Wetlands support some of the highest and
some of the lowest productivity values
recorded. At one extreme is reedswamp, whose productivity can be matched only
by tropical rainforests and the most intensively managed agricultural systems, while
at the other are ombrotrophic mires, some of whose productivities are on a par with
those of tundra or desert ecosystems.
The most productive wetlands are fringe
wetlands, bathed in a continually
replenished source of nutrients to replace those scavenged by bacteria or
incorporated into sediment. Single-species stands, such as reedswamp or papyrus
swamp, are dominated by species which, despite being perennials with the
capacity for year-round growth, are not woody, so that little energy is expended on
growing or maintaining non-photosynthetic structural features. Such wetland
dominants are essentially giant leaves. Furthermore, living in an environment where
water is plentiful, they have no need to limit transpiration rates. Seasonally
inundated flood wetlands can also be highly productive, the flood period bringing in
sediments and nutrients while the dry season allows aerobic decomposition of
detritus and consequent nutrient release.
In the floodplains along the Amazon, the
grass Echinochloa polystachya, which
grows very rapidly and can achieve productivities approaching lOOOOg m2,
absorbs large quantities of nutrients from the river. Dry-season decomposition of
this species, which grows at the lowest points on the floodplain, releases nutrients
which are carried by rising floodwaters to higher levels so that, while one
generation is absorbing nutrients from the river, the decomposition products of the
previous generation ensure that flood waters which penetrate further into the
floodplain remain nutrient-enriched.
The least productive wetlands - ombrotrophic
mires - are also dominated by non-
woody species, but are limited by permanently low nutrient availability. Despite this
limitation, they can have surprisingly high productivity values: a typical value being
800 g m~2yr~1for Sphagnum growing in pools in British bogs, somewhat higher
than the productivity of temperate grassland, although productivity on hummocks is
typically less than a quarter of this value.
High net productivity can be maintained
indefinitely in fringe wetlands if
decomposition rates or removal of detritus by the parent water body match
production, suppressing accumulation of organic matter. Blanket and raised mires
accumulate organic matter, as a result of extremely low rates of decomposition in
the cool temperate environments in which they usually form, but by raising the water
table as they develop, continued domination by Sphagnum is assured. In other
wetland environments, such as rheotrophic fens, accumulation of organic matter
can lead to relative lowering of the water table and an increased proportion of
species adapted to drier environments. Flood wetlands, too, experience reduced
water stress during the dry season, allowing more terrestrially adapted species to
persist. In such situations, total biomass may be higher than in fringe wetlands, but
net productivity is lower.
