Managing risk in clonal forestry.
2006
Burdon, R. D. | Aimers-Halliday, J.
Risks associated with clonal forestry are mainly biological. Among such risks, propagation failure, cultivar decline, and imperfect clonal evaluation all call for specific, targeted countermeasures; delayed failure of clones, however, calls mainly for risk spread. Propagation failures often reduce genetic gain and/or narrow the genetic base. Cultivar decline typically stems from maturation, especially in conifers, and can preclude successful clonal forestry. Maturation can be much delayed by hedging, serial propagation, coppicing (if possible), and cold-storage of tissue, but effectively halting maturation probably requires embryogenesis together with cryogenic storage. Totally restoring juvenility in vegetative material is an ideal, but it is still seldom achievable. Other possibleforms of cultivar decline include plagiotropism, and virus infection in hardwoods. Inadequate clonal evaluation can lead to delayed underperformance, especially where genotype-site interaction is important. Clonal failure can result from biotic and climatic events, which may be unpredictable. Climatic damage may affect only occasional clones, but biotic damage is often more comprehensive. Risk spread, however, will not counter agents that cause unacceptable failure rates among deployed clones. Theoretically, as the effective number of unrelated clones exceeds about 20, the additional protection conferred by risk spread becomes marginal, and genetic gain can become significantly eroded. How to deploy a portfolio of clones, in options ranging from intimate mixture to monoclonal mosaics, is debatable. The appropriate choice can depend on various factors, including the nature of likely damage, the epidemiology of the pathogen or insect pest, the silvicultural regime, and the logistics of possible salvage harvesting.
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