, the niche breadths and niche overlap of the old-field population of each species are greater than those in the prairie. Species distributions in the old field represent site preemption by early arrivals. Over time, selection pressure favors the better competitor in each area of the soil-moisture gradient. Empirical evidence presented here strongly suggests that changes in selective pressures on plants present on a site for a long period of time will result in predictable changes in both life-history characteristics and the niche relationships of co-occurring species.