3.1 Age classes
3.2 Volume increments
3.3 Rotation lengths
3.4 Planting rates
Discussion of roundwood production, and the key variables that determine it (age-class, increment and rotation) is primarily relevant for the production of industrial wood. The focus in this section is, consequently, on forest plantations with a primary purpose of supplying roundwood for sawntimber, veneer and pulp (industrial plantations). If all forest plantations in Europe and the former-USSR are assumed to be for industrial purposes, then global area of industrial forest plantations in 1995 is estimated at 103.3 million hectares or 83.5 percent of the total forest plantation area.
While analysis based on plantation forest areas can give very broad estimates of plantation wood production potential, the development of national age-class structures for plantation forests allows considerable refinement of these assessments. Age-class information enables more accurate assessment of the current level of wood production from plantations and likely future changes in production levels. FAO published a study that derives representative age-class structures on a country-by-country basis, consistent with published information (FAO 2000).
Figure 2: Derived industrial plantation age-class structure by region in 1995
Source: FAO, 2000Two dominant trends are apparent in the regional age-class data (Figure 2). Firstly, as noted earlier, the preponderance of Asian plantations compared with the other regions, is clearly evident. This is particularly the case for plantations established during the past decade. Asian plantations constitute 40 percent of the global total, and 57 percent of the plantations established since 1985.
Second, there is the very high proportion of forest plantations aged less than 15 years, particularly in developing countries (Figure 2). Overall, 54 percent of industrial plantations are less than 15 years of age, with 21 percent planted between 1990 and 1995.
Only 2.2 percent of plantations shown in Figure 2 are aged more than 50 years. A further 15.6 percent are between 30 and 50 years of age. This structure is largely the result of accelerating rates of new plantation establishment, but also reflects the harvesting of mature plantations in the older age-classes and a general shortening of rotation lengths in many countries.
A vast body of literature assesses increment for various species in limited research trials (see also Working Paper FP/1). The actual yields achieved in the field on a large scale are, however, generally lower, because land qualities are more variable, and the quality of establishment and silviculture is more difficult to control. Climate, altitude and geomorphology, matching of species to site, pests and diseases can all have marked effects on tree growth at local levels.
And, at a global level, any consistent bias in yield data can badly distort modelling results.
Tropical regions generally offer greater potential for future productivity gains than temperate regions. For example, while eucalyptus species in the tropics, rarely yield more than 25 cubic metres per hectare per annum in the field at present, significant advances may well be achieved in the not too distant future. In Brazil, for instance, hybrids of E. grandis with E. urophylla have, on some sites, have attained growth rates of 70 cubic metres per hectare per annum (Campinhos, 1994). The extent to which such results can translate to large-scale plantings, and particularly, whether other problems (for example, wood quality, or susceptibility to disease or windthrow) may arise from focusing mainly on growth and yield attributes remain important question-marks.
Figure 3: Indicative national-scale forest plantation yields by species (MAI)
Primary source: Leech (1998)Figure 3 shows a cross-section of estimated minimum and maximum yields, at a national scale, for major plantation species and genera. Thus the mean annual increment (MAI) for Pinus species plantations ranges from 3 m3/yr in some Scandinavian countries, to 24 m3/yr in New Zealand. Initiatives such as the Tree Growth Potential Information System (TROPIS) and the Sistema de Manejo de Informacion sobre Recursos Arboreos (MIRA) are making significant improvements to the availability and quality of growth and yield data, as well as the efficiency of research (see also Working Paper FP/1).
Rotation lengths in industrial wood plantations are determined by a number of factors: rate of wood and fibre production; desired wood and fibre properties; and maximisation of site productivity. The over-riding factor that generally determines rotation lengths is profitability. A properly designed forest plantation investment should encompass growth rates and wood properties in an investment equation that marries costs and prices to determine the optimal length of time that the plantation investment should be held. At present, the very shortest reported rotation lengths are five years, for coppiced Eucalyptus plantations in South America. Planted tree rotations of more than 100 years may occur for very high-value species, particularly in some temperate countries. Generally, management inputs under very long rotation regimes have to be very low to be financially viable, and the forest may revert to semi-natural status long before it is harvested.
FAO (2000) estimates current annual rates of new plantation establishment in temperate and boreal countries to be around 750,000 hectares, although this estimate does not include significant areas of enrichment planting and seeding in semi-natural forests. Pandey (1997) reported annual rates of plantation establishment in tropical and subtropical countries to be slightly more than 4 million hectares per annum in 1995, of which 1.65 million hectares was in the tropics and 2.36 million hectares was in the subtropics. Some of this planting (an unknown but, perhaps, large proportion) is actually replanting of harvested areas. Rates of plantation establishment are, however, subject to significant annual variance in most countries and are driven by a range of factors including government finances, general economic conditions, incentives offered to private sector interests, perceptions of forestry profitability, successes of previous planting programmes, and perceptions of future supply-demand imbalances.
The roles that forest plantations will play in meeting demands for wood and fibre in the immediate future are inextricably linked to past and current patterns of plantation establishment. Plantation production through to 2010 will be, almost exclusively, dependent on forests and woodlots that have already been planted. Beyond 2010, future rates of establishment and replanting become increasingly important in determining production from plantations.