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What is A Tree?
Trees are woody perennial plants with an upright trunk which holds the branches above other vegetation and the ground. In comparison, common shrubs do not have a distinct stem but tend to branch from ground level. Shrubs often grow under trees.
Trees can be grouped into two very broad categories:
• Hardwoods
• Softwoods
Hardwoods
Hardwoods include all those species which are members of the flowering plants (angiosperms) such as ash, oak, elm, poplar, eucalypts and willow. New Zealand native species of hardwood include pohutukawa, karaka, kowhai, kamahi and tawa.
Hardwoods often have harder, more durable wood but this is not always the case. They are usually characterised by a woody stem which divides into a number of larger branches and a rounded crown.
Softwoods
Softwoods include members of the gymnosperms (seeds produced from strobili rather than flowers), including conifers such as pine, spruce, fir and many others. New Zealand’s podocarps and kauri are softwoods. Softwoods tend to have a single stem which continues to grow upwards throughout the life of the tree with branches spread along the length of the stem. There are exceptions to this general description – kauri does not fit the pattern very well once it has grown above the canopy of other trees.
What is A Forest?
Forests in their natural state are communities of plants dominated by trees. They are mostly made up of a mixture of species growing together in the form of a stable living system. Some forests are almost pure stands of one or a few species such as beech forest and pockets of lowland forests of kahikatea.
Living in the forest are shrubs, ferns, mosses and fungi as well as birds, insects and other animals. Such communities of plants and animals living and working together are referred to as ‘ecosystems’.
In the forest, the trees and other plants are primary producers – using the sun’s energy to convert carbon into food (carbohydrates) and eventually plant tissues through the process of photosynthesis.
As leaves, twigs and bark fall to the ground, they form a bed of litter which is itself the food of many insects, fungi and other plants. Still other insects, birds and some parasitic plants feed directly on the living tissues of the trees to obtain energy. The whole grouping of plants and animals forms a recycling system, returning the vital substances used by living things so new organisms can use them again. This cycling of nutrients, water and energy within natural systems is often illustrated by diagrams known as the carbon cycle, nitrogen cycle and the water cycle.
As a forest matures, patterns of growth emerge. Certain successions may occur where the growth of one species allows another to develop in its shelter. Eventually a mature forest will exist and this will have a distinct structure. Within a system that is mature and in equilibrium, each species of plant or animal will occupy a specific niche to which it is particularly suited. For example, plants or animals on the forest floor which survive well in little light would quickly die out if the cover was removed. Such plants would include mosses and ferns and the kiwi is a good example of an animal that needs plenty of cover.
What is A Plantation
Plantation forests are usually created for commercial benefit and over a relatively short period. Plantation trees are selected for their wood properties, planted (usually as a single species) and encouraged to grow to a useful size as quickly as possible. To encourage growth fertiliser may be added, competing vegetation weeded out and slower growing trees thinned.
Plantations are not permanent ecosystems and their longevity is only achieved by man’s intervention. However, they do have many of the properties of natural forests: stabilising soils, recycling nutrients, controlling rainwater run-off and providing habitats for other plants and animals. The longer the period over which the tree crop is grown, the more effective plantation forests become in this regard.
The best examples in New Zealand of this beneficial effect are on degraded soils where land clearance for agriculture has been unsuccessful such as the East Coast (North Island) hill country, the coastal sand dunes and the badly degraded soils left by gum diggers in the Auckland/Northland regions.
Planting trees for other purposes such as shelter and landscaping has helped create the familiar urban and rural landscape and typically includes a wide mixture of natural and introduced species. |