TWO-STORIED HIGH FOREST

The system has been adopted at Dartington with Japanese larch(Larix leptolepis), and it has been suggested that it worksthere because Japanese larch grows fast. It is argued in thispaper, however, that the system is best adapted to rather slowlygrowing trees, and the reason for its use with Japanese larchis that this species is characterized by a rapid height growthbut a small current annual increment, so that, unless the treesare isolated, they make extremely narrow annual rings. WithDouglas fir (Pseudotsuga taxifolia) and Sitka spruce (Piceasitcbensis) the annual rings might become too broad if thistreatment were applied, except at a rather advanced age. Nevertheless,the system has certain other advantages and, to attain these,it may be useful to adopt it more extensively, even if the annualrings become broader than we like.     

THINNING GRADES BASED ON THE THICKNESS OF ANNUAL RINGS

This paper propounds a new method of calculating numerical thinningschedules for conifers which are designed to give, on the average,any reasonable pattern of ring width at breast height in the100 largest trees in a plantation. It starts from the ForestryCommission's Revised yield Tables for Conifers in Great Britainand is based on the principle that, if the current annual incrementin a plantation is unaffected by the grade of thinning, thering widths must be inversely proportional to the bole areaper acre, i.e. the total area of cambium on the main stems ofthe trees. Sources of error in the method of calculation arediscussed, and it is considered that such errors are not soserious as to endanger the use of the method as a means of calculatingapproximate thinning schedules. As an approximate yield tablecan be constructed for each thinning grade, the method can beused to estimate the relative costs of growing trees of variousdiameters with various ring width patterns.