Trees - basic classification, growth and structure
NZFFA Information leaflet No. 25 (2005).
Botanists classify all trees, other than tree ferns, as either gymnosperms (= naked seeds) or angiosperms (= seeds in a container). The differences between the two groups are not obvious to the naked eye. In gymnosperms, the female reproductive organs are rather like those in mammals – the unfertilised ovule sits in a little container, and a passage connects this container to the outside world. In mammals, a sperm swims down the passage to fertilise the ovule, whereas in gymnosperms, the pollen grain stays on the outside, and a tube grows from the pollen grain down the passage to meet the ovule. Angiosperms have a similar structure, but the whole lot is enclosed in solid tissue, and the pollen tube has to grow through this tissue before it reaches the passage with ovule at the other end.
Gymnosperms are usually called conifers, because the pollen is always borne in cones, and in most cases the seeds are also borne in cones, although sometimes the cones become soft (junipers) or the female ‘flowers’ are solitary (yews, and podocarps such as rimu, miro, matai and kahikatea). The counterpart to the term conifers, is flowering plants. This is a good distinction, provided we don’t regard solitary female structures on gymnosperms as flowers, and provided we don’t regard male catkins on trees such as alders, poplars, oaks, and hazels etc. as cones!
In forestry circles, gymnosperms are sometimes called needle-leaved trees, while angiosperms are called broadleaved trees, but this isn’t particularly helpful, as many gymnosperms have broad leaves, e.g. kauri, while many angiosperms have needles, e.g. Casuarina. Even more confusing, is that foresters often refer to gymnosperms as softwoods, and angiosperms as hardwoods. This was quite a valid distinction in the northern temperate forests, but elsewhere many gymnosperms have hard timbers, e.g. matai, and many angiosperms have very soft timbers, e.g. paulownia and balsa. In New Zealand, foresters usually distinguish the two groups as either conifers or broadleaved trees.
In all conifers, the male and female reproductive structures (cones) are borne separately, either on separate plants (dioecious = ‘two houses’), or on the same plant (monoecious = ‘one house’). Broadleaved trees can be either dioecious or monoecious, or both male and female structures can be in the same flower (hermaphrodite). Most tropical species are insect or bird pollinated, and have hermaphrodite flowers, while most temperate trees are monoecious and wind pollinated. Dioecious trees are wind pollinated and not very common, but notable examples are the poplars and willows, and our own podocarps.
The distinction between dioecious and monoecious trees is not absolute. For instance, amongst our podocarps, some male trees will bear a few seeds while some female trees will bear a little pollen. Likewise, amongst the monoecious pines for instance, some individuals bear mostly female cones, while others bear mainly pollen cones. In general, in monoecious conifers, the female cones are borne mainly on the leading shoots which are actively growing, and can therefore best provide nutrients to the developing seeds, while male cones are mostly borne on side shoots. The same is often true in some broadleaved trees.
All trees consist of cells, which are little containers of living matter where the walls are made of cellulose which is often reinforced with a natural glue called lignin. The growing point of a tree shoot is called a terminal bud, and it consists of a number of similar cells which keep dividing. As cells get left behind by growth of the terminal bud, most stop dividing and expand lengthwise to produce shoot growth. Some cells however, in a ring near the outside circumference of the stem, never stop dividing, and they form the cambium, which is responsible for diameter growth.
Cambial cells form wood cells (xylem) on the inside, and live bark (phloem) on the outside. Xylem is used mainly for transporting water and minerals from the roots to the leaves, while phloem is used to transport food (mainly sugars) from the leaves to the cambium and growing roots. As the cambium consists of unspecialised cells which are quite weak because they are continually dividing, bark can often easily be peeled from the wood, particularly in spring when the cambium is very active.
As the phloem cells age, i.e. get further away from the cambium, there develops a zone of weakness within the phloem called a bark cambium, and cells outside the bark cambium die and are made waterproof to protect the tree from moisture loss. As the tree grows, the phloem continues to form new bark cambiums inside the old ones. If the phloem produces lots of bark cambiums, and these extend most of the way round the tree, the older bark comes away in thin sheets as we see in paperbark species of maples, birches and cherries, and in the gum-barked eucalypts. If the bark cambiums are much smaller and overlap one another, the older bark comes off in flakes and has a hammered appearance as we see in kauri and miro. If the bark cambiums are smaller and don’t overlap, the older bark doesn’t come free, but persists and splits lengthwise as the tree grows in diameter, as we see in pines and redwoods.
The xylem produced on the inside of the cambium consists of several types of specialised cells. In broadleaved trees, the most conspicuous of these are vessels, which are large diameter cells which can often be seen with the naked eye or with a small hand lens. These are the main water conducting cells. Also present in large numbers are thick-walled small-diameter cells called fibres, and these give the wood its strength. Scattered throughout the wood, there are tracheids, which are rather like fibres except that they are also efficient water transporters. Conifers don’t have vessels, and have very few fibres. Instead mostm of their wood consists of tracheids.
Both conifers and broadleaved trees have other types of cell scattered through their wood, including thin-walled storage cells, and canals filled with resin or gums. Also present are bands of thin-walled cells called rays, which extend from the cambium towards the centre of the tree. These rays can be very conspicuous in some species such as oak, casuarina, rewarewa and lacebark, but in conifers they are not visible to the naked eye.
In some broadleaved trees, vessels are abundant in wood produced in spring when growth commences, and are relatively sparse in wood produced later in the season. These species are said to have ring-porous wood, and the bands where the vessels are concentrated is called springwood, while where the vessels are sparser, is called summerwood. Summerwood is denser than springwood and it is the contrast between the springwood and summerwood that makes these trees have conspicuous annual growth rings. In other broadleaved trees, the vessels are more evenly spread through the wood, and these are said to have diffuse-porous wood.
Wood cells formed in the autumn have smaller diameters, and the walls of fibres and tracheids tend to be much thicker, than those of cells formed in the main growing season. This wood is called latewood, and is denser than the earlywood formed in spring and summer. In conifers and diffuse-porous broadleaved trees, it is the contrast between early wood and latewood that shows up as annual rings, and the contrast is particularly great in species such as Douglas fir, larches and spruces.
Because ring-porous trees tend to have much the same amount of springwood whether they are fast or slow grown, fast grown trees have proportionately more summerwood, and the timber is denser and stronger than in slow grown trees. In trees where the annual rings are defined by the earlywood/latewood boundary, fast grown trees tend to produce proportionately more earlywood than latewood than slow grown trees, and therefore the wood tends to be less dense and weaker than in slow grown trees.
Photographs reproduced from New Zealand Tree Grower, February 2002.