From the poles to the tropics water table levels and their fluctuations are powerful discriminators in plant distribution. Any body of water - a lake, a river or just a small stream - strongly influences the zonation of neighbouring plant communities (Figs. 8.1-8.3). The fringes of willow, alder, reeds, rushes and sedges that flourish at the water's edge provide striking evidence of the ability of certain plant species to survive in these marginal situations with ever-fluctuating water tables.
The flowering plants that are able to inhabit the border between dry and wetland can be thought of as the botanical equivalent of the amphibious animals as they possess an ability to live in both aerobic and anaerobic environments. Flooding can take many forms and in doing so imposes different types of stress. It can be seasonal, varying between both winter and summer. Flooding can also be of short or long duration. It may be confined to just the rooting zone or it can be a total immersion of shoot and root. Consequently, surviving under such a range of diverse inundations requires differing adaptations. Particularly noticeable are the various strategies that have evolved for surviving long-term waterlogging in winter as opposed to short-term inundation in summer. During the growing season, flooding is generally brief and escape mechanisms based on growth and development are predominant. In winter, however, water-saturated soils have a high probability of persisting for months and, consequently,
survival frequently depends on physiological tolerance of low-oxygen regimes.
Tolerance of flooding as measured by tolerance of oxygen deprivation (anoxia tolerance) can vary both quantitatively, in terms of duration, and qualitatively in relation to depth of flooding and whether the water is stagnant or flowing. In addition the temperature of the water (seasonal), and whether or not it contains potentially toxic ions can also have an influence on plant survival. Anoxia tolerance can be assessed experimentally by the length of time either whole plants or just certain tissues can survive in an anaerobic chamber (Table 8.1).
There are many ways in which flooding can vary ecologically and which impact on various aspects of plant physiology and development and ultimately influence the capacity of the plant to survive. The multifaceted nature of these qualitative effects is illustrated in
Figs. 8.4-8.5 and Table 8.2. It is therefore not surprising that the varied characteristics of flooding in relation to duration, temperature, substrate, and season of the year create an environmental diversity in wetlands which is reflected in the variety of amphibious plant communities that manage to inhabit land by the water's edge. From the mangrove forests of the tropics, to the salt marshes, swamps and bogs of the temperate zones and the Arctic, the interface between dry land and water appears to provide both a challenge and an opportunity for niche specialization that has been exploited by a wide range of plant species.
Location also alters the nature of flooding stress. The shores of lakes with fluctuating water levels are the exclusive sites for truly aquatic and amphibious species. Some species, as in the case of the shoreweed (Littorella uniflora), normally live vegetatively while
Table 8.1. Maximum anoxia tolerance of underground stems and rhizomes as observed in an anaerobic incubator at 20-25 ° C. Survival was measured in having an ability to grow new shoots after the period of anaerobic incubation
Group 1. Species with minimal tolerance of anoxia, surviving only 1-4 days
Group 2. Species with moderate tolerance of anoxia, surviving 4-21 days
Carex rostrata Mentha aquatica Juncus effusus Juncus conglomeratus Eriophorum angustifolium Saxifraga oppositifolia Saxifraga caespitosa Ranunculus repens Iris germanica Eleocharis palustris Carex papyrus Filipendula ulmaria Tussilago farfara Phalaris arundinacea Glyceria maxima Festuca vivipara (Iceland)
Group 3. Species with a high tolerance of anoxia, surviving 1-3 months
Iris pseudacorus Spartina anglica Phragmites australis Typha latifolia Schoenoplectus lacustris Bolboschoenus maritimus Deschampsia beringensis (N. Alaska) Acorus calamus
Source: Crawford & Braendle (1996).
totally submerged but flower when lake levels recede and the flowering shoots can emerge into air (see Fig. 8.12). By contrast, bogs provide more predictable conditions in relation to water table fluctuations.
When wetlands are viewed on a worldwide basis, there is no major plant life-form that does not have some species that are adapted to the amphibious environment. The water's edge appears under varying circumstances to offer an ecological niche that is suitable for annual and perennial herbaceous species as well as bushes, and even trees. There are, however, marked regional differences in the relative success of plant forms in relation to wetland colonization. Marshlands in tropical and subtropical climates appear to be more conducive to tree establishment than those in cooler oceanic regions. In the tropics and warm temperate regions, extensive stands of timber grow in wet bottomland forests, while on sheltered tropical coastlines mangroves flourish in a regime of daily tidal seawater inundation. In the southern United States, swamps are typically wooded, while in Europe they are commonly treeless mineral mires. Forested wetlands in the British Isles are usually restricted to patches of willow and alder carr. In the hyperoceanic conditions of Ireland bog has replaced much of the forest that once flourished across the entire island. Forested wetlands that do remain are to be found in areas with only periodic flooding, as along the banks of the River Shannon and neighbouring waterways where alluvial woodlands prone to winter flooding support willow, alder, downy birch, and oak (Kelly & Iremonger, 1997).
The sensitivity of trees to changes in soil hydration is clearly seen around pools in forests and in raised bogs that are fringed with trees (Fig. 8.3). At the edge of raised bogs (the rand), the depth of the upper layer of aerated peat (the acrotelm) increases and facilitates the penetration of the peat by tree roots. Below the acrotelm lies unaerated peat (the catotelm) into which there is very limited root penetration, and where the water content is constant and conditions are permanently anaerobic, with microbial activity much reduced (Ingram, 1978). Throughout the Holocene, warm dry periods at various times have favoured the advance of trees onto the surface of both raised and Atlantic blanket bogs, often to such an extent that one or more layers of birch or pine stumps can be located at specific depths in the peat.
The widespread occurrence of these sub-fossil tree layers has been cited as evidence for large-scale periods of climatic change during the Boreal and sub-Boreal periods. With a return to wetter conditions, peat
Deprivation of Production in soil Changes in plant Facilitation of microbial oxygen to plant of reduced toxic ions hormone distribution pathogens organs
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