1.3 Strategy 1: Remaining active through the winter ('tough it out')
1.3.1 Evergreen plants
In temperate regions, the most prominent evergreen plants are coniferous trees, or conifers (phylum Coniferophyta). Conifers dominate large portions of the Earth's land area, particularly at northern latitudes and high altitudes. This distribution reflects their ability to withstand long periods of cold weather. The major problem faced by conifers in winter is lack of water. Water that has turned into ice is not available to plants and, at freezing temperatures, plant roots are able to absorb such water as is available only very slowly. If conifers are not to die of desiccation they must, therefore, reduce the rate at which they lose water. The needle-like shape of conifer leaves reduces the rate at which water is lost from their surfaces and so reduces a tree's requirement for water.
Though very small compared to the leaves of most deciduous trees, pine needles are relatively thick, in comparison to the broad, flat leaves of deciduous trees. Consequently, their surface area is small relative to their volume, reducing water loss. Evaporation is further reduced by a thick, waxy cuticle that forms the outer surface of needles and by the stomata (pores through which leaves exchange gases with the air) being positioned in sunken pits.
Not all conifers are evergreens, and not all evergreens are conifers. There are some ten species of larches (genus Larix) that live mostly at high altitude in the Northern Hemisphere; all are deciduous, dropping their needles at the onset of winter. The holm oak (Quercus ilex), also known as the evergreen oak, is not a conifer, but retains its leaves through the winter. The holm oak is a native of continental Europe, from the Mediterranean to Brittany, that has been introduced into Britain.
As explained in Section 1.2.4, life histories involve trade-offs between many factors and this principle is well illustrated by a comparison between deciduous and evergreen trees. By retaining their leaves through the winter, evergreens do not bear the cost, as deciduous trees do, of reconstructing their entire photosynthetic apparatus each spring. However, the adaptations that enable their leaves to survive the winter make them less efficient in the spring and summer than those of deciduous trees. Another trade-off is that conifers have a simpler system of water-conducting cells which is less efficient when water is plentiful, but better (because it is less likely to block) when water is scarce and freezing occurs.