The major environmental changes that bryophytes are facing (Chapter 10) have increasingly brought about concerns for their conservation. Conservation biology is a fairly new, multidisciplinary science that has developed to deal with the crisis confronting biological diversity (Primack 1993). As a crisis discipline, conservation biology appeared in response to a rising political demand to face the dramatic loss of biodiversity and the need to take steps to anticipate, prevent and reverse the trend (Heywood & Iriondo 2003). Subsequent ratification of the ‘Convention on Biological Diversity’ at the United Nations conference held in Rio in 1992 by most of the world's governments has placed the subject of biodiversity firmly on the political agenda.

The development of the concept of conservation biology is closely interrelated with that of biodiversity. Biodiversity is defined in the Convention on Biological Diversity as ‘the variability among living organisms, from all sources … and the ecological complexes of which they are part’.

In this broad conservation picture, bryophytes were initially completely overlooked. Cryptogams in general were, for example, the focus of only about 4% of published papers between 2000 and 2005 in leading conservation journals (Hylander & Jonsson 2007). The situation has, however, been changing recently and there has been an increasing awareness of the necessity to include cryptogams in general, and bryophytes in particular, in conservation programmes (Hylander & Jonsson 2007).

Bryophytes are familiar and attractive components of many natural landscapes such as peatlands, tundra environments and moist forests, where their shaggy coverings on branches, crags and boulders, add distinction to the landscape (Figs. 2.1,2.2). In particular, epiphytic bryophytes make up a conspicuous portion of many rain forest canopies, hanging from branches (Fig. 8.6) and densely festooning tree trunks (Wolf 1993, Kürschner & Parolly 2004).

Less appealingly, they also occasionally grow on unexpected substrates, like the leather of a discarded boot, or a rusty iron pipe. Even in modern cities where air pollution and the man-made environment may seem unrelenting, bryophytes are able to colonize crevices in masonry (Bates 2000). The occurrence of bryophytes in a wide range of terrestrial and freshwater habitats suggests intriguing questions on how they manage to colonize, adapt and develop in such environments. The occurrence of bryophytes in nearly every ecosystem on earth (all but salt water and permanently frozen ecosystems) also raises questions about their ecological significance in those ecosystems. This is the focus of the present chapter, where we will explore the contribution of bryophytes to ecosystem functioning and, more precisely, how they may influence vegetation dynamics, soil formation and characteristics, global geochemical cycles and hence, climate regulation.

Water and biogeochemical cycles

Bryophytes contribute to a substantial proportion of the global biomass in a range of ecosystems (Fig. 2.3) and hence play a major role in the cycling of carbon and nutrients through growth and decomposition.

The common ancestor to the Marchantiophyta arose immediately after the conquest of land, some 450 mya. Their evolutionary history is, however, also characterized by a more recent diversification during the early Tertiary Period. This diversification may coincide with the establishment of modern rainforests and resulted in species rich crown groups such as the Lejeuneaceae (Heinrichs et al. 2007). With approximately 5000 extant species currently recognized, liverworts compose a diverse lineage of land plants, represented on every continent and in nearly all ecosystems. Some species are virtually invisible to the naked eye, whereas others rival mosses in their stature. Although the experienced eye may easily distinguish liverworts from other bryophytes in the wild, not a single conspicuous unifying morphological vegetative character defines them. The architecture of the gametophyte varies from thallose, with or without internal differentiation, to leafy, with leaves divided in some lineages into two or more lobes. It is only after sexual reproduction is completed that liverworts reveal their shared ancestry: the sporangium is enclosed by an epidermis lacking stomata and elevated above the maternal protective tissue on a translucent seta that elongates after sporogenesis. Furthermore, the capsule lacks a columella and the sterile cells within the sporangium develop into elaters. Except for the Blasiales, liverworts lack endophytic cyanobacteria, but most species establish symbiotic associations with mycorrhizal fungi. Although inconspicuous at the landscape level, liverworts form diverse assemblages in moist habitats such as temperate and tropical rainforests, where they thrive on trunks and leaves.

The concept for a book providing an introduction to the biology of bryophytes emanates from our passion for these land plants and the lack of recent manuals offering general insights into their fascinating diversity and evolutionary history. Throughout its history, bryological research has contributed significantly to the field of plant sciences, starting with the discovery of sex chromosomes in plants and culminating most recently with the assembly of the Physcomitrella genome, fundamental for the study of the evolution of genes and their function during the diversification of land plants. Bryophytes are, in fact, pivotal in land plant evolution, and the recent advances in molecular phylogenetics and genomics have allowed for a clearer picture of land colonization and subsequent evolution to emerge. Ecologically, the significance of bryophytes for the regulation of global biogeochemical cycles, especially carbon, has long been acknowledged and has gained much attention in the present context of global changes. Finally, there has been an increasing concern for the conservation biology of cryptogams, and bryophytes in particular.

The aim of the present book is to offer a stepping-stone to anyone interested in discovering the fundamentals of the biology of bryophytes, making the bridge with more comprehensive treaties such as Goffinet and Shaw's Bryophyte Biology (2009) and Glime's Bryophyte Ecology (2007a). In Introduction to Bryophytes, we make an attempt at summarizing and explaining the recent advances made across the various aspects of bryophyte biology at a level that would be palatable to beginners.

In the previous chapter, we focused on the mechanisms that explain how bryophyte species are able to ‘travel’ across oceans and continents. The success in colonizing a new area, or the ability to persist within the same area in the long-term, depend, of course, on the ability of the species to cope with local environmental conditions. Bryophyte distributions are influenced by a variety of factors operating over a range of temporal and spatial scales (Rydin 2009) (Fig. 7.1), which are the focus of the present chapter.

Global ecology

A central thesis to plant ecology is that climate exerts the dominant control on the distribution of plants at the continental scale. In bryophytes, changes in communities depending on major climatic shifts during the Quaternary Era are evident from the succession of macro-remains preserved in peat. Each climatic phase had its characteristic range of habitats and these appear to have been colonized by bryophytes at a remarkable speed as soon as they developed and became available (Jonsgard & Birks 1995). Two macrofossil taxa are of particular significance in the interpretation of macro-climatic changes: Sphagnum spp. and Racomitrium lanuginosum.

The local distribution of R. lanuginosum is controlled by its preference for cold, wet climates (Tallis 1995). By contrast, the occurrence of Sphagnum largely depends on water availability. In the Holarctic, for example, Sphagnum bogs are strictly restricted to areas experiencing an annual moisture balance > 0 (Gignac et al. 2000).

Vascular plants, particularly seed plants, dominate vegetation throughout much of the world today, from the lush rainforests of the tropics harbouring a vast diversity of angiosperms, to the boreal forests of coniferous trees, draping the northern latitudes of the globe. This dominance in the landscape is the result of a long evolutionary history of plants conquering land.

Evolution is the result of a suite of incessant attempts to improve fitness and take advantage of opportunities, such as escaping competition and occupying a new habitat. Pilgrims, fleeing the biotic interactions in the aquatic habitat, faced severe abiotic selection forces on land. How many attempts were made to conquer land is not known, but at least one of them led to the successful establishment of a colony. At least one population of one species had acquired a suite of traits that allowed it to complete its life cycle and persist on land. The ancestor to land plants was born. It may have taken another 100 million years for plants to overcome major hurdles, but by the Devonian Period (approximately 400 mya), a diversity of plants adapted to the terrestrial environment and able to absorb water and nutrients, and transport and distribute them throughout their aerial shoots, occupied at least some portions of the land masses. Soon thereafter, plants were freed from the necessity of water for sexual reproduction, by transporting their sperm cells in pollen grains carried by wind or insects to the female sex organs, and seeds protected the newly formed embryo.

Mosses (Division Bryophyta) are generally seen as small plants confined to humid habitats, avoiding exposure to direct sunlight. Yet, an alert naturalist will quickly notice their presence in virtually every ecosystem. In parts of the world where short growing seasons limit plant growth, mosses may dominate the vegetation. Similarly, in temperate and tropical rain forests, mosses compose luxuriant epiphytic communities that play important ecological functions, especially in terms of water and nutrient flow. Approximately 12 000 species of mosses are currently recognized, reflecting a broad morphological diversity. In fact, a unifying macroscopic definition is impossible due to both fundamental architectural differences among major lineages and extensive reduction, and hence character loss, across the phylogenetic tree of mosses. A sporophyte composed of a robust stalk elevating the capsule diagnoses most mosses, but is lacking in peat mosses and various ephemeral taxa inhabiting seasonally dry ecosystems. In many cases, closer examination further reveals one or two rings of teeth lining the capsule mouth. The vegetative (gametophyte) body always consists of a terete axis with sessile leaves, rather than a flattened thallus as in hornworts and some liverworts. Although associations with fungi are common, evidence for a symbiotic nature of the relationship benefiting the moss is lacking.

Biogeography is the science that aims to describe the spatial distributions of biota (a pattern) and understand the means by which these distributions were achieved (a process). Biogeography is a field that existed long before evolutionary biology and indeed helped in founding the evolutionary ideas of Charles Darwin and Alfred Wallace, among others (Humphries & Parenti 1999). Biogeography and evolutionary biology therefore interface with each other, as the discovery of the mechanisms regulating species distributions involves an understanding of species dispersal ability, evolutionary rates and diversification mode, which are among the main foci of the sciences of evolution.

In general, bryophyte species have broad geographic ranges that often span more than one continent (e.g. Figs. 6.1–6.3). Some, termed as ‘cosmopolitan’, are even widespread across all continents. Bryophyte species thus tend to show wider distributions than vascular plants. In fact, many bryophyte species exhibit the same disjunctions that are well known in flowering plants at the generic level. For example, 43% of the moss species found in North America are also found in Europe, while 70% of the species found in Europe also occur in North America (Frahm & Vitt 1993). By contrast, 48% of the genera, but only 6.5% of the species, are shared between the North American and European vascular flora (Qian 1999). Two competing hypotheses, namely repeated intercontinental dispersal and continental drift, have traditionally been proposed to explain the broad and highly disjunctive distributions typical of bryophyte species.

The natural corollary of the narrow relationship between bryophytes and their environment is that a great deal of variation in community composition and physiological behaviour is to be expected whenever changes in environmental conditions occur. In the current context, global change brought about by human activities has increasingly been an area of concern. Bryophytes, perhaps more than many other organisms, are highly sensitive. Their response to climate change is obvious from the reconstruction of paleovegetation based on the analysis of macrofossils (see Section 7.1), so that a great deal of changes in species distribution and community composition is to be expected in the current context of global warming (Box 7.1). The response of bryophytes to an increasingly polluted world is no less dramatic. This is because bryophytes, being poikilohydric, mostly lack a protective, highly hydrophobic cuticle and are, therefore, directly exposed to pollutants. As a result, pollutants may bind on cell walls, eventually pass through the cell membrane and affect metabolism.

In this chapter, we review how pollution affects bryophyte ecophysiology and community composition. We focus on airborne pollution, which arguably constitutes the main threat to ecosystems (Lee 1998). We then assess how understanding the response of bryophytes to these changes can be used as one indicator to monitor the quality of the environment. Finally, we examine the mechanisms by which bryophytes are able to adapt and survive in a changing environment.