by equipewp | 22 February 2019
Published in Reforesta 6: 60-70. https://dx.doi.org/10.21750/REFOR.6.05.58. Stock types used in reforestation projects can influence plantation success, as they determine the morphological attributes of the planted seedlings. They can also interact with silviculture treatments to influence early seedling survival and growth. As nurseries develop and produce new stock types in response to -and in interaction with- manager needs, research efforts must be pursued to validate early seedling performance and long-term growth and yields. In this context, we aimed to evaluate the main and interactive effects of mechanical site preparation and stock type on planted black (<i>Picea mariana</i> [Mill.] BSP) and white spruce (<i>P. glauca</i> [Moench.] Voss) seedling dimensions at 16-y, and estimate the long-term impact of stock type on the merchantable volume at rotation age for white spruce. We hence compared medium (200 cm<sup>3</sup> root plug) and large (350 cm<sup>3</sup> root plug) containerized seedlings, as well as large bare-root seedlings of both species, in a field experiment established in Quebec (Canada), where there is a ban on the use of chemical herbicides for vegetation management treatments. Our results confirm that there is a significant, although limited impact of stock type on the size of black and white spruce at the juvenile stage, when medium and large stock types are compared, but that these small differences have a negligible effect on the estimated merchantable volume produced at rotation age (60 years). Mechanical site preparation does not promote seedling growth on these rich sites with thin humus. Therefore, selection of a medium or larger stock type for reforestation projects and application of mechanical site preparation in ecosystems similar to the one studied here should be based on other considerations than growth and yield, such as seedling availability, production and planting costs, or operational constraints.
by Marie-Claude Boileau | 30 January 2019
Published in Ecohydrology 11(7): e2014. https://doi.org/10.1002/eco.2014
Sapwood area is an important parameter for estimating transpiration at whole-tree and watershed scales, given that it is used for scaling up point sap flow measurements. However, its determination may be difficult, particularly for large-diameter trees of diffuse-porous, broad-leaved species. By delineating only the highly conductive sapwood, electrical resistivity tomography (ERT) may be more appropriate than sapwood dyeing for estimating tree sapwood area. We compared sapwood area measurements made with ERT with those that were obtained from wood dyeing, assuming that the more adequate measure was that which was best correlated with tree leaf area. To achieve this objective, we sampled 31 sugar maple (Acer saccharum Marsh.) and 20 yellow birch (Betula alleghaniensis Britt.) trees covering a large range of tree diameters and leaf areas from one site located in eastern Canada. Also, 79 sugar maple trees were sampled in two other sites to document the estimation of tree leaf area over a large territory. The sapwood area that was determined from ERT appeared to be the best method for scaling up point sap flow measurements to whole-tree transpiration. The predictive ability of ERT was particularly greater than that of wood dyeing when they were both measured at 1 m. If the objective is to estimate tree leaf area rather than sapwood area, the crown surface area would provide a useful substitute for the ERT method because it is rapid and easy to measure while serving a robust predictor applicable over a large territory.
by Marie-Claude Boileau | 30 January 2019
Published in Scandinavian Journal of Forest Research 33(8). https://doi.org/10.1080/02827581.2018.1495254
Commercial thinning enables forest managers to meet timber production objectives. Thinning reduces tree density to alleviate competition for resources and favour growth of selected tree species. However, in doing so, thinning can homogenize the composition of mixed-species forests and raise biodiversity issues. There is increasing evidence that species richness can lead to higher productivity through a complementarity effect. Hence, thinning that would maintain species diversity of mixed-species forests could enhance stand productivity and help forest managers to reconcile timber production objectives and biodiversity issues. The objective of this study was to compare post-thinning stand production, experimentally over 10 years, in mixed and monospecific stands of black spruce (Picea mariana [Mill.] B.S.P.) and jack pine (Pinus banksiana Lamb.). The post-thinning stand production curve of the mixed stand converged toward that of the unthinned mixed stand while the production curves of the thinned and unthinned monospecific stands remained parallel. The convergent productivity of the mixed stand could be explained by a positive interaction between effects of thinning and niche complementarity. We propose that thinning that maintains species diversity of mixed stands could help forest managers who are implementing ecosystem management to reconcile timber production objectives with biodiversity issues.
by Claire Morin | 30 January 2019
Published in Global Change Biology 24(10): 4797-4815. https://doi.org/10.1111/gcb.14365
The accumulation of soil carbon (C) is regulated by a complex interplay between abiotic and biotic factors. Our study aimed to identify the main drivers of soil C accumulation in the boreal forest of eastern North America. Ecosystem C pools were measured in 72 sites of fire origin that burned 2–314 years ago over a vast region with a range of ∆ mean annual temperature of 3°C and one of ∆ 500 mm total precipitation. We used a set of multivariate a priori causal hypotheses to test the influence of time since fire (TSF), climate, soil physico-chemistry and bryophyte dominance on forest soil organic C accumulation. Integrating the direct and indirect effects among abiotic and biotic variables explained as much as 50% of the full model variability. The main direct drivers of soil C stocks were: TSF >bryophyte dominance of the FH layer and metal oxide content >pH of the mineral soil. Only climate parameters related to water availability contributed significantly to explaining soil C stock variation. Importantly, climate was found to affect FH layer and mineral soil C stocks indirectly through its effects on bryophyte dominance and organo-metal complexation, respectively. Soil texture had no influence on soil C stocks. Soil C stocks increased both in the FH layer and mineral soil with TSF and this effect was linked to a decrease in pH with TSF in mineral soil. TSF thus appears to be an important factor of soil development and of C sequestration in mineral soil through its influence on soil chemistry. Overall, this work highlights that integrating the complex interplay between the main drivers of soil C stocks into mechanistic models of C dynamics could improve our ability to assess C stocks and better anticipate the response of the boreal forest to global change.
by Claire Morin | 30 January 2019
Published in Canadian Journal of Forest Research 47(8): 1066-1074. https://doi.org/10.1139/cjfr-2016-0498
Survival analysis methods make better use of temporal information, accommodate multiple levels of explanatory variables, and are meant to deal with interval-censored data. In a context of harvest modeling, this approach could improve some known limitations. In this study, we used data from a network of permanent plots in the province of Quebec, Canada, as a real-world case study. We tested the potential of survival analysis to predict plot-level harvest probabilities from plot- and regional-level variables. The approach also included random effects to account for spatial correlations. The results showed the potential of survival analysis to provide annual predictions of harvest occurrence. Both regional and time-varying variables, as well as spatial patterns, had important effects on the probability of a plot to be harvested. Respectively, reductions in the annual allowable cut volumes led to a decrease in the harvest probabilities. Greater harvest probabilities were associated with the broadleaved dynamics class and higher values of basal area. In contrast, they were decreased by stem density and slope classes. The spatial random effect resulted in an improvement of the model fit. Our plot-level model improved some limitations reported in previous studies by taking the effect of a time-varying regional variable into account.
