HR: 0830h
AN: H51E-1134    [PDF]
TI: Lichenometry's Black Box: Demographic Characteristics of {\it Rhizocarpon geographicum} and {\it Pseudophebe pubescens} Inferred from Whole Population Size-Frequency Distributions
AU: * Loso, M G
EM: mloso@es.ucsc.edu
AF: Earth Sciences, Univ. of California, Santa Cruz, CA 95064 United States
AU: Doak, D F
EM: doak@biology.ucsc.edu
AF: Ecology and Evolutionary Biology, Univ. of California, Santa Cruz, CA 95064 United States
AB: For decades, geologists have used the inferred ages of crustose lichens to date geomorphic landforms. Traditionally, lichenometry relies upon the assumption that the size of the largest lichen thallus in a population reliably predicts the age of the landform. But in a given population, both the largest lichens and the probabilities of finding them are determined by the size frequency distribution of the population, a demographic characteristic controlled by the population's particular rates of colonization, growth, and mortality. These rates, and their variability, are driven by biological processes that are difficult to observe directly in the slow-growing lichen species favored by lichenometrists. To improve our understanding of these demographic characteristics- lichenometry's "black box"- we documented the size-frequency distributions of two lichen species, {\it Rhizocarpon geographicum} and faster-growing {\it Pseudophebe pubescens}, in populations growing on well-dated lakeshores in southcentral Alaska. We sampled whole populations on 5 lakeshores ranging in age from 53 to 172 years old, assembling a dataset of over 15,000 measurements. The results reveal distinct trends shared by both species. On lakeshores of all ages, they consistently maintain a small modal size class ($\sim$ 3mm for {\it Rhizocarpon}, $\sim$ 8 mm for {\it Pseudophebe}), and frequency declines exponentially above the mode, extending to the largest size classes on the oldest surfaces. These data serve to constrain numerical demographic models of the full lichen population. Model results imply that colonization does not occur as a single pulse after substrate exposure, but instead occurs steadily after some initial lag. These lichens also undergo substantial mortality. For both species, models using steady mortality of 2%/year fit the data significantly better than models assuming zero mortality. This result could explain the discrepancy between lichenometrists' "great growth," a common empirical observation that largest lichen sizes increase most rapidly with age on the youngest substrates, and the seemingly contrary observation that careful repeated measurements of individual lichen thalli often show lichen radial growth rates increasing with thallus age. More importantly, this and other results offer the opportunity to assess quantitatively the accuracy and utility of lichenometric dating in a variety of contexts.
DE: 0400 Biogeosciences
DE: 1824 Geomorphology (1625)
DE: 1851 Plant ecology
SC: Hydrology [H]
MN: 2003 Fall Meeting