Across nearly half of the world's land surface, the original vegetation is forest. In some places it is easy to tell that forest is the natural vegetation because the landscape is still covered by it. For example, the eastern USA and southeastern Canada are at present mostly forested. Even areas in eastern North America that arc kept as farmland will go back to forest if left for several decades. Within just a few years an abandoned field will become covered with tree seedlings that have dispersed in from patchcs of forest nearby. In other parts of the world, it is not always so easy to tell what the natural vegetation would be like. In northwestern Europe, eastern China and Bangladesh, for example, a high human population density and intensive agriculture have removed almost all the natural forest cover in many areas, particularly in the fertile, flat lowlands that arc most desirable for farming. In parts of northern Europe and eastern China there is barely a patch of forest in sight, except perhaps a few scattered plantations of poplars all planted in rows. Nevertheless, where the landscape is hilly and difficult to farm, forests arc abundant even in these regions, indicating what the lowlands would probably look like if they were not now under cultivation. And we have independent corroboration from the pollen record dug up in the mud of lakes and back-swamps; this shows, for example, that up until a few thousand years ago all the most densely inhabited areas of Europe and China were densely forested.
However, there are many other parts of the world that arc presently almost treeless and could not support forest, even before human interference. This is because the climate is simply unsuitable for a dense tree cover. To have forest one needs two things: (1) enough warmth and (2) enough water. Below a certain threshold of temperature or water availability, the only plants that can survive are low shrubs or herbaceous plants.
So trees need more warmth and moisture than other growth forms of plants such as low shrubs and grasses. In places with a mean summer temperature less than about 7 or 8 C, trees cannot grow in the wild, although they can sometimes survive if they are pampered by humans and protected from competition with other smaller plants. Cool summers prevent trees from growing beyond a limit known as the "tree line" in the Arctic of Siberia or Canada, or in high mountains around the world. Even if a place has quite mild winters but very cool summers below 8°C it will not have any wild trees, proving the point that it is summer temperature not winter cold that prevents trees from growing. One example is the Faeroe Islands in the north Atlantic, with their cloudy cool oceanic climate.
Why is it that trees do badly in cooler summer conditions, when many small shrubs and herbaceous plants can do just fine? The problem for trees is that they are in a sense relatively inefficient at growing. They put aside a lot of their energy into woody tissues. They arc essentially in for the long haul, to overtop competitors and then reproduce. In many climates, this strategy pays off* and trees dominate the landscape. But if conditions are always fairly cool, trees cannot photosynthesize and metabolize fast enough to sustain basic living processes and also put aside materials into wood. Under such circumstances, their ambitious strategy is rather like trying to pay a mortgage while on a student income! The result of these compct-
ing demands is that the trees will simply fail to grow, or they grow so slowly (because they are trying to lay down wood) that the smaller faster-growing shrubs and herbs they arc competing with kill them oil'. This is why tundra (low shrubs and grasses) extends into colder summer temperature zones than the most cold-tolerant boreal (high-latitude) forests. Another likely reason that trees are replaced by shrubs in cold climates is that shrubs are better at "holding in" the heat from the sun on a cold day. A tree has a loose growth pattern that allows the wind to blow through it and carry heat away: whereas the dense mass of branches of a shrub holds in heat asainst the wind (more on this in Chapter 4 on microclimates).
The tree growth form drops out sooner along a temperature gradient in certain specialized habitats. In the tropics, in places where mud brought down by rivers accumulates along a shoreline, there arc usually mangroves—trees from various evolutionary lineages that are adapted to grow in the salty mud. They need special salt-excreting glands to prevent salt building up in their tissues, and prop roots that prevent the trees from falling over in the soft mud. Yet, whereas mangroves are almost ubiquitous on muddy shores in the tropics (at least when not cleared away by humans), they die out in the subtropics. At mean annual temperatures cooler than about 20 C, there arc no mangrove trees and their place is taken by low shrubs, reeds and small herbaceous plants—a vegetation known as salt marsh. Why are there no mangroves in cooler climates, when trees can grow easily on land and further inland in freshwater swamps? Most likely this temperature limit has something to do with the need for a mangrove tree to continuously adjust its anchorage to cope with wave action and the erosion and shifting of mud beneath its roots. In climates that have cool climates, tree growth stops in winter but the wave action and movement of mud does not. Thus, any tree that tried to grow as a mangrove in a climate with a winter would lose its footing and be washed away. In the marine shoreline habitat, the temperature limits on the tree growth form are different because of the peculiar demands of this environment.
So we can understand why trees need more warmth, but why do they need more moisture, only existing in the moistest climates? This relates to the fact that trees are large—their strategy in life being to overtop and out-compete other plants. They need more water than shrubs and grasses because they have a lot of evaporative leaf area that is essentially placed on top of a single pole, the trunk. But plants are limited in how much water their roots can take from the soil underneath them where they are growing, and the bigger the top parts of a plant, the more likely it is to run out of water. So in an arid environment although the tree may start to grow during a rainy spell, it eventually dies when its water supply runs out during a drought period. A low-shrubby plant has fewer leaves that evaporate water, relative to the size of its root system and relative to the patch of soil it is rooted into. Thus it makes less water demand on its own area of soil, and it is less likely to die of drought. The result is that along a line of decreasing annual rainfall, forest disappears way before scrub and grasslands do.
So in climate regions beyond the drier and colder limits of forest and woodland biomcs, the problem for trees is that they are in a sense "too ambitious" in trying to grow big and overtop everything else, even though this pays oil* for them in climates which arc warm and moist enough.
If fires or grazing are frequent, this can kill off the trees too. Trees cannot easily survive having their expensive top parts bitten or burnt off; they are most susceptible when they are seedlings or saplings—replacing old trees that have died off. or trying to establish in open habitat. Low shrubs can easily re-grow their relatively flimsy inexpensive branches. In contrast, the trees just die as a result of the damage, or become so crippled that they are out-competed by the shrubs and grasses. In a climate that is relatively dry, fires are more frequent and trees also grow more slowly and recover from damage with more difficulty. So climate and other factors can work in parallel against trees.
The advantage usually turns from trees to low shrubs and herbaceous plants quite gradually along a climate gradient. The trees may get sparser and smaller, until there are just isolated individuals, and then none at all. Even where trees are mostly rare in the landscape, often there will be small stands of them here and there where there is a pocket of favorable conditions; for example, a little sunlit cliff in a cold climate, or in a dry climate where a spring emerges, or perhaps where there is a pocket of deep moist soil.
However, sometimes there is a very sharp transition from forested or wooded vegetation to "open country" such as grass or scrub. This may occur at the edge of a
Figure 2.13. Treeline on a mountain. Source: Ciianluca Piovesan frequent grass lire zone, for instance: dead grass standing during the drier part of the year promotes the spread of fires set by lightening. The grass can tolerate fire becausc it just grows back from underground shoots after a lire, but any young trees establishing amongst the grass arc usually killed. The grass fire will burn right up to the edge of the forest, beyond which the lack of fuel and the moist cool conditions prevent the fire from spreading into the forest. So the landscape will either be "all grass"—where any trees are killed by fire—or "all trees" where the grass is shaded out by the dense closed canopy, with no gradation between the two. Such sudden transitions are often seen in savanna zones in South America and Africa, where islands of forest arc surrounded by a grassy landscape, and one can step straight from dark moist forest to the dry heat of open grassland in a matter of yards. Forest islands also used to be seen at the edges of the prairie zone in North America, before settlers ploughed up the entire landscape for agriculture.
Another place where the transition from tree cover to grassland can be very rapid is on mountains. Often there is a well-defined "tree line", above which no trees grow. The transition zone from forest, where trees become smaller and sparser until there arc none at all, can be as little as 30 metres. Such a sharp boundary is in part possible on a mountain becausc there is a far more rapid decline in temperature with altitude than there would be when traveling towards higher latitudes (Figure 2.13*). In the high latitudes, the "tree line" is much more gradual with the trees becoming smaller, sparser and more patchy over many tens of kilometers. Adding to the suddenness of the disappearance of trees along a temperature gradient is that their ability to "hold in" the warm air they need collapses as the canopy begins to thin (see Chapter 4).
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