Tree branches as populations of twigs

Abstract

Populations of twigs (1-year-old, current branch segments) had similar frequency distributions of twig lengths on branches of five tree species (Quercus coccinea, Acer rubrum, Picea rubens, P. abies, Pinus strobus). The modal number of twigs for each of the species was in the 1, 2, 3, or 4 cm length class, with an exponential decrease in twig number through the 6- to 10-cm class plus a few longer twigs. These characteristic twig frequency distributions developed within the first decade of the life of a branch and were retained as the twig population size changed. Old branches had twigs only less than 5 cm long. The number of new twigs increased as their parent's (2-year-old segments, last year's growth) lengths increased. On parents with more than one twig the terminal twig was longest. Length of the shortest twig, furthest from the terminal, generally increased as parent length increased, but, in Acer rubrum, stayed less than 1 cm. Frequency distributions were determined for the lengths of terminal twigs from parents less than 5 cm long. Mortality of parents in conifers was first indicated when they produced no twigs. A simple computer model simulated growth of twig populations using equations and probabilities to calculate the number and lengths of new twigs formed by parents.