Ageing and decline of trembling aspen stands in Quebec

The onset of stand decline is a critical stand development characteristic that can affect both harvesting schedule and annual allowable cutting volume. The age at which stand decline begins was assessed in trembling aspen (Populus tremuloides Michx.) stands in the province of Quebec, Canada, by three different methods. The first used a data source consisting of 1081 temporary sample plots from which a segmented regression model was fitted to detect any deviation from the self-thinning line. This type of deviation was observed below densities of 720 stems/ha, which is normally attained around age 60 on good-quality sites. The second used a data source composed of 34 permanent sample plots that allowed us to calculate the age at which the periodic annual increment was equal to zero. The estimated age at which stands began to decline averaged 64 years, but the large variation around this mean was poorly explained by site factors. The third method used a data source consisting of 98 inventory plots. Volume of tree mortality and of wood decay showed an important increase when stand age was around 60. Hence, the results from three independent sources of data converge toward a generalized loss of aspen volume around age 60. However, the prediction of the age at which decline begins in any particular aspen stand is imprecise and requires an on-site measurement of the stand state.