Introduction

Forest ecosystems cover little more than one-quarter of the earth's land surface (3.5 billion ha; FAO, 1995). However, approximately 85% of plant carbon (C) and 35% of soil C is found in forests (Kirschbaum and Fischlin, 1996). As such, they contain over 60% of the C stored in the terrestrial biosphere. Thus, forest ecosystems constitute a major terrestrial C reservoir, which may exert a significant impact on the atmospheric CO2 concentrations ([CO2]). Within the framework of the changing global C balance, there is a large potential for forests to act as a sink for atmospheric CO2. This could happen either through reforestation or through enhanced tree growth rates. Enhanced tree growth rates have been attributed to the net CO2 fertilization effect, to increased nitrogen (N) deposition rates (Spiecker et al., 1996) and to a prolongation of the growing season (Myneni et al., 1997). However, forests may also act as a net source and increase the atmospheric [CO2] through changes in land use such as, deforestation for timber products or the creation of pasture and rangeland.

Although most of the terrestrial biomass resides in forests, we live in a world where wood and woody products are increasingly scarce. Natural forests have been reduced from occupying nearly 46% of the earth's terrestrial ecosystems in pre-industrial times to only 27% today (Winjum and Schroeder, 1997). In recent years the need to intensify forestry has focused worldwide attention on the economic importance of tree plantations, and in particular fast-growing tree plantations. Forest plantations in the world now total approximately 130 Mha, with annual rates of establishment of about 10.5 Mha. Plantations of fast-growing trees are usually managed as short-rotation plantations (with growing cycles of less than 15 years) or as agro-forestry systems, often for the production of industrial raw materials or biomass for energy. Under the designation of fast-growing tree plantations, or

┬ęCAB International 2000. Climate Change and Global Crop Productivity (eds K.R. Reddy and H.F. Hodges)

short-rotation silviculture, one may find ecosystems managed for different economic objectives, with different intensities of cultural management and different levels of productivity. They may include any of a wide range of species grown under various environmental conditions. A common factor is the greater possibility that exists, relative to conventional forestry, for manipulation of both the environment and the genetics of the trees. Another obvious distinction between natural forests and agro-forestry plantations is in the processes and adaptive strategies available to managers of managed and unmanaged ecosystems.

A crop could be defined as a biological system tailored to give certain products. While planted forest trees meet the criteria of this definition, they are also economically important, whether in a managed or unmanaged stage. Global production of woody forest products is about 4 billion m3, and that of paper and pulp is 0.5 billion tonnes (Table 11.1). Timber and pulp are not the only economic tree products. Over 33 Mha of the world's terrestrial surface is covered with fruit, palm, rubber tree, or vineyard plantations (Goudriaan, Wageningen, 1998, personal communication). Together these produce a very significant part of all human food and other consumer products (Table 11.2).

Modern silviculture is designed to handle forest stands in such a manner that the saleable yield is maximized. Productivity and yield of cultivated, planted forests (whether short-term or medium-term) are influenced by genetic and physiological factors (Tigerstedt et al., 1985); so to maximize forest yields, both aspects have to be optimized in the silvicultural system. In the 1960s, agricultural crop physiologists realized drastic increases in, for example, grain production by improving the genotype/environment interactions. Tree physiologists and breeders are some 20-30 years behind their agricultural and horticultural colleagues, but in all parts of the world the forest tree is being turned into a cultivated plant. Basically, forest trees are not that different from other (agricultural or horticultural) crops.

Table 11.1. World production of wood and wood-derived products. (FAO, 1995.)

Product

Production

Woody materials (106 m3 year-1)

Fuelwood and charcoal

1920

Industrial roundwood

1430

Sawnwood and sleepers

430

Wood based panels

140

Total

3920

Wood-derived products (106 t year-1)

Wood pulp

170

Other fibre pulp

20

Recovered waste paper

100

Paper and paper board

290

Total

580

Table 11.2. World production of economic non-woody tree products (in 106 t year-1). (FAO, 1995.)

Product

Production

Multi-seed and stone fruits3

114

Citrus fruits

89

Bananasb

84

Coconut

45

Palm oil

15

Coffee and cacao beans

7

Coprah

6

Grapesc

6

Rubber

6

Tree nuts

4

aIncludes apples, peaches, plums, pears, apricots, cherries, olives, avocados, mangoes and papayas.

bIncludes banana fruits and plantains. cData from OIV (1998).

aIncludes apples, peaches, plums, pears, apricots, cherries, olives, avocados, mangoes and papayas.

bIncludes banana fruits and plantains. cData from OIV (1998).

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