Commenwealth Forestry Institute, University of Oxford
Abstract
Forest management, particularly plantation development, arose largely in response to the erosion of natural forest resources by agriculture. The objectives, skills and methods of farmer and forester have often conflicted yet the rapid rate of improvement possible with annual crops provides lessons and guidance for the development of perennial crops... The farmer is concerned mainly with intensive management of single crops on good sites yielding one product in which uniformity is at a premium and for which the value per unit of volume is high. In contrast the forester commonly acts as an ecologist maintaining annual yields of low value products by extensive management of variable crops on poorer sites with several objectives including protection and amenity as well as production which itself includes a range of properties for a range of uses. The evolution, domestication and some 50 years of intensive breeding of many annual crops have reduced the genetic base, attaining a selection plateau beyond which further gain requires introduction of genes from wild populations in the .natural origins. Forest trees are only two or three generations removed from the wild type and, with the exceptions of some intra-specific, natural population variants and some restricted national breeding populations, tree species have considerable genetic variation available. Nevertheless conservation of genetic resources is essential and, for many species, it is in process. The comparative testing of populations or individual genotypes is more difficult in perennial crops because the extended rotation period and large size of individuals complicate the design, management, assessment and analysis of field trials. The long rotation period and the physical size provide many opportunities in time and space for a range of pests to attack trees, and trees contain large proportions of dead material, such as heartwood, which are subject to types of pest that do not occur in annual crops. Cost-benefit analysis of perennial crops does not encourage the use of artificial pest control measures and the extended commercial rotations and reproductive cycles of forest trees make it essential to develop early testing methods for heritable pest resistance, within the constraints of breeding for several other traits. The bulk of forest plantations are based on introductions of exotic species for which there may be delays in the adaptation of local pests and in the accidental introduction of pests from the trees' natural source. Growth characters, however, may be rapidly changed in new and often sub-optimal environments. Also the adoption of monocultures, even with indigenous species, has provided ideal conditions for establishment and spread of pests. The practice of selective tree breeding and the use of clonal plantations intensify the risk of outbreak. The development of heritable resistance requires knowledge of the biology and life-cycle of the pest (often lacking in tree pests) and it requires at least several years in perennial crops. For trees it is thus of little value in an emergency. Further, prediction of pest problems is less certain with trees than with agricultural crops, since pests of exotic plantations are far from being stabilized. A much higher proportion of important tree pests is likely to be due to facultative, unspecialised organisms. Further, the nature of the problems force the forester to seek resistance mechanisms that are often more complex than those already understood and exploited for annual crops. The possibility of breakdown of resistance and the long generation interval of trees makes vertical resistance unattractive, even though several kinds of major gene resistance may occur in a single host and even with recycling and clonal mixtures. The most appropriate type of resistance is the polygenic horizontal resistance with its conceptual extension to generalised resistance to a wide range of pest species.</p