Climatic thresholds shape northern high‐latitude fire regimes and imply vulnerability to future climate change

Boreal forests and arctic tundra cover 33% of global land area and store an estimated 50% of total soil carbon. Because wildfire is a key driver of terrestrial carbon cycling, increasing fire activity in these ecosystems would likely have global implications. To anticipate potential spatiotemporal variability in fire‐regime shifts, we modeled the spatially explicit 30‐yr probability of fire occurrence as a function of climate and landscape features (i.e. vegetation and topography) across Alaska. Boosted regression tree (BRT) models captured the spatial distribution of fire across boreal forest and tundra ecoregions (AUC from 0.63–0.78 and Pearson correlations between predicted and observed data from 0.54–0.71), highlighting summer temperature and annual moisture availability as the most influential controls of historical fire regimes. Modeled fire–climate relationships revealed distinct thresholds to fire occurrence, with a nonlinear increase in the probability of fire above an average July temperature of 13.4°C and below an annual moisture availability (i.e. P‐PET) of approximately 150 mm. To anticipate potential fire‐regime responses to 21st‐century climate change, we informed our BRTs with Coupled Model Intercomparison Project Phase 5 climate projections under the RCP 6.0 scenario. Based on these projected climatic changes alone (i.e. not accounting for potential changes in vegetation), our results suggest an increasing probability of wildfire in Alaskan boreal forest and tundra ecosystems, but of varying magnitude across space and throughout the 21st century. Regions with historically low flammability, including tundra and the forest–tundra boundary, are particularly vulnerable to climatically induced changes in fire activity, with up to a fourfold increase in the 30‐yr probability of fire occurrence by 2100. Our results underscore the climatic potential for novel fire regimes to develop in these ecosystems, relative to the past 6000–35 000 yr, and spatial variability in the vulnerability of wildfire regimes and associated ecological processes to 21st‐century climate change.     

Succession in sub‐boreal forests of West‐Central British Columbia

Abstract. Structural and compositional changes were analysed over the course of 400+ yr of post‐fire succession in the sub‐boreal forests of west‐central British Columbia. Using a chronosequence of 57 stands ranging from 11 to 438 yr in age, we examined changes in forest structure and composition with complementary PCA and DCA ordination techniques. To determine stand ages and timing of tree recruitment, approximately 1800 trees were aged. Most early successional forests were dominated by Pinus contorta, which established rapidly following fire. Abies lasiocarpa and Picea glauca × engel‐mannii were also able to establish quickly, but continued to establish throughout the sere. Few Pinus contorta survived beyond 200 yr, resulting in major changes in forest structure. In some stands P. contorta never established, which led to considerable variation among stands less than 200 yr old. The oldest forests converged on dominance by Abies lasiocarpa. Vascular plant diversity decreased during succession whereas canopy structure became more complex as gap dynamics developed. Although these sub‐boreal forests contain few tree species, successional changes were pronounced, with structure changing more than composition across the chronosequence.     

Dispersal limitation drives successional pathways in Central Siberian forests under current and intensified fire regimes

Fire is a primary driver of boreal forest dynamics. Intensifying fire regimes due to climate change may cause a shift in boreal forest composition toward reduced dominance of conifers and greater abundance of deciduous hardwoods, with potential biogeochemical and biophysical feedbacks to regional and global climate. This shift has already been observed in some North American boreal forests and has been attributed to changes in site conditions. However, it is unknown if the mechanisms controlling fire‐induced changes in deciduous hardwood cover are similar among different boreal forests, which differ in the ecological traits of the dominant tree species. To better understand the consequences of intensifying fire regimes in boreal forests, we studied postfire regeneration in five burns in the Central Siberian dark taiga, a vast but poorly studied boreal region. We combined field measurements, dendrochronological analysis, and seed‐source maps derived from high‐resolution satellite images to quantify the importance of site conditions (e.g., organic layer depth) vs. seed availability in shaping postfire regeneration. We show that dispersal limitation of evergreen conifers was the main factor determining postfire regeneration composition and density. Site conditions had significant but weaker effects. We used information on postfire regeneration to develop a classification scheme for successional pathways, representing the dominance of deciduous hardwoods vs. evergreen conifers at different successional stages. We estimated the spatial distribution of different successional pathways under alternative fire regime scenarios. Under intensified fire regimes, dispersal limitation of evergreen conifers is predicted to become more severe, primarily due to reduced abundance of surviving seed sources within burned areas. Increased dispersal limitation of evergreen conifers, in turn, is predicted to increase the prevalence of successional pathways dominated by deciduous hardwoods. The likely fire‐induced shift toward greater deciduous hardwood cover may affect climate–vegetation feedbacks via surface albedo, Bowen ratio, and carbon cycling.     

Differential declines in Alaskan boreal forest vitality related to climate and competition

Rapid warming and changes in water availability at high latitudes alter resource abundance, tree competition, and disturbance regimes. While these changes are expected to disrupt the functioning of boreal forests, their ultimate implications for forest composition are uncertain. In particular, recent site‐level studies of the Alaskan boreal forest have reported both increases and decreases in productivity over the past few decades. Here, we test the idea that variations in Alaskan forest growth and mortality rates are contingent on species composition. Using forest inventory measurements and climate data from plots located throughout interior and south‐central Alaska, we show significant growth and mortality responses associated with competition, midsummer vapor pressure deficit, and increased growing season length. The governing climate and competition processes differed substantially across species. Surprisingly, the most dramatic climate response occurred in the drought tolerant angiosperm species, trembling aspen, and linked high midsummer vapor pressure deficits to decreased growth and increased insect‐related mortality. Given that species composition in the Alaskan and western Canadian boreal forests is projected to shift toward early‐successional angiosperm species due to fire regime, these results underscore the potential for a reduction in boreal productivity stemming from increases in midsummer evaporative demand.     

Predicting the composition of Canadian southern boreal forest in different fire cycles

Abstract. Post-fire succession was reconstructed for a sector located in the southern part of the Québec boreal forest. Forest composition for different periods since fire was evaluated using a stand initiation map together with ecological maps representing both site conditions and stand types. Nine fires covering at least 100 ha and representing a chronosequence of more than 230 yr were used. Although a relatively clear successional pattern from deciduous to coniferous composition relating to time-since-fire was observed, Pinus banksiana stands showed an erratic distribution not related to succession but possibly to the pre-fire stand composition. A comparison with forest cover maps produced after a recent spruce budworm outbreak, showed that succession toward coniferous dominance appeared to be interrupted by spruce budworm (Choristoneura fumiferana) outbreaks which, by killing Abies balsamea, lead to a mixed deciduous forest composition. A simple empirical model based on a negative exponential distribution of age classes was developed to evaluate how changes in the fire cycle would affect the composition of the forest mosaic. The transition between deciduous dominance and coniferous dominance occurs in a fire cycle > 200 yr. Although pure deciduous stands tend to disappear during long fire cycles, the proportion of mixed stands remains relatively constant. Prediction of the forest composition for longer fire cycles is complicated by the interaction between post-fire composition and stand vulnerability to spruce budworm outbreaks.     

Changes in fire regime break the legacy lock on successional trajectories in Alaskan boreal forest

Predicting plant community responses to changing environmental conditions is a key element of forecasting and mitigating the effects of global change. Disturbance can play an important role in these dynamics, by initiating cycles of secondary succession and generating opportunities for communities of long‐lived organisms to reorganize in alternative configurations. This study used landscape‐scale variations in environmental conditions, stand structure, and disturbance from an extreme fire year in Alaska to examine how these factors affected successional trajectories in boreal forests dominated by black spruce. Because fire intervals in interior Alaska are typically too short to allow relay succession, the initial cohorts of seedlings that recruit after fire largely determine future canopy composition. Consequently, in a dynamically stable landscape, postfire tree seedling composition should resemble that of the prefire forest stands, with little net change in tree composition after fire. Seedling recruitment data from 90 burned stands indicated that postfire establishment of black spruce was strongly linked to environmental conditions and was highest at sites that were moist and had high densities of prefire spruce. Although deciduous broadleaf trees were absent from most prefire stands, deciduous trees recruited from seed at many sites and were most abundant at sites where the fires burned severely, consuming much of the surface organic layer. Comparison of pre‐ and postfire tree composition in the burned stands indicated that the expected trajectory of black spruce self‐replacement was typical only at moist sites that burned with low fire severity. At severely burned sites, deciduous trees dominated the postfire tree seedling community, suggesting these sites will follow alternative, deciduous‐dominated trajectories of succession. Increases in the severity of boreal fires with climate warming may catalyze shifts to an increasingly deciduous‐dominated landscape, substantially altering landscape dynamics and ecosystem services in this part of the boreal forest.     

Simulation modeling of the effects of site conditions and disturbance history on a boreal forest landscape

Abstract. Computer simulations were used to elaborate hypotheses about controls on forest structure and composition in a 0.7 km ≤ area of boreal forest in Central Sweden. DBH and species of all adult trees and stand conditions were recorded for 57–10 m radius plots. Ordination of these data suggested that nutrient-availability and time-since-disturbance were the main controls of forest composition and structure within the area. The simulation model couples equations representing the effect of tree canopy structure and biomass on light and soil conditions with equations representing the effect of these conditions on reproduction, growth and mortality in height cohorts of trees on a 0.1 ha patch. Nitrogen-availability levels for each modeled plot were simulated by species-specific growth multipliers. The model was run for 400 simulated yr at six levels of N availability. Age and N status of each study plot were inferred by matching with the most similar model output. Inferred ages agreed with what is known of the disturbance history, and site factors related to soil fertility were correctly correlated with the inferred N status. The consequences of size-selective disturbance were explored by model experiments. Biomass was removed from large or small size classes at 100 - 200 yr and the simulations were run for an additional 300 yr. Disturbed stands of high N status often became similar to undisturbed stands of different N status. Size-selective disturbances produced stands that were different from any in the undisturbed succession, but these differences disappeared within 50 - 100 yr, implying successional convergence in stand structure and composition. Plots of simulated basal area against time and nitrogen-availability for the four species illustrate the time dependence of species performance along a fertility gradient.     

气候变化和不同强度造林对大兴安岭主要树种林分信息和地上生物量的长期影响

气候变化及相应火干扰在不同尺度上影响着我国大兴安岭地区森林动态,且在未来的影响可能继续加剧。为了提高森林生态功能和应对气候变暖,国家在分类经营基础上全面实施抚育采伐和补植造林,效果较好,但抚育采伐对森林主要树种的长期影响知之甚少,其在未来气候下的可持续性也有待进一步评估,同时,探讨造林措施对未来森林的影响也显得尤为重要。本文运用森林景观模型LANDIS PRO,模拟气候变化及火干扰、采伐和造林对大兴安岭地区主要树种的长期影响。结果表明:1)模型初始化、短期和长期模拟结果均得到了有效验证,模拟结果与森林调查数据之间无显著性差异(P0.05),基于火烧迹地数据的林火干扰验证亦能够反映当前火干扰的效果,模型模拟结果的可信度较高;2)与当前气候相比,气候变暖及火干扰明显改变了树种组成、年龄结构和地上生物量,B1气候下研究区森林基本上以针叶树种为主要树种,A2气候下优势树种向阔叶树转变;3)与无采伐预案相比,当前气候下,抚育采伐使落叶松的林分密度和地上生物量分别降低了(165±94.9)株/hm~2和(8.5±5.1) Mg/hm~2,增加了樟子松、白桦和云杉等树木株数和地上生物量(3.3—753.4株/hm~2和0.2—4.0 Mg/hm~2),而对山杨的影响较小;B1和A2气候下抚育采伐显著改变林分密度,降低景观尺度地上生物量,进而表现为不可持续;4)B1气候下,推荐实施中低强度造林预案(10%和20%强度),在A2气候下,各强度造林均可在模拟后期增加树种地上生物量。     

Effects of stand composition on fire hazard in mixed‐wood Canadian boreal forest

Abstract. Surface fuels were examined in 48 stands of the Canadian mixed‐wood boreal forest. Tree canopy was characterized with the point‐centred quadrant method and stands were characterized as deciduous, mixed‐deciduous, mixed‐coniferous or coniferous according to the percentage of conifer basal area. Woody debris loadings were measured with the line intersect method and the litter, duff, shrub loads and depths or heights were sampled with various quadrats. No significant difference was found among stand types for total woody debris load, large basal diameter shrub loads and load or depth of litter and duff. However, conifer stands had significantly heavier loads of small diameter elements (twigs and shrubs) and conifer pieces were more numerous within these stands than in deciduous stands. The BEHAVE prediction system was used to evaluate the impact of these differences on the potential of fire ignition in situations where topography and weather were constant. The qualitative and quantitative changes in fuels, resulting from species replacement and fast decay rates, influence fire hazard. Simulations of fire behaviour showed that in the mixed‐wood boreal forest fires were less intense and spread more slowly in deciduous stands than in mixed or coniferous stands. Moreover, spring fires were more intense than summer fires, and differences between seasons increased with the increase of deciduous basal area.     

Variation in Carbon Storage and Its Distribution by Stand Age and Forest Type in Boreal and Temperate Forests in Northeastern China

The northeastern forest region of China is an important component of total temperate and boreal forests in the northern hemisphere. But how carbon (C) pool size and distribution varies among tree, understory, forest floor and soil components, and across stand ages remains unclear. To address this knowledge gap, we selected three major temperate and two major boreal forest types in northeastern (NE) China. Within both forest zones, we focused on four stand age classes (young, mid-aged, mature and over-mature). Results showed that total C storage was greater in temperate than in boreal forests, and greater in older than in younger stands. Tree biomass C was the main C component, and its contribution to the total forest C storage increased with increasing stand age. It ranged from 27.7% in young to 62.8% in over-mature stands in boreal forests and from 26.5% in young to 72.8% in over-mature stands in temperate forests. Results from both forest zones thus confirm the large biomass C storage capacity of old-growth forests. Tree biomass C was influenced by forest zone, stand age, and forest type. Soil C contribution to total forest C storage ranged from 62.5% in young to 30.1% in over-mature stands in boreal and from 70.1% in young to 26.0% in over-mature in temperate forests. Thus soil C storage is a major C pool in forests of NE China. On the other hand, understory and forest floor C jointly contained less than 13% and <5%, in boreal and temperate forests respectively, and thus play a minor role in total forest C storage in NE China.     

Multiple successional pathways of boreal forest stands in central Canada

Predicting forest composition change through time is a key challenge in forest management. While multiple successional pathways are theorized for boreal forests, empirical evidence is lacking, largely because succession has been inferred from chronosequence and dendrochronological methods. We tested the hypotheses that stands of compositionally similar overstory may follow multiple successional pathways depending on time since last stand‐replacing fire (TSF), edaphic conditions, and presence of intermediate disturbances. We used repeated measurements from combining sequential aerial photography and ground surveys for 361 boreal stands in central Canada. Stands were measured in 8–15 yr intervals over a ~ 60 yr period, covering a wide range of initial stand conditions. Multinomial logistic regression was used to analyze stand type transitions. With increasing TSF, stands dominated by shade‐intolerant Pinus banksiana, Populus sp., and Betula papyrifera demonstrated multiple pathways to stands dominated by shade‐tolerant Picea sp., Abies balsamea, and Thuja occidentalis. Their pathways seemed largely explained by neighborhood effects. Succession of stands dominated by shade‐tolerant species, with an exception of stands dominated by Picea sp., was not related to TSF, but rather dependent on edaphic conditions and presence of intermediate disturbances. Varying edaphic conditions caused divergent pathways with resource limited sites being dominated by nutrient‐poor tolerant species, and richer sites permitting invasion of early successional species and promoting species mixtures during succession. Intermediate disturbances promoted deciduous persistence and species diversity in A. balsamea and mixed‐conifer stands, but no evidence was detected to support “disturbance accelerated succession”. Our results demonstrate that in the prolonged absence of stand‐replacing disturbance boreal forest stands undergo multiple succession pathways. These pathways are regulated by neighborhood effects, resource availability, and presence of intermediate disturbance, but the relative importance of these regulators depends on initial stand type. The observed divergence of successional pathways supports the resource‐ratio hypothesis of plant succession.     

Implications of floristic and environmental variation for carbon cycle dynamics in boreal forest ecosystems of central Canada

Abstract. Species composition, detritus, and soil data from 97 boreal forest stands along a transect in central Canada were analysed using Correspondence Analysis to determine the dominant environmental/site variables that differentiate these forest stands. Picea mariana stands were densely clustered together on the understorey DCA plot, suggesting a consistent understorey species composition (feather mosses and Ericaceae), whereas Populus tremuloides stands had the most diverse understorey species composition (ca. 30 species, mostly shrubs and herbs). Pinus banksiana stands had several characteristic species of reindeer lichens (Cladina spp.), but saplings and Pinus seedlings were rare. Although climatic variables showed large variation along the transect, the CCA results indicated that site conditions are more important in determining species composition and differentiating the stand types. Forest floor characteristics (litter and humus layer, woody debris, and drainage) appear to be among the most important site variables. Stands of Picea had significantly higher average carbon (C) densities in the combined litter and humus layer (43530 kg‐C.ha^‐1) than either Populus (25 500 kg‐C.ha^‐1) or Pinus (19 400 kg‐C.ha^‐1). The thick surface organic layer in lowland Picea stands plays an important role in regulating soil temperature and moisture, and organic‐matter decomposition, which in turn affect the ecosystem C‐dynamics. During forest succession after a stand‐replacing disturbance (e.g. fires), tree biomass and surface organic layer thickness increase in all stand types as forests recover; however, woody biomass detritus first decreases and then increases after ca. 80 yr. Soil C densities show slight decrease with ages in Populus stands, but increase in other stand types. These results indicate the complex C‐transfer processes among different components (tree biomass, detritus, forest floor, and soil) of boreal ecosystems at various stages of succession.     

Fire severity mediates climate‐driven shifts in understorey community composition of black spruce stands of interior Alaska

Question: How do pre‐fire conditions (community composition and environmental characteristics) and climate‐driven disturbance characteristics (fire severity) affect post‐fire community composition in black spruce stands? Location: Northern boreal forest, interior Alaska. Methods: We compared plant community composition and environmental stand characteristics in 14 black spruce stands before and after multiple, naturally occurring wildfires. We used a combination of vegetation table sorting, univariate (ANOVA, paired t‐tests), and multivariate (detrended correspondence analysis) statistics to determine the impact of fire severity and site moisture on community composition, dominant species and growth forms. Results: Severe wildfires caused a 50% reduction in number of plant species in our study sites. The largest species loss, and therefore the greatest change in species composition, occurred in severely burned sites. This was due mostly to loss of non‐vascular species (mosses and lichens) and evergreen shrubs. New species recruited most abundantly to severely burned sites, contributing to high species turnover on these sites. As well as the strong effect of fire severity, pre‐fire and post‐fire mineral soil pH had an effect on post‐fire vegetation patterns, suggesting a legacy effect of site acidity. In contrast, pre‐fire site moisture, which was a strong determinant of pre‐fire community composition, showed no relationship with post‐fire community composition. Site moisture was altered by fire, due to changes in permafrost, and therefore post‐fire site moisture overrode pre‐fire site moisture as a strong correlate. Conclusions: In the rapidly warming climate of interior Alaska, changes in fire severity had more effect on post‐fire community composition than did environmental factors (moisture and pH) that govern landscape patterns of unburned vegetation. This suggests that climate change effects on future community composition of black spruce forests may be mediated more strongly by fire severity than by current landscape patterns. Hence, models that represent the effects of climate change on boreal forests could improve their accuracy by including dynamic responses to fire disturbance.     

The impacts and implications of an intensifying fire regime on Alaskan boreal forest composition and albedo

Climate warming and drying are modifying the fire dynamics of many boreal forests, moving them towards a regime with a higher frequency of extreme fire years characterized by large burns of high severity. Plot‐scale studies indicate that increased burn severity favors the recruitment of deciduous trees in the initial years following fire. Consequently, a set of biophysical effects of burn severity on postfire boreal successional trajectories at decadal timescales have been hypothesized. Prominent among these are a greater cover of deciduous tree species in intermediately aged stands after more severe burning, with associated implications for carbon and energy balances. Here we investigate whether the current vegetation composition of interior Alaska supports this hypothesis. A chronosequence of six decades of vegetation regrowth following fire was created using a database of burn scars, an existing forest biomass map, and maps of albedo and the deciduous fraction of vegetation that we derived from Moderate Resolution Imaging Spectroradiometer (MODIS) satellite imagery. The deciduous fraction map depicted the proportion of aboveground biomass in deciduous vegetation, derived using a RandomForest algorithm trained with field data sets (n=69, 71% variance explained). Analysis of the difference Normalized Burn Ratio, a remotely sensed index commonly used as an indicator of burn severity, indicated that burn size and ignition date can provide a proxy of burn severity for historical fires. LIDAR remote sensing and a bioclimatic model of evergreen forest distribution were used to further refine the stratification of the current landscape by burn severity. Our results show that since the 1950s, more severely burned areas in interior Alaska have produced a vegetation cohort that is characterized by greater deciduous biomass. We discuss the importance of this shift in vegetation composition due to climate‐induced changes in fire severity for carbon sequestration in forest biomass and surface reflectance (albedo), among other feedbacks to climate.     

Tree species dynamics and disturbance in three Swedish boreal forest stands during the last two thousand years

Abstract. Data from three forest stands for the past 2000 yr show how the shade-intolerant species Pinus sylvestris and Betula pubescens maintain significant populations in the Swedish boreal landscape. Age structure data from a northern stand close to the range limits of Picea abies and Pinus complement a local pollen diagram, and reveal cyclic population fluctuations which can be related to periods of climatic stress and fire. Pollen data from two southern stands show that high fire frequencies in the past prevented the expansion of Picea populations. Reduction of the fire frequency during the last 200 yr has favoured Picea. A long time perspective reveals the population dynamics of long-lived species and indicates the controlling factors. Such knowledge permits assessment of the current status and likely future of forest stands.     

Projected climate change effects on Alberta's boreal forests imply future challenges for oil sands reclamation

Climate change will drive significant changes in vegetation cover and also impact efforts to restore ecosystems that have been disturbed by human activities. Bitumen mining in the Alberta oil sands region of western Canada requires reclamation to “equivalent land capability,” implying establishment of vegetation similar to undisturbed boreal ecosystems. However, there is consensus that this region will be exposed to relatively severe climate warming, causing increased occurrence of drought and wildfire, which threaten the persistence of both natural and reclaimed ecosystems. We used a landscape model, LANDIS‐II, to simulate plant responses to climate change and disturbances, forecasting changes to boreal forests within the oil sands region. Under the most severe climate forcing scenarios (representative concentration pathway [RCP] 8.5) the model projected substantial decreases in forest biomass, with the future forest being dominated by drought‐ and fire‐tolerant species characteristic of parkland or prairie ecosystems. In contrast, less extreme climate forcing scenarios (RCPs 2.6 and 4.5) had relatively minor effects on forest composition and biomass with boreal conifers continuing to dominate the landscape. If the climate continues to change along a trajectory similar to those simulated by climate models for the RCP 8.5 forcing scenario, current reclamation goals to reestablish spruce‐dominated boreal forest will likely be difficult to achieve. Results from scenario modeling studies such as ours, and continued monitoring of change in the boreal forest, will help inform reclamation practices, which could include establishment of species better adapted to warmer and drier conditions.     

气候变化和林火干扰对大兴安岭林区地上生物量影响的动态模拟

预测森林地上生物量对气候变化和林火干扰的响应是陆地生态系统碳循环研究的重要内容,气温、降水等因素的改变和气候变暖导致林火干扰强度的变化将会影响森林生态系统的碳库动态.东北森林作为我国森林的重要组成部分,对气候变化和林火干扰的响应逐渐显现.本文运用LANDIS PRO模型,模拟气候变化对大兴安岭森林地上生物量的影响,并比较分析了气候变暖对森林地上生物量的直接影响与通过林火干扰强度改变所产生的影响.结果表明: 未来气候变暖和火干扰增强情景下,森林地上生物量增加;当前气候条件和火干扰下,研究区森林地上生物量为(97.14±5.78) t·hm^-2;在B1F2预案下,森林地上生物量均值为(97.93±5.83) t·hm^-2;在A2F3预案下,景观水平第100～150和150～200年模拟时期内的森林地上生物量均值较高,分别为(100.02±3.76)和(110.56±4.08) t·hm^-2.与当前火干扰相比,CF2预案(当前火干扰增加30%)在一定时期使景观水平地上生物量增加(0.56±1.45) t·hm^-2,CF3预案(当前火干扰增加230%)在整个模拟阶段使地上生物量减少(7.39±1.79) t·hm^-2.针叶、阔叶树种对气候变暖的响应存在差异,兴安落叶松和白桦生物量随气候变暖表现为降低趋势,而樟子松、云杉和山杨的地上生物量则随气候变暖表现出不同程度的增加;气候变暖对针阔树种的直接影响具有时滞性,针叶树种响应时间比阔叶树种迟25～50年.研究区森林对高CO₂排放情景下气候变暖和高强度火干扰的共同作用较为敏感,未来将明显改变研究区森林生态系统的树种组成和结构.     

Long‐term post‐fire changes in the northeastern boreal forest of Quebec

Abstract. Natural dynamics in the boreal forest is influenced by disturbances. Fire recurrence affects community development and landscape diversity. Forest development was studied in the northeastern boreal forest of Quebec. The objective was to describe succession following fire and to assess the factors related to the changes in forest composition and structure. The study area is located in northeastern Quebec, 50 km north of Baie‐Comeau. We used the forest inventory data gathered by the Ministère des Ressources naturelles du Québec (MRNQ). In circular plots of 400 m², the diameter at breast height (DBH) of all stems of tree species greater than 10 cm was recorded and in 40 m² subplots, stems smaller than 10 cm were measured. A total of 380 plots were sampled in an area of 6000 km². The fire history reconstruction was done based on historical maps, old aerial photographs and field sampling. A time‐since‐fire class, a deposit type, slope, slope aspect and altitude were attributed to each plot. Each plot was also described according to species richness and size structure characteristics. Traces of recent disturbance were also recorded in each plot. Changes in forest composition were described using ordination analyses (NMDS and CCA) and correlated with the explanatory variables. Two successional pathways were observed in the area and characterized by the early dominance of intolerant hardwood species or Picea mariana. With time elapsed since the last fire, composition converged towards either Picea mariana, Abies balsamea or a mixture of both species and the size structure of the coniferous dominated stands got more irregular. The environmental conditions varied between stands and explained part of the variability in composition. Their effect tended to decrease with increasing time elapsed since fire, as canopy composition was getting more similar. Gaps may be important to control forest dynamics in old successional communities.     

Carbon distribution of a well- and poorly-drained black spruce fire chronosequence

The objective of this study was to quantify carbon (C) distribution for boreal black spruce (Picea mariana (Mill.) BSP) stands comprising a fire chronosequence in northern Manitoba, Canada. The experimental design included seven well‐drained (dry) and seven poorly‐drained (wet) stands that burned between 1998 and 1850. Vegetation C pools (above‐ground + below‐ground) steadily increased from 1.3 to 83.3 t C ha^?1 for the dry chronosequence, and from 0.6 to 37.4 t C ha^?1 for the wet chronosequence. The detritus C pools (woody debris + forest floor) varied from 10.3 to 96.0 t C ha^?1 and from 12.6 to 77.4 t C ha^?1 for the dry and wet chronosequence, respectively. Overstorey biomass, mean annual biomass increment (MAI), woody debris mass, and litterfall were significantly greater (α = 0.05) for the dry stands than for the wet stands, but the bryophyte, understorey, and forest floor C pools were significantly less for the dry than for the wet stands. The root mass ratio decreased with stand age until 37 years after fire, was fairly constant thereafter, and was not significantly affected by soil drainage. The C pools of the overstorey and bryophyte tended to increase with stand age. Foliage biomass, litterfall, and MAI (for the dry stands) peaked at 71 years after fire and declined in the oldest stands. The results from this study illustrate that the effects of disturbance and edaphic conditions must be accounted for in boreal forest C inventories and C models. The appropriateness of using chronosequences to examine effects of wildfire on ecosystem C distribution is discussed.     

Understorey vegetation development in North Finnish Picea forests after disturbance: re‐analysis of Sirén's data

Abstract. Sirén (1955) studied understorey species composition, tree stand properties and humus‐layer thickness in 64 unlogged forest stands on topographically and pedologically comparable sites. The stands were of even age (6 – 300 yr), stocked with the first or second tree generation after wildfire. The view of Sirén and several authors after him, that the vegetation of old‐growth boreal Picea forests is homogeneous on a broad scale, was examined by applying, in parallel, the partial variants of two ordination methods (DCA and PCA) to Sirén's vegetation data. Two main vegetation gradients were found: a major gradient running from recently burnt plots with prominence of pioneer species to plots with stand age > 100 yr, a well stocked tree layer and a thick humus layer, dominance of feather‐mosses and ample occurrence of shade‐tolerant as well as light‐preferring vascular plant species, and a second gradient along which first‐ and second‐generation plots segregate. The more prominent element of Betula trees in first‐ than in second‐generation stands < 100 yr contributed to the latter. A minor third gradient related to humus‐layer thickness was recovered by partial DCA only. The main vegetation gradient reappeared in separate ordinations of data from 47 mature forest stands (> 100 yr), but without being correlated with forest age. Variation among mature‐forest stands in the importance of pioneer species is considered mainly to be brought about by fine‐scale disturbance processes such as tree uprooting. Increasing importance of factors operating on within‐stand scales [development of a varied gap structure and stronger gradients in tree influence (radiation at ground level), soil moisture, soil depth and nutrient availability] with time is also reflected in the second and third mature‐forest ordination axes. Possible implications of the results for conservation of biological diversity and monitoring of changes in boreal forests are discussed.