In some areas forests keep their leaf cover all year round. In others they drop their leaves part of the year and grow a new set after a few weeks or months. So one finds "temperate deciduous forests" in the northern temperate zone, but "temperate evergreen forests" in eastern Australia, southern China, New Zealand and parts of Chile (Figure 2.1). In some parts of the tropics, mainly near the equator, the forests are evergreen. In other places—mainly the outer tropics—the forests are deciduous. The evolutionary "decision" as so whether the leaves should stay on all year round depends on the energy, nutrient and water economy of the trees. Under some circumstances, there is no benefit to the tree in hanging on to leaves if they are going to be a burden during hard times. It is best to get rid of them and grow a new set when favorable conditions return. In other cases, dropping the leaves would waste an opportunity for photosynthesis, so they are retained all through the year.
In the moistest forests close to the equator, the climate is warm and there is plenty of rainfall all year round. In this environment there is no reason for the trees to drop their leaves at any particular time of year, so the forest stays green year round. The broad laurel-like leaves are held on the trees for several years, before they reach the end of their useful lifespan and are shed.
However, in parts of the tropics where there is a regular dry season (e.g. Thailand), holding onto leaves during the dry months presents a risk of killing the tree by drought. All leaves lose some water, even if the tiny stomatal pores—see Chapter 4— in the leaf are kept shut: the only way to ensure that water loss is eliminated is to shed the leaves. Also if trees keep their leaves during the dry season, they risk losing nutrients unnecessarily through general wear and tear of the leaves, plus herbivores chewing away leaf tissue. This is at a time of year when the stomata must be kept shut so there is no photosynthesis and no benefit to the tree from having the leaves present. Under these conditions the trees will do best by re-absorbing nutrients and dropping leaves for the dry season. They then grow a brand new set of leaves which will photosynthesize rapidly when the wet season returns. So, the reason that forests are deciduous in the monsoonal outer tropics is that this is the best solution to an environmental challenge. To anyone from the northern mid-latitudes, it is a strange experience to walk through the dry deciduous forest with trees bare and leaf litter crunching under foot, looking just like a temperate forest in winter. Yet instead of frost and snow, the air will be roasting hot—often up around 40° C.
Just beyond the reach of the outer dry seasonal tropics, evergreen forests appear again in the warm temperate zone. For example, in southernmost China and the southeastern USA, trees tend to have leathery, long-lived leaves. Evergreen forests also occur in warm Mediterranean climates (such as southern Europe and California) with a relatively dry summer, where the summer drought is not normally long or intense enough to require the trees to shed their leaves.
Temperate evergreen forests can also occur in oceanic climates with quite cool summers—such as in New Zealand and southernmost South America—so long as the winters are mild. Here there is no reason to drop leaves at any particular time of year. There is enough moisture year round, and the winters are mild, so photosynthesis is possible at any time of year. Through most of the mid-latitudes, colder winters mean that there would be a disadvantage in holding on to leaves all through the year. It would be too cold during winter for them to work effectively, and they would just lose water, get tattered and torn, their cells damaged by frost, and chewed by herbivores. With all these damaging influences, they would be thoroughly ineffective by the end of the winter season. Having no strong reason to keep their leaves, and several good reasons not to keep them, the trees shed them as the cold season sets in. An orderly process of dismantling the cell contents of the leaf ensures that nearly all the most valuable substances (such as nitrogen and phosphate-containing molecules, and ions such as potassium and magnesium) are drawn back into the tree. Chlorophyll is broken down for its magnesium ion early on in the process, whereas other less useful pigments in the leaf such as carotenoids and anthocyanins are discarded with the leaf.
Figure 2.17. Autumn leaves in a northern temperate deciduous tree, Norway maple (Acer platanoides). Source: Author.
Unmasking the colors of these other pigments after the chlorophyll has gone is partly what gives the brilliant colors of autumn leaves in the mid-latitudes of the northern hemisphere (Figure 2.17*). However, adding to the brightness of autumn colours, senescing leaves also increase their levels of red anthocyanin pigments. This is thought to protect the leaf tissue from damaging sunburn while it is still dismantling itself to release nutrients from the cells.
Trees that lose their leaves during part of the year and then re-grow them must take a fairly precise cue from their environment. In the mid and high latitudes, if they put the leaves out too early in the year, these may be damaged by frost and valuable nutrients lost, because the tree cannot easily reclaim nutrients from a frost-bitten leaf. Or in a seasonally dry climate, the tree may die of drought from putting its leaves out too soon. If the leaves are dropped too early, time that could be spent photosynthesiz-ing is wasted. A tree must in effect take a gamble as to the best time to drop its leaves, using the best cues that it has available. Usually in temperate deciduous forests—as temperatures dip close to freezing in autumn—the tree starts to break down the contents of cells in its leaves, and withdraw them back in to be stored in the trunk, branches and roots. A further cue is taken from the declining day length as summer ends. Often trees right next to street lights retain their leaves a few weeks longer because they are "fooled" that there is still more daylight around. If winter-deciduous trees do not receive any cues, they may simply keep their leaves going. In my own experiments, young deciduous white oak trees (Quercus alba) grown in a greenhouse in warm temperatures and long artificial day lengths (due to lighting) retained their leaves healthy and green all winter long, and then grew an additional set after several months at roughly the time that corresponded to spring. On the other hand, red maples (Acer rubrum) still dropped their leaves just about on cue despite the lack of environmental stimuli.
In the mid-latitude temperate forests of Europe, North America and eastern Asia (extending between around 30 and 50°N, though it depends on the locality), there is an "autumn wave'' of leaf shedding that starts earlier in the north and moves progressively southwards as each latitudinal band reaches colder autumn temperatures. Although the timing is tuned by climate, there is evidence that populations of trees of the same species from northern and southern parts of their ranges are genetically programmed to take environmental cues differently. When they are planted farther north, more "southerly" populations tend to keep their leaves longer.
In dry-season deciduous forests in the tropics, it is drought stress that begins the process of leaf drop. If a particular year is unusually wet, the trees retain their leaves longer until the supply of soil moisture is used up, and only then do they drop them.
When good growing conditions return, deciduous trees must also take cues from their environment to regain leaves at the best time. As I mentioned above, for temperate deciduous trees, it is particularly critical not to start producing young spring leaves too early because their soft tissues can easily be damaged by frost. Trees take their cue for the arrival of spring from exposure to a certain number of days of warm temperatures. Increasing day length can also help to act as a trigger for leafing out, and cold temperatures during the winter help to prepare ("vernalize") the buds for breaking with the arrival of spring. Without these requirements, the tree might start leafing out during periods of a few warm days in early winter, only to have all its leaves killed when the true winter cold returns. Just as with the timing of autumn leaf drop, there is evidence that different populations of trees (e.g., elms Ulmus in Europe) of the same species can be quite finely adapted in their cooling or day length requirements for leafing out, according to the length of the winter where they come from, to ensure the best balance between leafing out early enough to exploit the arrival of spring temperatures, and leafing out reluctantly enough to avoid being misled by short-lived warm periods during winter. In my greenhouse-grown red maples, populations from warmer climates like Florida leafed out several weeks earlier than ones from colder climates like New England, despite being kept under identical conditions. In the seasonally dry tropics, the most common cue for leafing seems to be the uptake of water by the roots once the rains start. However, some trees that lose their leaves during the dry season may start to produce new leaves just before the rains arrive. It is thought that in this case the cue is an initial drop in temperatures that accompanies the arrival of moist air before the monsoon although in some species, beyond about 10° from the equator, the very slight change in day length that precedes the tropical wet season also seems to be a cue for leafing out.
Figure 2.18. The relationship between January temperature and leafing out date in a range of North American trees. The vertical axis is the week of the year, starting from January 1st. From Borchert et al. (2005).
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