Distribution of Thalli in a Population of the Epiphytic Lichen Lobaria oregana and a Model of Population Dynamics and Production

Abstract

Data on weight-class distribution, growth rate and asexual propagule (lobule) release are used to analyze populations of the nitrogen-fixing lichen Lobaria oregana in old-growth Douglas fir canopies of western Oregon. A life-table model prepared from observed data is used to produce an estimate of net annual production of biomass, and to analyze the response of a population of this lichen to possible environmental perturbations. The weight-class distribution is similar in each offour trees studied and at different levels within the trees, but differed among the different components of the trees. Survivorship curves (log survivorship versus estimated thallus age) are similar for populations in all trees studied. Estimated net annual production is 31.1% of the standing crop of Lobaria oregana biomass. Mathematical manipulation of matrix formulations of the life-table model indicates that populations would be stable if they recruited 10% of the lobules released each year. In such a configuration, the model population returns to its original size and distribution within 30 years following a simulated storm that removes 90% of the largest thalli and momentarily increases lobule production. Study of growth and annual production of lichens has emphasized individual thalli (reviewed by Hale 1973 and Armstrong 1976) or total standing crops of several species in one habitat (reviewed in Pike 1978). There are virtually no studies of the structure of whole lichen populations or the dynamics within them. Pike (197 1) looked at relative numbers and total weight of thalli of Usnea subfloridana Stirt. on oak and ash twigs of different ages in Oregon in order to estimate annual production, but did not look at the dynamics of growth of individual thalli. Yarranton (1975) examined the change in distribution of biomass and numbers of podetia of Cladina stellaris (Opiz.) Brodo in successional populations on burnt sites 25 to 55 years of age in Ontario, Canada. Data on the distribution of size classes of podetia were not analyzed because of difficulty in estimating the effect of clonal fragmentation. Armstrong (1978) has examined the pattern of dispersion of Parmelia glabratula ssp. fuliginosa (Fr. ex Duby) Laund. on slate rock surfaces. He found larger thalli at the top than bottom of the substrate, but identical growth rates among thalli, concluding that colonization had taken place from top to bottom. Further 007-2745/83/309-331 $2.35/0 This content downloaded from 157.55.39.45 on Tue, 19 Jul 2016 05:14:23 UTC All use subject to http://about.jstor.org/terms 310 THE BRYOLOGIST [Volume 86 studies by Armstrong (1981) have shown that substrate topography plays a major role in the entrapment of propagules and their establishment as thalli, and that only a small percentage of propagules become established as thalli. The dynamics of populations of organisms (growth, mortality, reproduction, potential stability and potential response to environmental perturbations) can be examined by analysis of survivorship curves and life tables (Harper 1977). These types of analyses traditionally have been used to study organisms (insects, birds, seed plants) whose populations show a natural division into ageable groups, and lichens have not been studied in this manner. Lichens are long-lived, reproduce by fragmentation and do not have life histories that fall into specific age classes (Harper & White 1974). Thus, it has been difficult to establish age-specific functions for mortality and reproduction in such populations. Watson's (1979) study of the variability of populations of mosses is unique, using survivorship curves to analyze the age-class distribution of cryptogams. She was able to show differences among species and among populations of the same species in different environments. Lefkovitch (1965) showed that a modified formulation of life-table analysis developed by Leslie (1945) can be used for a population of organisms grouped in unequal-aged stages. Hartshorn (1975) has used this method in describing the population dynamics of a tropical tree species. Leslie (1945) showed that the dominant root (XI) of a matrix of (age-specific) coefficients is equal to er, where r is the intrinsic rate of natural increase in the exponential equation of population growth,