Forest Carbon Balance Research Articles

HISTORY OF RESEARCH ON FOREST PRODUTIVITY IN KARELIA

The article deals with the history of research on the biological productivity of forests in Republic of Karelia as a methodological basis for the modern field of research into the components of carbon balance in forests. Particular attention is paid to the review of the studies of the researchers of the Forest Institute of the Karelian Research Center of the Russian Academy of Sciences from the moment of its formation to the present. The studies of the structure and stocks of phytomass, organic matter and carbon in different components of forest ecosystems, as well as biological aspects of the productivity of the republic’s forests are considered. Some results and prospects of studying the carbon balance in the forests of Eastern Fennoscandia at the current stage of the Institute’s activities are discussed. The references list the main studies in this research field.

Differences between tree stem CO2 efflux and O2 influx rates cannot be explained by internal CO2 transport or storage in large beech trees.

Tree stem respiration (RS ) is a substantial component of the forest carbon balance. The mass balance approach uses stem CO2 efflux and internal xylem fluxes to sum up RS , while the oxygen-based method assumes O2 influx as a proxy of RS . So far, both approaches have yielded inconsistent results regarding the fate of respired CO2 in tree stems, a major challenge for quantifying forest carbon dynamics. We collected a data set of CO2 efflux, O2 influx, xylem CO2 concentration, sap flow, sap pH, stem temperature, nonstructural carbohydrates concentrationand potential phosphoenolpyruvate carboxylase (PEPC)capacity on mature beech trees to identify the sources of differences between approaches. The ratio of CO2 efflux to O2 influx was consistently below unity (0.7) along a 3-m vertical gradient, but internal fluxes did not bridge the gap between influx and efflux, nor did we find evidence for changes in respiratory substrate use. PEPC capacity was comparable with that previously reported in green current-year twigs. Although we could not reconcile differences between approaches, results shed light on the uncertain fate of CO2 respired by parenchyma cells across the sapwood. Unexpected high values of PEPC capacity highlight its potential relevance as a mechanism of local CO2 removal, which merits further research.

Basin-wide variation in tree hydraulic safety margins predicts the carbon balance of Amazon forests

Tropical forests face increasing climate risk1,2, yet our ability to predict their response to climate change is limited by poor understanding of their resistance to water stress. Although xylem embolism resistance thresholds (for example, varPsi 50) and hydraulic safety margins (for example, HSM50) are important predictors of drought-induced mortality risk3–5, little is known about how these vary across Earth’s largest tropical forest. Here, we present a pan-Amazon, fully standardized hydraulic traits dataset and use it to assess regional variation in drought sensitivity and hydraulic trait ability to predict species distributions and long-term forest biomass accumulation. Parameters varPsi 50 and HSM50 vary markedly across the Amazon and are related to average long-term rainfall characteristics. Both varPsi 50 and HSM50 influence the biogeographical distribution of Amazon tree species. However, HSM50 was the only significant predictor of observed decadal-scale changes in forest biomass. Old-growth forests with wide HSM50 are gaining more biomass than are low HSM50 forests. We propose that this may be associated with a growth–mortality trade-off whereby trees in forests consisting of fast-growing species take greater hydraulic risks and face greater mortality risk. Moreover, in regions of more pronounced climatic change, we find evidence that forests are losing biomass, suggesting that species in these regions may be operating beyond their hydraulic limits. Continued climate change is likely to further reduce HSM50 in the Amazon6,7, with strong implications for the Amazon carbon sink.

Anthropogenic activities dominated tropical forest carbon balance in two contrary ways over the Greater Mekong Subregion in the 21st century.

The tropical forest carbon (C) balance threatened by extensive socio-economic development in the Greater Mekong Subregion (GMS) in Asia is a notable data gap and remains contentious. Here we generated a long-term spatially quantified assessment of changes in forests and C stocks from 1999 to 2019 at a spatial resolution of 30 m, based on multiple streams of state-of-the-art high-resolution satellite imagery and in situ observations. Our results show that (i) about 0.54 million square kilometers (21.0% of the region) experienced forest cover transitions with a net increase in forest cover by 4.3% (0.11 million square kilometers, equivalent to 0.31 petagram of C [Pg C] stocks); (ii) forest losses mainly in Cambodia, Thailand, and in the south of Vietnam, were also counteracted by forest gains in China due mainly to afforestation; and (iii) at the national level during the study period an increase in both C stocks and C sequestration (net C gain of 0.087 Pg C) in China from new plantation, offset anthropogenetic emissions (net C loss of 0.074 Pg C) mainly in Cambodia and Thailand from deforestation. Political, social, and economic factors significantly influenced forest cover change and C sequestration in the GMS, positively in China while negatively in other countries, especially in Cambodia and Thailand. These findings have implications on national strategies for climate change mitigation and adaptation in other hotspots of tropical forests.

Strict forest protection: A meaningful contribution to Climate-Smart Forestry? An evaluation of temporal trends in the carbon balance of unmanaged forests in Germany

The role of unmanaged forests is discussed controversially in the context of climate change. One of the key questions is, whether they can contribute to the mitigation of climate change as a carbon sink and storage. However, carbon dynamics of forests after the cessation of management are not well understood. We analyzed a set of 64 forest sites, covering wide gradients of time since abandonment (0–68 years) and stand age (65–261 years) in even-aged, unmanaged beech forests. Five sites that were unmanaged for >100 years complemented the main dataset. We compiled site-specific carbon balances, distinguishing six carbon-compartments: Carbon in aboveground living and dead biomass, carbon in belowground living and dead biomass, and carbon in the organic layer and the mineral topsoil (0–30 cm). We found positive effects of increasing TSA on the carbon stock in living biomass and aboveground dead biomass for up to 50 years after management ceased. The average increase of the total carbon stock over 50 years of TSA was ≈ 80 Mg C ha–1. The effect of stand age on aboveground living biomass showed a convex relation. Aboveground dead biomass increased logistically with TSA, while belowground dead biomass decreased. On average, the five sites unmanaged for >100 years held lower total carbon stocks compared to the observed biomass peak around 50 years of TSA. However, they contained considerably higher amounts of deadwood. Carbon in the mineral soil did neither change with TSA nor with stand age and was driven by pH. Carbon stocks in newly unmanaged forests increased almost linearly for approximately 50 years after cessation of management. Subsequently, a stabilization or medium-term decrease in carbon stock was observed, likely due to the initiating transition from even-aged to multi-aged structures. We conclude that, besides their value for biodiversity and ecosystem functions, the potential of naturally developing forests as a medium-term carbon sink and long-term stable carbon storage should be considered as a valuable contribution to Climate-Smart Forestry.

Natural disturbances as drivers of tipping points in forest ecosystems under climate change – implications for adaptive management

AbstractAs climate continues to change, disturbances may increasingly navigate forest ecosystems towards tipping points, causing irreversible state shifts and a loss of biodiversity and ecosystem services. In this review, I elaborate the Special Issue topic ‘Natural disturbances as tipping points of forest ecosystems under climate change’ featured by Forestry and suggest a suit of adaptive measures to mitigate increasing disturbances and their impacts on forest ecosystems. The Special Issue contains seven case studies assessing drought, fire, wind and bark beetle disturbances in Europe, North America and Africa. Despite high severities and/or frequencies, disturbances have not yet induced a shift of the investigated forest ecosystems towards an alternative state. Instead, forests have shown high resistance, resilience or both. For instance, one case study reveals that short-interval fires in European beech forests may not even significantly alter tree species composition due to the high resprouting ability of beech. Yet, other case studies identify distinct differences in resilience amongst ecoregions, and indicate that climate change and disturbance interactions might tip the future carbon balance of forests from sink to source. Many forest ecosystems are likely able to cope with future increases in disturbance activity to some degree. However, tipping points might vary across ecosystems dominated by different disturbance agents. While wind and bark beetles mainly affect canopy trees, fire and drought also directly impair tree regeneration, thus reducing the capacity of ecosystems to recover and reorganize. Hence, forest ecosystems in which fire and drought are the dominant disturbance agents might approach tipping points earlier than forests dominated by other agents if disturbance activity continues to increase. Several proactive and reactive adaptive measures are available to mitigate increasing disturbances, but more research is needed to develop robust and region-specific strategies to prevent tipping points in forest ecosystems.

Evaluating Carbon Sink Potential of Forest Ecosystems under Different Climate Change Scenarios in Yunnan, Southwest China

Nature-based Solutions (NbS) can undoubtedly play a significant role in carbon neutrality strategy. Forests are a major part of the carbon budget in terrestrial ecosystems. The possible response of the carbon balance of southwestern forests to different climate change scenarios was investigated through a series of simulations using the forest ecosystem carbon budget model for China (FORCCHN), which clearly represents the influence of climate factors on forest carbon sequestration. Driven by downscaled global climate model (GCM) data, the FORCCHN evaluates the carbon sink potential of southwestern forest ecosystems under different shared socioeconomic pathways (SSPs). The results indicate that, first, gross primary productivity (GPP), ecosystem respiration (ER) and net primary productivity (NPP) of forest ecosystems are expected to increase from 2020 to 2060. Forest ecosystems will maintain a carbon sink, but net ecosystem productivity (NEP) will peak and begin to decline in the 2030s. Second, not only is the NEP in the SSP1-2.6 scenario higher than in the other climate change scenarios for 2025–2035 and 2043–2058, but the coefficient of variation of the NEP is also narrower than in the other scenarios. Third, in terms of spatial distribution, the carbon sequestration potential of northwest and central Yunnan is significantly higher than that of other regions, with a slight upward trend in NEP in the future. Finally, GPP and ER are significantly positively correlated with temperature and insignificantly correlated with precipitation, and the increasing temperature will have a negative and unstable impact on forest carbon sinks. This study provides a scientific reference for implementing forest management strategies and achieving sustainable development.

Comments on Soimakallio et al. (2022) “Closing an open balance: The impact of increased tree harvest on forest carbon”

Soimakallio et al. (2022) reviewed literature to draw leveraged conclusions on how forest carbon balances are affected by increased harvest rates. The conclusions are based on a carbon balance indicator that is not transparent and loses information when averaged over multiple factors. While potentially useful for comparing fossil fuel displacement by wood in alternative harvest scenarios of a given forest resource, the indicator should not be used to compare scenarios based on different forest and forestry conditions.

Tree volume estimation with terrestrial laser scanning — Testing for bias in a 3D virtual environment

Tree volume is a key feature in forest monitoring, delivering information, such as wood availability or forest carbon balance. To date, tree volume, i.e. the total volume of the above ground woody parts of a tree, cannot be measured directly with conventional tools. Terrestrial laser scanning (TLS) offers the potential to directly measure tree volume. However, its application in forest monitoring requires a profound understanding of the precision and accuracy of retrieval approaches. In this study, we present a simulation environment for evaluating TLS application in forest inventories. We investigate the influence of understorey density, scanner placement and TLS sensor type on volume estimation of tree parts of varying diameters. Using information from 30 sample plots from the Swiss NFI to simulate 197 sample trees, we evaluate three understorey densities, five scanner locations and their combinations and three realistic and one hypothetic (geometric scanning) TLS sensor types. We show that tree volume estimates from point clouds are biased to certain extent: from about 25% for small trees to a few percent for larger trees above 40 cm diameter at breast height (DBH). Especially small tree parts (diameters < 7 cm) lack accurate and precise estimation. In small trees with 12 cm DBH they are overestimated by 110% in average with a high variation, whereas they are underestimated in large trees, i.e. with DBH ≥ 75 cm, by 50% in average. Volume estimation of small tree parts is subject to physical limits of TLS, however the estimation of volume of large tree parts could be feasible with appropriate TLS settings and field protocols. Nevertheless, tree volume estimation using TLS must be understood in greater depth before it can be applied regularly in forest inventories.

Periodic harvests, rather than passive conservation, increase the carbon balance of boreal forests much more in Northeast China

Proper forest management can significantly contribute to global or regional carbon neutrality goals; however, the effects of different harvesting strategies on the carbon balances of forest sectors are still not clear. The motivation of this study was to develop a simulation–optimization system that can concurrently maximize the total economic benefits from timber production, biofuel production, and carbon balancing under four alternative harvesting scenarios. The current strict protection strategy (NoCutting) in this region was set as the reference scenario, and the NoConstraints and EvenFlow scenarios were generated based on whether they included restrictions on the flow of harvest volume over time. A final scenario (PreDefine) was inspired by the management policy that was implemented in this region prior to the 2010s. The scenarios were applied to a 123,400 ha boreal forest in Northeast China over a 100 year horizon. All relevant carbon pools within the forestry sector, as well as the avoided emissions related to the substitution effects of using wood products and biofuels, were considered. The results indicated that increasing harvesting intensities significantly improved the carbon balances (1.6 to 5.1 times greater), carbon stock (1% to 7.1% greater) and profitability (1.33 to 4.95 times greater), during which scenario EvenFlow was the first-rank harvest scenario. Hereinto, the carbon balances of the forest under EvenFlow were increased by 0.57 ton ha−1 yr−1 in total, during which the net atmospheric benefits from forest ecosystems, wood productions and substitutions were approximately 0.05, 0.24, and 0.38 ton ha−1 yr−1, respectively, while approximately 0.09 and 0.01 ton ha−1 yr−1 were lost by using the bio and fossil fuels, respectively, during the planning period when compared with NoCutting. The sensitivity analysis revealed that both carbon prices and substitution rates have positive effects on the total carbon balances, while significant negative effects were observed for discount rates. Overall, our results highlighted that periodic harvests rather than passive conservation should be a priority for the sustainable management of boreal forests in Northeast China.

From source to sink – recovery of the carbon balance in young forests

We analyzed ecosystem carbon fluxes from eddy-covariance measurements in five young forests in southern Sweden where the previous stand had been harvested by clear-cutting or wind-felled: three stands with Norway spruce (Picea abies (L.) Karst.), one with Scots pine (Pinus sylvestris) and one with Larch (Larix x eurolepis A. Henry). One of the spruce stands had the stumps harvested, one was fertilized and one without any special treatments. These stands returned from positive (sources) to negative (sinks) annual carbon fluxes 8–13 years after disturbance, depending on site productivity and management. This corresponds to approximately 15% of the rotation periods at these sites. Extrapolation in combination with chronosequence data suggests that conventionally regenerated stands reach a neutral carbon balance after approximately 30% of the rotation period. The lowest carbon emissions and shortest recovery time was observed in a stand where the stumps of the trees, in addition to the stems and logging residues, were removed after harvest. This stand not only returned to a carbon sink within this time period but the total carbon gains since disturbance also equaled the total losses after only 11 years. These results stress that production stands in southern Sweden are carbon sources during a relatively small part of the rotation period, and that this part can be considerably shortened by measures that increase productivity or reduce the amount of woody debris left after disturbance.

A review of research on land use/cover change and its impact on forest carbon balance

A review of research on land use/cover change and its impact on forest carbon balance

Windthrow characteristics and their regional association with rainfall, soil, and surface elevation in the Amazon

Windthrows (trees uprooted and broken by winds) are common across the Amazon. They range in size from single trees to large gaps that lead to changes in forest dynamics, composition, structure, and carbon balance. Yet, the current understanding of the spatial variability of windthrows is limited. By integrating remote sensing data and geospatial analysis, we present the first study to examine the occurrence, area, and direction of windthrows and the control that environmental variables exert on them across the whole Amazon. Windthrows are more frequent and larger in the northwestern Amazon (Peru and Colombia), with the central Amazon (Brazil) being another hot spot of windthrows. The predominant direction of windthrows is westward. Rainfall, surface elevation, and soil characteristics explain the variability (20%–50%) of windthrows but their effects vary regionally. A better understanding of the spatial dynamics of windthrows will improve understanding of the functioning of Amazon forests.

A stand level scenario model for the Norwegian forestry – a case study on forest management under climate change

Carbon sequestration and income generation are competing objectives in modern forest management. The climate commitments of many countries depend on forests as carbon sinks which must be quantified, monitored, and projected into the future. For projections we need tools to model forest development and perform scenario analyses to assess future carbon sequestration potentials under different management regimes, the expected net present value of such regimes, and possible impacts of climate change. We propose a scenario analysis software tool (GAYA 2.0) that can assist in answering these types of questions using stand level simulations, detailed carbon flow models and an optimizer. This paper has two objectives: (1) to describe GAYA 2.0, and (2) demonstrate its potential in a case study where we analyze the forest carbon balance over a region in Norway based on national forest inventory sample plots. The tool was used to map the optimality front between the carbon benefit and net present value. We observed changes in net present value for different levels of carbon benefit as well as changes in optimal management strategies. We predicted future changes in several forest carbon pools as well as albedo and illustrated the impact of gradual increase in forest productivity (i.e., due to climate warming). Having been updated and modernized from its previous version with increased attention to forest carbon and energy fluxes, GAYA 2.0 is an effective tool that offers multiple opportunities to perform various types of scenario analyses in forest management.

Above-ground biomass allocation and potential carbon sink of black pine – a case study from southern Poland

Biomass allocation is a key factor for understanding the forest carbon balance and reflects plants’ ecological strategies in different environmental conditions. Allocation patterns and biomass models outside of the native range of black pine have not been analyzed in the context of the observed climate changes. The study's goals were to develop biomass equations for mature black pine from southern Poland and assess biomass and carbon allocation patterns and the potential of trees of different social statuses for carbon sequestration. A total of 129 felled black pine trees were measured, among which 14 were destructively sampled to determine biomass and carbon content in tree components. The developed set of biomass equations provided allocation patterns and accumulation of trees of different social statuses. Biomass and carbon allocation patterns were different but related to tree social status. The introduction of diameter at crown base significantly improved the accuracy of the developed models. The analyzed trees allocated relatively more in stem than in crown in comparison with that observed in other studies. Biomass and carbon allocation patterns of the analyzed black pines differ from those of the native range. They should be considered in biomass modeling with factors influencing social status structure.

Siberian carbon sink reduced by forest disturbances

Siberian forests are generally thought to have acted as an important carbon sink over recent decades, but exposure to severe droughts and fire disturbances may have impacted their carbon dynamics. Limited available forest inventories mean the carbon balance remains uncertain. Here we analyse annual live and dead above-ground carbon changes derived from low-frequency passive microwave observations from 2010 to 2019. We find that during this period, the carbon balance of Siberian forests was close to neutral, with the forests acting as a small carbon sink of $$+ 0.02_{ + 0.01}^{ + 0.03}$$ PgC yr−1. Carbon storage in dead wood increased, but this was largely offset by a decrease in live biomass. Substantial losses of live above-ground carbon are attributed to fire and drought, such as the widespread fires in northern Siberia in 2012 and extreme drought in eastern Siberia in 2015. These live above-ground carbon losses contrast with ‘greening’ trends seen in leaf area index over the same period, a decoupling explained by faster post-disturbance recovery of leaf area than live above-ground carbon. Our study highlights the vulnerability of large forest carbon stores in Siberia to climate-induced disturbances, challenging the persistence of the carbon sink in this region of the globe. Carbon sequestration by Siberian forests has been low over the past decade due to disturbances that have decreased live biomass and increased dead wood, according to passive microwave observations.

A dataset of biomass estimation models and parameters of spruces in Xinjiang from 1980 to 2021

Spruces are an important tree species in Xinjiang. Accurate estimation of spruce biomass plays an important role in evaluating forest ecological structure and forest carbon balance in Xinjiang. In this study, we collected the relevant literature of Xinjiang spruce biomass estimation model from CNKI, Web of Science and the National Public Service Platform for Standards Information by constructing a retrieval method. And we collated the literature and used CiteSpace software for visualized mapping, so as to create a dataset of biomass estimation models and parameters of spruces in Xinjiang from 1980 to 2021. Biomass estimation models consist of biomass estimation models based on measurable factors and biomass estimation models based on remote sensing, machine learning algorithm. The former has 13 types, 101 fitting formulas, applicable to the estimation of different spruce organs, and the ground biomass; the latter has 3 types, 12 fitting formulas, suitable for the estimation of spruce forest biomass. This dataset can provide model data support for the study on spruce biomass estimation in Xinjiang.

Boosts in leaf-level photosynthetic capacity aid Pinus ponderosa recovery from wildfire

Forests mitigate climate change by sequestering massive amounts of carbon, but recent increases in wildfire activity are threatening carbon storage. Currently, our understanding of wildfire impacts on forest resilience and the mechanisms controlling post-fire recovery remains unresolved due to a lack of empirical data on mature trees in natural settings. Here, we quantify the physiological mechanisms controlling carbon uptake immediately following wildfire in mature individuals of ponderosa pine (Pinus ponderosa), a wide-spread and canopy-dominant tree species in fire-prone forests. While photosynthetic capacity was lower in burned than unburned trees due to an overall depletion of resources, we show that within the burned trees, photosynthetic capacity increases with the severity of damage. Our data reveal that boosts in the efficiency of carbon uptake at the leaf-level may compensate for whole-tree damage, including the loss of leaf area and roots. We further show that heightened photosynthetic capacity in remaining needles on burned trees may be linked with reduced water stress and leaf nitrogen content, providing pivotal information about post-fire physiological processes. Our results have implications for Earth system modeling efforts because measurements of species-level physiological parameters are used in models to predict ecosystem and landscape-level carbon trajectories. Finally, current land management practices do not account for physiological resilience and recovery of severely burned trees. Our results suggest premature harvest may remove individuals that may otherwise survive, irrevocably altering forest carbon balance.

Reassessment of carbon emissions from fires and a new estimate of net carbon uptake in Russian forests in 2001–2021

Russia has the largest forest area on earth. Its boreal forests officially store about 97 Pg C, which significantly affect the global carbon cycle. In recent years, forest fires have been intensifying on the planet, leading to increased carbon emissions. Here we review how differences in fire control management of Russian forests affect fire related emissions. Carbon emissions due to fire were estimated using satellite data and compared to official reports for 2001–2021. We found that the relative areas affected by fire did differ between different fire protection zones, and 89 % of the area burnt was in forests controlled by fire-fighting aircraft or areas without protection. As a result, 417.7 Mha of poor or unprotected Russian forests (42 % of total) account about a half of total carbon emissions. According to our estimates, the average area of burnt forests in Russia was about 8.3 Mha per year between 2016 and 2021, resulting in annual carbon emission of 193 million metric tons (Mt) C emissions, and 53 % of them were from unprotected forest. These estimated carbon emissions are significantly higher than official national reports (79 Mt C yr−1). We estimated that net carbon uptake for Russia for 2015–2021 was about 333 ± 37 Mt C, which is roughly double the official estimates.Our results highlight large spatial differences in fire protection and prevention strategies in fire related emissions. The so-called control zone which stretches across large parts of Eastern Russia has no fire control and is the region of major recent fires. Our study shows that to estimate the Russian forest carbon balance it is critical to include this area. Implementation of some forest management in the remote areas (i.e., control zone) would help to decrease forest loss and resulting carbon emissions.

Disentangling carbon uptake and allocation in the stems of a spruce forest

Although forests store significant amounts of carbon in tree stems, the extent to which stem growth depends on carbon assimilation and environmental factors is poorly understood. In this study, gross primary productivity (GPP) and net ecosystem productivity (NEP) are compared for the first time at daily resolution with stem carbon allocation (SCA) in a spruce forest. We found substantial differences in the seasonal patterns of all variables as a function of environmental conditions, leading to three important findings. First, carbon uptake was only marginally invested in wood growth during the first half of the growing season. This means that there were other priorities for carbon allocation than stem growth at this time of year. Second, our results show that SRI at the beginning of the stem growth period, which mainly involves cell division and expansion, is a process that requires less carbon than the process of xylem lignification and thickening that follows. And third, NEP was generally less sensitive to drought than SCA. This suggests that the carbon uptake balance is less sensitive to dry periods than growth, particularly because the carbon uptake period is much longer than the growth window. These results may change the way we perceive the effects of climate change on forests, as tree carbon dynamics are primarily explained by the seasonal timing of dry periods rather than the intensity of these events. • Novel daily-resolved stem carbon allocation data of spruce trees. • Carbon uptake and allocation are partially decoupled. • Drought limits stem growth of spruce before carbon uptake is affected. • Timing of extreme events determines the impact on the carbon balance of forests. • Spruce trees increasingly suffer under drought stress.