Eventually, global warming may get to the point where the present distribution ranges of species of plants and animals are left far behind the areas that they could potentially live in. For example, the potential range of sugar maple (Acer saccharum) might shift hundreds of kilometers to the north, way up into Canada towards the
Figure 3.13. Sugar maple extends from southeastern Canada to the south-central USA (a). By 2090, sugar maple may be able to grow way up to the southern Hudson Bay (lighter gray area on map). Meanwhile its natural range in the southern USA will tend to be lost as temperatures there become too warm for it (b). Source: Redrawn from IPCC.
edges of the Hudson Bay (Figure 3.13). When this situation of a much warmer climate arises, the new vegetation zones and communities won't just snap into place overnight. Seeds of plants will have to physically spread over the landscape, across hundreds of kilometers.
From what we can see of present-day plant distributions, many species will be left growing in temperatures that are too warm for them, at least in the lower-latitude parts of their distribution ranges. Sugar maple, for example, extends down to Tennessee and if the warming causes climates to shift north it may be left out-of-sorts in the south of its range. If the southern range limits of northern temperate species contract faster than they can expand at their northern limits, they may ultimately go extinct. It is unlikely that a widely distributed species such as sugar maple, which occurs across a very broad range of temperatures, will ever be completely driven extinct by global warming. However, there are many other species of plants that have a much more restricted geographical range, and presumably a much narrower climate tolerance range. These are the ones most likely to lose their original ranges when the climate changes. If they cannot migrate quickly enough, they will also be unable to reach any new potential range created by the warming—and that will be the end of them. One example is the southern Torreya (Torreya taxifolia), a coniferous tree which occurs in the wild in just a few small areas of northern Florida.
If we assume a very glum scenario in which nearly all the world's plant species are unable to move their ranges over the coming decades (for whatever reason), then extinctions are likely to begin. For example, Thuiller and colleagues (2005) took the current distributions of 1,350 wild plant species in Europe, and assumed each has a range that is limited by the combination of temperature and rainfall conditions in which it occurs. Then as a "what if" scenario, they used various different climate model predictions for Europe between around 2050 and 2080, with altered temperature and rainfall patterns. In all of these scenarios they found that if species don't manage to migrate, many of them will be left with hardly anywhere to live in a few decades' time. As many as a fifth of plant species in Europe were predicted to become critically endangered, having lost 80% or more of their previous range. Up to 2% (around 30) of Europe's plant species seemed likely to go extinct, having nowhere at all left to live. This is of course not a mass extinction for Europe's plants, but it does seem to set the scene for later disaster if temperature keeps on rising after 2080.
If species do manage to migrate in response to climate, there will be fewer extinctions—and the faster they migrate, the better they will be able to save themselves. But in European landscapes broken up by farming and towns, it is especially uncertain how fast these wild plants will be able to shift themselves around as the climate changes.
How long will it take for the plant communities of the greenhouse world to take shape? Will it be decades, centuries, millennia; or will species just never manage to shift themselves this far? And what will happen to the communities and vegetation types that exist in each place now, when climate warms. For example, will the trees in our forests just die, before others can spread north and replace them?
Questions such as these are very hard to answer, but there are some clues from the past, when warming events—of comparable speed and magnitude to that which we are anticipating over the next century—actually occurred (see above). Such events were associated with the immensely unstable climates of the last 2.4 million years. There were repeated sudden warming and cooling stages, apparently taking only decades in many cases. For instance, around 11,500 years ago at the end of the cold phase known as the Younger Dryas, a very sudden warming event around the North Atlantic was largely completed in 75 years. This sudden jump is comparable in size and speed with the projected "greenhouse effect'' warming over the next 100 years.
From the evidence of responses to past sudden climate changes, it looks like vegetation will remain out of equilibrium with climate for hundreds and indeed thousands of years following the onset of greenhouse effect warming. For example, in Britain, after a similar sudden warming event 14,500 years ago, vegetation remained out of balance with climate for hundreds of years (above). Trees seem to have been unable to spread north into Britain fast enough to exploit a warm climate which would have suited them, and the landscape remained covered in a sort of meadow vegetation. We might find similarly strange situations arising in a future greenhouse world over the next several centuries: vegetation types that no longer match the climate, without warm-climate plants having spread in from the south to take their place. When warmer-climate plants do eventually start to arrive, particular species may start to dominate out of all proportion, just as the hazel tree did in England after the warming event 11,500 years ago (above).
We can estimate how fast trees spread after past warming events, from the time delay between them turning up in the pollen record at each lake and then the next ones slightly farther north. For Europe, when trees spread north after sudden warming events, they moved at peak rates of between 0.02 and 2 km a year, depending on the species. In North America the rates of movement were rather slower, between 0.08 and 0.4 km per year. Going simply by these past figures for migration rates, it seems that the geographical ranges of a few species in Europe might almost be able to keep track within a moderate greenhouse warming scenario, where climate moves north at about 5 km a year. They would perhaps show a migration lag of a few decades. The future rate of warming may be expected to vary with latitude, according to climate model predictions, so the rate at which tree species' ranges will need to shift to keep step with climate warming will be greatest at the more northerly latitudes. If the reported migration rates from the past are representative of the northern coniferous and temperate zones of the world, it appears that at all latitudes most tree species would be left far behind, but might catch up on a timescale of centuries or millennia if the warming stabilized.
However, just relying on reported migration rates of trees in the pollen record is a very simplistic way of trying to forecast their future responses to climate change. For one thing, we can never quite be sure that each species arrived in northern Europe exactly when it shows up in the fossil pollen record. Some species might have arrived hundreds of years earlier as very isolated individual trees, and then spent a long time building up their numbers to the point where their pollen wqs abundant enough to get noticed in lake sediments. If these trees really did turn up in the north so much earlier than we find them, it would mean that their capacity to migrate fast in response to climate is even greater than we had thought. On the other hand, part of the reason so many trees turn up in northern Europe so fast after the end of the ice age may be that they actually had small, scattered ice age populations that survived quite far north into central Europe all the way through the cold of the ice ages (Stewart and Lister, 2007), instead of being confined to southern Europe. If trees were really already living so far north, it is no surprise they turn up quickly after the end of ice age because they did not have so far to move. So, the estimates on how fast trees can potentially migrate with global warming depend on having a rough idea when each species made it north and where it was coming from, and at present there is some uncertainty about this. Because of these, we should regard the potential future migration speeds of trees based on pollen evidence with some caution, and not make too many firm pronouncements about what this means for forests under greenhouse effect warming. It is difficult to figure out all the other factors that could have affected the rate of movement of trees. In some areas, such as Europe, there were no pre-existing forests in place before the warming, and this probably allowed trees to move faster than they would through the now-forested landscapes. However, nowadays humans often harvest trees from the woods, even clear-cutting whole swathes of forest. Such open areas may provide an ideal opportunity for migrating trees to establish themselves. On the other hand, the open spaces of fields or suburban land between forest patches could present a new barrier to migration.
In North America, there was already forest covering the eastern USA at the time of the sudden warming 11,500 years ago, but its species composition altered in response to the change in climate. The pollen record from lakes shows that many different tree species spread north, but it generally took between several hundred and several thousand years for them to reach their final limits under the new warmer climate (above). It is likely that, if left to themselves, forests in the mid-latitudes will take a similar period of time to adjust to greenhouse effect warming.
What about the cooler-climate species of trees that already made up these forests before the warming event? Did those trees already in place die in response to the change in climate? Reassuringly, there is relatively little evidence that the rapid warming event at the end of the last ice age in North America was associated with any sudden death of the forests. It seems that the trees already present at the time were tolerant enough of warmer temperatures to survive. Where they disappeared from the forests it seems to have been a gradual process over hundreds of years brought about by competition from other trees that moved in from the south, allowing them the chances to spread their ranges northwards. This makes it likely at least that the forests we see in the present world will not all die when the climate suddenly warms by several degrees. There is an exception to this in southern New England, where a study by Dorothy Peteet and colleagues shows that various cold climate conifers suddenly vanish from the pollen record at the end of the Younger Dryas. Their disappearance seems to take less than 30 years suggesting that these tree populations just died with the warming. This is an ominous sign for the fate of at least some of the world's forests under global warming.
We do not really know just how much warming will occur, especially if some of the feedbacks mentioned in the later chapters in this book start to kick in. Eventually, the temperature rise might start to exceed what is survivable by some species. The temperature increases are likely to be particularly drastic in the high latitudes, where various positive feedbacks (see Chapters 5 and 6) will tend to amplify the greenhouse warming. At least one major tree species in Siberia—the Siberian larch (Larix siberica)—seems unable to cope with mild winters regardless of competition from other species and will simply die in place. If it is grown in the mild climates of western Europe, Siberian larch thrives for about 25 years and then suddenly dies, apparently unable to defend itself against attack by fungi in its environment. Whether winters in Siberia will ever become as mild as they now are in western Europe is a moot point, but it does show that there are limits to what cold climate plants can tolerate, beyond which they will simply die. For all we know, other plants from the north might turn out to be even less tolerant of warmth than Siberian larch is.
3.7.1 Movement of biomes under greenhouse effect warming
The climate predictions of GCMs coupled to vegetation schemes provide some clues to what the final distribution of vegetation types in the greenhouse world might look like. Warming of several degrees C is enough to push the ranges of northern temperate trees hundreds of kilometers polewards beyond their present limits. At the same time, range limits in the south are likely to contract as well (although the picture from the last glacial suggests that this may be a slow process, dependent on other warmer-climate competitors moving northwards to out-compete them).
Movement of many different temperature-limited biomes outwards from the equator seems likely. Some areas that now have temperate climates with frosts are predicted to become tropical. For example, in a moderate warming scenario, by 2100 tropical rainforest is predicted to be the "right" vegetation type for southern Louisiana. However, even if they are expanding at the edges, the core areas of tropical rainforest that exist at present might start to suffer under global warming. A model study by Peter Cox and colleagues suggested that, as sea surface temperatures warm due to the greenhouse effect, the Amazon rainforest will experience severe droughts added to by a collapse of the rainwater recycling mechanism within the Amazon Basin (see Chapter 6). This collapse of water recycling is intensified by decreased transpiration from the leaves of forest trees under the higher C02 levels of the future (Chapter 8). There is also a widespread fear amongst ecologists that tropical forest trees might not be able to stand temperatures that are much warmer than at present, such that even a fairly slight warming in climate will cause them to die. Already in years of warmer-than-average temperatures, the world's tropical forests seem to grow less and take in less carbon: a sustained warming could be enough to push them over the edge. Generally, though, we do not know much about the upper heat tolerance limits of tropical rainforest, so it is hard to say exactly what will happen. As if to prove this point, a year after Cox et al.'s study was published, the Amazon region suffered an unprecedented drought associated with a sudden warming in the equatorial Atlantic.
In the mid-latitudes of the USA, one study using various climate model scenarios by Bachelet and colleagues suggested that with a certain moderate amount of warming there will be a net increase in forest, spreading out over desert and grassland areas as a result of increased rainfall. But they also suggested that if the temperature keeps on increasing above a certain limit, the climate will get drier overall and forest will retreat. Something that complicates many of these modeled future scenarios is that they also include a direct effect of increased C02 on the physiology of plants. As we will explore in Chapter 8 of this book, the influence of higher C02 on the growth and water balance of plants is a big uncertainty that adds to the difficulty of forecasting effects from climate change alone.
The greatest shifts in vegetation are predicted to be seen in the high latitudes where warming is predicted to be strongest, and where the most dramatic warming is in fact already under way.
Changes in the amount of rainfall and snow, and in the precipitation/evaporation balance, are seen as being more difficult to predict than temperature. Different GCMs come up with very different conclusions for the amount and distribution of change in rainfall based on only slight differences in their assumptions. 0verall, it looks like the changes in moisture balance will not be dramatic over the next century as the global climate warms, with perhaps more rainfall giving slightly less arid vegetation overall across the greenhouse world.
The way in which biome-based models divide up the world tends to give the impression that the only changes which occur during warming are at the boundaries between biomes. However, there are major differences in species composition and physical form of vegetation within each biome, and it is important to remember that we can expect changes in these just as much as at the boundaries.
All the biome-predicting vegetation schemes we have considered here so far are "static": they simply state what vegetation types will be in balance with a changed climate. They do not tackle the problem of how long it will take for the new vegetation to arrive in a new place and then grow to maturity. We know from the history of past change that vegetation can remain out of balance with climate for hundreds or even thousands of years. To get a better idea of the time course of changes in vegetation as the earth warms over the next century, ecologists have come up with dynamic vegetation schemes, which gradually "grow" new vegetation suited to the changed climate produced by a GCM. Dynamic schemes do not just assume that forest can spring up in grassland or desert areas fully grown overnight; they recognize that it will take decades to mature from seedlings. Examples of such schemes are the MAPSS scheme, and the DOLY scheme.
Although they are likely to give a more realistic simulation of the time course of events as climate warms, these dynamic schemes do not simulate the complex processes of migration of species which will be necessary in order to alter biome distributions. They simply assume that the vegetation of the future is already in place as seedlings, waiting to grow when the climate changes. Yet, as we know from the aftermath of sudden warming events in the earth's recent history, the time taken for migration can cause a major delay in the adjustment of vegetation to climate.
In the modern world, the process of migration could take even longer than it did after ice ages. The distributions of many plant species are broken up by agricultural landscapes, making it hard for them to move across sterile fields that are regularly ploughed and sprayed with herbicides. For instance, in western Europe many types of plants that normally only live in forest would somehow have to hop between isolated woods that may be kilometers apart from one another. The problems of migration may be particularly great for species of European and North American wild flowers known as "ancient woodland species'', because they only seem to be found in very old, established fragments of forest, seeming unable to colonize young forest. One example is the bluebell, Hyacinthoides non-scripta (Figure 3.14*), which forms a beautiful blue carpet in English and Welsh woodlands in the spring. Others include the various types of Trillium of the forests of eastern North America. It is not clear whether such species would ever be able to migrate in response to climate change under present circumstances, given the extra handicap that they suffer due to their restrictive requirements.
However, it is possible that humans can come to the rescue, helping many wild plants to overcome what is in the first place a human-made problem. In the northern mid-latitudes, which are so intensively farmed, deliberate planting of species north of their previous range could allow them to exploit the warmer climate, and make up for loss of range at their southern boundaries. It may require a concerted mass movement of volunteers to plant young trees and flowers farther north in their new potential climate range. However, it is also important to remember that many species of trees and shrubs are already planted well outside their natural ranges in parks, gardens and
Figure 3.14. A bluebell (Hyacinthoides non-scripta) woodland in spring, southwestern England. Being a flower confined to ancient forests, the bluebell is unlikely to migrate easily across human-influenced landscapes as the climate changes. Credit: Keith Hulbert.
forest plantations. Beyond their ranges they may exist as poorly-performing and poorly-reproducing individuals, unable to compete with the wild species around them in the current climate. Yet, as climate warms they may come into their own and form a natural part of the vegetation. In effect, part of the flora of the future greenhouse world may already be in place, waiting for the warming to happen.
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