Recent studies in many parts of the world have demonstrated the importance of maintaining ancient forests if woodlands of the highest conservation value, known as woodland key habitats, are to be both identified and retained (see Sections 6.4.1-2). However, as demand for timber grows, it is inevitable that plantations will increase in area. More of our commercial timber already comes from plantations (34% - despite their comparatively small area; Table 1.1) and managed secondary growth forests - i.e. felled and regrowing (22%) -than from less managed forests (34%) (Sedjo and Botkin, 1997). Such plantations often lack the biodiversity of unmanaged forests. In temperate and northern forests, the field and ground layer vegetation are key components in maintaining biodiversity. Fungi, lichens, herbs and shrub species differ considerably between plantation forests grown on former pasture and the sites of felled older woodlands, though all are subject to shade as the woodland matures, so plants characteristic of open sites are gradually lost, often with some of the associated animals. Most, but not all, comparisons of unmanaged forests and plantations show an impoverishment of plants and animals in the plantations. For example, in Britain, lowland Norway spruce Picea abies plantations contain less than 10 species of lichen while lowland oakwood can contain over 40 species (Humphrey et al, 2002) while in montane rain forest 81 species of vascular epiphyte were found in untouched forest compared with 13 species in a hardwood plantation, made up of fewer ferns and orchids but more bromeliads (Barthlott et al., 2001). Similar examples can be given for animals: in tropical forests, Carlson (1986) found forest-interior birds (i.e. living away from forest edges) to be 2-3 times as abundant in intact tropical forest as in radiata pine Pinus radiata plantations, and Vallan (2002) observed more than twice the number of amphibian species in intact Madagascan forests than in eucalypt plantations. Foresters sometimes argue, with reason, that plantations should be viewed in the same way as an agricultural crop (to which they compare very favourably) and not as a replacement for natural forests. Plantations usually include non-native species, are even-aged and often monocultures and so are very distinct from natural forests. But as plantations become a larger part of many countries it is inevitable that they become more vital as reservoirs of forest species.
It is important to appreciate that like any other forest type, plantations vary from even-aged exotic monocultures to much more structurally diverse stands of native species, and so some are better substitutes for natural forest than others. A study in Britain by Ferris et al. (2000) showed that lowland conifer plantations were least like natural forests but mature and over-mature (see Section 10.1.3) pine and spruce stands in the uplands closely matched native pine and oak woodlands. Older, more open stands allow more light to reach the ground, encouraging the vital field and ground vegetation. There is plenty of evidence that the management of any sort of plantation can be altered so that it better conserves biodiversity and is more aesthetically appealing without losing much profitability. This is the idea behind multi-use forests discussed below.
Plantations have a distinct value not only in what species they hold but in their position in the landscape. Plantations can be used as a buffer between natural forests and an agricultural landscape and may be useful in connecting existing forest patches, allowing species to migrate. In England and Wales 83% of ancient woodlands (amounting to 31% of ancient woodland total area) are less than 20 ha in size and there is a great need to facilitate movement of species to prevent local extinctions and inbreeding. Moreover, plantations planted against older woodlands will more speedily acquire species. This is all the more important now that we know that the rates at which many of the key species migrate from ancient sites to more recent woodland are often exceedingly slow (see Section 6.4).
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