Conifers are gymnosperm plants that produce seeds in female cones, formed by scales arranged around an axis. The female cones contain one or more egg-bearing scales on the outer scales. These, after fertilization, are transformed into seeds. Pollen is found in the male cones that are born at the end of the young branches, usually in winter and mature during the spring months. The male cones are smaller since they can be between 2 and 12 mm in length.
In the most primitive trees they are isolated or little gathered, whereas in the more evolved species, like the pines, they form a species of cluster. Each cone consists of a series of scales containing large amounts of pollen. in spring. When moving the branches of the conifers, if the flowers are ripe, they produce a cloud of pollen. Pollination is done through the air, that is why most of the seeds of the conifers contain winglike extensions, which facilitates air glide.
The majority of the species are monoecious, means they present the reproductive structures in the same tree, although we also have other species that are dioecious like the Araucarias, with some masculine and other feminine trees.
Conifers are woody plants. Most are in the form of trees, although there are many shrubs. Leaves usually have the form of needles or scales and are perennial, except those of the genus Larix and Taxodium that are deciduous. The leaves of the genus Agathis and some species of Podocarpus and Araucaria are ovate and those of the genera Sequoia and Metasequoia are lanceolate.
History of conifers
There are approximately 1,000 species of conifers. They appeared on the earth about 300 million years ago in the Carboniferous period and they formed along the Mesozoic, which is the age between about 250 and 65 million years ago. It is believed that at the beginning of the Tertiary Age, that is to say about 65 million years ago, the present species already existed.
They were extremely important in the past occupying areas that today are dominated by angiosperms. Their little capacity to adapt to the temperature increase that our planet has been experiencing progressively has relegated them mainly to the colder zones, where they form large forests nowadays still in places where most of the flowering plants have not been able to adapt.
Although the number of species of conifers is very low compared to angiosperms, their high quantity in some areas of the world, as well as their economic importance in the production of wood and pulp, make them a well-known group.
The conifers live mainly in northern areas of America, Europe and Asia between 50º and 65º or 70º north latitude in the area known in Europe and Asia as taiga, forming the largest forests on the planet.
Above this area is the shrub tundra, where bryophytes dominate along with some shrubs such as the arctic willow (Salix artica) or the dwarf birch (Betula nana).
In Europe and North America, the deciduous forests of beech, oak, linden and ash are found in the south of these forests. Within this area we also have a very characteristic conifer, the yew. As the climate becomes less extreme the conifers are progressively replaced by the birches and, subsequently, by the trees mentioned above.
In Asia, below the vegetation formed by conifers, we find a steppe zone in which grasses live mainly.
Thus, coniferous forests dominate the northern soils from the Atlantic coast of North America, to Scandinavia, including Siberia. In this way they form a dense belt of about 10,000 kilometers of length and with more than 2000 km of amplitude in some places.
Other conifers are located in more northern latitudes with warm or temperate climates. Particularly noteworthy are the temperate coniferous forests found only in certain areas of the world with hot summers and cold winters and with a very high rate of precipitation.
Among the most important are the redwood forests of California (Sequoia sempervirens) or giant sequoia (Sequoiadendron giganteum) with such bulky specimens as General Sherman. No less important are the Douglas fir forests (Pseudotsuga menziesii), also called Oregon pines, in the northwestern United States.
In southeastern South America in Chile and Argentina are the larch or forest formed by the lahuán (Fitzroya cupressoides) with trees similar to the larch that can surpass 50 meters of height. In the north of New Zealand are the forests of kauri (Agathis australis).
These trees present very thick trunks, that reaches the diameter of 4 meters. They are characterized by their high trunks devoid of branches that constitute an authentic tourist attraction. They grow in a very humid environment along with podocarpos, another type of conifer.
How are conifers distributed?
Fir trees are the dominant species in many northern regions, such as Canada, the United States and Siberia. In addition, we can find conifers as dominant trees in mountainous areas of temperate or even warm climates, where climatic conditions resemble the cold northern areas.
In south we can find conifers in warmer places. In these latitudes, pines may constitute the dominant species of the forest or form part of it together with other species, however, the aspect of these conifers, as we shall see later, is different from those found in colder areas.
In general, in the northern hemisphere we find conifers belonging to the Pinaceae, and Taxodiaceae, while the Podocarpaceae and Araucariaceae are mainly distributed in the southern hemisphere. We can also find Podocarpaceae in East and Central America. The most widespread family is the Cupresáceas that can be found in both the north and the South.
Adaptations of conifers
Most northern conifers are adapted to the climatic conditions of cold areas as they are able to withstand the low winter temperatures that often exceed -40 ºC. The main problem that these plants have is that, when winter arrives, the soil freezes in a way that they can not absorb water, so in practice, it is as if they lived in desert areas.
Conifers have adapted to the scarce availability of water, the hard cold of winter and the short duration of the vegetative season. Among the main adaptations of the leaves of these plants we must highlight the following:
- Its duration: The leaves of the conifers are perennial to take advantage of the limited duration of the vegetative season. When the good weather arrives they can begin to "work" in the photosynthesis without having to wait for the tree to produce them again as it happens in the deciduous trees. In this way these plants remain green throughout the year. A leaf can hold up to seven years on the tree, so that the glass is gradually renewed.
In areas with very dry summers the perennial leaves of the conifers are also advantageous to resist drought.
Only certain conifers do not respond to this scheme. These are larches (Larix). These trees grow on very poor and dry land. Their availability of water is much lower than in the rest of the conifers, so they can not afford to lose the slightest. To avoid evaporation the larches, when the bad weather arrives, they drop the leaves.
- Its shape and size: Most are shaped like a needle, so its total surface, compared to perennial leaves, is small and can carry less weight on top. A typical perennial leaf would accumulate more snow and would break more easily with its weight. The volume of the leaf is small, so it contains less sap and consequently has less liquid that can be frozen in the cold. Its smaller surface gives it less exposure to solar radiation, which means less evaporation.
In addition to the leaves, the general shape of the conifers is adapted to the snow. Its pyramidal form and its sloping branches downwards facilitates the sliding of the snow towards the ground so that its weight does not spoil the tree.
This same reason would explain why the conifers located in hot zones do not have the same structure, but they have more open and wide crowns and branches directed upwards. They do not need to get rid of the snow and, at the same time, their more extended form allows them to catch the light better. It is enough to compare the pyramidal shape of the fir (Abies alba) with that of a carrasco (Pinus halepensis) and, above all, of the pine (Pinus pinea) with its typical form of parasol to realize these two different ways of adapting to medium.
- Color: The leaves of the conifers are generally very dark which is used during the short summer to capture more light.
- Resins and natural antifreeze: The leaves of conifers are impregnated with resins that prevent the loss of water. The outer cells contain a kind of antifreeze that protects the tissues against low temperatures.
Stomas: Stomas are a series of pores or holes located in the leaves or young branches where the exchange of gases between plants and the atmosphere occurs. Most stomas are located on the underside of the leaves.
Plants, when they open the stomata, to absorb carbon dioxide and expel oxygen, also lose water.
The stomata in the leaves of conifers are adapted to lose the minimum possible both because they have a smaller amount than other plants, and because they are protected within a species of furrow on the underside of the leaf. This furrow is filled with air and forms a kind of layer that reduces evaporation.
Conifers and fire
Conifers are better adapted to fire than most plants. They even favor fires by eliminating competition from other species.
To be able to adapt to the fire, many of these plants have developed a thick crust that prevents the fire burns the vital parts. Among the best known and adapted are the Canary pine (Pinus canariensis) whose trunk and branches is covered with a very thick layer able to isolate the inner layers. Even if the trunk is affected, this tree can regrow from the ground or regenerate the part of the damaged trunk from special tissues that are not found in any other conifer.
Other conifers do not have this regrowth ability but are also favored by fires. Many pines or cypresses have pine cones that remain closed until the heat of a fire opens them and expels the seeds. These can grow on land where the fire has eliminated competing species. Many soils are even more productive after the fire, because the heat breaks down the leaves and extracts components that enrich it.
Thus natural fires caused by solar rays or storms have in many places favored the presence of vegetation dominated by conifers.