The Regional Impacts of Climate Change
5.3. Key Impacts
5.3.1. Ecological Systems
Europe has few genuine natural ecosystems. Natural ecosystems generally are
confined to small areas; agriculture and forestry occupy most soils. Many sites
with intermediate soils are occupied by seminatural vegetation types.
Vegetation responds to climate change directly and indirectly. Direct effects
include responses to temperature; indirect effects occur primarily as soil-mediated
phenomena, such as the influence of precipitation on soil moisture regimes.
Indirect effects also may occur as a result of the responses of herbivores and
pests to climate changes, changes in soil fauna, and changes in the frequency
and severity of disturbances such as fire. In addition, vegetation responds
directly and indirectly to atmospheric CO2 concentrations. The responses are
species-dependent; no two taxa respond to climatic change in exactly the same
way (e.g., Stirling et al., 1997). The main response of individual taxa to climatic
change consists of changes in distribution; adaptive evolutionary changes are
very rare (Huntley, 1991). At the ecosystem level, the impact of CO2 on ecosystem
processes remains very uncertain. Zaller and Arnone (1997) have documented an
increase in surface-casting earthworm activity in Swiss grasslands exposed to
610 µl CO2/l, but such studies are extremely rare.
In a warmer climate, the pattern of species response will be extremely complex
within Europe (Grime, 1996) because a variety of temperature and moisture gradients
exist. Although there is a general temperature gradient from south to north
and from low to high altitudes, east-west gradients in temperature and precipitation
also exist; the latter are associated with increasing continentality toward
central Europe. Further complications arise in the prediction of species responses
to temperature changes because of the importance of the nature of the change.
For example, a rise in late-summer temperatures will have different impacts
than a rise in early-spring temperatures or an average rise in temperatures
spread evenly throughout the year (Fitter et al., 1995). Similarly, the occurrence
of late frost may play an important role in restricting the responsiveness of
small-genome species to mean temperature changes (MacGillivray and Grime, 1995).
This factor makes the prediction of species responses difficult; a general northward
shift in species distributions is now recognized as too simplistic a hypothesis.
Successful migration depends on a number of factors—in particular, the range
of tolerance of a given plant or tree to heat and moisture stress, environmental
conditions at the new location, the rate of migration, the presence of competing
species, and natural and human barriers to migration (Thompson, 1994; Malanson
and Cairns, 1997). Anthropogenic barriers are especially important for large
portions of western and central Europe, where land use is dominated by direct
Short-term experiments suggest that many types of plants will respond positively
to increases in CO2 concentrations in the atmosphere (the so-called CO2 fertilization
effect), whereby their photosynthetic rates increase if other factors remain
constant; this is particularly the case for C3 plant types and all important
European crops except maize (Semenov et al., 1996; Wolf et al., 1996). Most
experiments have been undertaken using isolated plants with optimum nutrient
supply. Such experimental conditions are relevant to horticultural and agro-industrial
situations but are inapplicable to natural and seminatural plant communities
(Körner, 1996). A wide range of vegetation types may show little or no response
to increasing CO2 concentrations under field conditions (Körner, 1996). The
responses, however, will be species-specific, especially when other factors
such as enhanced nitrogen deposition are taken into account (Hättenschwiler
and Körner, 1996a). The net primary productivity of other plants may increase
(provided that they are not water limited), but there are many uncertainties
regarding the long-term responses of plants to increased CO2; studies around
natural sources of CO2 have not revealed any gradients in the growth rates or
biomass of Mediterranean grassland species (Körner and Miglietta, 1994). Studies
of trees growing around natural sources of CO2 revealed no changes in stomatal
density, but the guard cells were reduced in size (Miglietta and Raschi, 1993).
Downy oak (Quercus pubescens Willd.) growing close to a CO2 source had lower
stomatal conductance than those further away, but Holm oak (Quercus ilex L.)
showed no such trend (Tognetti et al., 1996). In both species, the osmotic potential
and apoplasmic fraction of water was elevated close to the CO2, indicating that
these trees were more tolerant of drought conditions (Chaves et al., 1995; Tognetti
et al., 1996).
There is increasing evidence that traits other than photosynthetic metabolism
are more important in determining the response to elevated CO2 of different
species under field conditions (e.g., Körner, 1993; Körner et al., 1995; Diaz,
1995, 1996). For example, increased levels of CO2 are likely to result in increased
water-use efficiency in many species. Increased water-use efficiency may help
many plants and trees resist the extremes of heat and drought that may occur
more frequently in southern Europe and the Mediterranean region.
Studies of model ecosystems exposed to enhanced CO2 and nitrogen deposition
suggest the presence of nonlinear system-level adjustments (Hättenschwiler and
Körner, 1996b). These adjustments include physiological down-regulation of photosynthesis
at the leaf level, reduced leaf area index, and increasing strength of below-ground
carbon sinks. At the same time, no aboveground growth stimulation was observed.
Changes were observed between CO2 concentrations of 280 and 420 µl/l; major
changes in coniferous forest ecosystems may be underway already in response
to increasing CO2 concentrations.
The impact of climate change on biodiversity and the composition of ecosystems
in Europe is extremely difficult to predict. A great deal depends on the impacts
on ecosystem processes, such as the rates and magnitudes of disturbance. The
resilience of many ecosystems to change also is very uncertain; many Norway
spruce forests, for example, are likely to persist for several hundred years
in the absence of any major disturbance (Sykes and Prentice, 1996). This inertia,
along with the possibility of species acclimation to changed environmental conditions
(Kellomäki and Wang, 1996), may delay the onset of many changes in natural ecosystems
(Woodward, 1993). Models that currently are available for predicting such changes
generally are restricted to the most important components of vegetation (e.g.,
trees in forests); little research has been done on possible future interactions
between these and other ecosystem components under changed climatic and CO2
conditions. The models suggest that in some cases, species arrivals may compensate
for species losses, whereas in others, human-induced changes in land use may
delay the arrival of new species, causing a reduction in diversity. Consequently,
there is a need to combine models that simulate changes in species distributions
with models developed to look at species turnover at specific sites (compare
van der Maarel and Sykes, 1993; Økland, 1995a,b; Fröborg and Eriksson, 1997).