In most studies of crop canopies or of the foliage of single plants, all leaves are treated as if they have the same properties. This is done so that we may make generalisations about the ways in which plant or crop growth rates may be interpreted as a function of leaf area. There is no gainsaying that this approach with its underlying assumption has been profitable. Concepts such as leaf area index (LAI) and net assimilation rate (NAR) have contributed greatly to our understanding of how a photosynthetic surface contributes to determining the growth rate of plants. However, the assumption is false.

The leaves on a plant or in a crop form a population, an assemblage of things that can be counted, and they are manifestly not all the same. Their heterogeneity derives in part from the fact that they (like a population of rabbits in a field or of blue tits in a woodland) are not of the same age and change their properties as they age. They are also borne in different positions relative to each other and their positions determine which leaves shade which. The positions that they occupy in a canopy are also related to their age – in general, young leaves are found in the fringes of a canopy with older ones in their shade.

Population biologists have much experience of studying the behaviour of age-structured populations and the aim of this chapter is to explore how far the study of populations (demography) may contribute to the study of plant canopies.

Introduction

Plant canopy structure is the spatial arrangement of the above-ground organs of plants in a plant community. Leaves and other photosynthetic organs on a plant serve both as solar energy collectors and as exchangers for gases. Stems and branches support these exchange surfaces in such a way that radiative and convective exchange can occur in an efficient manner. Canopy structure affects radiative and convective exchange of the plant community, so information about canopy structure is necessary for modelling these processes.

In addition to considering how canopy structure and environment interact to affect the processes occurring in the plant community, the influence of the canopy on the environment should also be considered. The presence and structure of a canopy exert a major influence on the temperature, vapour concentration, and radiation regime in the plant environment. Interception and transmission of precipitation are also affected, as are soil temperature and soil heat flow. Canopy structure can therefore be important in determining the physical environment of other organisms within the plant community, and can strongly influence their success or failure. Plant canopy structure can indirectly affect such processes as photosynthesis, transpiration, cell enlargement, infection by pathogens, growth and multiplication of insects, photomorphogenesis, and competition between species in a plant community. The indirect influence on soil moisture and temperature can also affect root growth, evaporative water losses from the soil, residue decomposition and other soil microbial processes.

Growth analysis – old and new

When a canopy of leaves is sunlit, photosynthesis proceeds at a rate which depends on how photons are distributed over individual elements of the foliage and on the relationship between photosynthetic rate and irradiance for each foliage element. In principle, therefore, photosynthesis by a canopy, expressed per unit of ground area rather than per unit leaf area, can be estimated from a statistical description of irradiance as a function of leaf disposition. In many models of productivity, this is a central and complex component. In practice, however, modelling can often be greatly simplified with little sacrifice of precision by exploiting the observation that, at least during vegetative growth, uniform stands produce dry matter at a rate which is almost proportional to the amount of radiant energy intercepted by the canopy. In this chapter we consider the theoretical basis of this relationship, its experimental verification, and its usefulness for exploring the dependence of growth on environmental variables in general.

Traditional growth analysis is based on the observation that, when single plants are exposed to a more or less constant environment, their rate of growth is approximately proportional to their weight and to their leaf area until a significant fraction of older foliage is shaded by younger foliage. Consequently, relative growth rate (RGR) and net assimilation rate (NAR) are conservative indices of growth initially.

Introduction

Over the past 15 years a number of studies have focused on characterising diurnal leaf movements that occur in a variety of plants in response to the sun's movement across the sky. It is now clear that these solar tracking leaf movements are triggered by a directional light stimulus and that these movements result in at least a partial regulation by the leaf of the intensity of the incident photon irradiance. The purpose of this chapter is to review what is known about the different kinds of leaf solar tracking movements, their impact on primary productivity, and the potential ecological roles of these phenomena.

Solar tracking is an expression applied to describe the heliotropic movements of both leaves and flowers; it denotes the ability of these structures to move in response to the diurnal change in the sun's position in the sky. Heliotropic movements are distinguishable from other directional types of growth by their rapidity, the reversibility and by the overnight resetting to face the morning sun (Yin, 1938). Two main kinds of diurnal movements are recognised: diaheliotropic movements in which the leaf lamina remain oriented perpendicular to the sun's direct rays and paraheliotropic movements in which the leaf lamina are oriented obliquely to the sun's direct rays (Ehleringer & Forseth, 1980). In the extreme cases of paraheliotropism, the leaf lamina may change from nearly perpendicular to the sun's rays to an orientation parallel to the sun's rays.

Environmental control of carbon dioxide exchange

Methods

A plant survives only where the annual environmental regime includes periods favourable for metabolism and growth and where, in the case of perennials, there are no periods of lethal stress. Most polar plants, vascular and non-vascular, are perennials, and must therefore show resistance to both elastic and plastic stress. Levitt (1980) defines a biological stress as ‘any environmental factor capable of inducing a potentially injurious strain in living organisms’: a plastic stress is one producing irreversible chemical or physical change, e.g. frost damage, while an elastic stress results in a reversible change such as a major reduction in net assimilation rate (NAR) under suboptimal conditions. This chapter begins to examine the features that enable bryophytes and lichens to survive under the apparently severe stress of Arctic and Antarctic environments. Such features include, first, general characteristics of these plants that confer fitness under polar conditions and, second, adaptations specific to polar species or populations which may therefore have evolved in response to local selection pressure.

Carbon dioxide exchange has been widely investigated to determine how polar cryptogams maintain positive carbon and energy budgets under adverse conditions. The results are not fully comparable due to differences in experimental procedure. Most methods have involved infrared gas analysis (IRGA) in the laboratory, either in open systems at ambient CO2 concentration (Oechel, 1976) or in sealed cuvettes (Larson & Kershaw, 1975a). The latter method allows greater replication but has the disadvantage that CO2 concentration inevitably fluctuates somewhat during the period of measurement. The relative merits of these systems have been discussed by Lange & Tenhunen (1981) and Kershaw (1985).

Energy flow

Production and phytomass

Definition and estimation Bryophytes and lichens are major elements in the tundra communities described in Chapter 2 in terms of cover and also energy flow, mineral nutrient cycling and other dynamic aspects of polar ecosystems. The part played by bryophytes has been briefly reviewed (Longton, 1984). Here we shall explore the role of both groups in greater detail, beginning with the contribution of cryptogams to production and phytomass.

Phytomass is the amount of plant material present, and thus potentially available to consumers and decomposers. Vascular plant phytomass comprises above- and below-ground components, the latter including living and dead, and the former green, other living, standing dead and litter. Only photosynthetic (green) and non-photosynthetic components can realistically be distinguished in many cryptogams because of difficulties in determining how far living tissue extends into the colonies (Chapter 6), and most published estimates of living phytomass probably represent the green component. Mosses and lichens decompose from the base with little surface litter deposition. Phytomass varies seasonally, particularly in flowering plants; the value recorded near the end of the growing season is that most commonly reported.

Above-ground phytomass is determined by harvesting and dry-weighing plants from sample plots of known area, those for cryptogams normally comprising cores through colonies of one or an assemblage of species. The results are extrapolated to phytomass per unit area assuming continuous cover, and this value multiplied by percentage cover indicates phytomass of the species concerned in the community. Below-ground phytomass is assessed from cores through the phanerogamic rooting zone.

Solar radiation

Variation with latitude

Solar radiation exerts a pervasive influence on plant–environment relationships, supplying the energy available for photosynthesis and controlling temperature and water regimes in the microclimate of lowgrowing cryptogamic vegetation. Microclimatic conditions differ dramatically from those indicated by standard meteorological recording, and must be analysed in many types of ecophysiological investigation. Walton (1984) has recently provided a critical review of microclimatic studies in the Antarctic.

All energy received at the earth's surface, apart from a small amount of geothermal heat, originates as solar radiation. The solar constant, i.e. the irradiance of a plane perpendicular to the sun's rays at the outer edge of the atmosphere and at the earth's mean distance from the sun, is approximately 8.4 J cm−2 min−1 (= 2.0 cal cm−2 min−1 or 1402 W m−2). Radiation receipt at the earth's surface varies widely in response to changes with latitude in the angle of solar elevation, in daylength regimes, and in attenuation of radiation during passage through the atmosphere.

The angle of solar elevation (Fig. 4.1), and therefore maximum irradiance, fall progressively from the tropics towards the poles. Daylength varies with latitude because the earth's axis of rotation is inclined in relation to its plane of revolution around the sun (Fig. 4.1), resulting in pronounced seasonality and relatively low diurnal fluctuation in solar irradiance in polar regions. At the northern summer solstice, regions north of the Arctic circle lie in the path of direct solar radiation 24 hours per day whereas south of the Antarctic circle there is no direct insolation, the converse being true at the northern winter solstice.

Patterns of growth in relation to assimilation and translocation

Bryophytes

Growth and net assimilation Positive net photosynthesis tends to increase dry weight due to accumulation of assimilate. In this chapter we are concerned with the translation of this process into growth, in the sense of increase in plant size. Adaptations that permit positive net assimilation at substantial rates under severe environmental conditions are commonly viewed as the key to plant success in polar regions (e.g. Mooney, 1976), and considerable effort has been directed towards investigating environmental relationships of CO2 exchange in mosses and lichens. As discussed in Chapter 5, the results confirm that polar species in situ are able to photosynthesise at reasonable rates, but the assumption that assimilation would be maximised by a close correspondence between optimum conditions for net photosynthesis and the most frequently prevailing environmental conditions has not been fully substantiated (page 160; Lechowicz, 1981b). However, the parallel assumption, that survival is favoured by maximum rates of photosynthesis and growth, is not necessarily valid for plants with essentially opportunistic growth responses in environments where competition is not everywhere intense, and low stature may be advantageous.

Moreover, conditions promoting maximum NAR are often very different from those favouring growth in size. This is particularly true in polar species in which maximum NAR may occur at temperatures low enough to cause a severe depression of respiration, and probably other processes essential to growth. The latter may then be restricted more by direct limitation than by availability of assimilate, and adaptation further increasing net assimilation would be superfluous.

Reproductive processes and propagules

Bryophytes

In this final chapter we shall consider first the reproductive biology of polar cryptogams, as an introduction to a discussion of life strategies and their significance in the origin and evolution of the polar floras. The emphasis is of necessity on bryophytes, as there have been few relevant studies on lichens (Smith, 1984a).

The bryophytes life history involves heteromorphic alternation of a haploid gametophyte and a diploid sporophyte, the latter permanently attached to and, nutritionally, partially dependent upon the gametophyte (Fig. 8.1). Gametophytes are monoecious, dioecious or occasionally heteroecious, depending on the species. In mosses, antheridia and archegonia develop in groups at the apices of leafy shoots in acrocarpous species, or of reduced lateral branches in pleurocarps. Groups of gametangia with their surrounding bracts are termed inflorescences, and are bisexual, or more commonly exclusively male (perigonia) or female (perichaetia). Female bracts in leafy liverworts fuse to form tubular perianths that eventually surround the developing sporophytes.

Each antheridium liberates many biflagellate sperm, and a single, nonmotile ovum develops in each archegonium. Where fertilisation occurs, repeated mitotic division of the zygote and its derivatives gives rise to the sporophyte, normally one per perichaetium. The mature sporophyte comprises a haustorial foot embedded in the gametophyte, and a slender stalk, the seta, on which the sporangium, or capsule, is borne several centimetres above the gametophyte. Meiosis occurs during sporogenesis, and several hundred thousand spores commonly develop synchronously within each capsule. Bryophytes with sporophytes are traditionally, if loosely, described as fruiting.

Topography and geological history

This book considers the biology of bryophytes and lichens in polar tundra and adjacent, open woodland, their contribution to the vegetation, their role in polar ecosystems, and their adaptations to the rigours of life in regions generally regarded as among the least hospitable on earth. Tundra is used here in the broad sense of treeless regions beyond climatic timberlines. It occurs in the polar regions as defined in Figs. 1.1 and 1.2, and in some alpine and oceanic areas at lower latitudes.

The polar regions are diverse in topography and climate. Arctic lands comprise substantial parts of the North American and Eurasian continents which, with Greenland and smaller islands, encircle a polar ocean (Fig. 1.1). The terrain ranges from extensive flat-bedded plains and plateaus to folded mountains, often high and imposing as in the Brooks Range, Alaska, where elevations reach over 2800 m (Figs. 1.3 and 1.4). Except in Greenland, contemporary glaciation is localised, and largely confined to Spitzbergen and other far-northern islands. In contrast, the Antarctic continent is centred over the pole and is surrounded by the vast expanse of the Southern Ocean, with a minimum width of over 850 km between the Antarctic Peninsula and Cape Horn (Fig. 1.2). The continent is fringed by coastal mountains, and rises to an inland ice-plateau at elevations of 1800 to 3800 m. Over 98% is currently buried beneath an ice sheet more than 4 km thick in places. Islands lying close by the mainland or widely scattered in the Southern Ocean thus provide the major terrestrial habitats in Antarctic regions, where vegetation is restricted to the almost universally rugged terrain of coastal regions.

Environmental relationships of polar cryptogamic vegetation

Evidence concerning environmental control over the distribution of polar bryophytes and lichens is largely circumstantial as it is derived principally from observed correlations between vegetation and environment, which do not necessarily imply direct control by the factors concerned. Moreover, distribution is influenced by complex combinations of climatic and other variables so that correlations may be difficult to elucidate.

Ordination provides a means of alleviating the latter difficulty, as demonstrated by Webber's (1978) study of Alaskan tundra. Webber showed that species richness among both bryophytes and lichens is highest under mesic to dry conditions with moderate soil phosphate levels, but that species richness for lichens shows a stronger correlation with good soil aeration than does that for mosses (Fig. 3.1). Different patterns were shown by species indices based on frequency and cover (Fig. 3.2), which reveal that bryophytes are most abundant under relatively wet conditions with poor soil aeration and low phosphate, whereas lichen abundance is greatest on drier, well aerated soils with moderate phosphate.

The analysis also provided valuable insight into the habitat preferences of individual species, but such techniques have not been widely applied to polar vegetation. In some instances, however, relationships between plant distribution and environment stand out so clearly that a controlling influence for the factors concerned may reasonably be inferred from simple observation. Other evidence comes from physiological studies, as discussed in Chapters 5 and 6.

This book is about bryophytes and lichens in the Arctic and Antarctic. It considers the evolution and adaptations of the polar floras, and the role of these plants in the vegetation and in the functioning of tundra ecosystems. The study of plant ecology in the polar regions has advanced dramatically in recent years as a result of work initiated in the Antarctic during the International Geophysical Year (1957–58), and in both Arctic and Antarctic as part of the International Biological Programme Tundra Biome investigations. Much attention has been focused on bryophytes and lichens because of their obvious abundance in local communities. The work has been broad in scope, ranging from phytogeography to physiological ecology, and from vegetation ecology to reproductive biology. The results, as synthesised here, are of relevance far beyond the polar regions, because they make a substantial contribution towards a general understanding of the environmental relationships of bryophytes and lichens. It should be noted, however, that mosses are of considerably greater ecological significance in the tundra than liverworts; they have consequently received more emphasis in research and therefore in the present text.

Authorities for most of the plant names cited in the text can be found in the following: mosses – D. M. Greene (1986), Steere (1978a); hepatics – Schuster (1966–80), Grolle (1972); lichens – Thomson (1984), Lindsay (1974); vascular plants – Greene & Walton (1975), Scoggan (1978–79). Where differences occur, the nomenclature used by the first-mentioned authority for each group has been followed.

Vegetation classification

Boundaries between communities based on the distribution and abundance of cryptogams and of flowering plants may only partially coincide, as Alpert & Oechel (1982) demonstrated in Alaska. The most useful vegetation accounts thus consider all the major plant groups. The procedures of the Braun-Blanquet school have been adopted to define vegetation types at isolated Antarctic localities (Kappen, 1985a), and more comprehensively on Svalbard (Philippi, 1973), in the Soviet cold-Arctic (Aleksandrova, in press), in Greenland (Daniëls, 1982, 1985), and on Marion and Prince Edward Is (Gremmen, 1982). In a rather different approach, some Arctic lichen communities have been delimited by principal component analysis (Kershaw & Rouse, 1973; Richardson & Finegan, 1977), or by an agglomerative technique that indicates diagnostic species (Sheard & Geale, 1983). Although appearing with increasing frequency (e.g. Bliss & Svoboda, 1984), such quantitative analyses cannot yet form the basis for a comprehensive, objective classification of polar vegetation because of the limited database and differences in methodology. Thus Brossard, Deruelle, Nimis & Petit (1984) used small sample plots (100cm2) to study lichen-dominated vegetation on Svalbard, and as a consequence recognised communities on a smaller scale than those indicated by the conventional, larger quadrats.

In view of these problems, the present description of Antarctic vegetation is based on a hierarchical classification, formulated subjectively, with the major units defined by growth form. The two formations (Table 2.1) include vegetation dominated respectively by vascular and nonvascular plants, while subformations are based on growth form of the community dominants.

Introduction

The numbers of known fungi are vast in comparison with the numbers in other groups of microorganisms used in biotechnological and other industries. Around 64 200 species (including yeasts) are currently known (Hawksworth, Sutton & Ainsworth, 1983), with new species being described at the rate of about 1500 each year. The number being described is limited only by the available mycologists, and the actual number of fungal species in the world may well exceed 250 000.

While only 7000 of these species have yet been grown successfully in pure culture, fungi hitherto known only on a specific host or natural substratum continue to be encountered or cultured for the first time; with appropriate techniques it is clear that many more species could also be cultured. This, together with the appreciation that many of the new species discovered each year are grown in pure culture, is exciting to the biotechnologist in search of strains with significant novel properties.

With such large numbers of fungal species, and a dispersed systematic literature growing at the rate of around 1200 titles each year (Bibliography of Systematic Mycology 1943 on), the extent to which a nonspecialist can expect to identify isolates to species with confidence is limited. However, some culture collections have specialists in identification on their staff or are associated with scientists able to assist in their identification of isolates and further information on collections providing these services can be obtained by reference to Chapter 1.

Considerable care is needed in identification if confusion is not to be created in the commercial and scientific literature. There are many cases of elegant biochemical, chemical, cytological or ultrastructural studies which have used incorrectly identified material.