Predicting site index from climatic, edaphic, and stand structural properties for seven plantation-grown conifer species in Québec

by Marie-Claude Boileau | 30 January 2019

Published in Canadian Journal of Forest Research 41: 682-693

Models were built to relate site index to biophysical variables and stand structure for seven conifer species grown in plantation. Reduced models without stand structure were also derived to allow the prediction of site index even when information on the stand is unavailable. Biophysical variables tested were degree-days, aridity, summer precipitation, vapour pressure deficit, and soil water-holding capacity.

The stand structure was accounted for using a Shannon evenness index of diameter at breast height distribution. Both full and reduced models had low to moderate R² values and were slightly biased for most of the species. The results indicate that while correlations between phytometric and biophysical site indices are rather weak, those between plantation yield predicted using both indices and volume observed in the field are reasonably high (above 0.80). The biophysical site index models derived in this study could be used for strategic planning to estimate plantation yield or allowable cut for each of the seven conifer species.

Sugar maple (Acer saccharum Marsh.) growth is influenced by close conspecifics and skid trail proximity following selection harvest

by Marie-Claude Boileau | 30 January 2019

Published in Forest Ecology and Management 258: 823-831

In this study, we quantified the effects of local neighbourhood competition, light availability, and proximity to skid trails on the growth of sugar maple (Acer saccharum Marsh.) trees following selection harvest. We hypothesized that growth would increase with decreasing competition and increasing light availability, but that proximity to skid trails would negatively affect growth. A total of 300 sugar maples were sampled ~10 years after selection harvesting in 18 stands in Témiscamingue (Québec, Canada). Detailed tree and skid trail maps were obtained in one 0.4 ha plot per stand. Square-root transformed radial growth data were fitted to a linear mixed model that included tree diameter, crown position, a neighbourhood competition index, light availability (estimated using the SORTIE light model), and distance to the nearest skid trail as explanatory variables. We considered various distance-dependent or -independent indices based on neighbourhood radii ranging from 6 to 12 m. The competition index that provided the best fit to the data was a distance-dependent index computed in a 6 m search radius, but a distance-independent version of the competition index provided an almost equivalent fit to data. Models corresponding to all combinations of main effects were fit to data using maximum likelihood, and weighted averages of parameter estimates were obtained using multimodel inference. All predictors had an influence on growth, with the exception of light. Radial growth decreased with increasing tree diameter, level of competition and proximity to skid trails, and varied among crown positions with trees in suppressed and intermediate positions having lower growth rates than codominants and dominants. Our results indicate that in selection managed stands, the radial growth of sugar maple trees depends on competition from close (< 6 m) conspecific neighbours, and is still affected by proximity to skid trails ~10 years after harvesting. Such results underscore the importance of minimizing the extent of skid trail networks by careful pre-harvest planning of trail layout. We also conclude that the impact of heterogeneity among individual-tree neighbourhoods, such as those resulting from alternative spatial patterns of harvest, can usefully be integrated into models of post-harvest tree growth.

Using longitudinal survival probabilities to test field vigour estimates in sugar maple (Acer saccharum Marsh.)

by Marie-Claude Boileau | 30 January 2019

Published in Forest Ecology and Management 256: 1771-1779

Tree mortality is a major force driving forest dynamics. To foresters, however, tree mortality is often considered a loss in productivity. To reduce tree mortality, silvicultural systems, such as selection cuts, aim at removing trees that are more likely to die. In order to identify trees with higher risks of mortality, field classifications are employed that assess vigour based on external characteristics of trees. We used a novel longitudinal approach for estimating survival probabilities based on ring-width measurements, initially developed by Bigler and Bugmann [Bigler, C., Bugmann, H., 2004. Predicting the time of tree death using dendrochronological data. Ecol. Appl. 14 (3), 902–914], to parameterize a survival probability model for sugar maple (Acer saccharum Marsh.) and to test whether field-assessed tree vigour classes are corroborated by survival probabilities determined from radial growth history. Data from 56 dead and 321 live sugar maples were collected in stands in western Quebec (Canada) that had undergone a selection cut ~10 years prior to sampling. Our results showed that tree vigour established from external defects and pathological symptoms, using the classification of Boulet [Boulet, B., 2005. Défauts externes et indices de la carie des arbres: guide d’interprétation. Publication du Québec, Sainte-Foy, Québec. 291 pp.], is partially corroborated by growth-driven survival probabilities. Moribund trees had lower survival probabilities than vigorous trees over several years in the period prior to vigour assessment. Intermediate vigour classes showed less obvious tendencies, but this may be due to the growth-independent nature of some defects used for their classification. Although the timing of tree death may not be correctly predicted by the vigour classification (i.e., our results suggest that time of death generally was overestimated), its general agreement with survival probabilities determined from growth series make it a useful tool for tree selection in sugar maple stands under selection management.