Annual Periodicity of Incremental Growth in the Moss Polytrichum commune

Abstract

In the temperate moss species Polytrichum commune Hedw. field studies demonstrated that growth segments which are formed by changing patterns of leaf morphology are repeated annually. Evidence for exceedingly smallscale microhabitat variability in available water was found in the scattered distribution of desiccated stems in space and time, throughout the population under study. Moss turfs were extremely sensitive to disturbance which apparently altered the water-regulating capacity of the normally unbroken moss canopy. Mortality rates reported were undoubtedly elevated due to disturbance of the moss canopy during the course of this investigation. The last few years have witnessed a growing interest in the study of demography of plant populations (cf. Harper & White, 1974 for review). Allied to this interest has been a need to identify plant species in which ages of individual plants may be easily determined on the basis of direct morphological assessment. Formation of annual growth rings in woody perennials is the best known example of this type of interpretable morphological periodicity. Tree ring analysis has frequently been used in the study of historical events within a region or as an aid in the construction of management schedules for commercially harvested stands. Only recently, however, has this type of information been used in the construction of life tables for the study of the dynamics of natural forest stands (Hett & Loucks, 1971, 1968; Hett, 1971). Even less information is available for perennial herbs, which in general lack reliable morphological criteria for assigning age. Work by Tamm (1948, 1956, 1972a, 1972b) on life histories of woodland herbs relied exclusively upon mapping of individuals over years of observation in permanent plots. Similar methodology was employed in the recent work of Sarukhain and Harper (1973), in which successive cohorts of three Ranunculus spp. were mapped and observed over a three year period in order to elucidate the differences among the three species in life history patterns. Several herbaceous plant species do, however, possess sound morphological cues upon which a life history analysis may be structured. For example Kerster (1968), 1This work was done in partial fulfillment of requirements for the degree of Doctor of Philosophy, Yale University, 1974. It was supported in part by USPHS Predoctoral Training Grant in Developmental Biology and a Sigma Xi Grant-in-Aide of Research. I would like to thank the New Haven Water Company for access to their land. 2 Department of Biology, University of Utah, Salt Lake City, Utah 84112. This content downloaded from 207.46.13.115 on Sat, 08 Oct 2016 05:20:07 UTC All use subject to http://about.jstor.org/terms 1975] WATSON: GROWTH PERIODICITY IN POLYTRICHUM 415