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Net ecosystem gas exchange (NEE), a balance between gross ecosystem primary productivity (GPP) and ecosystem respiration (ER), is an important indicator of terrestrial ecosystem CO2 sink or source. Increasing frequency in droughts during different periods of the growing season may affect terrestrial ecosystem carbon (C) balance. However, detecting how drought timing controls ecosystem C processes is insufficiently explored because it is a challenge to accurately monitor and forecast drought dynamics. In a 5-year (2015–2019) precipitation manipulation experiment in a temperate steppe in northern China, we imposed a 60% decrease in precipitation in the early (April–June, DEP) and late (July–September, DLP) growing seasons in plots under rainout shelters to simulate drought occurrence timing. The responses of GPP, ER and NEE to DEP and DLP were examined to determine how the timing of decreased precipitation affects CO2 fluxes. Both DEP and DLP reduced GPP and ER. Decreased precipitation in the late growing season reduced NEE due to a greater decline in GPP than ER. In contrast, the lack of an effect of DEP on NEE can be attributed to a proportional decline in GPP and ER. Reduced normalized difference vegetation index (NDVI) and GPP induced by decreased precipitation explained the decline in GPP and ER respectively. Drought in the late growing season weakened the ecosystem C sink. The result indicates a low resistance in net C uptake to DLP. However, we did not find evidence that previous precipitation decreases resulted in a lower overall rate of ecosystem C exchange, indicating a high resilience in C processes in this semi-arid grassland. Synthesis and applications. Our study provides solid evidence that drought in the late growing season weakens ecosystem C sink in the semi-arid grassland. The findings suggest that management practices aimed at enhancing water availability during the late growing season should be preferred for increasing C sequestration.

Numerous studies have reported that treelines are moving to higher elevations and higher latitudes. Most treelines are temperature limited and warmer climate expands the area in which trees are capable of growing. Hence, climate change has been assumed to be the main driver behind this treeline movement. The latest review of treeline studies was published in 2009 by Harsch et al. Since then, a plethora of papers have been published studying local treeline migration. Here we bring together this knowledge through a review of 142 treeline related publications, including 477 study locations. We summarize the information known about factors limiting tree-growth at and near treelines. Treeline migration is not only dependent on favorable growing conditions but also requires seedling establishment and survival above the current treeline. These conditions appear to have become favorable at many locations, particularly so in recent years. The review revealed that at 66% of these treeline sites forest cover had increased in elevational or latitudinal extent. The physical form of treelines influences how likely they are to migrate and can be used as an indicator when predicting future treeline movements. Our analysis also revealed that while a greater percentage of elevational treelines are moving, the latitudinal treelines are capable of moving at greater horizontal speed. This can potentially have substantial impacts on ecosystem carbon storage. To conclude the review, we present the three main hypotheses as to whether ecosystem carbon budgets will be reduced, increased or remain the same due to treeline migration. While the answer still remains under debate, we believe that all three hypotheses are likely to apply depending on the encroached ecosystem. Concerningly, evidence is emerging on how treeline migration may turn tundra landscapes from net sinks to net sources of carbon dioxide in the future.

In the subtropics, the influence of water on the net ecosystem carbon exchange (NEE) is critical, subsequently determining net primary productivity (NPP). Water also contributes the majority of int...

Differences in dominant tree species affect ecosystem carbon budgets in forests, but their independent impacts have not been fully recognized. We aimed to clarify the interannual variation in net p...

Forest stand age can affect ecosystem carbon (C) cycling and net ecosystem productivity (NEP). In Canada, establishment of short-rotation plantations on previously agricultural lands has been ongoing, but the effect of stand development on soil respiration (Rs) and NEP in such plantations is poorly understood. These types of data are essential for constraining ecosystem models that simulate C dynamics over the rotation of a plantation. We studied Rs (including autotrophic, Ra, and heterotrophic, Rh) and NEP in 2008 and 2009 in a chronosequence of 5-, 8-, 14-, and 16-yr-old (ages in 2009) hybrid poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantations in northern Alberta. The highest Rs and NEP were generally found in the 14-yr-old stand. Seasonal variations in Rs were similar among the plantations, with most of the variation explained by soil temperature at the 10 cm depth in 2008 with far less explained in 2009, a much drier year. In diurnal measurements, hysteresis was found between soil respiration and soil temperature, with the patterns of hysteresis different among stand ages. Soil respiration in the 14-yr-old plantation had the greatest sensitivity to temperature changes. Stand age did not affect the Rh:Rs ratio, whereas the NEP exhibited strong inter-annual variability. We conclude that stand age was a major factor affecting Rs and NEP, and such effects should be considered in empirical models used to simulate ecosystem C dynamics to evaluate potentials for C sequestration and the C source–sink relationship in short-rotation woody crop systems.

AIM: Ecosystem carbon use efficiency (CUEe) is a core parameter of ecosystem process models, but its relationships with climate are still uncertain, especially for ecosystems with harsh environments. Large inconsistencies in climate impacts on the CUEe have been reported among various spatial scales. The goal of this study was to examine whether warming promotes or restricts the CUEe and whether the CUEe responds to a warming gradient in a linear or nonlinear manner. LOCATION: Tibetan Plateau. TIME PERIOD: 2000–2018. MAJOR TAXA STUDIED: Alpine grassland ecosystem. METHODS: We integrated multiple‐source data of carbon fluxes and CUEe, including warming experiments at a site scale, eddy covariance observations at a landscape scale and synthesized warming experiments and ecosystem process models at a regional scale. Next, we deployed a statistical model to examine the warming impacts on the CUEe across scales; the effects of biotic and abiotic factors on the CUEe and its components were summarized based on the results of standardized major axis tests and routines, structural equation modelling and nonlinear models. RESULTS: This study reported a suppressive warming impact on the CUEe, which followed a nonlinear curve with severe inhibition in the high‐level warming treatment. With a warming threshold of 1.5–2.0°C, CUEe response patterns transitioned from no change to a significant decrease. The restriction effects can be ascribed to the joint adverse and asymmetric effects of warming on CUEe components under multiple‐level warming. Warming‐modified relationships among CUEe components and the nonlinear effects of biotic and abiotic factors led to the nonlinear responses of CUEe to warming. MAIN CONCLUSIONS: This study revealed suppressive and nonlinear effects of warming on the CUEe, including especially dramatic CUEe decreases with high‐level warming. These findings are critical for optimizing model parameters and improving predictions of the carbon sequestration capacity of alpine grasslands.

Estimates of plant biomass are helpful for many applications in invasive plant science and management, but measuring biomass can be time-consuming, costly, or impractical if destructive sampling is inappropriate. The objective of this study was to assess feasibility of developing regression equations using a fast, nondestructive measure (cover) to estimate aboveground biomass for red brome (Bromus rubens L.), a widespread nonnative annual grass in the Mojave Desert, USA. At three study sites, including one measured for three consecutive years, B. rubens cover spanned 0.1% to 85% and aboveground biomass 1 to 321 g m−2. In log10-transformed linear regressions, B. rubens cover accounted for 68% to 96% of the variance in B. rubens biomass among sites, with all coefficients of determination significant at P < 0.05. For every doubling of percent cover, biomass was predicted to increase by 78%, 83%, and 144% among the three sites. At the site measured for three consecutive years, which ranged in rainfall from 65% to 159% of the long-term average, regression slopes each year differed from other years. Regression results among sites were insensitive to using cover classes (10 classes encompassing 0% to 100% cover) compared with simulated random distribution of integer cover within classes. Biomass of B. rubens was amenable to estimation in the field using cover, and such estimates may have applications for modeling invasive annual plant fuel loads and ecosystem carbon storage.

The Tibetan Plateau (TP) has a variety of vegetation types that range from alpine tundra to tropic evergreen forest, which play an important role in the global carbon (C) cycle and is extremely vulnerable to climate change. The vegetation C uptake is crucial to the ecosystem C sequestration. Moreover, net reduction in vegetation C uptake (NRVCU) will strongly affect the C balance of terrestrial ecosystem. Until now, there is limited knowledge on the recovery process of vegetation net C uptake and the spatial-temporal patterns of NRVCU after the disturbance that caused by climate change and human activities. Here, we used the MODIS-derived net primary production to characterize the spatial-temporal patterns of NRVCU. We further explored the influence factors of the net reduction rate in vegetation C uptake (NRRVCU) and recovery processes of vegetation net C uptake across a unique gradient zone on the TP. Results showed that the total net reduction amount of vegetation C uptake gradually decreased from 2000 to 2015 on the TP (Slope = −0.002, P < 0.05). Specifically, an increasing gradient zone of multi-year average of net reduction rate in vegetation carbon uptake (MYANRRVCU) from east to west was observed. In addition, we found that the recovery of vegetation net C uptake after the disturbance caused by climate change and anthropogenic disturbance in the gradient zone were primarily dominated by precipitation and temperature. The findings revealed that the effects of climate change on MYANRRVCU and vegetation net C uptake recovery differed significantly across geographical space and vegetation types. Our results highlight that the biogeographic characteristics of the TP should be considered for combating future climate change.

Soil respiration (Rs) measurements in relation to agricultural management and environmental variables are necessary to improve our understanding of net ecosystem carbon balance (NECB) and to predic...

We welcome the letter of Song and Wang (1) regarding aquatic carbon (C) export on the Tibetan Plateau (TP) and appreciate the opportunity to clarify our thinking. The net ecosystem C balance (NECB) includes CO2 and CH4 exchange, volatile organic C loss, and particulate and aquatic C transport. By contrast, our work focused on terrestrial CO2 exchange (2), a subset of the NECB. We partly agree with Song and Wang regarding the importance of aquatic C loss, but their conclusion was reached based upon a catchment-scale case study (3). In an attempt to clarify how much the terrestrial CO2 sink has been compromised at a regional scale, we expanded the NECB to incorporate rivers, lakes, thermokarst, wetlands, grasslands, and … [↵][1] 1To whom correspondence may be addressed. Email: wxd{at}imde.ac.cn. [1]: #xref-corresp-1-1

Monitoring net ecosystem carbon dioxide (CO2) exchange (NEE) using eddy covariance (EC) flux towers is quite common, but the measurements are valid at the scale of tower footprints. Alternative way...

In the northern hemisphere, terrestrial ecosystems transition from net sources of CO2 to the atmosphere in winter to net ecosystem carbon sinks during spring. The timing (or phase) of this transiti...

As an essential part of terrestrial ecosystems, convenient and accurate reconstruction of the past carbon sequestration capacity of forests is critical to assess future trends of aboveground carbon storage and ecosystem carbon cycles. In addition, the relationship between climate change and carbon sequestration of forests has been vigorously debated. In this study, dynamic change of carbon sequestration capacity in aboveground biomass of Picea schrenkiana (hereinafter abbreviated as P. schrenkiana) in the Tianshan Mountains, northwestern China, from 1850–2017, were reconstructed using dendrochronology. The main climate drivers that affected carbon sequestration capacity in aboveground biomass of P. schrenkiana were then investigated. The results showed that: (1) tree-ring width and diameter at breast height (DBH) of P. schrenkiana obtained from different altitudes and ages were an effective and convenient estimation index for reconstructing the carbon sequestration capacity of P. schrenkiana. The carbon storage of P. schrenkiana forest in 2016 in the Tianshan Mountains was 50.08 Tg C calculated using tree-ring width and DBH, which was very close to the value determined by direct field investigation data. (2) The annual carbon sequestration potential capacity of P. schrenkiana exhibited an increasing trend from 1850–2017. Temperature, especially minimum temperature, constituted the key climatic driver resulting in increased carbon sequestration capacity. The contribution rates of temperature and minimum temperature to the change of P. schrenkiana carbon sequestration capacity was 75% and 44%, respectively. (3) The significant increase of winter temperature and minimum temperature led to warming in the Tianshan Mountains, resulting in a significant increase in carbon sequestration capacity of P. schrenkiana. The results indicate that, with the continuous increase of winter temperature and minimum temperature, carbon sequestration of P. schrenkiana in the Tianshan Mountains is predicted to increase markedly in the future. The findings of this study provide a useful basis to evaluate future aboveground carbon storage and carbon cycles in mountain systems possessed similar characteristics of the Tianshan Mountains.

Ecosystem carbon dioxide (CO2), water vapor, and heat exchanges in alpine wetlands on the Qinghai–Tibetan Plateau are not comprehensively understood. Thus, we studied variability of net ecosystem CO2 exchange (NEE), ecosystem respiration (Re), gross primary production (GPP), evapotranspiration (ET), and heat fluxes over a headwater wetland ecosystem in the Qinghai Lake region. Results showed that the headwater wetland ecosystem was net CO2 absorption on the annual scale, in which monthly NEE, GPP, and Re in two consecutive years varied from −165.16 to 93 g CO2 m−2 mo−1, 6.66 to 384.45 g CO2 m−2 mo−1, and 6.9 to 232.02 g CO2 m−2 mo−1, respectively. The monthly ET from June to September was smaller than precipitation; these results reversed in the remaining months. Annual ET was 362.1 mm in 2015 and 324.96 mm in 2016. The net radiation (Rn), sensible heat (H), latent heat (LE), and ground heat (G) fluxes showed similar monthly patterns. Values of monthly average half-hour Rn, H, LE, and G at the daytime showed Rn > LE > H > G, and the time of the monthly half-hour G peak obviously lagged the Rn, H, and LE. Monthly average Bowen ratios were

Vegetation–soil systems differentially influence the ecosystem processes related to the carbon cycle, particularly when one tree species is dominant over wide geographic regions that are undergoing climate change. The objective of this study was to quantify the stocks of ecosystem carbon in three vegetation–soil systems along a highland elevational gradient in central Mexico. The vegetation–soil systems, from lower to higher elevation, were dominated by Alnus jorullensis Kunth, Abies religiosa (Kunth) Schltdl. & Cham., and Pinus hartwegii Lindl., respectively. Above- and below-ground tree biomass was determined in each system, along with the litter, coarse woody material, roots, and litterfall. The A. religiosa system had the greatest stock of aboveground biomass carbon (216 ± 31 Mg C·ha−1). The A. jorullensis system had the greatest production of litterfall (3.1 ± 0.08 Mg·ha−1·year−1); however, the carbon content of this litter layer (1.2 ± 0.32 Mg C·ha−1) was lower than that of P. hartwegii (10.1 ± 0.28 Mg C·ha−1). Thus, the litter layer in the A. jorullensis system had markedly the shortest residence time (8 years), suggesting high rates of litter decomposition. The soil carbon stock (at soil depth of 1 m) was greater in A. jorullensis (189 Mg C·ha−1) and P. hartwegii (137 Mg C·ha−1) than in A. religiosa (68 Mg C·ha−1). The A. religiosa and A. jorullensis systems had the highest and lowest total ecosystem C content (301 and 228 Mg C·ha−1, respectively). Upward migration of the A. religiosa system in response to global climate change, however, could cause losses by 2030 of 187 Mg C·ha−1 associated with aboveground biomass.

Eddy covariance (EC) is an important measurement technique used in physical geography and atmospheric sciences to measure the exchange of carbon dioxide between an ecosystem and the atmosphere at a specific location. However, EC produces a net exchange of carbon dioxide yet research questions require an understanding of component fluxes, carbon dioxide uptake by plants through photosynthesis and carbon dioxide emissions due to plant and soil respiration. There are two major methods to partition EC measurements into these component fluxes: night-time and day-time partitioning methods. In the night-time method, nighttime measurements are used to estimate daytime respiration and calculate photosynthesis as a residual and in the daytime method, a light response curve is created to estimate daytime respiration and photosynthesis. This study investigates the benefits and drawbacks of these partitioning methods on two carbon dioxide exchange datasets from ecosystems in Canada. The research sites were a) Mer Bleue, a peatland bog near Ottawa, Ontario and b) Cape Bounty, a high arctic tundra in Nunavut. By using a combination of the REddy-Proc software package, developed by the Max Planck Institute for Biogeochemistry, along with additional Matlab processing, the differences in photosynthesis and respiration due to partitioning methods are presented and discussed.

Peatland ecosystems are integral to the mitigation of climate change as they represent significant terrestrial carbon sinks. In Ireland, peatlands cover ~20% of the land area but hold up to 75% of the soil organic carbon stock however many of these ecosystems (~85% of the total area) have been degraded due to anthropogenic activities such as agriculture, forestry and extraction for horticulture or energy. Furthermore, the carbon stocks that remain in these systems are vulnerable to inter-annual variation in climate, such as changes in precipitation and temperature, which can alter the hydrological status of these systems leading to changes in key biogeochemical processes and carbon and greenhouse gas exchange. During 2018 exceptional drought and heatwave conditions were reported across Northwestern Europe, where reductions in precipitation coupled with elevated temperatures were observed. Exceptional inter-annual climatic variability was also observed at Clara bog, a near natural raised bog in the Irish midlands when data from 2018 and 2019 were compared. Precipitation in 2018 was ~300 mm lower than 2019 while the average mean annual temperature was 0.5°C higher. The reduction in precipitation, particularly during the growing season in 2018, consistently lowered the water table where ~150 consecutive days where the water table was >5cm below the surface of the bog were observed at the central ecotope location. The differing hydrological conditions between years resulted in the study area, as determined by the flux footprint of the eddy covariance tower, acting as a net source of carbon of 53.5 g C m-2 in 2018 and a net sink of 125.2 g C m-2 in 2019. The differences in the carbon dynamics between years were primarily driven by enhanced ecosystem respiration (Reco) and lower rates of Gross Primary Productivity (GPP) in the drier year, where the maximum monthly ratio of GPP:Reco during the growing season was 0.96 g C m-2 month in 2018 and 1.14 g C m-2 month in 2019. This study highlights both the vulnerability and resilience of these ecosystems to exceptional inter-annual climatic variability and emphasises the need for long-term monitoring networks to enhance our understanding of the impacts of these events when they occur.

The ecosystem carbon (C) dynamics after permafrost thaw depends on more than just climate change since soil nutrient status may also impact ecosystem C balance. It has been advocated that the potential nitrogen (N) release upon permafrost thaw could promote plant growth and thus offset soil C loss. However, compared with the widely accepted C-N interactions, little is known about the potential role of soil phosphorus (P) availability. Here we combined two-year field observations along a permafrost thaw sequence (constituted by four thaw stages, i.e., non-collapse and 5, 14, and 22 years since collapse) with an in-situ fertilization experiment (included N and P additions at the level of 10 g N m-2 yr-1 and 10 g P m-2 yr-1, respectively) in a Tibetan swamp meadow to evaluate ecosystem C-nutrient interactions upon permafrost thaw. Our results showed that changes in soil P availability rather than N availability played an important role in regulating the increases in gross primary productivity and the decreases in net ecosystem exchange along the thaw sequence. The fertilization experiment further confirmed that P addition had stronger effects on plant growth than N addition in this permafrost ecosystem. These two lines of evidence highlight the crucial role of soil P availability in altering the trajectory of permafrost C cycle under climate warming.

In summer 2018, Europe experienced a record drought, but it remains unknown how the drought affected ecosystem carbon dynamics. Using observations from 34 eddy covariance sites in different biomes across Europe, we studied the sensitivity of gross primary productivity (GPP) to environmental drivers during the summer drought of 2018 versus the reference summer of 2016. We found a greater drought-induced decline of summer GPP in grasslands (−38%) than in forests (−10%), which coincided with reduced evapotranspiration and soil water content (SWC). As compared to the ‘normal year’ of 2016, GPP in different ecosystems exhibited more negative sensitivity to summer air temperature (Ta) but stronger positive sensitivity to SWC during summer drought in 2018, that is, a stronger reduction of GPP with soil moisture deficit. We found larger negative effects of Ta and vapour pressure deficit (VPD) but a lower positive effect of photosynthetic photon flux density on GPP in 2018 compared to 2016, which contributed to reduced summer GPP in 2018. Our results demonstrate that high temperature-induced increases in VPD and decreases in SWC aggravated drought impacts on GPP. This article is part of the theme issue ‘Impacts of the 2018 severe drought and heatwave in Europe: from site to continental scale’., Philosophical Transactions of the Royal Society B: Biological Sciences, 375 (1810), ISSN:0962-8436, ISSN:1471-2970, ISSN:0080-4622

The ecosystem carbon (C) balance in permafrost regions, which has a global significance in understanding the terrestrial C-climate feedback, is significantly regulated by nitrogen (N) dynamics. However, our knowledge on temporal changes in vegetation N limitation (i.e., the supply of N relative to plant N demand) in permafrost ecosystems is still limited. Based on the combination of isotopic observations derived from a re-sampling campaign along a ~3000 km transect and simulations obtained from a process-based biogeochemical model, here we detect changes in ecosystem N cycle across the Tibetan alpine permafrost region over the past decade. We find that vegetation N limitation becomes stronger despite the increased available N production. The enhanced N limitation on vegetation growth is driven by the joint effects of elevated plant N demand and gaseous N loss. These findings suggest that N would constrain the future trajectory of ecosystem C cycle in this alpine permafrost region., Massive stores of carbon and nutrients in permafrost could be released by global warming. Here the authors show that though warming across the Tibetan alpine permafrost region accelerates nitrogen liberation, contrary to expectations the elevated nutrients do not alleviate plant nitrogen limitation.

Accurately representing the response of ecosystems to environmental change in land surface models (LSM) is crucial to making accurate predictions of future climate. Many LSMs do not correctly capture plant respiration and growth fluxes, particularly in response to extreme climatic events. This is in part due to the unrealistic assumption that total plant carbon expenditure (PCE) is always equal to gross carbon accumulation by photosynthesis. We present and evaluate a simple model of labile carbon storage and utilisation (SUGAR), designed to be integrated into an LSM, that allows simulated plant respiration and growth to vary independently of photosynthesis. SUGAR buffers simulated PCE against seasonal variation in photosynthesis, producing more constant (less variable) predictions of plant growth and respiration relative to an LSM that does not represent labile carbon storage. This allows the model to more accurately capture observed carbon fluxes at a large-scale drought experiment in a tropical moist forest in the Amazon, relative to the Joint UK Land Environment Simulator LSM (JULES). SUGAR is designed to improve the representation of carbon storage in LSMs and provides a simple framework that allows new processes to be integrated as the empirical understanding of carbon storage in plants improves. The study highlights the need for future research into carbon storage and allocation in plants, particularly in response to extreme climate events such as drought.

The recent research and policy efforts on climate change mitigation highlight the need for proper understanding of the effects of many types of land management interventions on greenhouse gas exchange processes. The complexity of carbon and nitrogen cycles, which is the case also for agricultural ecosystems, call for model-based research approaches. These can make the decision-making applications easier as well. The agricultural use of sewage sludge is widespread in many countries. There are a number of case studies about its possible effects on greenhouse gas fluxes under different climatic conditions, but there are not many experiences in relevant model-based assessments. In our contribution, the Biome-BGC MuSo (v.6.) model was used for the investigation of the main characteristics of ecosystem exchange of carbon in arable land of warm dry temperate climate in the Great Plain of Hungary. The Biome-BGC is one of the most widely used biogeochemical models, it is capable of handling different land management activities, have a multilayer soil module and enable a quite detailed ecophysiological parameterization, which make it suitable for the targeted study. The results of laboratory analyses of soil profiles of the study area were used for the parameterization (element contents, organic matter, etc.). The poster presents the first results of the integrated measurement and modelling research work.

In the Sand County Almanac (Leopold 1949), Aldo Leopold writes of the odyssey of element X and thus of the circulation of all nutrient elements as they cycle through the Earth’s forests, rangelands, lakes, and oceans. In mid-century, G.E. Hutchinson, while always one to praise aesthetic values, commented disparagingly about the quantitative science of element cycling, specifically that ecosystem carbon (C) data were “wretchedly inadequate” (Hutchinson 1954). Hutchinson’s comment was not only a complaint but also a challenge to all ecosystem scientists who followed to quantify the Earth’s biogeochemical cycles, for he understood that the resilience and functioning of ecosystems was entirely dependent on how plants, animals, and decomposers used and reused the chemical elements that we call nutrients.

The importance of forests and soil in carbon (C) sequestration and storage is continually increasing with climate change. Disturbances, such as storms and insect outbreaks, are the drivers of forest functioning, composition and structure, and many of them are predicted to become more common in the future. However, environmental factors that predispose forests to disturbance as well as the diverse effects of disturbances on forest C cycling are not fully known. In this dissertation, stand, site and soil characteristics predisposing forest stands to outbreaks of two common insect species that can cause tree damage and mortality—the common pine sawfly (Diprion pini L.) and the European spruce bark beetle (Ips typographus L.)—were examined, and the impacts of storm and I. typographus disturbance on soil respiration, tree and soil C stocks, and microbial community composition and associated C contents were investigated in forests located in eastern and southern Finland. The level of tree damage by D. pini and I. typographus in managed Scots pine (Pinus sylvestris L.) and urban Norway spruce (Picea abies (L.) Karst.) forests, respectively, were associated with various site and soil characteristics. Defoliation of P. sylvestris by D. pini was more severe on sites with soil properties indicating greater fertility (e.g. lower soil C/N ratio and finer textured). Highest cumulative probabilities for severe I. typographus infestation of P. abies were associated with trees growing on sites having an east-facing aspect and the most fertile site types combined with either moderately steep slopes, shallow till soil or high soil C/N ratio. The effects of storm and I. typographus (5–7 years and circa 1–4 years after tree mortality, respectively) disturbance on forest C were studied in P. abies dominated forests that had been left unmanaged after disturbance. Soil surface total and heterotrophic CO2 effluxes, and topsoil C stocks of storm and I. typographus disturbed and undisturbed sites differed little, despite the shift in tree C stocks from living to dead after both disturbances and greater litter detritus C stocks on the I. typographus disturbed sites. Soil surface autotrophic CO2 effluxes were mostly lower at the disturbed sites than at undisturbed ones. The most distinct differences in the humus layer microbiology were the lower abundances of tree-symbiotic ectomycorrhizal fungi, and consequently slightly lower microbial and fungal biomasses in the storm and I. typographus disturbed sites in comparison to the undisturbed sites. The remaining living trees on or in close proximity to the disturbed sites probably mitigated the belowground response to disturbance to some extent. This dissertation shows that certain site and soil characteristics predispose trees and forest stands to D. pini and I. typographus infestations, which could help in identifying sites that are susceptible to insect disturbance. Furthermore, it provides new information about the short-term effects of natural disturbance on boreal forest C cycling and soil microbiology, which is important for improving understanding of the complexity of the possible impacts of climate change on forest C sequestration. Metsien ja maaperän merkitys hiilen sidonnassa ja varastoina korostuu jatkuvasti. Luontaiset häiriötekijät, kuten myrskyt ja hyönteisten joukkoesiintymät, ovat metsän toiminnan oleellisia muokkaajia. Joidenkin puita tappavien häiriöiden on arvioitu yleistyvän ilmastonmuutoksen myötä, millä voi olla merkittäviä paikallisia vaikutuksia metsien hiilinieluun. Metsiä häiriöille altistavista ympäristötekijöistä, sekä häiriöiden monimutkaisista vaikutuksista metsän hiilen kiertoon ei kuitenkaan tiedetä tarpeeksi. Tässä tutkimuksessa tarkasteltiin maaperä-, puusto- ja pinnanmuototekijöitä, jotka altistavat metsiköitä kahden hyönteislajin — pilkkumäntypistiäisen ja kirjanpainajan — joukkoesiintymille ja niistä aiheutuville puustotuhoille Etelä- ja Itä-Suomen metsissä. Lisäksi työssä tarkasteltiin myrsky- ja kirjanpainajahäiriöiden vaikutusta maahengitykseen (maasta vapautuva hiilidioksidivuo), puiden ja maaperän hiilivarastoihin sekä maaperän mikrobiyhteisöjen koostumukseen ja hiilen fraktioihin Etelä-Suomen metsissä. Pilkkumäntypistiäisen aiheuttaman metsämännyn neulaskadon havaittiin olevan talousmetsissä pääosin voimakkaampaa alueilla, joiden maaperätekijät viittasivat korkeampaan ravinteikkuuteen (esim. pienempi hiili/typpi suhde ja suurempi hienojen maapartikkelien osuus). Suurimmat todennäköisyydet kirjanpainajan voimakkaalle esiintymiselle metsäkuusella löydettiin kaupunkimetsissä puille, jotka kasvoivat itään viettävillä, ravinteikkailla kasvupaikoilla (lehto tai lehtomainen kangas) ja joihin liittyi joko kaltevimpia pinnanmuotoja, ohut moreenimaa tai maaperän korkea hiili/typpi suhde. Myrskyn (5–7 vuoden jälkeen) ja kirjanpainajan (noin 1–4 vuotta puustokuoleman jälkeen) vaikutuksia metsän hiileen ja maaperän mikrobeihin tarkasteltiin metsäkuusivaltaisessa myrskyn jälkeen suojellussa metsässä. Kokonaismaahengitys, heterotrofinen maahengitys ja pintamaan hiilivarastot eivät juuri eronneet myrskyn tappamien puiden, kirjanpainajan tappamien puiden ja häiriöttömien (elävät puut) alueiden välillä, vaikka molemmilla häiriöalueilla puuston hiilivarastot koostuivat pääosin kuolleista puista ja kirjanpainajan tappamien puiden alueilla oli suurimmat pintakarikkeen hiilivarastot. Autotrofinen maahengitys oli pääosin vähäisempää häiriöalueilla kuin elävien puiden alueilla. Selkeimmät erot maan humuskerroksen mikrobiologiassa olivat pienempi puiden symbionttisten mykorritsasienten rihmaston kasvu, sienibiomassa ja mikrobibiomassahiili myrskyn sekä kirjanpainajan tappamien puiden alueilla verrattuna elävien puiden alueiseen. Kuolleiden puiden joukossa sekä lähialueilla olevat elävät puut todennäköisesti hieman lievensivät häiriöiden aiheuttamia muutoksia metsämaassa. Tämä tutkimus osoittaa, että tietyt maaperä-, pinnanmuoto- ja kasvupaikkatekijät altistavat puita ja metsiköitä pilkkumäntypistiäis- ja kirjanpainajahäiriöille, mikä voi tarvittaessa auttaa kyseisten hyönteisten joukkoesiintymille alttiiden metsäalueiden tunnistamisessa. Lisäksi tutkimus tarjoaa uutta tietoa luontaisten häiriöiden lyhytaikaisista vaikutuksista boreaalisen metsän hiilen kiertoon ja maaperän mikrobiologiaan, mikä on oleellista, jotta ilmastonmuutoksen mahdollisia monimutkaisia vaikutuksia metsien hiilensidontaan voidaan ymmärtää paremmin.

Ecosystems responded asymmetrically to the 2018 European drought., In summer 2018, central and northern Europe were stricken by extreme drought and heat (DH2018). The DH2018 differed from previous events in being preceded by extreme spring warming and brightening, but moderate rainfall deficits, yet registering the fastest transition between wet winter conditions and extreme summer drought. Using 11 vegetation models, we show that spring conditions promoted increased vegetation growth, which, in turn, contributed to fast soil moisture depletion, amplifying the summer drought. We find regional asymmetries in summer ecosystem carbon fluxes: increased (reduced) sink in the northern (southern) areas affected by drought. These asymmetries can be explained by distinct legacy effects of spring growth and of water-use efficiency dynamics mediated by vegetation composition, rather than by distinct ecosystem responses to summer heat/drought. The asymmetries in carbon and water exchanges during spring and summer 2018 suggest that future land-management strategies could influence patterns of summer heat waves and droughts under long-term warming.

Ecosystem carbon (C) transit time is a critical diagnostic parameter to characterize land C sequestration. This parameter has different variants in the literature, including a commonly used turnover time. However, we know little about how different transit time and turnover time are in representing carbon cycling through multiple compartments under a non-steady state. In this study, we estimate both C turnover time as defined by the conventional stock over flux and mean C transit time as defined by the mean age of C mass leaving the system. We incorporate them into the Community Atmosphere Biosphere Land Exchange (CABLE) model to estimate C turnover time and transit time in response to climate warming and rising atmospheric [CO2]. Modelling analysis shows that both C turnover time and transit time increase with climate warming but decrease with rising atmospheric [CO2]. Warming increases C turnover time by 2.4 years and transit time by 11.8 years in 2100 relative to that at steady state in 1901. During the same period, rising atmospheric [CO2] decreases C turnover time by 3.8 years and transit time by 5.5 years. Our analysis shows that 65 % of the increase in global mean C transit time with climate warming results from the depletion of fast-turnover C pool. The remaining 35 % increase results from accompanied changes in compartment C age structures. Similarly, the decrease in mean C transit time with rising atmospheric [CO2] results approximately equally from replenishment of C into fast-turnover C pool and subsequent decrease in compartment C age structure. Greatly different from the transit time, the turnover time, which does not account for changes in either C age structure or composition of respired C, underestimated impacts of warming and rising atmospheric [CO2] on C diagnostic time and potentially led to deviations in estimating land C sequestration in multi-compartmental ecosystems.

The Beijing-Tianjin Sand Source Control Project (BTSSCP), a national ecological restoration project, was launched to construct an ecological protection system in the Beijing-Tianjin sand source areas to reduce dust hazards. The carbon sequestration dynamics can be used to assess the ecological effects of an ecological restoration project. Here, we conducted vegetation and soil study to assess the carbon sequestration in the plantations with 10 years old stands in Beijing-Tianjin sand source areas. The results at the site scales indicated that the average net increase of plantation ecosystem carbon stock was 33.8 Mg C ha−1, with an annual increase rate of 3.38 Mg C ha−1 yr−1. The average net increase of carbon varied among regions, vegetation types, and forest management activities. Soil bulk density in the top soil decreased slightly after 10-year implementation of the project. Coniferous forests and shrubs are suitable plant species for sand source areas. Natural restoration in the plantations is a practical and feasible and promising approach for enhancing ecosystem carbon sequestration potential.

Western Hubei is the most concentrated area of forest resources in Hubei Province, and the knowledge of the distribution characteristics of ecosystem carbon density is important to understand the regional characteristics of carbon density and its mechanism of formation. Carbon density and factors influencing different layers in the ecosystem were studied by using field data. The average carbon density of ecosystems in western Hubei was 159.05 t/hm2; the carbon density of different forest types in descending order was Abies fargesii forests (362.25 t/hm2), mixed broadleaf-conifer forests (154.13 t/hm2), broad-leaved forests (146.09 t/hm2), and coniferous forests (135.76 t/hm2), and ecosystem carbon density increased with increasing age. The carbon density of the arborous layer, shrub layer, and soil layer of A. fargesii forests was significant higher than that of the other forests (P 100 mm.