Forest productivity can be improved in various ways, but is most easily done using plantation trees. Suitable choices of species and origin are absolutely essential in this process. The terms provenance and origin have very precise meanings in forestry. A provenance is the place where any group of trees, whether native or not, is growing. Origin, on the other hand, is the place in which a stand of native trees is growing, or that from which a non-native stand was originally introduced.
There are many examples of trees that have been introduced to new areas with great benefits to the local population. The neem tree Azadirachta indica, which may well have originated in Assam and Myanmar (Burma), and is important in India where it is considered sacred, is amongst the species likely to have much greater significance in the future (Shultz et al., 1992). Numerous uses are made of its products. Neem oil extracted from its seeds has long been used as a hair tonic, to treat skin diseases, and a source of soap, while an extract from the bark is used in preparations that reduce dental caries and inflammation of the mouth. A wide variety of complex organic compounds can be extracted from this tree and many are proving of value as medicines, insecticides, nematicides (killing nematodes) and fungicides. Its timber is also valuable and this substantial evergreen tree of the mahogany family (Meliaceae) grows rapidly in the lowland tropics, reaching a height of 30 m and a dbh of 80 cm. The tree is often established by planting seeds or seedlings, but saplings, root suckers and tissue culture have also been employed. It is now grown in many parts ofthe world including much ofAsia, Saudi Arabia, South America, parts of the USA and many African countries, being used even in dry infertile sites on the southern margin of the Sahara. Its deep and extensive root system is very effective in preventing erosion and it survives well in droughts. In contrast the tree cannot resist frost and dies quickly if the soil becomes waterlogged. Neem pollards are the source of much firewood in Africa.
Plantation forests consisting of non-native monocultures are often extremely successful as timber crops as in the case of the Monterey or radiata pine Pinus radiata. This is a Mediterranean coastal species native to California and Baja California where it can exceed 40 m in height, which flourishes as an alien in many countries and particularly New Zealand. The foresters here have spent a great deal of time selecting and improving particular strains and clones of this fast-growing tree. A 25-year-old plantation at Rotorua, North Island which had been pruned and grown for saw logs was in 1999 stocked at 400 stems with a mean diameter of 50 cm and a total volume of 940 m3 of timber ha~\ This is equivalent to a yield class of nearly 38 (see Fig. 10.4). Seed from this plantation still has the origin of California, but the provenance is Rotorua. Some plantations, incidentally, now have even greater growth rates. This is not at major expense to soil nutrients; studies of these plantations in New Zealand have shown that their annual uptake of major nutrients is only a thirtieth to a hundredth of that of major agricultural crops.
Topography has a major influence on the forest industry. New Zealand is always thought of as a major forestry centre and most of its wood production is exported, yet half the country slopes at an angle greater than 28 degrees and two-thirds is over 12 degrees. Three-quarters has an altitude greater than 200 m. Shallow soils, long wind runs, frosts during the growing season, and browsing by the tree-climbing marsupial possum Trichosurus vulpecula introduced from Australia, all limit the possibilities for commercial forestry in many areas. Yet with the choice of the right provenance of the right species forestry is very successful.
Forests can also be improved by tree breeding, crossing trees that have useful characteristics to try and get all the good traits into one tree. Seeds collected from these breeding trials would be grown and the best trees, usually referred to as plus or superior trees are identified. Seed from these superior trees would then be used for the next crop. In this way it is possible to select for desirable growth characteristics (fast growth, straight trunk, low taper, etc.) and for other desirable traits such as disease resistance. This sort of breeding programme has been used in agriculture for millennia and is certainly not a new technique. The major drawback with such programmes is that the seed collected from superior trees is genetically mixed (each seed has two parents) so not all the seeds will perform as well as the superior parent. Various solutions to this problem are possible. Forest geneticists have used seed orchards where superior trees are grafted onto established root stocks. Since the resulting trees are close together, they will most likely exchange pollen increasing the probability of a greater number of the seeds being superior. This is an expensive process and tends to be used for valuable fruit or nut trees rather than for timber trees. Genetic engineering techniques can be used to transfer desirable genes directly from one individual to another but this requires a knowledge of which genes are responsible for the desired trait. Gene transfer is currently being used to address a range of issues (Giri et al., 2004) including:
* reducing the time until onset of seed production;
* modifying tree hormones to manipulate tree shape and growth;
* modifying wood to allow easier separation of cellulose and lignin during papermaking.
Another way to reduce genetic variability is to take cuttings from a superior tree, root them and use them as seedlings for the next plantation. Since all the new trees are genetically identical they are clones and this has given rise to clonal forestry. Again, this is not a new process in the wider world. The vast cultivated olive Olea europaea ssp. europaea plantations of the Mediterranean region are based upon the repeated careful selection and subsequent cloning by cuttings of favoured genotypes of the wild tree O. europaea ssp. sylvestris, most populations of which have wider leaves, thornier branches and smaller fruits than the commercial varieties. The ultimate result is the production of large numbers of genetically identical and highly productive trees. Commercial cloning of timber trees has been carried out for a number of decades now, involving poplars Populus spp., Norway spruce Picea abies and many other trees in temperate areas, and eucalypts throughout the tropics.
Foresters initiating plantations of clonal trees often use several clones in order to reduce the risk of disease devastating the whole plantation. Cloning works well for a number of generations (usually around five) but thereafter does not work indefinitely. Eventually there is a build-up of topophysis, the long-term persistence of age or position, such that new trees are physiologically old, and cuttings from the side of a tree continue to grow sideways when rooted. This and the high costs have dogged the long promised and much awaited clonal forestry programme in Europe using Norway spruce, and its use has been more or less abandoned in northern Europe (Anon., 2002). A solution to topophysis, and a way of increasing the number and cost effectiveness of producing clones, is to use somatic embryogenesis. Tissue from selected trees is cultured under laboratory conditions (tissue culture) with hormones and nutrients, stimulating them to quickly produce a number of bare embryos. These can be separated and grown into whole young clonal trees. The stock of tissue from a superior tree can be stored in liquid nitrogen which prevents it from physiologically ageing. This technique is still being developed but Giri et al. (2004) list more than 50 trees that are under investigation.
Trees can often be improved by using polyploids (having extra sets of chromosomes). Polyploids are rare amongst conifers but common amongst hardwoods (angiosperms). While many of these are of no great commercial value, others such as the polyploid forms of white birch Betula papyrifera and Japanese alder Alnus japonica grow larger and faster than their normal diploid cousins (which have the normal two sets of chromosomes in each cell). Hybrid offspring from the crossing of two separate species frequently show more rapid growth than either parent. Besides this hybrid vigour, they often also bring together useful features of the parent species into one new plant. For example, the hybrid larch Larix x eurolepis (the x preceding the species name denotes a hybrid) brings together the good shape and fast growth of the European larch L. decidua and the disease resistance of the Japanese larch L. kaempferi.
Global warming will undoubtedly cause major changes in tree planting practice worldwide. Just how much change partly depends upon the sensitivity of trees to climate. To test how different clones of common European species would cope with climate change, Kramer (1995) relocated clones of European larch Larix decidua, Downy birch Betula pubescens, small-leaved lime Tilia cordata, grey poplar Populus canescens, English oak Quercus robur, beech Fagus sylvatica and Norway spruce Picea abies over a large latitudinal range in Europe. He compared the timing of spring events such as bud opening and leaf opening in these transplanted clones with trees of the same species already growing in the areas which were assumed to have adapted to their local climates. The response of the transplants was very similar to the indigenous individuals showing that these species possess considerable plasticity in their response to climate and should be able to cope with changes in their local climate. However, with long-lived trees, it is difficult to predict exactly what might happen to a valuable crop over decades or centuries and economic prudence will require caution. Beech Fagus sylvatica has grown splendidly for millennia in the south-east counties of England and in Sussex particularly. How will this native English tree fare in the increasingly dry and droughty conditions brought about by climate change? If left to nature beech woods might well be increasingly invaded by oak and ash that can withstand drought far more effectively. Perhaps the answer is to encourage more planting of beech in northern England and Scotland. The effect of climate change on forests is considered further in Section 11.3.1.
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