Growth and the limitations to it are of general interest to forest ecologists and of particular concern to foresters who grow trees as a crop. All tree species influence the nutrient content and other aspects of the soils in which they grow. Whether this results in an overall advantage or disadvantage to themselves, other plants, animals and the soil often depends upon local conditions and forestry practice. This chapter investigates how growth is measured, how it accumulates as biomass and the distinction between how much a tree can grow each year versus how much biomass the whole forest ecosystem can accumulate. The second part deals with nutrient flows and the problems of gains, losses and limitations. Much of what we know about temperate hardwood forests stems from the work carried out at Hubbard Brook (see Box 8.1).

Growth of forests

Biomass and productivity

The sheer size of forest and woodlands is what people often comment upon. The weight or mass of organic material present is referred to as the biomass (or sometimes the standing crop). It should be borne in mind that this can be a somewhat loose term since it may or may not include dead wood or litter. Table 8.1 shows that the biomass above ground increases from the boreal forest towards the tropics, starting from very low levels at the Arctic treeline and reaching in excess of 940 t ha-1 in the Amazon basin. However, there are exceptionally large forests outside the tropics, the record-holders being the temperate forests of the Pacific Northwest of North America including stands of huge Douglas fir (1600 t ha-1) and coastal redwoods, the tallest trees in the world (trunk biomass of 3450 t ha-1 with total net primary productivity (NPP) possibly approaching a staggering 4500 t ha-1 y-1).