Radiative forcing of natural forest disturbances

Forest disturbances are major sources of carbon dioxide to the atmosphere, and therefore impact global climate. Biogeophysical attributes, such as surface albedo (reflectivity), further control the climate‐regulating properties of forests. Using both tower‐based and remotely sensed data sets, we show that natural disturbances from wildfire, beetle outbreaks, and hurricane wind throw can significantly alter surface albedo, and the associated radiative forcing either offsets or enhances the CO₂ forcing caused by reducing ecosystem carbon sequestration over multiple years. In the examined cases, the radiative forcing from albedo change is on the same order of magnitude as the CO₂ forcing. The net radiative forcing resulting from these two factors leads to a local heating effect in a hurricane‐damaged mangrove forest in the subtropics, and a cooling effect following wildfire and mountain pine beetle attack in boreal forests with winter snow. Although natural forest disturbances currently represent less than half of gross forest cover loss, that area will probably increase in the future under climate change, making it imperative to represent these processes accurately in global climate models.     

温度和降水对森林生物量分配的影响研究进展

森林生物量分配策略是全球变化背景下群落保持生产力的重要机制.温度和降水会影响森林生物量的分配格局.文章基于文献分析,总结了增温、低温和降水对森林地上、地下生物量分配的影响机制,以及温度和降水对森林生物量分配的交互作用,并对未来温度和降水影响森林生物量分配的研究进行了展望,提出该领域今后的研究重点为:(1)加强生物量分配...     

Holocene radiative forcing impact of northern peatland carbon accumulation and methane emissions

Throughout the Holocene, northern peatlands have both accumulated carbon and emitted methane. Their impact on climate radiative forcing has been the net of cooling (persistent CO₂ uptake) and warming (persistent CH₄ emission). We evaluated this by developing very simple Holocene peatland carbon flux trajectories, and using these as inputs to a simple atmospheric perturbation model. Flux trajectories are based on estimates of contemporary CH₄ flux (15–50 Tg CH₄ yr⁻¹), total accumulated peat C (250–450 Pg C), and peatland initiation dates. The contemporary perturbations to the atmosphere due to northern peatlands are an increase of ∼100 ppbv CH₄ and a decrease of ∼35 ppmv CO₂. The net radiative forcing impact northern peatlands is currently about −0.2 to −0.5 W m⁻² (a cooling). It is likely that peatlands initially caused a net warming of up to +0.1 W m⁻², but have been causing an increasing net cooling for the past 8000–11 000 years. A series of sensitivity simulations indicate that the current radiative forcing impact is determined primarily by the magnitude of the contemporary methane flux and the magnitude of the total C accumulated as peat, and that radiative forcing dynamics during the Holocene depended on flux trajectory, but the overall pattern was similar in all cases.     

Disturbance history mediates climate change effects on subtropical forest biomass and dynamics

The responses of forest communities to interacting anthropogenic disturbances like climate change and logging are poorly known. Subtropical forests have been heavily modified by humans and their response to climate change is poorly understood. We investigated the 9‐year change observed in a mixed conifer‐hardwood Atlantic forest mosaic that included both mature and selectively logged forest patches in subtropical South America. We used demographic monitoring data within 10 1 ha plots that were subjected to distinct management histories (plots logged until 1955, until 1987, and unlogged) to test the hypothesis that climate change affected forest structure and dynamics differentially depending on past disturbances. We determined the functional group of all species based on life‐history affinities as well as many functional traits like leaf size, specific leaf area, wood density, total height, stem slenderness, and seed size data for the 66 most abundant species. Analysis of climate data revealed that minimum temperatures and rainfall have been increasing in the last few decades of the 20th century. Floristic composition differed mainly with logging history categories, with only minor change over the nine annual census intervals. Aboveground biomass increased in all plots, but increases were higher in mature unlogged forests, which showed signs of forest growth associated with increased CO₂, temperature, and rainfall/treefall gap disturbance at the same time. Logged forests showed arrested succession as indicated by reduced abundances of Pioneers and biomass‐accumulators like Large Seeded Pioneers and Araucaria, as well as reduced functional diversity. Management actions aimed at creating regeneration opportunities for long‐lived pioneers are needed to restore community functional diversity, and ecosystem services such as increased aboveground biomass accumulation. We conclude that the effects of climate drivers on the dynamics of Brazilian mixed Atlantic forests vary with land‐use legacies, and can differ importantly from the ones prevalent in better known tropical forests.     

Time‐dependent effects of fertilization on plant biomass in floating fens

Abstract. A cross‐over fertilization experiment was carried out in Dutch floating fens to investigate effects on biomass production in the same and the following years. In total 16 fertilizer treatments were applied, combining four treatments in 1999 with four treatments in 2000 (addition of 20 g.m^?2 N, 5 g.m^?2 P, both elements and unfertilized control). The above‐ground biomass production of vascular plants was co‐limited by N and P in both years. However, in plots that were only fertilized in 1999 the effects of individual nutrients differed between the two years: N‐fertilization slightly increased the amount of biomass produced in the same year (1999), whereas P‐fertilization did so in the following year (2000). Fertilizer applied in 1999 also influenced the effects of fertilizer applied in 2000. One year after N‐fertilization vascular plant growth was still co‐limited by N and P, but one year after P‐fertilization, vascular plant growth was only limited by N. Bryophyte biomass responded weakly to fertilization. Nutrient concentrations in plant biomass, nutrient standing crops and measurements of N and P availability in the soil indicated that one year after fertilization, the N‐fertilizer had mostly ‘disappeared’ from N‐fertilized plots, whereas the availability of P remained markedly enhanced in P‐fertilized plots. In addition, P‐fertilization enhanced the uptake of N by plants the following year. The time‐dependence of fertilizer effects was probably caused by (1) higher addition of P than of N relative to the requirements of plants; (2) longer retention of P than of N in the system; (3) positive effect of P‐fertilization on the availability of N; (4) contrasting effects of N‐ and P‐fertilization on nutrient losses by plants and/or on their responses to subsequent nutrient addition; (5) changing interactions between vascular plants and mosses (mainly Sphagnum spp.); (6) nutrient export through the repeated harvest of above‐ground biomass. To determine which nutrient limits plant growth fertilization experiments should be short, avoiding that indirect effects of a non‐limiting nutrient influence results. To indicate how changed nutrient supply will affect an ecosystem longer‐term experiments are needed, so that indirect effects have time to develop and be detected.     

CO2 fertilization contributed more than half of the observed forest biomass increase in northern extra-tropical land

The existence of a large-biomass carbon (C) sink in Northern Hemisphere extra-tropical ecosystems (NHee) is well-established, but the relative contribution of different potential drivers remains highly uncertain. Here we isolated the historical role of carbon dioxide (CO₂) fertilization by integrating estimates from 24 CO₂-enrichment experiments, an ensemble of 10 dynamic global vegetation models (DGVMs) and two observation-based biomass datasets. Application of the emergent constraint technique revealed that DGVMs underestimated the historical response of plant biomass to increasing [CO₂] in forests (${\beta }_{\text{Forest}}^{\mathrm{Mod}}$) but overestimated the response in grasslands (${\beta }_{\text{Grass}}^{\mathrm{Mod}}$) since the 1850s. Combining the constrained ${\beta }_{\text{Forest}}^{\mathrm{Mod}}$ (0.86 ± 0.28 kg C m⁻² [100 ppm]⁻¹) with observed forest biomass changes derived from inventories and satellites, we identified that CO₂ fertilization alone accounted for more than half (54 ± 18% and 64 ± 21%, respectively) of the increase in biomass C storage since the 1990s. Our results indicate that CO₂ fertilization dominated the forest biomass C sink over the past decades, and provide an essential step toward better understanding the key role of forests in land-based policies for mitigating climate change.     

耕作措施和施肥方式对麦田杂草密度和生物量的影响

为明确我国华北地区麦玉轮作系统小麦免耕的推广和施肥方式的改变对麦季杂草生长的影响,在免耕实施5年后,调查了麦田杂草密度、单株质量和生物量。结果表明:华北地区小麦生长早期,免耕有降低麦田杂草总密度和优势种播娘蒿密度趋势,但差异并不显著;相对于传统耕作,免耕秸秆覆盖和不覆盖处理总杂草生物量显著降低,其中播娘蒿生物量分别降低了57%和73%;免耕也使播娘蒿单株质量降低了27%～53%;免耕秸秆覆盖和不覆盖处理播娘蒿的株高分别比传统耕作降低了25%和19%;但一般情况耕作方式并没有显著影响离子草和麦家公生长;相对于分次施肥,集中施肥杂草生物量降低了21%～68%,播娘蒿生物量降低了58%～65%,麦家公降低91%;免耕在一定程度上抑制了某些杂草的生长,但追肥促进了杂草的快速生长。     

On the role of dynamic atmosphere–vegetation interactions under increasing radiative forcing

• 1 A zero‐dimensional model of local atmosphere–vegetation interaction is presented. The model includes essentials of water related two‐way feedbacks, such as the influence of vegetation on evapotranspiration, and the impact of temperature and drought on biomass growth and mortality. The simple model serves as a framework for the preliminary investigation of vegetation related feedbacks under climate change scenarios.
  • 2 Model simulations for a mid‐latitude forest area for an increasing external forcing indicate a transient growth of biomass up to a critical forcing, where drought stress begins to dominate the response. Beyond, biomass decreases, reinforced by the reduced evapotranspiration of a diminished vegetation, leading to an additional temperature increase (biomass–evapotranspiration feedback).
    • 3 The implementation of an additional feedback loop based on the hypothesis that drought stress implies not only a reduction in above‐ground biomass, but also a net reduction in roots and therefore a reduction of the amount of water accessible to the plants for transpiration, leads to the occurrence of a second stable state in the atmosphere–vegetation system. In the bistable regime, a moderate perturbation can trigger an abrupt change of state.
      • 4 The present conceptual investigations underline the importance of a dynamic vegetation subsystem in transient climate change, and stress in particular the possible role of feedbacks related to root dynamics.

     

Genotypic effects of fertilization on seedling sweetgum biomass allocation,N uptake,and N use efficiency

Screening and selecting tree genotypes that are responsive to N additions and that have high nutrient use efficiencies can provide better genetic material for short-rotation plantation establishment. A pot experiment was conducted to test the hypotheses that (1) sweetgum ( Liquidambar styraciflua L.) families have different patterns in biomass production and allocation, N uptake, and N use efficiency (NUE), because of their differences in growth strategies, and (2) sweetgum families that are more responsive to N additions will also have greater nutrient use efficiencies. Seedlings from two half-sib families (F10022 and F10023) that were known to have contrasting responses to fertility and other stress treatments were used for an experiment with two levels of N (0 vs. 100 kg N/ha equivalent) and two levels of P (0 vs. 50 kg P/ha equivalent) in a split-plot design. Sweetgum seedlings responded to N and P treatments rapidly, with increases in both size and biomass production, and those responses were greater with F10023 than with F10022. Growth response to N application was particularly strong. N and P application increased the proportional allocation of biomass to leaves. Under increased N supply, P application increased foliar N concentration and content, as well as total N uptake by the seedlings. However, NUE was decreased by N addition and was higher in F10023 than in F10022 when P was not limiting. A better understanding of genotype by fertility interactions is important in selecting genotypes for specific site conditions and for optimizing nutrient use in forestry production.     

Impacts of human alteration of the nitrogen cycle in the US on radiative forcing

Nitrogen cycling processes affect radiative forcing directly through emissions of nitrous oxide (N₂O) and indirectly because emissions of nitrogen oxide $(\hbox{NO}_{x})$ and ammonia (NH₃) affect atmospheric concentrations of methane (CH₄), carbon dioxide (CO₂), water vapor (H₂O), ozone (O₃) and aerosols. The emissions of N₂O are mostly from agriculture and they contribute to warming on both short and long time scales. The effects of $\hbox{NO}_{{x}}$ and NH₃ on CH₄, O₃, and aerosols are complex, and quantification of these effects is difficult. However, the net result on time scales of decades is likely one of cooling, which becomes less significant on longer time scales. Deposition of N onto ecosystems also affects sources and sinks of N₂O, CH₄, and CO₂, but the dominant effect is changes in carbon (C) stocks. Primary productivity in most temperate ecosystems is limited by N, so inputs from atmospheric deposition tend to stimulate plant growth and plant litter production, leading in some cases to significant C sequestration in biomass and soils. The literature reviewed here indicates a range of estimates spanning 20–70 kg C sequestered per kg N deposited in forests, which are the dominant C sinks. Most of the sequestration occurs in aboveground forest biomass, with less consistency and lower rates reported for C sequestration in soils. The permanency of the forest biomass sink is uncertain, but data for the fate of forest products in the US indicate that only a small fraction of enhanced forest biomass C is sequestered in long-term harvest products or in unmanaged forests. The net effect of all of these N cycle processes on radiative forcing in the US is probably a modest cooling effect for a 20-year time frame, although the uncertainty of this estimate includes zero net effect, and a modest warming for a 100-year time frame. We know that N-cycling processes are important and that biotic feedbacks to climate change are unlikely to be properly modeled or assessed without including C–N interactions. However, due to the complexity of biological processes involving C–N–climate interactions, biogeochemical models are still poorly constrained with respect to ecosystem responses to impacts of N deposition and climate change. Only recently have N-cycling processes been incorporated into Earth system models for C–N interactions. The robustness of these models remains to be demonstrated. Much work remains for improving their representation in models used to simulate climate forcing scenarios.     

模拟分类经营对小兴安岭林区森林生物量的影响

运用空间直观景观模型LANDIS 7.0 PRO,模拟了在当前采伐模式和无采伐两个预案下,小兴安岭林区森林生物量及主要树种生物量在2000—2200年间的动态。模拟结果如下:(1)无采伐预案下,森林生物量由最初的93.6 t/hm2逐渐升高,90a后达到最大值258 t/hm2,之后森林生物量在245 t/hm2上下小幅波动;(2)前100a采伐预案会明显降低森林生物量,与无采伐预案相比森林生物量最大可降低21.4 t/hm2,平均减少14.7 t/hm2;后100a采伐对森林生物量的影响逐渐减弱,森林生物量平均减少2.6 t/hm2;(3)当前采伐模式促进保护树种红松和紫椴生长,其生物量分别最大可提高9.0 t/hm2和0.53 t/hm2,占到无采伐预案生物量的56%和15%;(4)采伐预案对云冷杉生物量影响较小,主要降低先锋树种(白桦、山杨)和一些阔叶树种(枫桦、春榆)的生物量。研究结果表明现行采伐模式在未来100 a内会显著影响森林生物量,之后其影响逐渐减小,并且保护政策能提高所保护树种(红松、紫椴)的生物量,但要保持较高的总生物量,仍需要降低目前的采伐强度。     

Fertilization effects on fineroot biomass, rhizosphere microbes and respiratory fluxes in hardwood forest soils

Fertilizer-induced reductions in CO(2) flux from soil ((F)CO(2)) in forests have previously been attributed to decreased carbon allocation to roots, and decreased decomposition as a result of nitrogen suppression of fungal activity. Here, we present evidence that decreased microbial respiration in the rhizosphere may also contribute to (F)CO(2) reductions in fertilized forest soils. Fertilization reduced (F)CO(2) by 16-19% in 65-yr-old plantations of northern red oak (Quercus rubra) and sugar maple (Acer saccharum), and in a natural 85-yr-old yellow birch (Betula allegheniensis) stand. In oak plots, fertilization had no effects on fine root biomass but reduced mycorrhizal colonization by 18% and microbial respiration by 43%. In maple plots, fertilization reduced root biomass, mycorrhizal colonization and microbial respiration by 22, 16 and 46%, respectively. In birch plots, fertilization reduced microbial respiration by 36%, but had variable effects on root biomass and mycorrhizal colonization. In plots of all three species, fertilization effects on microbial respiration were greater in rhizosphere than in bulk soil, possibly as a result of decreased rhizosphere carbon flux from these species in fertile soils. Because rhizosphere processes may influence nutrient availability and carbon storage in forest ecosystems, future research is needed to better quantify rhizo-microbial contributions to (F)CO(2).     

Loss of aboveground forest biomass and landscape biomass variability in Missouri,US

Disturbance regimes and forests have changed over time in the eastern United States. We examined effects of historical disturbance (circa 1813 to 1850) compared to current disturbance (circa 2004 to 2008) on aboveground, live tree biomass (for trees with diameters ≥13 cm) and landscape variation of biomass in forests of the Ozarks and Plains landscapes in Missouri, USA. We simulated 10,000 one-hectare plots using random diameters generated from parameters of diameter distributions limited to diameters ≥13 cm and random densities generated from density estimates. Area-weighted mean biomass density (Mg/ha) for historical forests averaged 116 Mg/ha, ranging from 54 Mg/ha to 357 Mg/ha by small scale ecological subsections within Missouri landscapes. Area-weighted mean biomass density for current forests averaged 82 Mg/ha, ranging from 66 Mg/ha to 144 Mg/ha by ecological subsection for currently forested land. Biomass density of current forest was greater than historical biomass density for only 2 of 23 ecological subsections. Current carbon sequestration of 292 TgC on 7 million ha of forested land is less than half of the estimated historical total carbon sequestration of 693 TgC on 12 million ha. Cumulative tree cutting disturbances over time have produced forests that have less aboveground tree biomass and are uniform in biomass compared to estimates of historical biomass, which varied across Missouri landscapes. With continued relatively low rates of forest disturbance, current biomass per ha will likely increase to historical levels as the most competitive trees become larger in size and mean number of trees per ha decreases due to competition and self-thinning. Restoration of large diameter structure and forested extent of upland woodlands and floodplain forests could fulfill multiple conservation objectives, including carbon sequestration.     

Decadal vegetation changes in a northern peatland, greenhouse gas fluxes and net radiative forcing

Thawing permafrost in the sub‐Arctic has implications for the physical stability and biological dynamics of peatland ecosystems. This study provides an analysis of how permafrost thawing and subsequent vegetation changes in a sub‐Arctic Swedish mire have changed the net exchange of greenhouse gases, carbon dioxide (CO₂) and CH₄ over the past three decades. Images of the mire (ca. 17 ha) and surroundings taken with film sensitive in the visible and the near infrared portion of the spectrum, [i.e. colour infrared (CIR) aerial photographs from 1970 and 2000] were used. The results show that during this period the area covered by hummock vegetation decreased by more than 11% and became replaced by wet‐growing plant communities. The overall net uptake of C in the vegetation and the release of C by heterotrophic respiration might have increased resulting in increases in both the growing season atmospheric CO₂ sink function with about 16% and the CH₄ emissions with 22%. Calculating the flux as CO₂ equivalents show that the mire in 2000 has a 47% greater radiative forcing on the atmosphere using a 100‐year time horizon. Northern peatlands in areas with thawing sporadic or discontinuous permafrost are likely to act as larger greenhouse gas sources over the growing season today than a few decades ago because of increased CH₄ emissions.     

The effects of NPK fertilization for nine years on boreal forest vegetation in northwestern Canada

Abstract. Plant productivity is limited by mineral nutrient availability in many boreal forest ecosystems. This study is an analysis of the growth responses of components of a boreal plant community (cryptogams, herbaceous and woody perennials, the dominant shrubs Salix glauca (grey willow) and Betula glandulosa (bog birch) and the dominant tree Picea glauca (white spruce), to the addition of an NPK fertilizer over a nine-year period. The study was carried out in a low-nutrient boreal forest ecosystem in the Yukon territory in northwestern Canada. The following predictions were tested: (1) that there would be an overall increase in abundance (measured either as cover, density, or dry mass) of all components of the vegetation, (2) that vegetation composition would change as more competitive species increased in abundance and (3) that initial community changes in response to fertilization would be transient. In general, all predictions were found to be true. Species composition changed rapidly in response to fertilizer. Graminoids (e.g. Festuca altaica) and some dicots (e.g. Mertensia paniculata and Achillea millefolium) increased in cover, while other dicots (e.g. Anemone parviflora), dwarf shrubs (e.g. Arctostaphylos uvaursi), bryophytes and lichens declined. There was a significant increase in the growth rate of the two dominant shrubs and of Picea, but not in the cone crop or seed production by Picea. Surveys after 1 or 2 years showed responses by the vegetation but more stable patterns of response did not emerge until after 5 or 6 years. There were consistent and directional changes in the percent cover of some of the herbaceous species on control plots. Growth rates of Salix and Betula varied considerably from year to year, independently of treatment. Long-term studies are essential if we are to understand the role of nutrient limitation in this ecosystem.     

沈阳城市森林绿量测算

借助ARC/GIS地理信息系统,以“平面量推算平面量”的方法,测算了沈阳市城市森林的绿量.结果表明:沈阳市城市森林不同类型单位面积绿量以风景游憩林最高,为3·86m2·m-2;生态公益林最低,为2·27m2·m-2;城市森林分布区单位面积绿量为2·99m2·m-2;城区单位面积绿量为0·25m2·m-2.沈阳城市森林总体绿量约为1·13×108m2·其中,附属林为4·15×107m2,占36·64%;生态公益林2·72×107m2,占23·99%;风景游憩林2·20×107m2,占19·38%;道路林1·84×107m2,占16·20%;生产经营林0·43×107m2,占3·79%.经检验,平面量推算平面量方法测算精度达到91·81%(α=0·05).     

Short- and long-term greenhouse gas and radiative forcing impacts of changing water management in Asian rice paddies

Fertilized rice paddy soils emit methane while flooded, emit nitrous oxide during flooding and draining transitions, and can be a source or sink of carbon dioxide. Changing water management of rice paddies can affect net emissions of all three of these greenhouse gases. We used denitrification–decomposition (DNDC), a process‐based biogeochemistry model, to evaluate the annual emissions of CH₄, N₂O, and CO₂ for continuously flooded, single‐, double‐, and triple‐cropped rice (three baseline scenarios), and in further simulations, the change in emissions with changing water management to midseason draining of the paddies, and to alternating crops of midseason drained rice and upland crops (two alternatives for each baseline scenario). We used a set of first‐order atmospheric models to track the atmospheric burden of each gas over 500 years. We evaluated the dynamics of the radiative forcing due to the changes in emissions of CH₄, N₂O, and CO₂ (alternative minus baseline), and compared these with standard calculations of CO₂‐equivalent emissions using global warming potentials (GWPs). All alternative scenarios had lower CH₄ emissions and higher N₂O emissions than their corresponding baseline cases, and all but one sequestered carbon in the soil more slowly. Because of differences in emissions, in radiative forcing per molecule, and in atmospheric time constants (lifetimes), the relative radiative impacts of CH₄, N₂O, and CO₂ varied over the 500‐year simulations. In three of the six cases, the initial change in radiative forcing was dominated by reduced CH₄ emissions (i.e. a cooling for the first few decades); in five of the six cases, the long‐term radiative forcing was dominated by increased N₂O emissions (i.e. a warming over several centuries). The overall complexity of the radiative forcing response to changing water management could not easily be captured with conventional GWP calculations.     

浙江省森林生物量动态

以浙江省1976至2004年森林资源连续清查资料为数据源,采用基于生物量与蓄积之间关系的生物量转换因子连续函数法,对全省林分生物量和包括林分在内的森林生物量动态进行估计。森林生物量为包括林分、疏林、灌木林、竹林、经济林和四旁树在内的所有林木生物量之和。结果表明,浙江省1976至2004年间森林生物量从1.00828×10^8Mg上升到2.44426×10^8Mg;其中,林分生物量由0.5712×10^8Mg上升到1.51128×10^8Mg。森林生物量和林分生物量的年平均增长速度分别为5.1%和9.1%。在1999至2004年间,森林生物量和林分生物量增长速度均明显加快,分别达到8.6%和10.1%。在1976至2004年间,全省森林面积年均增长速度为1.0%,森林平均生物量从16.50Mg·hm^-2上升到36.59Mg·hm^-2。但是,在森林资源总量不断增加的同时,全省林分质量仍维持较低水平。2004年全省林分单位面积生物量为38.40Mg·hm^-2,远低于全国平均水平（77.40Mg·hm^-2）。研究还表明,利用森林资源连续清查数据和基于单株测树因子的森林生物量模型能够估计大尺度范围内的森林生物量及其动态,但亟待在统一标准下建立和完善覆盖所有树种的生物量模型。