The Hazards of Interpretation of Static Age Structures as Shown by Stand Reconstructions in a Pinus Contorta -- Picea Engelmannii Forest

1 Stand reconstruction methods which involved the dating of live and dead trees were used to demonstrate the major flaws inherent in the common approach of using static stand structure to infer stand dynamics. To illustrate the problems with this approach we tested two hypotheses on conifer forest succession: (i) that the empty middle age classes in static stand age-class distributions are the result of a period in stand development in which recruitment was prevented, and (ii) that understorey trees have replaced canopy trees. 2 Reconstructions of age-class distributions at 20-year intervals into the past for five stands ranging in age from 58 to 222 years revealed a short postfire period of high recruitment of both Pinus contorta and Picea engelmannii followed by low and sporadic recruitment for the rest of the life of the stands. The initial postfire cohort also had a lower mortality rate than the subsequent cohorts. This pattern of low recruitment and high mortality for the understorey cohort resulting in the disappearance of these cohorts over time explains the false impression of a period of prevented recruitment in the middle age class of the static stand age distribution. 3 Height-date curves for individual standing live and dead trees, obtained by sectioning trees at 1 -m intervals and ageing the disks, showed little, if any, canopy replacement by trees which were once in the understorey. Again, the combination of low recruitment and high mortality precludes any trees in the subsequent (understorey) cohorts from surviving and growing into the canopy. 4 This study of the dynamics of Pinus contorta Picea engelmannii forest points out that forests must be viewed as a collection of species populations of stacked cohorts and that succession (dynamics) can be understood by studying the recruitment and mortality rates of these cohorts.