Changing sources of soil respiration with time since fire in a boreal forest

Radiocarbon signatures (Δ¹⁴C) of carbon dioxide (CO₂) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ¹⁴C of soil respiration and soil CO₂ in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ¹⁴C with Δ¹⁴C of CO₂ evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ¹⁴C of respired CO₂ indicated marked variation in respiration sources in space and time. The ¹⁴C signature of respired CO₂ respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ¹⁴C greater (averaging ～120‰) than autotrophic respiration. The Δ¹⁴C of autotrophically respired CO₂ in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004). CO₂ respired by black spruce roots in stands >40 years old had Δ¹⁴C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants. Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ～50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO₂ had Δ¹⁴C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ¹⁴C of soil respiration in younger successional stands dropped below those of the atmospheric CO₂.     

Impact of fire on active layer and permafrost microbial communities and metagenomes in an upland Alaskan boreal forest

Permafrost soils are large reservoirs of potentially labile carbon (C). Understanding the dynamics of C release from these soils requires us to account for the impact of wildfires, which are increasing in frequency as the climate changes. Boreal wildfires contribute to global emission of greenhouse gases (GHG—CO₂, CH₄ and N₂O) and indirectly result in the thawing of near-surface permafrost. In this study, we aimed to define the impact of fire on soil microbial communities and metabolic potential for GHG fluxes in samples collected up to 1 m depth from an upland black spruce forest near Nome Creek, Alaska. We measured geochemistry, GHG fluxes, potential soil enzyme activities and microbial community structure via 16SrRNA gene and metagenome sequencing. We found that soil moisture, C content and the potential for respiration were reduced by fire, as were microbial community diversity and metabolic potential. There were shifts in dominance of several microbial community members, including a higher abundance of candidate phylum AD3 after fire. The metagenome data showed that fire had a pervasive impact on genes involved in carbohydrate metabolism, methanogenesis and the nitrogen cycle. Although fire resulted in an immediate release of CO₂ from surface soils, our results suggest that the potential for emission of GHG was ultimately reduced at all soil depths over the longer term. Because of the size of the permafrost C reservoir, these results are crucial for understanding whether fire produces a positive or negative feedback loop contributing to the global C cycle.     

林火干扰对北方针叶林林下植被的影响

林下植被在北方针叶林植被群落中的物种多样性最高, 且具有较高的生物量周转率和地上部分净初级生产力, 对北方针叶林生态系统功能起着重要作用。火干扰是决定北方针叶林林下植被结构与功能的一个重要景观过程。该文综述了火干扰是如何通过与地形、火前林冠组成的交互作用而影响环境资源和林下植被的。最近的研究表明: 林下植被能够影响火后树木更新苗的定植、重建速率及森林演替轨迹; 林下植被还会通过影响元素的生物地球化学过程(凋落物降解和养分循环)影响林下环境资源的数量与异质性。因此, 研究火后初期北方针叶林林下植被的动态变化, 对于物种多样性保护和森林管理具有重要意义。     

Spatial and temporal variations in boreal forest fire frequency in northern Alberta

Abstract. Spatial and temporal variations in fire frequency in the boreal forest of Wood Buffalo National Park (WBNP) were assessed using forest stand age, fire scar and historical data. I test the hypotheses that (1) fire frequency is higher in jack pine forests and aspen forests than in black spruce forests and white spruce forests, (2) these variations in fire frequency can be related to the mean waterbreak distance (MWD) around a site and (3) fire frequency has changed over the past 300 years. The fire cycles (the time required to burn an area equal in size to the entire study area) in jack pine forests (39 years) and in aspen forests (39 years) were significantly shorter than those in black spruce forests (78 years) and in white spruce forests (96 years). The length of the fire cycle varies inversely with the MWD around a site, and the MWD was significantly higher in jack pine and aspen forests than in black or white spruce forests. It is suggested that covariations between soil type and the MWD influence, respectively, variations in forest dominant and fire frequency. A change in fire frequency at 1860 was apparent in the fire history for all of WBNP, the black spruce dominated stands, and the near and medium MWD classes. The fire cycle estimates for these classes were all significantly shorter during the period 1750 to 1859 (fire cycles = 25–49 years) than they were in the period 1860 to 1989 (fire cycles = 59–89 years). The possible roles of changes in climate and aboriginal burning practices in causing the temporal change in fire frequency are discussed.     

Major disturbances test resilience at a long‐term boreal forest monitoring site

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林火干扰对大兴安岭主要林分类型地上生物量预测的影响模拟研究

林火干扰是北方森林最主要的自然干扰之一,对北方森林地上生物量影响是一个长期的过程。因此,在预测地上生物量动态变化时需要考虑林火的影响。运用空间直观景观模型LANDIS PRO,模拟大兴安岭林区林火对不同树种地上生物量预测的影响。选取研究区5种主要树种林分(兴安落叶松、樟子松、云杉、白桦和山杨),以无干扰情景为参考预案,在验证模型模拟结果的基础上,模拟林火在短期(0—50a)、中期(50—150a)和长期(150—300a)对地上生物量的定量化影响,及其对不同立地类型地上生物量的动态变化。结果表明:(1)基于森林调查数据参数化的2000年森林景观模拟结果能够较好地代表2000年真实森林景观,模拟的2010年森林林分密度和胸高断面积与2010年森林调查数据无显著性差异(P0.05),当前林火干扰机制模拟结果能够较好地与样地调查数据匹配,说明林火模拟能够代表当前研究区林火发生情况;(2)与无干扰预案相比,整个模拟时期内景观水平上林火减少了1.7—5.9 t/hm2地上生物量;(3)与无干扰预案相比,林火预案下主要树种生物量在短期、中期和长期变化显著(P0.05);(4)在不同模拟时期,林火显著地改变了地上生物量空间分布,其中以亚高山区地上生物量降低最为明显。研究可为长期森林管理以及森林可持续发展提供参考。     

Stand- and plot-level changes in a boreal forest understory community following wildfire

Background: Boreal forest understory plant communities are known to be resilient to fire – the species composition of stands after a fire is quite similar to the pre-fire composition. However, we know little about recovery of individual plants within particular locations in forest stands (i.e. plot-level changes) since we usually do not have pre-fire data for plots.

Aims: We wanted to determine whether species recruited into the same or different locations in a Pinus banksiana stand that experienced a severe wildfire.

Methods: We used pre-existing permanent plots to evaluate the cover of understory after an unplanned wildfire.

Results: Across the entire stand nine of 47 species showed a significant change in cover. The largest change in a plant functional group was in the mosses, with all species present before fire being eliminated. There was no change in species diversity or total cover. At the plot level, species composition showed a much greater change. An average of 47% of the species present in a plot before the fire were absent in the same plot after the fire, and the total species turnover in plots was 88% of the species present before the fire. The plots showed a similar shift in species composition.

Conclusions: These results confirm that boreal forest communities show a high degree of resilience to fire, but within a forest stand species will be found in different locations following fire, potentially exposing them to a different set of biotic and abiotic conditions in these new locations.     

我国北方针叶林人为火发生的预测模型

我国北方针叶林带是重要的森林资源储藏地,也是林火发生的重灾区,其自然火和人为火所占比例相当. 气象因子、地形特征、植被条件、人为基础设施等因素对人为火发生具有显著影响,国内目前应用空间分析技术对北方针叶林带人为火影响因子的研究还存在一定不确定性. 本文基于1974—2009年间人为火的空间地理坐标,结合研究地的气象因子、基础地理信息及矢量化林相图,应用ArcGIS 10.0中的空间分析工具和SPSS 19.0的逻辑斯蒂回归模型对影响人为火发生的主要驱动因子进行分析,并建立人为火发生的概率模型. 利用HADCM2模式下研究区域未来气象数据对塔河地区2015年人为火发生情况进行计算.结果表明： 距离铁路距离（x₁）和平均相对湿度（x₂）对研究区域人为火发生具有显著影响,并得到火险概率模型P=1/［1+e^{-(3.026-0.00011x1-0.047x₂)}］. 模型校验结果显示,模型的准确度可达到80%.林火发生预测结果表明,塔河地区2015年 4—6月、8月为人为火高发期,其中,4—5月的林火发生概率最高.从火险空间分布来看,高火险主要集中在塔河西部和西南部,铁路线路主要包含在此区域.     

Effects of forest fire and drought on acidity of a base-poor boreal forest stream: similarities between climatic warming and acidic precipitation

In a boreal forest catchment in the Experimental Lakes Area in northwestern Ontario, wildfire caused an increase in the concentrations of strong acid anions and base cations of the stream. In the naturally base-poor Northwest (NW) Subbasin, a 1980 wildfire caused exports of strong acid anions to increase more than export of base cations, causing a 2.5 fold increase in the acidity of the stream. Mean annual stream pH declined from 5.15 prior to fire to 4.76 two years after fire. Acid-neutralizing capacity (ANC), calculated as the difference between total base cations and strong acid anions, decreased to 20% of pre-fire values. Sulfate and chloride were the strong acid anions responsible for the decline in ANC, increasing four-fold. While nitrate increased eleven-fold, concentrations were too low to significantly affect ANC. There was a significant correlation between weekly sulfate concentration and base cation concentration (r ² = 0.83) in the two years after fire. Recovery of ANC was caused by the more rapid decline in concentration of sulfate than by changes in base cations. Drought produced a similar but weaker response than fire, with increased sulfate concentrations and decreased stream pH. Climatic warming that increases drought and fire frequency would have effects that mimic the impacts of acidic precipitation (i.e. higher sulfate concentrations and acidic stream waters). Areas which have higher concentrations of stored S from past acid precipitation or have large areas of peatlands in the watershed may have aggravated losses of S and H⁺ after drought and fire.     

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.     

Paired-tower measurements of carbon and energy fluxes following disturbance in the boreal forest

Disturbances by fire and harvesting are thought to regulate the carbon balance of the Canadian boreal forest over scales of several decades. However, there are few direct measurements of carbon fluxes following disturbances to provide data needed to refine mathematical models. The eddy covariance technique was used with paired towers to measure fluxes simultaneously at disturbed and undisturbed sites over periods of about one week during the growing season in 1998 and 1999. Comparisons were conducted at three sites: a 1‐y‐old burned jackpine stand subjected to an intense crown fire at the International Crown Fire Modelling Experiment site near Fort Providence, North‐west Territories; a 1‐y‐old clearcut aspen area at the EMEND project near Peace River, Alberta; and a 10‐y‐old burned, mixed forest near Prince Albert National Park, Saskatchewan. Nearby mature forest stands of the same types were also measured as controls. The harvested site had lower net radiation (R_n), sensible (H) and latent (LE) heat fluxes, and greater ground heat fluxes (G) than the mature forest. Daytime CO₂ fluxes were much reduced, but night‐time CO₂ fluxes were identical to that of the mature aspen forest. It is hypothesized that the aspen roots remained alive following harvesting, and dominated soil respiration. The overall effect was that the harvested site was a carbon source of about 1.6 gC m^?2 day^?1, while the mature site was a sink of about ?3.8 gC m^?2 day^?1. The one‐year‐old burn had lower R_n, H and LE than the mature jackpine forest, and had a continuous CO₂ efflux of about 0.8 gC m^–2 day^?1 compared to the mature forest sink of ? 0.5 g C m^?2 day^?1. The carbon source was likely caused by decomposition of fire‐killed vegetation. The 10‐y‐old burned site had similar H, LE, and G to the mature mixed forest site. Although the diurnal amplitude of the CO₂ fluxes were slightly lower at the 10‐y‐old site, there was no significant difference between the daily integrals (? 1.3 gC m^?2 day^?1 at both sites). It appears that most of the change in carbon flux occurs within the first 10 years following disturbance, but more data are needed on other forest and disturbance types for the first 20 years following the disturbance event.     

CO2 stimulation and response mechanisms vary with light supply in boreal conifers

北方针叶树CO₂ 的刺激和响应机制随光强而变化 黑云杉(Picea mariana [Mill.] B.S.P.)和白云杉(Picea glauca [Moench] Voss.)是同属物种,两者都是适度耐阴,并且在北美北方针叶林中广泛分布。为了了解光照对CO₂ 浓度升高的生理生态反应的影响,在三种光照条件下(温室中光照设置为100%、50%和30%)将一年生的两种幼苗暴露在360和720 µmol mol^–1 浓度的CO₂环境中,测定了其中后期叶面气体交换量。研究结果表明,CO₂的浓度升高提高了净光合速率 (P_n)和光合水分利用效率,但降低了气孔导度和蒸腾作用。CO₂对光合作用的刺激在50%光照下最大, 在100%光照下最小。光合作用、最大羧化速率(V_cmax)和光饱和电子传递速率(J_max)均随光照强度的 降低而降低。升高的CO₂在所有光照处理中显著降低了V_cmax,在生长季节中期,两种云杉的V_cmax均显著 降低,但在生长季节后期,当光照达到30%时,这一影响变得不明显,而且黑云杉的响应大于白云杉。CO₂ 浓度升高也降低了白云杉的J_max,但在生长季后期30%光照时,这种影响变得不显著。但CO₂ 浓度升高对 黑云杉的影响随时间而变化。在所有光照处理中,CO₂ 浓度升高降低了黑云杉生长中期的J_max,且在生长后 期30%光照时影响不显著,但在100%和50%光照时,J_max升高。这些研究结果表明,两个树种植物都受益于CO₂ 浓度的升高,但它们的响应机制随着光照的增加而变化：即在100%和50%光照下,它们的响应主要是生理上的,而在30%光照下,它们的响应主要是形态上的。     

Globally consistent climate sensitivity of natural disturbances across boreal and temperate forest ecosystems

Disturbance regimes are changing in forests across the world in response to global climate change. Despite the profound impacts of disturbances on ecosystem services and biodiversity, assessments of disturbances at the global scale remain scarce. Here, we analyzed natural disturbances in boreal and temperate forest ecosystems for the period 2001–2014, aiming to 1) quantify their within- and between-biome variation and 2) compare the climate sensitivity of disturbances across biomes. We studied 103 unmanaged forest landscapes with a total land area of 28.2 × 10⁶ ha, distributed across five continents. A consistent and comprehensive quantification of disturbances was derived by combining satellite-based disturbance maps with local expert knowledge of disturbance agents. We used Gaussian finite mixture models to identify clusters of landscapes with similar disturbance activity as indicated by the percent forest area disturbed as well as the size, edge density and perimeter–area-ratio of disturbed patches. The climate sensitivity of disturbances was analyzed using Bayesian generalized linear mixed effect models and a globally consistent climate dataset. Within-biome variation in natural disturbances was high in both boreal and temperate biomes, and disturbance patterns did not vary systematically with latitude or biome. The emergent clusters of disturbance activity in the boreal zone were similar to those in the temperate zone, but boreal landscapes were more likely to experience high disturbance activity than their temperate counterparts. Across both biomes high disturbance activity was particularly associated with wildfire, and was consistently linked to years with warmer and drier than average conditions. Natural disturbances are a key driver of variability in boreal and temperate forest ecosystems, with high similarity in the disturbance patterns between both biomes. The universally high climate sensitivity of disturbances across boreal and temperate ecosystems indicates that future climate change could substantially increase disturbance activity.     

Rapid expansion of lichen woodlands within the closed-crown boreal forest zone over the last 50 years caused by stand disturbances in eastern Canada

Aim Our two main goals are first to evaluate the resilience of the boreal forest according to latitude across the closed‐crown forest zone using the post‐disturbance distribution and cover of lichen woodlands and closed‐crown forests as a metric, and second to identify the disturbance factors responsible for the regeneration and degradation of the closed‐crown forest according to latitude since the 1950s. Location The study area extends between 70°00′ and 72°00′ W and throughout the closed‐crown forest zone, from its southern limit near 47°30′ N to its northern limit at the contact with the lichen woodland zone at around 52°40′ N. Methods Recent (1972–2002) and old (1954–1956) aerial photos were used to map the distribution of lichen woodlands across the closed‐crown forest zone. Forest disturbances such as fire, spruce budworm (Choristoneura fumiferana (Clemens)) outbreak, and logging were recorded on each set of aerial photos. Each lichen woodland and stand disturbance was validated by air‐borne surveys and digitized using GIS software. Results Over the last 50 years, the area occupied by lichen woodlands has increased according to latitude; that is, 9% of the area that was occupied by closed‐crown forests has shifted to lichen woodlands. Although logging activities have been concentrated in the same areas during the last 50 years, the area covered by logging has increased significantly. Outbreaks by the spruce budworm occurred predominantly in the southern (47°30′ N to 48°30′ N) and central (48°53′ N to 50°42′ N) parts of the study area, where balsam fir stands are extensive. In the northern part of the study area (51°–52°40′ N), extensive fires affected the distribution and cover of closed‐crown forests and lichen woodlands. Main conclusions Over the last 50 years, the area occupied by closed‐crown forests has decreased dramatically, and the ecological conditions that allow closed‐crown forests to establish and develop are currently less prevalent. Fire is by far the main disturbance, reducing the ability of natural closed‐crown forests to self‐regenerate whatever the latitude. Given the current biogeographical shift from dense to open forests, the northern part of the closed‐crown forest zone is in a process of dramatic change towards the dominance of northern woodlands.