Stand Structure and Allometry of Trees During Self-Thinning of Pure Stands

Abstract

SUMMARY (1) Stand structure and mean weight-density relations of nearly pure, dense, even-aged, natural stands of Prunus pensylvanica and Abies balsamea in the north-eastern U.S.A. were examined and related to allometric growth. Values for the exponents of allometric and mean weight-density equations were estimated by principal components analysis of logarithmically transformed data. (2) It is proposed that soon after a stand of woody plants becomes established the sizefrequency distribution is a negatively skewed, bell-shaped curve; the distribution subsequently becomes positively skewed, and eventually approaches normality after substantial thinning. Maximum positive skewness occurs at the tiine self-thinning begins. (3) The weight-frequency distribution undergoes a parallel series of stages: roughly normal at first, but quickly changing to lognormal, with maximum skewness attained at the tiine thinning begins. These curve-forins are proposed only as approximations to empirically observed distributions. A consistent tendency toward bimodality is one commonly-observed departure from the idealized distributions. (4) The calculated exponents of the inean weight-density equations for Prunus pensylvanica and Abies balsamea were - 1 46 and - 1 22, as compared to the proposed value of - 15 (- 3/2 power 'law' of self-thinning). In general, weight changes in plant parts during self-thinning did not parallel those for whole plants; in particular, the mean weight-density exponent for foliage was approxiinately - 1 0. The exponents of inean weight-density equations for total roots and total shoots, however, approximately equalled the exponent for whole plants. (5) It is concluded that observed patterns of alloinetric growth are incompatible with inean weight-density equations for whole plants, unless a mutual adjustment between allometry and stand structure is assumed.