Creaming the best, or creatively transforming? Might felling the biggest trees first be a win-win strategy?

摘要

There are several possible strategies for diversifying the age-class structure of an even-aged forest, each with different economic implications. An experiment in a privately owned woodland at Trallwm, Mid-Wales, is examining five possibilities. The treatment of particular interest, "creaming", derives from a strictly financial objective. The largest trees are removed successively; but not, as in some experimental treatments, at a desired individual target diameter. Instead, it aims to remove enoughof the largest trees in mid-rotation to achieve a target crop basal area. The immediate result should be greater revenue per cubic metre harvested, compared with alternative treatments, and harvesting cost per cubic metre less than from all other transformation strategies. The longer-term issues, which are addressed by means of a growth model, are what effect this will have on revenue and increment in the remainder of the transformation period; whether gaps in the canopy - smaller than in group felling,but larger than in classical selection - will suffice for adequate natural regeneration, and so allow reduced regeneration cost; whether regeneration can be economically managed so as to concentrate increment onto stems that will eventually be harvestedat large size; whether early removal of the largest trees will cause a significant dysgenic effect. The possible outcomes are explored by means of a simple model of future stand growth. Parameters are derived from a time study of harvesting at Trallwm.On the whole, the results of creaming are quite favourable, even in comparison with continued clear felling, and much more favourable than those of group felling.