5.8.3. Impacts on Wetland Services
Generally, climatic warming is expected to start a drying trend in wetland
ecosystems. According to Gorham (1991), this largely indirect influence of climate
change, leading to alteration in the water level, would be the main agent in
ecosystem change and would overshadow the impacts of rising temperature and
longer growing seasons in boreal and subarctic peatlands. Monsoonal areas are
more likely to be affected by more intense rain events over shorter rainy seasons,
exacerbating flooding and erosion in catchments and the wetlands themselves.
Similarly, longer dry seasons could alter fire regimes and loss of organic matter
to the atmosphere (Hogenbirk and Wein, 1991).
220.127.116.11. Habitat and Biodiversity
Climate change may be expected to have clear impacts on wetland ecosystems,
but there are only a few studies available for assessing this. There are some
laboratory studies concerned with the responses of individual plant species
or groups of species (Jauhiainen et al., 1997, 1998a,b; van der Heijden et al.,
1998). Based on these studies, however, it is difficult to predict the responses
of plant communities formed by species with somewhat varying environmental requirements.
The response of wetland plant communities to drought has received some attention
in temperate freshwater wetlands (Greening and Gerritsen, 1987; Streng et al.,
1989). Stratigraphical studies in peatlands have shown hydroseral succession
whereby swamp and fen communities gradually develop into bog communities (Tallis,
1983). These changes are largely autogenic, connected to growth of wetland communities
and caused by past climatic variability or artificial drainage. An alternative
approach to observe the vegetation-environmental change succession has been
to use space as a time substitute by mapping different plant communities onto
climatic and hydrological surfaces (Gignac et al., 1991). This approach has
shown tight coupling between various peatland plants, climate, hydrology, and
resultant chemistryand even for trace gas exchange (Bubier, 1995)and
has been used to infer certain aspects of peatland development through macrofossil
analysis (Gorham and Janssens, 1992; Kuhry et al., 1993).
Much is known about how vegetation changes as a result of water-level drawdown
following drainage for forestry in northwestern Europe. Drying of surface soil
initiates a secondary succession whereby original wetland species gradually
are replaced by species that are typical of forests and heathlands (Laine and
Vanha-Majamaa, 1992; Vasander et al., 1993, 1997; Laine et al., 1995). Plants
living on wet surfaces are the first to disappear, whereas hummock-dwelling
species may benefit from drying of surface soil. In nutrient-poor peatlands,
bog dwarf-shrubs dominate after water-level drawdown; at more nutrient-rich
sites, species composition develops toward upland forest vegetation (Laine and
Vanha-Majamaa, 1992; Minkkinen et al., 1999).
The effect of sea-level rise on wetlands has been addressed in several assessments.
In northern Australia, extensive seasonally inundated freshwater swamps and
floodplains are major biodiversity foci (Finlayson et al., 1988). They extend
for approximately 100 km or more along many rivers but could be all but displaced
if predicted sea-level rises of 10-30 cm by 2030 occur and are associated
with changes in rainfall in the catchment and tidal/storm surges (Bayliss et
al., 1997; Eliot et al., 1999). Expected changes have been demonstrated by using
information collected from the World Heritage-listed Kakadu National Park, but
the scenario of massive displacement of these freshwater wetlands can be extended
further afield given similarities in low relief, monsoonal rainfall, and geomorphic
processes (Finlayson and Woodroffe, 1996; Eliot et al., 1999). In fact, the
potential outcome of such change can be seen in the nearby Mary River system,
where saline intrusion, presumably caused by other anthropogenic events, already
has destroyed 17,000 ha of freshwater woodland and sedge/grassland (Woodroffe
and Mulrennan, 1993; Jonauskas, 1996).
Mechanisms by which environmental factors and biotic interactions control wetland
biodiversity are not well understood (Gorham, 1994b). The effects of water-level
drawdown after drainage for forestry indicate that the shift in species composition
from bog and fen species to forest species only slightly affects plant species
richness of individual sites (Laine et al., 1995). In regions dominated by forests,
there would be clear reduction in regional diversity as landscapes become homogenized
after drainage (Vasander et al., 1997).
The inherent changeability of wetland communities, resulting from spatial and
temporal variability in water supply (Tallis, 1983), may be the key factor in
the response of wetland communities to climate change. Because there may be
differences between species in adaptation potential, community structures would
change, and there would be profound effects on the nature of the affected wetlands,
as discussed by Gorham (1994a). The response of wetland plant communities to
changing environment may have fundamental effects on the species diversity of
Because of spatial and temporal variability in ecosystem processes, development
of systems models for wetlands is becoming an important assessment tool. A fully
coupled peatland-climate model has not yet been developed, but there have been
some significant advances in modeling various components of the peatland and/or
wetland biogeochemical system (Harris and Frolking, 1992; Roulet et al., 1992;
Christensen and Cox, 1995; Christensen et al., 1996; Walter et al., 1996; Granberg,
1998), and several process-level models are now used at the global scale (Cao
et al., 1996; Potter et al., 1996; Potter, 1997).
Elevated CO2 levels will increase photosynthetic rates in some types
of vegetation (e.g., C3 trees and emergent macrophytes) (Bazzaz et
al., 1990; Idso and Kimball, 1993; Drake et al., 1996; Megonigal and Schlesinger,
1997; see also Section 18.104.22.168). Responses of nonvascular
vegetation, such as sphagna, have been less clear (Jauhiainen et al., 1994,
1997, 1998a; Jauhiainen and Silvola, 1999). A study in Alaskan tussock tundra
found that photosynthetic rates in Eriophorum vaginatum quickly adjusted downward
such that rates with elevated and ambient CO2 were similar after
1 year (Tissue and Oechel, 1987). However, a sustained increase in net ecosystem
carbon sequestration was observed when elevated CO2 treatments were
combined with a 4°C increase in temperature (Oechel et al., 1994).
Many C3 plants respond to elevated CO2 with a decrease
in stomatal conductance (Curtis, 1996), which could reduce transpiration rates.
Because transpiration is an important pathway for water loss from many ecosystems
(Schlesinger, 1997), including wetlands (Richardson and McCarthy, 1994), reductions
in transpiration rate could affect the position of the aerobic-anaerobic interface
in wetland soils (Megonigal and Schlesinger, 1997).