Temporal, spatial, and structural patterns of adult trembling aspen and white spruce mortality in Quebec's boreal forest

Temporal, spatial, and structural patterns of adult trembling aspen (Populus tremuloides Michx.) and white spruce (Picea glauca (Moench) Voss) mortality were studied in intact 150-year-old stands in the southwestern boreal forest of Quebec. For both species, mortality decreases (number of dead trees/total number of trees) with distance from the lake edge until 100-150 m, from which point it slightly increases. Strong peaks in mortality were found for 40- to 60-year-old aspen mainly between 1974 and 1992. Such mortality in relatively young aspen is likely related to competition for light from the dominant canopy trees. Also, the recruitment of this young aspen cohort is presumably the result of a stand breakup that occurred when the initial aspen-dominated stand was between 90 and 110 years old. For spruce, strong peaks in mortality were found in 110- to 150-year-old trees and they occurred mainly after 1980. No clear explanation could be found for these peaks, but we suggest that they may be related to senescence or weakening of the trees following the last spruce budworm outbreak. Suppressed and codominant aspen had a much higher mortality ratio than spruce in the same height class, while more surprisingly, no difference in mortality rate was found between dominant trees of the two species. Most spruce trees were found as standing dead, which leads us to reject the hypothesis that windthrow is an important cause of mortality for spruce in our forests

Evaluating the performance of morphological models

Evaluating the performance of numerical models of coastal morphology against observations is an essential part of establishing their credibility. In the past, this has usually been done by comparing predicted with observed behavior, and the modeler making a subjective judgement of the goodness of fit. By contrast, meteorological models have been tested for many years against observations using objective, scientifically rigorous methods. Drawing on the analogy between distributions of atmospheric pressure (and similar scalars) and coastal bathymetry, the meteorological statistical methods are here adapted to coastal applications. A set of criteria is proposed that identify the desirable attributes of statistical measures of the performance of coastal morphological models. Various statistical parameters used in the meteorological field are described, dealing both with cases in which the model outputs comprise a small number of categories (e.g., advance/equilibrium/retreat of shoreline), and those in which they are continuous (e.g., bathymetry). Examples of the application of these methods are given, in terms of both hypothetical illustrations, and real field data and model predictions. Following meteorological practice, it is shown that measuring the skill of a model (i.e., its performance relative to a simple baseline predictor) is a more critical test than measuring its absolute accuracy. The attributes of the different performance measures are compared with the proposed desirable criteria, and those that match them best are selected. These are the LEPSOB test for categorical data, and the Brier Skill Score (BSS) for continuous data. Routine use of these measures of performance would aid inter-comparability of models, and be a step toward strengthening user confidence in the predictions of coastal numerical models