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Understorey vegetation hosts the most species diversity in temperate and boreal forests and contributes substantially to ecosystem functions. Despite its ecological importance, the dynamics of understorey biomass, production and turnover rates following stand-replacing disturbance and throughout forest succession remain poorly understood. Using a replicated chronosequence spanning 8, 16, 34, 98, 146 and 210 years following fire, we examined the dynamics of understorey biomass, production and turnover associated with stand development and overstorey types (broadleaf, mixedwood, and conifer) in the central boreal forest of Canada. Across all ages and overstorey types, biomass, production and turnover rates of woody plants, herbs, and bryophytes were on average 2.71, 0.10, and 0.13 Mg ha −1 ; 0.24, 0.13, and 0.08 Mg ha −1 year −1 and 9.7%, 127.3%, and 67.6% year −1 , respectively. Total understorey biomass and production increased continuously with stand age and reached the maximum in 146-year-old stands. Herbaceous biomass and production, peaked in 16-year-old stands, and remained stable thereafter with a slight increase in 146-year-old stands; in contrast, woody plant and bryophyte biomass and production peaked in 98- and 146-year-old stands. Herbaceous and woody vegetation turnover rates were higher in young stands, and those of bryophytes were higher in older stands. Total, woody and herbaceous biomass, production and turnover rates were higher under deciduous broadleaf overstorey, while values for bryophytes were higher in conifer stands, with mixedwood being intermediate. Overstorey mixture had no overall effect on total, woody, herbaceous biomass and production other than the expected from the averages of their broadleaf and conifer stands, but reduced bryophyte biomass and woody turnover. Our findings suggest that understorey biomass, production and turnover rates in the boreal forest are mainly driven by time since colonization and the resource availability and heterogeneity mediated via overstorey composition. Our results highlight the roles of stand development and overstorey composition in understorey biomass, production and turnover and their importance for modeling total forest ecosystem contribution to the global forest carbon. [ABSTRACT FROM AUTHOR]

Copyright of Environmental Reviews is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Measures of soil nutrient availability such as concentrations of nitrogen and phosphorus ([N] and [P]) are important indicators of terrestrial productivity. Optimal plant growth and ecosystem functioning are also strongly correlated with nutrient ratios in soils. Long-term trends in soil [C], [N], [P], and their stoichiometric ratios during secondary succession in the fire-driven boreal forest remain unclear. We used replicated 7- to 209-year chronosequences to examine the influence of stand age and overstory composition on [C], [N], and [P] and their ratios in the forest floor, surface (0–15 cm), and subsurface (15–30 cm) mineral soil in the boreal forest of central Canada. In the forest floor, [C] and [N] increased rapidly during the first three decades following fire, after which they fluctuated, but remained larger than in the youngest stands. Surface soil [C] and [N] increased from young to intermediate-aged stands, but decreased in the oldest stands. Subsurface [C] and [N] followed a similar trend, but was higher in the 7-year-old stands. Forest floor [P] followed a gradual, linear increase throughout stand development but of smaller magnitude than [C] and [N]. The temporal pattern of [P] in both mineral soil layers was lower in the 33-year-old stands of all overstory types, suggesting that P resources may be outpaced by their demand during this highly competitive stage of advanced stem exclusion during forest succession. In the forest floor and surface soil, C:N differed not only with stand age but also with overstory type, due to higher C:N in intermediate-aged conifer stands. Forest floor C:P and N:P were higher in the 33- to 146-year-old stands in all overstory types and soil layers, but particularly so in the conifer stands. In the mineral layers, C:N, C:P, and N:P all followed a similar trend. Our results demonstrate the important influence of stand age, overstory composition (as influenced by succession), and soil depth on forest soil nutrient dynamics in boreal forest ecosystems. [ABSTRACT FROM AUTHOR]

The recovery of forests following stand-replacing disturbance is of widespread interest; however, there is both a lack of definitional clarity for the term “recovery” and a dearth of empirical data on the rates of forest recovery associated with different disturbance types. We conducted a quantitative review of literature to determine recovery times following wildfire and timber harvest and to evaluate variation in recovery rates across Canada’s diverse forest ecosystems. Recovery was assessed according to the rate of change associated with certain forest structural attributes that have traditionally been used as indicators of forest growth and productivity. The recovery of forest canopy cover, tree height, and stand basal area varied at rates that depended on disturbance type, forest biome, and ecozone. We found that, on average, it took 5–10 years, depending on factors such as location and species, for most forest ecosystems of Canada to attain a benchmark canopy cover of 10% after wildfire or harvest. Similarly, regenerating stands in Canada’s boreal forests were capable of attaining average heights of 5 m within five to ten years after wildfire or harvest. Stands in the Boreal Plains ecozone post-harvest reached stand basal area, benchmarked at 10 m 2 ha −1 , faster than those in the Boreal Shield, attributable to differences in tree species composition and the rich mineral deposits of the Boreal Plains. Overall, recovery of canopy cover, tree height, and stand basal area was similar or more rapid following wildfire than harvest. Our review provides temporal benchmarks for gauging recovery times after disturbance. Building upon these temporal benchmarks, and conditioned by disturbance type, site conditions, and location, we present opportunities for using dense time series of remotely sensed data to inform on regional and national trends in forest recovery following disturbance. [ABSTRACT FROM AUTHOR]

Copyright of Environmental Reviews is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

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]

Mixed-species forest stands are generally thought to be more productive than single-species stands. However, empirical evidence is lacking. We tested the effects of species mixture on productivity based on variable compositions of western hemlock (Tsuga heterophylla (Raf.) Sarg.) and western redcedar (Thuja plicata Donn ex D. Don) stands in southern coastal British Columbia. We selected 27 single-storied, even-aged stands that established naturally following clearcutting and slash burning, were at the advanced stem exclusion stage, 50–70-year-old at breast height (bh), and on equivalent sites with fresh soil moisture regime and medium soil nutrient regime. We used simple and multiple regression analyses to relate stand aboveground overstory biomass, understory biomass, and annual net primary productivity (ANPP) to tree species composition, stand age, stand density, and hemlock site index. Overstory biomass increased linearly with percentage of hemlock and stand age. No quadratic relationship was found between overstory biomass and percent hemlock, revealing that productivity was not higher in mixed-species stands. Understory biomass accounted for less than 0.1% of total stand biomass and decreased with stand density, increased with stand age and percent hemlock. Percent hemlock composition was positively related to stand age and negatively to stand density. Simple linear regression analysis showed no relationship between ANPP and percent hemlock. ANPP peaked at an age of 60 years at bh, increasing with stand density, and decreasing as hemlock site index increased. After separating the correlation among stand attributes, multiple regression analysis indicates higher ANPP with increasing hemlock percentage within a given stand age and density. We concluded that mixed-species composition in western hemlock and western redcedar stands in southern coastal British Columbia did not increase aboveground productivity. [Copyright &y& Elsevier]

Productivity potential of high-elevation forests in British Columbia is poorly understood despite continuing harvesting operations. We examined how site index of three principal tree species, subalpine fir (Abies lasiocarpa (Hook.) Nutt.), Engelmann spruce (Picea engelmannii Parry ex Engelm.), and lodgepole pine (Pinus contorta var. latifolia Dougl. ex Loud.), were affected by climatic and edaphic conditions in the Engelmann spruce–subalpine fir zone. We deliberately selected naturally established stands with a wide range in site quality, which are described by subzones, latitude, longitude, elevation, soil moisture regime (SMR), and soil nutrient regime (SNR). Site index varied among subzones for all three species, particularly between the parkland and the forested subzones. Increasing latitude and elevation resulted in a significant decrease in site index for all study species. Site index of subalpine fir and Engelmann spruce significantly increased with SNR from very poor to very rich while site index of lodgepole pine increased with SMR from moderately dry to moist and then decreased on very moist sites. The models developed accounted for 63–70% of variation of site index. These models, tested by independent data, were unbiased with mean prediction error of ±2.3, ±2.6, and ±1.8 m in predicting site index of subalpine fir, Engelmann spruce, lodgepole pine, respectively. They are recommended for estimating site index of the study species to assist in making silvicultural decisions in the interior subalpine forest. [Copyright &y& Elsevier]

• We modeled how tree mortality will affect wood supply in three regions of Canada. • We project strong declines in biomass due to increased drought and wildfire. • Maintaining long-term sustainable harvesting levels will be extremely challenging. • Canada's future wood supply is highly vulnerable to changes in climate. Climate change poses a serious risk to long-term sustainable forest management, particularly when projecting future wood supply for forestry operations. However, few studies have accounted for the cumulative and interacting impacts of climate change on forest productivity and natural disturbances; even fewer have considered drought impacts when projecting wood supply. We modeled how disturbance- and drought-induced tree mortality will affect wood supply in three regions of Canada's boreal forest over a 200-year period under three climate-forcing scenarios. We project strong declines in overall aboveground biomass due to increases in mortality caused mainly by increased drought and wildfire, particularly in drier, western regions. Using two management strategies, we show that maintaining current long-term sustainable harvesting levels will be extremely challenging depending on the level of anthropogenic climate forcing. Increased wildfire activity under severe climate forcing will strongly decrease the availability of harvestable stands. In western areas that are already water-limited, harvesting levels will need to be low to remain sustainable. Our results show that Canada's future wood supply and sustainable forest management practices are highly vulnerable to changes in climate; hence adaptation actions are needed to lower these vulnerabilities. [ABSTRACT FROM AUTHOR]