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Abstract: Soil organic carbon (SOC) in mineral soil accounts for a large portion of total ecosystem carbon (C) in boreal forests. We evaluated the effects of stand age and disturbance origin on SOC, soil aggregate stability, and aggregate-associated SOC in the boreal forests of Ontario, Canada. Mineral soils at 0–15 cm depth were sampled in 27 stands of six post-fire age classes (2- to 203-year-old) and three post-clearcut age classes (2- to 29-year-old), each with three replications. In post-fire stands, the SOC pool increased from 2- and 10-year-old to 29-, 85- and 140-year-old, and then decreased in 203-year-old stands. Aggregate-associated SOC showed a similar trend. Abundance of water stable aggregates (>0.25 mm in diameter) was the highest in 2-year-old stands. Compared with the same-aged post-fire stands, the SOC pool and aggregate-associated SOC were higher, and aggregate stability was lower in 2- and 10-year-old post-clearcut stands. But the differences in SOC pool, aggregate-associated SOC, and aggregate stability between the two stand origins diminished or became less dramatic in 29-year-old stands. Our results indicate that aggregate stability is more dependent on thermal modification of SOC by fire than on aggregate-associated SOC. Our results also show higher SOC pool and aggregate-associated SOC but lower aggregate stability in post-clearcut than post-fire stands shortly after disturbance; however, differences between the two stand origins diminish when stands become older. [Copyright &y& Elsevier]

Abstract: Although coarse root (diameter > 1 cm) biomass can account for approximately 30% of total biomass in forest ecosystems, coarse root biomass models are rarely available, especially for larger trees. In this study, we used a back-hoe excavator and pressurized water to excavate the coarse root systems of large size trees of Abies balsamea L., Picea mariana (Mill.) BSP, Pinus banksiana Lamb., and Populus tremuloides Michx. in the boreal mixedwood forest of central Canada in order to develop allometric equations that link stem diameter at breast-height (DBH) and height to coarse root biomass. All regressions for coarse root biomass using DBH or height alone, or both DBH and height as predictors were significant (R 2 ≥ 0.796, P < 0.05). The DBH – coarse root biomass models had higher R2 values than the height – coarse root biomass models for all four species, indicating that DBH was a better predictor of coarse root biomass than height. In addition, the DBH-height – coarse root biomass models did not have higher R2 values than the DBH – coarse root biomass models. All but one DBH – coarse root biomass model from the published literature with similar DBH range underestimated or overestimated coarse root biomass using the data from this study. It would appear that coarse root biomass allometric equations are probably site-specific because above- and below-ground biomass allocation differs with site condition. However, differences among studies may also be attributable to sampling methodologies. [Copyright &y& Elsevier]

Abstract: Tree diversity is an important component of biodiversity. Management intensification is hypothesized to affect tree diversity. However, evidence to support the relationship between management intensity and tree diversity in northern forests is lacking. This study examined the effects of fertilization, site preparation, and brush control on tree species diversity, shade tolerance diversity and size diversity of jack pine (Pinus banksiana Lamb.), black spruce (Picea mariana [Mill.] B.S.P.), white pine (Pinus strobus L.) and white spruce (Picea glauca [Moench] Voss) plantations, 15 years after planting in Ontario, Canada. Species diversity and shade tolerance diversity were highly correlated, so were diameter size diversity and height size diversity. Fertilization did not affect the tree diversity indices of any plantations. Species diversity and shade tolerance diversity was interactively influenced by site preparation and brush control in the black spruce, white pine, and white spruce plantations, showing that the highest diversity occurred on sites with intensive site preparation without brush control, whereas on sites with brush control, diversity was higher with least intensity of site preparation. However, in the jack pine plantation, neither species diversity nor shade tolerance diversity differed with management intensification, and is attributed to the fast capture of site resources by the planted crop trees of jack pine which minimized establishment of non-crop species. Tree size diversity increased with site preparation intensity in the jack pine and black spruce plantations, while it decreased with brush control in the white pine and white spruce plantations. We concluded that (1) the effects of management intensification on diversity of northern plantations differ with growth habit of planted crop tree species and (2) species diversity and tree size diversity tend to be highest at intermediate levels of silvicultural intensification during the stand establishment phase, supporting the intermediate disturbance hypothesis. [Copyright &y& Elsevier]

Stand structure diversity is hypothesized (i) to increase with stand development and (ii) to be greater in mixedwood stands than in conifer and broadleaf stands. We examined the effects of time since stand-replacing fire (TSF) and overstory type on stand volume, stand density, and tree-size variability, which is measured using Shannon’s diversity index (H′) and coefficient of variation, in fire-origin boreal forest stands. We sampled 36 stands representing conifer, mixedwood, and broadleaf overstory types, ranging in ages from 72 to 201 years TSF on upland mesic sites in northwestern Ontario, Canada. Stand volume decreased in older mixedwood and broadleaf stands, but followed a U-shaped pattern in conifer stands with TSF. Diameter-at-breast-height-based H′ followed an inverse U-shaped pattern with TSF for all overstory types. Height-based H′ decreased with TSF in conifer and mixedwood stands but peaked at the intermediate age class in broadleaf stands. Diameter-at-breast-height- and height-based coefficient of variation indices followed an inverse U-shaped distribution with TSF. Our results partially supported the two hypotheses, as (i) the 124- to 139-year-old stands were most diverse and (ii) mixedwood stands were more than or as equally diverse as conifer and broadleaf stands, depending on stand development stage and the diversity indices used. Nous posons les hypothèses que (i) la diversité de la structure des peuplements augmente au fil de leur développement et (ii) cette diversité est plus grande dans les peuplements mixtes que dans les peuplements résineux et feuillus. Dans des peuplements boréaux issus de feu, nous avons étudié les effets du temps écoulé depuis le dernier feu (TDF) et de la composition du couvert dominant sur le volume et la densité du peuplement et sur la variabilité de la taille des arbres, déterminée à l’aide de l’indice de diversité de Shannon (H′) et du coefficient de variation. Sur des stations mésiques et bien drainées du nord-ouest de l’Ontario, au Canada, nous avons échantillonné 36 peuplements représentant des peuplements résineux, mixtes et feuillus pour lesquels le TDF variait de 72 à 201 ans. Le volume à l’hectare diminuait dans les vieux peuplements mixtes et feuillus, mais la relation entre le volume et le TDF suivait une trajectoire en U dans les peuplements résineux. La relation entre la valeur de H′ basée sur le diamètre à hauteur de poitrine et le TDF suivait une trajectoire en U inversé pour tous les types de peuplement. La valeur de H′ basée sur la hauteur diminuait avec une augmentation du TDF pour les peuplements résineux et mixtes, mais a atteint un sommet à la classe d’âge intermédiaire dans les peuplements feuillus. Les relations entre le TDF et la valeur du coefficient de variation basée sur le diamètre à hauteur de poitrine et sur la hauteur suivaient des trajectoires en U inversé. Nos résultats n’ont que partiellement supporté les deux hypothèses puisque (i) les peuplements de 124 à 139 ans étaient les plus diversifiés et (ii) la diversité des peuplements mixtes était supérieure ou égale à celle des peuplements résineux et feuillus, dépendamment du stade de développement des peuplements et de l’indice de diversité utilisé. [ABSTRACT FROM AUTHOR]

• The change in temperature determined the performance of introduced sugar maple. • A moderate increase in precipitation would ameliorate heat stress. • The leaf level plastic responses influence the survival and growth of sugar maple. Global climate change will lead to new combinations of temperature and precipitation patterns. Extreme precipitation events will become more common by the late 21st century due to anthropogenic warming, particularly in high latitudes; however, the specific responses of high latitude species are mostly unknown. We employed an introduction trial by transplanting temperate sugar maples (Acer saccharum Marsh) from three provenances of their native ranges within Canada, which had a mean annual temperature (MAT) ranging from 3.0 to 9.4 ℃, and mean annual precipitation (MAP) from 520 to 1190 mm. These maples were transplanted to four sites in subtropical China with a MAT of 15.8–16.2 ℃ and a MAP of 913 mm, 1001 mm, 1172 mm, 1490 mm, respectively. We measured the survival and growth of the first-year seedlings of all provenances, as well as the survival, growth, and leaf morphological and physiological traits of the seedlings of Ontario provenance after four years of acclimation. We found that the first-year survival and growth of all provenances increased with the MAP at the planting sites, peaked at 1172 mm, and then decreased at the 1490 mm MAP site, with the seedlings of Ontario provenance had the best overall survival and growth rates across all of the sites. At the end of the 4th growing season, the seedlings of the Quebec and Manitoba provenances died off, and the survival- and growth-MAP relationships of the Ontario provenance mirrored those observed in the first year. For the Ontario provenance at year 4, the major vein density, stomatal density and size, and non-structural carbohydrates (NSC) changed significantly with MAP at the introduced sites. Principal component analysis indicated that seedling survival was negatively associated with stomatal size, density and starch content and that growth was positively related to leaf major vein density and negatively to soluble sugar and NSC. Our results indicate that changes in the precipitation range determined the performance of introduced sugar maple at the introduced sites. A moderate increase in precipitation was observed to ameliorate heat stress and benefit plant growth; however, excessive water could result in a fatal effect. Moreover, our findings suggest that the plasticity of leaf morphological and physiological traits influenced the survival and growth of the introduced sugar maple under future precipitation and warming. [ABSTRACT FROM AUTHOR]