Coldclimate Evergreenness

Deciduous forests are a feature of mid-latitude climates with cold winters. Yet at still higher latitudes with even colder winters (as in much of Canada or Russia), evergreen conifers (mostly of the pine family, such as Pinus, Abies, and Picea) are dominant instead. This seems to contradict the explanation for temperate trees losing their leaves—surely here the need to drop leaves in winter is even greater, and yet these are

Figure 2.20. Typical leaf of red maple (Acer rubrum) population in Canada (left) compared with one from Florida (right).

Scatter plot, shaded fay P

Map, shaded by P

Scatter plot, shaded fay P

Proportion entire leaves (P)

Map, shaded by P

West latitude

West latitude

Figure 2.21. The proportion (P) of species of trees with "entire" (non-toothed) leaves depends closely on the warmth of the climate. From Adams et al.

evergreens. However, another factor has entered the equation, the briefness of the growing season in the high latitudes. The several weeks in spring spent growing new leaves represent valuable time that could be spent photosynthesizing. Similarly, the process of shutting down a leaf ready for it to be shed in the autumn takes several weeks, at a time when temperatures may still be warm enough for photosynthesis. The short summers of the boreal climate may give the edge to plants that can sit tight and hang on to their leaves rather than having to regrow a new set each spring, which is a lengthy process. For leaves to survive the severe winter intact, they must be made tough to stand dehydration and frost; so these conifers have "needle leaves''—thin and hard with a thick waxy coating. Evergreen conifer forest is often also found above the deciduous belt on mountains in the mid-latitudes, where the same conditions of short summers and harsh winters are found.

In fact, although most of the high-latitude forest is evergreen, in the really cold continental parts of east-central Siberia and Canada the winters are so harsh (down to —60°C in some forested parts of Siberia) that even a tough conifer leaf would be damaged by the frost and dehydrated. So in the coldest forest areas on Earth, in north-central Siberia, forests are dominated by the deciduous conifer larch (Larix) and small deciduous broadleaved trees such as birch (Betula) and aspen (Populus). But in these extremely "continental" climates (Chapter 1) the brief summers are quite intensely warm, so the trees can just about do well enough by unfurling new leaves for the summer.

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Renewable Energy Eco Friendly

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