Abstract: The importance and distribution of spruce-lichen woodland in northern Canada is described and fire is documented as being centrally im- portant to the ecology of these woodlands. Thus the "cladonia (C. stellaris (Opiz) Pouz. & Vezda) woodland" simply represents a long-term phase during the recovery sequence following fire, leading ultimately, although rarely, to a closed canopy spruce woodland. The existence of a second lichen woodland type in Canada is described and the possible widespread occurrence of this "stereocaulon (S. paschale (L.) Hoffm.) woodland" north of latitude 600N in the Northwest Territories is discussed. Excessive grazing by caribou is thought not to be an important factor in the ecology of this woodland type. The slow development of the Stereocaulon paschale surface is correlated with the very extreme surface microclimate of burned woodland at this latitude. Although the pattern of net photosynthesis activity in Stereocaulon is closely adapted to the less stressful microclimate of the developing woodland, soil nutrients, pH and snow depth interactions are probably also involved in the ecology of this species.

Abstract: A field examination of representatives of the vascular plants of xeric sites and shallow soils in Australia disclosed about 25 species with foliage which can revive after dehydration. Most were grasses, or ferns in the genus Cheilanthes. Only one dicotyledonous species was found. The protoplasmic drought tolerance limits were predominantly in the range of water potentials equivalent to 0-5 % relative humidity at 28°C. Most grass species were too low-growing to have agricultural potential.

Abstract: Evidence that lichenized blue-green algae are among the principal agents of N2 fixation on drier terrain in the Arctic and Subarctic has prompted attempts to quantify N-input by lichens in these habitats. The nitrogenase activity in Stereocaulon paschale mats in spruce-lichen woodland of central subarctic Canada has been examined in relation to thallus water content, temperature and incident radiation. The results revealed that while the level of activity at any point in time is dependent upon operative levels of these environmental factors, it can also be strongly influenced by pretreatment. It is suggested that simple predictive models are not yet able to accurately describe levels of nitrogenase activity in nature and that estimates of N-input on an annual or seasonal basis may be precarious. Leaching of metabolites from, and decomposition of the thallus are the two principal potential pathways for nitrogen, subsequent to fixation by cyanophilic lichens. Quantitative information on the operation of these pathways in nature is required. Lichenized blue-green algae are probably among the principal agents of nitrogen (N2) fixation in drier terrestrial habitats of the Arctic and Subarctic (Schell & Alexander, 1973; Kallio & Kallio, 1975; Crittenden, 1975; Huss-Danell, 1977). Several investigators working in boreal-arctic systems have used measurements of nitrogenase activity in lichens obtained either under field conditions or in the laboratory to derive estimates of N-input per unit area by the lichen biomass in situ on a seasonal or annual basis. Estimates have been produced for cryptogamic communities on polygonal tundra at Barrow, Alaska, containing species of Peltigera and Stereocaulon (Alexander, 1974) and for Nephroma arcticum and Stereocaulon paschale in subarctic forests and heathlands in Fennoscandia (Kallio & Kallio, 1975; Huss-Danell, 1977). Similarly, Horne (1972) predicted annual N2 fixation by Collema pulposum (= C. tenax) on mossy gravel surfaces of Signy Island, Antarctica. The accurate determination of N-input is clearly of value in assessing the role of lichens in the nitrogen cycle and primary production in these habitats, a topic that is of considerable interest in view of sug1 Paper presented at the Second International Mycological Congress, Tampa, Florida, 1977, forming a part of the symposium entitled "The Role of Lichens in Ecosystems." 2 We are indebted to Dr. D. P. Stribley (University of Sheffield, U.K.) for his interest in this work and for permitting us to present his unpublished data. Thanks are also due to Dr. D. W. Larson for critically reading the original manuscript. This study was conducted while one of us (P.D.C.) held a British Natural Environmental Research Council Postdoctoral Fellowship and this support is gratefully acknowledged. 3 Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1. 4 Present address: Department of Botany, The University, Sheffield, U.K. S10 2TN. 0007-2745/78/258-267$1.25/0 This content downloaded from 157.55.39.17 on Fri, 02 Sep 2016 04:42:34 UTC All use subject to http://about.jstor.org/terms 1978] CRITTENDEN & KERSHAW: NITROGEN IN BOREAL-ARCTIC ECOSYSTEMS 259 gestions that soil nitrogen deficiency is frequently the factor that ultimately limits plant productivity in cold regions (Russell, 1940; Sorensen, 1941; Warren Wilson, 1957, 1966; Bliss, 1962; Haag, 1974). The major shortcoming of such estimates is that their accuracy is unknown: the results of in situ measurements of nitrogenase activity over long periods have not been compared with those of predictive procedures applied to the same period. We have recently been examining the N2 fixing capabilities of S. paschale mats in spruce-lichen woodland in the Abitau-Dunvegan Lakes region of the Northwest Territories (60?21'N, 106'54'W). In this paper the results of in situ measurements of nitrogenase activity are discussed with particular reference to the problems associated with predictive modelling of N2 fixation by lichens. The probable pathways for nitrogen subsequent to fixation by lichens are also considered. NITROGEN FIXATION BY STEREOCAULON PASCHALE IN SPRUCE-LICHEN WOODLAND--A CASE STUDY The importance of Stereocaulon paschale as a major component of spruce-lichen woodland in central subarctic Canada has been reviewed by Kershaw (1977). The maintenance of large areas of open Stereocaulon-spruce woodland in this region is dependent upon the high frequency of forest fire, the latter being a natural facet of the environment in the boreal forest (see, e.g. Johnson & Rowe, 1975). In the AbitauDunvegan Lakes region the periodicity of fire is such that spruce woodland older than 200 y is infrequently encountered on drier terrain and the average reburn interval is less than 100 y (Maikawa & Kershaw, 1976). Stereocaulon paschale usually dominates the lichen synusia in spruce-lichen woodland of between 60 and 130 y old where almost pure carpets of this lichen may occur (Fig. 1). The acetylene (C2H2-) reduction technique (Stewart, Fitzgerald & Burris, 1967) with modifications after Stewart et al. (1971) was used to examine diurnal variation in rates of nitrogenase activity in a S. paschale mat in relation to thallus water content, temperature and incident photosynthetically active radiation (PAR-photon flux density in the 400-700 nm wavelength range). The field studies were conducted in typical S. paschale woodland estimated by Maikawa (1976) to have been last burned in 1898. Sequential C2H2-reduction assays were performed during periods of natural diffuse radiation to avoid the acute problem of excessive elevation of thallus temperature within the incubation bottles that occurs at high levels of irradiance and each assay was begun within 2 min of removing the pseudopodetium from the lichen mat. Intensive monitoring programs of this kind were conducted on five occasions: 24-25 June, 6-7, 10-11 and 18-19 August 1976 and 10-11 May 1977. The results of two of these studies are presented here. On 17 August 1976 the lichen mat was wetted by rainfall commencing at 04.15 h and monitoring of nitrogenase activity began at 14.30 h the following day. Thallus temperature within the lichen mat was measured with micro-thermocouples (42 gauge wire) arranged in thermopiles and with the junctions embedded in the main stem of pseudopodetia within the upper 15 mm of the lichen canopy (Fig. 2a). A similar method was employed to monitor thallus temperature within the incubation bottles (Fig. 2b) thus providing a check on the extent of deviations from operative temperature in situ. Photosynthetically active radiation (Fig. 2b) was measured with a quantum sensor (Lambda Instruments Corporation), and pseudopodetia were weighed on an electrobalance immediately prior to incubation in order that water content could be determined after dry weight estimates had been obtained (Fig. 2c). In other respects This content downloaded from 157.55.39.17 on Fri, 02 Sep 2016 04:42:34 UTC All use subject to http://about.jstor.org/terms 260 THE BRYOLOGIST [Volume 81

Abstract: Antarctic terrestrial ecosystems can be divided into two main series: those of continental Antarctica and those of the maritime Antarctic. Within the former series, plant "communities" exist as isolated groupings of plants growing under a severe continental polar-desert climate and appear to be similar to communities of the High Arctic (e.g. the interior plateau of Devon Island). Communities in the maritime Antarctic region are more diverse, with less severe environmental conditions, and show affinities with lichen com- munities occupying similar habitats in boreal-Arctic zones of the northern hemisphere. Roles played by lichens in the two series of ecosystems are influenced considerably by environmental conditions and in continental Antarctica are often completely subordinated to physical processes. Weathering of substrata, for instance, in continental Antarctica, consists of the operation of several physical processes with lichens playing an insignificant role. It is only in the more suitable habitats found in the maritime Antarctic that lichens are able to play their full role in the development and functioning of ecosystems. The Antarctic regions present some of the most severe environmental conditions endured by plant life, involving habitats characterized by intense cold, physiological drought and seasonal limitation of insolation. Such conditions severely limit the range of life forms found in these regions. Compared with the Arctic, the Antarctic presents a series of terrestrial ecosystems characterized by a greatly reduced flora and fauna, isolated from other systems by extensive ocean barriers. These Antarctic ecosystems have thus been defined as being relatively simple, in which physical processes often dominate and subordinate biotic factors in the development and functioning of com- munities. Unlike the Arctic, vascular plants are scarce, being represented by only two species in the Antarctic Peninsula region. A second major difference is the almost complete lack of herbivores, vertebrate grazers being absent. Antarctic terrestrial eco- systems are thus rendered unique in that they are dominated by cryptogamic plants and invertebrate animals. Lichens are the most prominent macroscopic plants in terms of species, distribution and biomass, but before their roles can be outlined it is nec- essary to briefly describe the ecosystems present, a general review of which has been

Abstract: Although a number of snow-cover-soilmoisture plant distribution studies have been carried out in the Front Range of the Rocky Mountains of the United States, the emphasis of such studies has been on vascular plants, and lichens and bryophytes have received little attention. Bryophytes and lichens are seldom a conspicuous element of the plant community above timberline in the Front Range. They rarely form large masses but usually exist as small individual specimens hidden by the canopy of vascular plants or plant litter and are often overlooked or ignored. A study was made of the percentage cover and abundance of 92 lichen taxa and 39 bryophyte taxa in 42 stands in a 1-km2 area in the lower alpine zone of Niwot Ridge in the Colorado Front Range. The percentage of total area covered by individual species was seldom very high, but taken collectively the cover of lichens and bryophytes combined averaged 38%.

Abstract: The flora of Mawson Rock is fairly depauperate; the two commonest lichens are Buellia frigida and Caloplaca elegans and the only two mosses are Bryum algens and Grimmia lawiana. The lichens form open colonies on exposed rock, and closed patches on sheltered aspects. The mossfields are small and confined chiefly to gravels on northerly aspects receiving snow-bank melt waters. The moss colonies form cushions, turfs or hummocks and may be contorted by cryoturbic processes. Larger colonies thus contain greater cores of sand and gravel. Mature cushions of Bryum algens are up to 25 years old, but the deepest peats are certainly much older. The small biomass of green moss in the most sheltered sites at Mawson is similar to comparable mossfields on the exposed plateau of subantarctic Macquarie Island.

Abstract: Lichens are believed to be extremely resistant to high-temperature stress when desiccated. Results from a reexamination of this concept indicate that some air-dry lichen thalli can be extremely sensitive to even moderate levels of heat stress whereas others exhibit a considerable degree of heat resistance. These differential levels of thermal resistance correlate exactly with the ecology of these populations.

Abstract: The survival potential of lichens in a given habitat is determined by the response of CO2 exchange to photosynthetically active radiation (PhAR), thallus temperature, and thallus relative water content (RWC). Therefore morphologically similar lichens from contrasting climatic environments 1) should differ in their CO2 exchange responses, and 2) these differences should reflect adaptations to their climatic regimes. The CO2 exchange responses of a subarctic (55°N, 67°W) Cladina stellaris (Opiz) Brodo population and a temperate (29°N, 82°W) Cladina evansii (Abb.) Hale and W. Culb, population were used to test these two related hypotheses. Infrared gas analysis with lichens collected in September–October 1975 established that the two populations differed in their responses to incident PhAR, thallus temperature, and thallus RWC. Net photosynthesis in C. stellaris had an optimum at a lower temperature and a greater relative photosynthetic capacity at low temperatures than did C. evansii. Cladina evansii maintained net photosynthesis above 35°C thallus temperature; C. stellaris did not. In both species the optimum temperature for net photosynthesis increased with increasing irradiance. The C. stellaris light saturation point was consistently lower than that of C. evansii. Both species had maximal rates of net photosynthesis at 70–80% relative water content. In C. evansii the CO2 exchange rates, expressed as percentages of the maximum rate, declined more rapidly under suboptimal conditions. The absolute CO2 exchange rates of C. evansii were greater than those of C. stellaris. At 20°C and 90–95% RWC, resaturation respiration occurred in both species and continued until 6–7 h after wetting. Contrasts in the temporal patterns of thallus condition at each collection site suggest that not all differences in the two response surfaces reflect climatic adaptation. The two populations appear well adapted to incident PhAR and thallus temperature regimes but the 70–80% RWC optimum for net photosynthesis common to both species is puzzling since their water regimes differ markedly. The overall adaptedness of the CO2 exchange responses in the two species cannot be judged without a comprehensive quantitative analysis of carbon balance under differing climatic regimes.

Abstract: Studies carried out since 1959 and recently summarized by us have demonstrated higher land plant-type microfossils (trilete spores, spore tetrads, tracheid-like tubes, cuticle-like tissues) from rocks of latest Ordovician to latest Silurian age. Vascular plant body fossils are unknown until mid-Late Silurian (Ludlovian) time. Pre-Late Ordovician land plant-type fossils, either body fossils or microfossils, are not recognized with any assurance. The latest Ordovician may mark the first appearance of such materials and may coincide with the initial evolution of continental vegetation of higher photosynthetic land plants, although these may have been preceded by plant life lower on the evolutionary scale, such as algae, fungi, lichens, and plants of similar morphologic levels of organization. Because previously studied plant fossils occur in shallow water or nearshore marine depositional sites, unequivocal proof for their nonmarine origin is lacking, although their apparent functional morphology repudiates the possibility of wholly submerged plants. A recent report by Pratt, Phillips, and Dennison now removes lingering doubts about the minimum date when higher land plants occupied the continental environment; they reported definite nonmarine (freshwater or terrestrial), nonvascular higher land plant material of early Llandoverian (earliest Silurian) age near the north end of Massanutten Mountain, Virginia. Their discovery provides hitherto lacking unequivocal proof for nonmarine, pre-latest Silurian (Pridolian-early Downtonian) higher land plants. Coeval fossils that we have been studying are morphologically identical to theirs. It is now clear that there was a significant lag, at least the entire Silurian Period, before undoubted nonmarine animals evolved and were preserved in the fossil record.

Abstract: SUMMARY A simple three-compartment model is used to test the assertion that lichens show low incorporation of assimilate into structural material. Changing specific activities of three fractions equated with the compartments of the model are followed using carbon-14. It is concluded that lichens show tight control over the synthesis of structural material.

Abstract: SUMMARY Using Toluidine Blue-stained sections, metachromatic granules have been detected in the fungi of two different lichens. These are histochemically identical with similar granules found in the fungal component of mycorrhizas and, like those, appear to consist largely of poly-phosphate. Small numbers of polyphosphate granules have been previously reported from the algal component of a lichen, but not from the fungus. In the material examined here, the size and number of granules in the fungus greatly exceeded those in the algae. These granules merit further investigation to evaluate their significance in ion uptake and accumulation within this symbiotic system.

Abstract: Samples of lichen and moss species collected in Norway have been analyzed for total amount of mercury. On the average the Hg content of lichens and mosses was greater than that of other higher plants or plant prod- ucts. Division of the lichen thallus into three parts showed that the upper part was essentially richer in mercury than the middle or lower parts. The mercury content of the plant material was determined by neutron activation analysis and flameless atomic absorption spectrometry. Recent reports concerning the content of heavy metals in cryptogams, led us to investigate the values of the total mercury content in a number of lichen and moss species. As far as the authors are aware, no measurements of Hg in lichen material sampled in Norway have previously been reported. This paper gives the results of a systematic investigation of these plant groups during the years 1973-1976. It may be assumed that the Hg content of plants is collected through concentrations of the mercury in rainwater or supplied absorbed in dust particles. Since Hg is known to be absorbed by most surfaces, it might be expected that dust particles would carry mercury if it were present in the atmosphere. Through the action of rain or of organic acids, the constituents of dust are released and absorbed by the lichen and moss carpet. Elevated concentrations of Hg in living lichen or moss samples may not be a result of air pollution alone, but may differ according to the ability of genus and species to absorb Hg. From our study it is evident that these plants may tolerate high concentra- tions of Hg. Distinct from vascular plants, lichens and mosses have little or no ability to utilize minerals directly from the substrate. The Hg content of the most common herbaceous plants fluctuates in the range from about 0.1 to 5 pg per 100 g dried material, depending upon plant species, plant parts and the supply of Hg during growth (Odelien, 1974). Unpublished analyses at our laboratory have given the following Hg content values: straw 5, ryegrass 2, timothy 3, clover 5 and needles of evergreens (30 samples) 3-16 pg per 100 g. Level of Hg in soil, however, has been reported to be very low.

Abstract: An annotated list of 260 lichen taxa from 82 genera giving habitat information is recorded from several localities in Nelson Lakes National Park, New Zealand. All records are supported by herbarium specimens, with the following taxa being new to the New Zealand flora; Arthrorhaphis citrinella, Calicium viride, Clzaenotlzeca clzrysoceplzala, Everniastrum sorocheilum Hypotrachyna rockii, H. thysanota, Lecidea scalaris, Menegazzia enteroxantha, Pseudocyphellaria ardesiaca, P. berberina, P. hamata, P. hamataoides, P. hirsutula, P. neglecta.

Abstract: Several species of the genus Athelia show a close relationship with either free-living algae or algae within lichens. Although most taxa of Athelia live as saprophytes, a growing number of species is known to form basidiolichens and as such grow constantly together with algae which do not seem to be much damaged. Apart from the species which are known as parasites of algae and lichens, there are three species which live symbiotically with filamentous Cyanophyceae (Algae), viz. Athelia andina Julich, A. epiphylla Pers., and A. phycophila Julich. Athelia epiphylla occurs mostly as a saprophyte on a variety of substrates, whereas the other two species are only known as basidiolichens. Athelia andina and A. phycophila are devoid of clamps at every septum of the basidiocarp, but A. epiphylla shows some clamp-connexions on the basal hyphae.

Abstract: SUMMARY Though the grassland associated with the sugar limestone outcrops in Upper Teesdale supports the richest terricolous lichen assemblage of any calcareous site so far discovered in England or Wales it may not have acted as a significant refugium for late-glacial lichens. A possible reason for this is suppression by open canopy forest during the boreal and atlantic periods. The current high density of terricolous lichens may, to some extent, be a result of intensive grazing over the last few centuries which has favoured the development of an extremely short, open sward offering a wide range of microhabitats.

Abstract: Lichens are a constant, though variable, component of arctic tundra ecosystems (Wielgolaski, 1972, 1975). In the Barrow tundra lichens have been reported as a minor part of the vegetation (Britton, 1967; Spetzman, 1959; Wielgolaski, 1972). We have examined the lichens in the vicinity of Barrow and have attempted to quantify their occurrence, biomass, and productivity, as well as to discover the relationships of lichens to their microenvironments. We measured the elemental contents of selected lichens and their substrates in an attempt to discover if significant mineral accumulation was occurring in the lichens.

Abstract: The four new depsides, 2-O-methylsekikaic acid, 2,4'-di-O-methylnorsekikaic acid, 4'-O-methyl-paludosic acid and 4,4'-di-O- methylcryptochlorophaeic acid, have been prepared by unambiguous syntheses and shown to co-occur with usnic acid and boninic acid in several Ramalina species.

Abstract: Ultrastructural studies of lichens have not paralleled investigations on lichen physiology and, with one exception (Harris and Kershaw, 1971), morphological changes have not been studied under controlled environmental conditions. Moreover, contradictory results on the same topic have been published. For example, Brown and Wilson (1968) and Jacobs and Ahmadjian (1971a) claimed that under damp conditions lichen thalli frequently contained starch inclusions, whereas other results (Harris and Kershaw, 1971; Ascaso and Calvan, 1976) conflicted with this observation. According to Webber and Webber (1970), starch granules do not appear in wet thalli collected in the spring because of the increased metabolic requirement of the mycobiont. The pyrenoid of green phycobionts usually contains a variable amount of electron-dense particles named pyrenoglobuli; these seem to be lipid in nature and are considerad as secondary storage producís (Jacobs and Ahmadjian, 1969, 1971a, ¿>). Jacobs and Ahmadjian (1973) pointed out that the behaviour of pyrenoglobuli under different environmental conditions remained obscure. Dampness seemed to increase the number of globuli (Rudolph and Giesy, 1966; Peveling, 1969) except in one case (Brown and Wilson, 1968), and short-term desiccation was found to reduce their amount (Peveling, 1968) without detectable damage to the thylakoids. Other ultrastructural changes of the phycobiont that occur under dry conditions have been reported by Peveling and Galun (1976). The above reports have all failed to take into account several ecological factors (e.g. moisture, temperature) and thus a meaningful comparison of these results is not possible. In long-term experiments we have shown that, under controlled experimental conditions, dryness causes a large number of ultrastructural modifications in both the phycobiont and the mycobiont (Ascaso and Galván, 1976). The variations in the amounts of pyrenoglobuli and starch seem to be correlated with changes in the physiology of the algal component (Ahmadjian, 1974); such changes are likely to be induced by alterations in the ecological conditions. On the other hand, physiological studies on both the photosynthesis and respiration of lichens (Feige, 1972; Richardson, 1973; Larson and Kershaw, 1975; Kallio and Karenlampi, 1975) and the nutrient movement between symbionts (Richardson et al, 1967; Richardson, 1973; Armstrong, 1976; Hill, 1976) have not been complemented by ultrastructural investigations. The aim of the present work was to study, by electrón microscopy, the response of three lichens to moderately dry and wet conditions. The moisture levéis were selected so as to correspond to those for which physiological data are available.