Impacts Of Enhanced Ultraviolet-B Radiation On Mosses In A Subarctic Heath Ecosystem

INTRODUCTION

Radiation in the waveband region 280-320 nm (ultraviolet-B radiation [UV-B]) is increasing at the Earth's surface as a result of the stratospheric ozone depletion. Impacts of UV-B radiation on plant species, mainly vascular plants, have been studied on potted individuals grown in growth cabinets or greenhouses, but rarely outdoors (Caldwell et al. 1995). However, whether an enhancement of UV-B radiation will affect ecosystems consisting of a variety of life and growth forms is still an open question. The contribution by cryptogams to ecosystems, in terms of biomass and production, increases with increasing latitude (Richardson 1981), and it is at those high latitudes the highest relative ozone depletion has taken place (Madronich et al. 1995). The hitherto rare consideration of cryptogams in UV-B studies is thus surprising. This paper deals with effects of increased UV-B radiation on the bryophyte component of a subarctic heath.

For a number of reasons, it can be assumed that the majority of bryophytes are particularly susceptible to UV-B radiation.

1) In most bryophyte species, a leaf consists of only one cell layer. This may cause high internal UV-B fluence rates, as shown for vascular plants where internal UV-B was higher in thin than in thick leaves (Teramura 1983, Bornman and Vogelmann 1991).

2) Most bryophytes lack protection by a cuticle, though a corresponding UV-B shield may be provided by thickened cell walls.

3) Most bryophytes possess an undifferentiated leaf anatomy, which does not allow a plastic response of one type of cell layer to protect the underlying cells from high internal irradiation. In contrast, in vascular plants, an UV-B induced increase in leaf thickness has been found (Cen and Bornman 1990).

4) Leaf water content in bryophytes depends on air humidity. Periods of high solar (and UV-B) radiation lead to drought, i.e., physiological inactivity. This leaves sensitive targets, such as cell organelles, nuclei, or DNA, exposed to UV-B radiation without enabling immediate repair by photoreactivation. In contrast, vascular plants remain physiologically active because of their ability to control leaf water content.

5) The shoots of most bryophytes are rootless and thus lack buffering against aboveground stress, such as by UV-B radiation, through belowground storage of assimilates.

This paper compares the impacts of enhanced UV-B radiation on two bryophyte species possessing clear innate growth markers, Hylocomium splendens and Polytrichum commune. They are the dominating species in bryophyte patches within patches of dwarf shrubs in a subarctic birch-heath ecosystem. H. splendens is a species with typical bryophyte features. It has thin, undifferentiated leaves, most of its biomass is assimilating tissue, and it lacks both roots and vascular tissue and thus cannot translocate assimilates or water within the shoot. In contrast, P. commune rather resembles a vascular plant in terms of morphology and function. It possesses thick, cuticularized leaves with a differentiated anatomy (Bayfield 1973). It is able to control leaf water content to a considerable extent, both due to its ability to translocate water internally (Bowan 1931) and due to its leaf anatomy. The two species are different in two further respects. First, in H. splendens, a shoot consists of a sympodial chain of annual segments. The segments are arched and grow rather horizontally, which maximizes their exposure to solar radiation. In P. commune, a shoot appears segmented, as small leaves mark the beginning and end of a growing season, enabling distinctive separation of growth years. Because of the strictly vertical shoot growth, younger leaves shade the older leaves, thus diminishing light exposure with increasing leaf age. Second, photosynthates produced in older segments are used in different ways in the two species. In H. splendens, no translocation of photosynthates occurs between growth modules produced in different years (Callaghan et al. 1978). In contrast in P. commune assimilates are translocated within the shoot, and even between shoots, by rhizomes (Collins and Oechel 1974, Thomas et al. 1988), so that UV-B induced reactions in the top part of a shoot may be buffered via less exposed tissue.

Considering their contrasting physiological and morphological features, I hypothesize that H. splendens may be more sensitive to enhanced UV-B radiation than P. commune. In this study, growth variables were measured that describe and quantify the morphology of the two species. Additionally, supposedly UV-B-responsive variables were measured (photosynthetic pigments as an indication for physiological damage; UV-B screening compounds as a protective reaction). Ecological implications of the responses are discussed considering the contrasting intra- and interspecific relations in both species.

MATERIAL AND METHODS

Site and species

The experiment has been conducted in a subarctic heath close to the Abisko Scientific Research Station (68.35 [degrees] N, 18.82 [degrees] E; 360 m above sea level) in northern Swedish Lapland. A description of the edaphic conditions is given by Johanson et al. (1995). The mean annual air temperature at Abisko is -0.8 [degrees] C, with July being the warmest summer month (mean temperature, 11 [degrees] C) as calculated from data for 1961-1990. Of the rather low mean annual precipitation (304 mm, long-term mean for 1961-1990), approximately one-third occurs during the summer months June-August. The vegetation consists of a tree layer (Betula pubescens ssp. tortuosa), an ericaceous dwarf shrub layer mixed with scattered herbs (Rubus chamaemorus) and grasses (Deschampsia flexuosa), and a ground layer of lichens and mosses. The moss patches are dominated by Hylocomium splendens (feather moss) and Polytrichum commune (common haircap moss); see Plate 1.