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 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 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 cast-central Siberia and Canada the winters are so harsh (down to -60 C in 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 broad-leaved trees such as birch (Betula) and aspen (Popu/us). But in these extremely "continental" climates (Chapter 1) the brief summers arc quite intensely warm, so the trees can just about do well enough by unfurling new leaves for the summer.

2.8 THE LATITUDINAL BANDS OF EVERGREEN AND DECIDUOUS FOREST

So, moving away from the equator there are alternating bands of deciduous and evergreen forest vegetation (Figure 2.18). This pattern is only found in its most "perfect" form in eastern Asia, where the climate is moist enough to support trees all the way along a line from the equator to the high latitudes. However, the pattern is present in a more fragmentary way in many parts of the world.

POLAR

TROPICAL

POLAR

Figure 2.18. Latitudinal bands of alternating evergreen and deciduous forest. Idealized arrangement of evergreen vs deciduous forest types on the earth's surface. In reality, this is complicated by non-forest zones, oceans, and climate and soil differences affecting the evergreen strategy.

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POLAR

TROPICAL

POLAR

Figure 2.18. Latitudinal bands of alternating evergreen and deciduous forest. Idealized arrangement of evergreen vs deciduous forest types on the earth's surface. In reality, this is complicated by non-forest zones, oceans, and climate and soil differences affecting the evergreen strategy.

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Evergreen boreal

Deciduous temperate

Evergreen warm temperate Deciduous tropical tropical Deciduous tropical Evergreen warm temperate Deciduous temperate boreal

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Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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