The soil conditions, the risk of inundation
and, in flood wetlands, the seasonality of
the inundation cycle, all make a wetland an extreme environment for rooted plants.
The precise physiological tolerances of most wetland plants ensure that community
structure is intimately related to the water table. Conditions may be so extreme that
no species can be adapted optimally to more than a narrow range of requirements
and will be out-competed elsewhere.
Adaptations to permanently waterlogged
conditions can lead to problems living in
well drained soils. Some wetland species have little control over transpiration rates
from their leaves and will dry out rapidly in the absence of a constant supply of
water. Others, growing shallow roots to avoid anoxia, have little capacity for
extracting water from dry soils, but wetlands support many species, adapted to
waterlogging and low nutrient levels, which are physiologically able to survive or
even flourish in well drained soil. Most of these species are, however, confined to
wetlands by their poor competitive abilities elsewhere, as adaptation to
waterlogging leads to strong specialisation and therefore a competitive
disadvantage against terrestrial species if the environment dries up.
This is illustrated by changes wrought
to the nutrient-poor Pinelands wetlands in the
coastal region of New Jersey, USA, parts of which have undergone drying and
nutrient enrichment as a consequence of nearby urban development. Changes to
water quantity and quality have little effect upon the overall physical appearance of
the habitat, nor on the relative abundance of the various life forms of plants: trees,
shrubs and herbaceous vegetation.
What does occur, however, is a change
in species composition, as a result of two
processes. Lowering the water table allows the soil to dry, removing the constraints
of toxicity and low oxygen concentrations which had previously impeded invasion by
terrestrial species, while nutrient enrichment allows them, once established, to grow
rapidly.
Wetland plants, adapted to a normally
nutrient-poor environment, are low in stature
and slow-growing, and in the presence of nutrients are outcompeted by the more
vigorous terrestrial species, which have taller stems, broader leaves and more
rapid growth rates. It has been found that the trend following urbanisation was
towards increased species richness, but that this was actually hiding a loss of
wetland species and their replacement with terrestrial plants, rather than a simple
mixing of the two sets of species assemblages.
Sphagnum mires
Sphagnum species growing in peat-forming
mires are unusual in that they can
control and enhance the water table to their own competitive advantage. Sphagnum
mosses lack roots, whereas most of their potential competitors are rooted vascular
plants, so Sphagnum strategies for reducing competition involve creating adverse
conditions below ground, to suppress root growth. Waterlogging clearly impedes
non-wetland species, but Sphagnum also increases the acidity of its environment
and reduces nutrient availability, its leaves intercepting atmospheric nutrients
before they can penetrate the surface layers, and through slow mineralisation as a
result of reduced decay rates. Finally, its poor heat-conducting properties ensure
that the below-ground parts of the bog remain cool.
Suppression of vascular plants increases
light availability and reduces water loss
through transpiration, both of which maintain optimum conditions for Sphagnum
itself.
Sphagnum shoot growth can exceed 10 cm
yr~' in pools, but is typically 1-5 cm yr"1
in hummocks. Vascular plants growing with Sphagnum in peat-forming systems
must, therefore, match this growth to avoid being smothered. The growth form of
sundew (Drosera rotundifolia), for example, comprises a vertical stem and a series
of leaf rosettes; stem growth keeps pace with that of Sphagnum, but at the expense
of successive leaf rosettes, which are abandoned within the Sphagnum moss as
they become smothered. By adopting strategies such as this, vascular plants can
maintain their presence in Sphagnum bogs. In turn, they have little detrimental effect
upon Sphagnum itself, although its growth will be suppressed by shading and
particularly by accumulation of above-ground litter. The competitive advantage held
by Sphagnum does, however, require maintenance of the high water table.
Drought conditions will benefit vascular
plants, which have effective root systems for
gaining water. Paradoxically, those with shallow roots, including Drosera, rely on
the water-conducting capacity of Sphagnum for their water supply, so will also suffer
under drought conditions.
A Sphagnum bog consists of raised hummocks,
above the water table and often
relatively dry, hollows which are permanently waterlogged or even flooded, and flat
lawns. Although up to 10 different species of Sphagnum may coexist in a single
mire, most are confined to a narrow vertical range and can be identified as
'hummock species' (e.g. S. rubellum and S. fuscum) and hollow species (e.g. S.
balticum and S. tenellum), with a range of species from both groups occupying
lawns. Within this division, there is often further, more subtle zonation, in which, for
example, S. fuscum may occupy the upper part of hummocks and S. rubellum the
lower part.
Hummock species take advantage of their
high capillarity, greater tolerance of low
nutrient levels and more stable dead tissue to grow above the open water level and
form hummocks, the upper limit for each species being set by its physiological
tolerance of an increasingly drier environment and reduced nutrient levels. The
determinants of the lower limits to each species are, however, much less clear;
transplantation experiments have demonstrated that it is not simply a case of hollow
species being more competitive, as hummock species, when transplanted to
hollows, have persisted and even thrived. How so many species can coexist when
several apparently occupy each niche without any apparent habitat differentiation is
also a mystery.
The hummock pool sequence is very effective
at reducing runoff and therefore
retaining the water table at a level advantageous to Sphagnum. The hummocks,
however, are more oxygenated and therefore suitable for vascular plant species.
So they support the growth of bog shrubs, such as heather (Calluna vulgaris] and
even trees. Ironically, the roots of vascular plants form a firm matrix which supports
the spongy Sphagnum; without this matrix, there is a danger that hummocks would
collapse. The Sphagnum, therefore, relies upon vascular plants to maintain the high
water table which gives it the competitive edge over these same plants.
