The impact on ecosystems

A little over 10% of the world's land area is under cultivation - that was the area addressed in the last section. The rest is to a greater or lesser extent unman-aged by humans. In Figure 7.13 are illustrated the world's major ecosystems (or biomes) with their global areal extent showing how they have been transformed by land use.

Ecosystems are of great importance to human communities. They provide supplies for human communities in the provision of food, water, fuel, wood and biodiversity. They also provide important regulation especially for components of the hydrological cycle. Further they possess a wide range of important cultural value. All these together are commonly called ecosystem services.

The variety of plants and animals that constitute a local ecosystem is sensitive to the climate, the type of soil and the availability of water. Ecologists divide the world into regions characterised by their distinctive vegetation. This is well illustrated by information about the distribution of vegetation over the world during past climates (e.g. for the part of North America shown in Figure 7.14), which indicates the ecosystems most likely to flourish under different climatic regimes. Changes in climate alter the suitability of a region for different species (Figure 7.15), and change their competitiveness within an ecosystem, so that even relatively small changes in climate will lead, over time, to large changes in the composition of an ecosystem.

However, changes of the kind illustrated in Figure 7.14 took place over thousands of years. With global warming similar changes in climate occur over a few decades. Most ecosystems cannot respond or migrate that fast. Fossil records indicate that the maximum rate at which most plant species have migrated in the past is less than 1 km per year. Known constraints imposed by the dispersal process (e.g. the mean period between germination and the production of seeds and the mean distance that an individual seed can travel) suggest that, without human intervention, many species would not be able to keep up with the rate of movement of their preferred climate niche projected for the twenty-first century, even if there were no barriers to their movement imposed by land use.46 Natural ecosystems will therefore become increasingly unmatched to their environment. How much this matters will vary from species to species: some are more vulnerable to changes in average climate or climate extremes than others. But all will become more prone to disease and attack by pests. Any positive effect from added 'fertilisation' due to increased carbon dioxide is likely to be more than outweighed by negative effects from other factors.

Atmosphere ~2000 Atmosphere P-IND Atmosphere LGM

m oK

u tfl

00 0

Figure 7.13 The world's major ecosystems with their global areal extent (lower panel), transformed by land use in yellow, untransformed in purple and total carbon stores (upper panel) in plant biomass (green), soil (brown) and yedoma/permafrost (light blue). D, deserts; G & S(tr), tropical grasslands and savannas; G(te), temperate grasslands; ME, Mediterranean ecosystems; F(tr), tropical forests; F(te), temperate forests; F(b), boreal forests; T, tundra; FW, freshwater lakes and wetlands; C, croplands; O, oceans. Approximate carbon content of the atmosphere is indicated for last glacial maximum (LGM), pre-industrial (P-IND) and current (about 2000).

Forests cover about 30% of the world's land area and are among the most productive of terrestrial ecosystems. They represent a large store of carbon. Of terrestrial carbon, 80% of above-ground and 40% of below-ground is in forests, together storing about twice as much as is in the atmosphere (Figure 3.1). They are particularly important in the context of climate change. Current levels of deforestation are responsible for around 20% of the additional carbon dioxide emitted to the atmosphere each year due to human activities. What effect will climate change have on the world's forests and how in turn might that effect the climate?

Trees are long-lived and take a long time to reproduce, so cannot respond quickly to climate change. Further, many trees are surprisingly sensitive to the average climate in which they develop. The environmental conditions (e.g. temperature and precipitation) under which a species can exist and reproduce are known as its niche. Climate niches for some typical tree species are illustrated in Figure 7.16; under some conditions a change as small as 1 °C in annual average temperature can make a substantial difference to a tree's productivity. For the likely changes in climate in the twenty-first century, a substantial proportion of existing trees will be subject to unsuitable climate conditions. This will be particularly the case in the boreal forests of the northern hemisphere where, as trees become less healthy, they will be more prone

] Tundra

] Tundra

Figure 7.14 Vegetation maps of the southeastern United States during past climate regimes: (a) for 18 000 years ago at the maximum extent of the last ice age, (b) for 10 000 years ago, (c) for 5000 years ago when conditions were similar to present. A vegetation map for 200 years ago is similar to that in (c).

Figure 7.14 Vegetation maps of the southeastern United States during past climate regimes: (a) for 18 000 years ago at the maximum extent of the last ice age, (b) for 10 000 years ago, (c) for 5000 years ago when conditions were similar to present. A vegetation map for 200 years ago is similar to that in (c).

Conifer forest showing trees damaged by 'acid rain'. An important factor that will influence the future concentrations of sulphate particles is 'acid rain' pollution, caused mainly by the sulphur dioxide emissions. This leads to the degradation of forests and fish stocks in lakes, especially in regions downwind of major industrial areas.

Conifer forest showing trees damaged by 'acid rain'. An important factor that will influence the future concentrations of sulphate particles is 'acid rain' pollution, caused mainly by the sulphur dioxide emissions. This leads to the degradation of forests and fish stocks in lakes, especially in regions downwind of major industrial areas.

to pests, dieback and forest fires. One estimate projects that, under a doubled carbon dioxide scenario, up to 65% of the current boreal forested area could be affected.47

A decline in the health of many forests in recent years has received considerable attention, especially in Europe and North America where much of it has been attributed to acid rain and other pollution originating from heavy industry, power stations and motor cars. Not all damage to trees, however, is thought to have this origin. Studies in several regions of Canada, for instance, indicate that the dieback of trees there is related to changes in climatic conditions, especially to successions of warmer winters and drier summers.48 In some cases it may be the double effect of pollution and climate stress causing the problem;

Figure 7.15 The pattern of world biome types related to mean annual temperature and precipitation. Other factors, especially the seasonal variations of these quantities, affect the detailed distribution patterns (after Gates).

Figure 7.15 The pattern of world biome types related to mean annual temperature and precipitation. Other factors, especially the seasonal variations of these quantities, affect the detailed distribution patterns (after Gates).

Mean annual precipitation (cm)

Mean annual precipitation (cm)

trees already weakened by the effects of pollution fail to cope with climate stress when it comes. Stresses on the world's forests due to climate change (see box on page 208) will be concurrent with other problems associated with forests, in particular those of continuing tropical deforestation and of increasing demand for wood and wood products resulting from rapidly increasing populations especially in developing countries.

If a stable climate is eventually re-established, given adequate time (which could be centuries), different trees will be able to find again at some location their particular climatic niche. It is during the period of rapid change that most trees will find themselves unsatisfactorily located from the climate point of view. If, because of the rate of climate change, substantial stress and die-back occur in boreal and tropical forests (see box below) a release of carbon will occur. This positive feedback was mentioned in Chapter 3 (see the box on page 48-9). Just how large this will be is uncertain but estimates as high as 240 Gt over the twenty-first century for the above-ground component alone have been quoted.50

0 0

Post a comment