Note on Measurement of Woody Plant Diameter Distributions

Introduction Forest ecologists use the frequency distribution of d.b.h. (diameter at breast height, 1.35 m above ground) of the stems of woody species to describe age/size structures and draw inferences about regeneration strategies, synchrony of climatic events, mortality patterns and the effects of animal browsing (e.g. recent New Zealand studies by Allen, Payton and Knowlton, 1984; Mark, 1963; Mark and Sanderson, 1962; Stewart and Veblen, 1982; Veblen and Stewart, 1980; P. Wardle, 1978; J. Wardle, 1970; 1984). Even if it is assumed that diameter distributions are equivalent to age distributions (but see Harper, 1977; Ogden, 1978; Veblen and Stewart 1980), the d.b.h. convention is not particularly appropriate for demographic analysis. Most obviously, it cannot be applied to stems less than 1.35 m tall. One contemporary solution to the problem partitions woody forest plants into four groups and applies different protocols to each of them (Allen and McLennan, 1983): trees, defined as those > 3 cm d.b.h. and >2 m tall, are measured at breast height; saplings, < 3 cm d.b.h. and > 1.35 m, are counted; 'understorey' plants which exceed 15 cm tall are counted in sub-plots by 16-45 cm, 46-75 cm, 76-105 cm, 106-135 cm and > 135 cm height classes; plants less than 15 cm are recorded in subplots as present or absent. Although such measurements can be spliced into a statement of apparent demographic pattern (e.g. Allen, Payton and Knowlton, 1984), the procedure is complex and the criteria of size are not constant throughout the life cycle of plants. Accordingly, Batcheler and Craib (1985) suggested the use of a simple extension of the d.b.h. protocol by which stems ≥ 2.6 m are measured at breast height, whereas those < 2.6 m tall are measured at half their height. The measurement, d.b.h.h. (diameter at breast height or half height), yields a continuous range of size of all plants in a population. Second, in dealing with shrubby species, Batcheler and Craib recognized that recording each stem as a plant results in high estimates of density and low estimates of mean plant basal area. They therefore adopted a procedure by which where more than one stem occurs at the d.b.h.h. measurement point, each stem is measured and the size of the plant is represented by the diameter equivalent to the combined cross sections of the stems.