Temperature sensitivity of gross N transformation rates in an alpine meadow on the Qinghai-Tibetan Plateau

Purpose

Alpine ecosystems on the Qinghai-Tibetan Plateau are sensitive to global climatic changes. However, the effects of temperature change resulting from global warming or seasonal variation on soil N availability in those ecosystems are largely unknown.

Materials and methods

We therefore conducted a ¹⁵N tracing study to investigate the effects of various temperatures (5–35 °C) on soil gross N transformation rates in an alpine meadow (AM) soil on the Qinghai-Tibetan Plateau. A natural secondary coniferous forest (CF) soil from the subtropical region was chosen as a reference to compare the temperature sensitivity of soil gross N transformation rates between alpine meadow and coniferous forest.

Results and discussion

Our results showed that increasing temperature increased gross N mineralization and NH₄ ⁺ immobilization rates and overall enhanced N availability for plants in both soils. However, both rates in the CF soil were less sensitive to a temperature change from 5 to 15 °C compared to the AM soil. In both soils, different N retention mechanisms could have been operating with respect to changing temperatures in the different climatic regions. In the CF soil, the absence of NO₃ ^? production at all incubation temperatures suggests that in the subtropical soil which is characterized by high rainfall, an increase in N availability due to increasing temperature could be completely retained in soils. In contrast, the AM soil may be vulnerable to N losses with respect to temperature changes, in particular at 35 °C, in which higher nitrification rates were coupled with lower NH₄ ⁺ and NO₃ ^? immobilization rates.

Conclusions

Our results suggest that increased soil temperature arising from global warming and seasonal variations will most likely enhance soil N availability for plants and probably increase the risk of N losses in the alpine meadow on the Qinghai-Tibetan Plateau.

     

Sensitivity of Tundra Carbon Balance to Ambient Temperature

Summer field observations in thenorth-east European region of Russia (1995–1996) proved thatnatural local and temporal warming could cause similarreversible changes of carbon (C) balance pattern ofsouth tundra ecosystems. Thirty-six diurnal chambermeasurements of carbon dioxide fluxes at two differentsites (67°57′N, 64°40′E; 67°20′N,63°44′E) in 4 main types of ecosystems werestudied. Wide magnitude of ambient mean diurnal airtemperature (from +4.1 to +20.2 °C) allowed toobtain significant linear relationship between thisparameter and variation in ecosystems C net flux (from–2.6 (sink) to +2.0 (source) gC m^-2 d^-1, R² = 0.769). In the midst ofthe vegetative season, an increase of mean diurnal airtemperature in the plant canopy over the criticalvalue of +14 °C lead to a change in the C netflux from sink to source. This can revert with atemperature decreases below the critical value. Theabove-mentioned effect is mostly due to thetemperature influenced increase of gross ecosystemrespiration. The summer temperature induced changesmay result in positive (C source) annual C balance insome studied ecosystems. The annual C loss ofshrub-dwarf shrub community in 1996 was estimated at45 gC m^-2 yr^-1. We consider ourresults to be another evidence of possible positivefeed-back between climate warming and C emission tothe atmosphere on short-term and regional scales.     

Predicting the effects of climate change on fire frequency in the southeastern Canadian boreal forest

Although an increasing frequency of forest fires has been suggested as a consequence of global warming, there are no empirical data that have shown climatically driven increases in fire frequency since the warming that has followed the end of the “Little Ice Age” (～1850). In fact, a 300-year fire history (AD 1688–1988) from the Lac Duparquet area (48°28′N, 79°17′W) shows a significant decrease both in the number and extent of fires starting 100 years ago during a period of warming. To explore this relationship between climatic change and fire frequency we used daily data from the Canadian Atmospheric Environment Service's General Circulation Model to calculate components of the Canadian Forest Fire Weather Index (FWI) System for the 1xCO₂ and 2xCO₂ scenarios. The average FWI over much of eastern Canada, including the Lac Duparquet region, decreased under the 2xCO₂ simulation, whereas FWI increased dramatically over western Canada. According to these results, fire frequency would decrease over the southeastern boreal forest which is in agreement with the empirical data from the fire history. Our results stress the importance of large regional variability and call into question previous generalisations suggesting universal increases in the rate of disturbance with climate warming.     

Effects of global change and human disturbance on soil carbon cycling in boreal forest: A review

Increasing human demands for Earth’s resources are hastening many environmental changes and creating a need to incorporate the routine monitoring of ecosystem functions into forest management.Under global change and anthropogenic disturbances,soil carbon (C) cycling in terrestrial ecosystems is undergoing substantial changes that result in the transformation between soil C sources and sinks.Therefore,the forest C budget requires an understanding of the underlying soil C dynamic under environment...     

Temperature responses of nitrogen processes cannot be inferred from carbon turnover in a boreal Norway spruce forest

Cold season processes contribute substantially to annual carbon (C) and nitrogen (N) budgets in boreal forest ecosystems, but little is known about how decomposition processes are affected at temperatures prevalent during wintertime. The aim of this study was to evaluate temperature responses of soil C and N processes and to test the hypothesis that there is a switch towards decomposing N‐rich material when soil temperatures drop to near 0°C. In the laboratory, soils from a boreal forest long‐term nutrient fertilization experiment were exposed to different temperatures varying from +2 to +15°C, and C mineralization, gross as well as net N mineralization/immobilization were estimated. Carbon mineralization declined exponentially as temperature decreased, whereas the response of N processes to temperature varied, with some indication that soil C and N processes are decoupled at low temperatures. We could only partially confirm that the decoupling between C and N processes at low temperature was due to a switch to N‐rich material, i.e., a change in the material undergoing decomposition. Overall, our results clearly showed that temperature responses of N processes cannot be inferred from C processes in boreal forest ecosystems, and that there is a need to improve our understanding of the relationship between the two across the range of temperatures experienced throughout the year. In particular, further research is required to establish and evaluate appropriate proxies for modelling the relationship of C and N processes at temperatures close to the freezing point.     

Resistance of microbial and soil properties to warming treatment seven years after boreal fire

Boreal forests store a large fraction of global terrestrial carbon and are susceptible to environmental change, particularly rising temperatures and increased fire frequency. These changes have the potential to drive positive feedbacks between climate warming and the boreal carbon cycle. Because few studies have examined the warming response of boreal ecosystems recovering from fire, we established a greenhouse warming experiment near Delta Junction, Alaska, seven years after a 1999 wildfire. We hypothesized that experimental warming would increase soil CO₂ efflux, stimulate nutrient mineralization, and alter the composition and function of soil fungal communities. Although our treatment resulted in 1.20 °C soil warming, we found little support for our hypothesis. Only the activities of cellulose- and chitin-degrading enzymes increased significantly by 15% and 35%, respectively, and there were no changes in soil fungal communities. Warming resulted in drier soils, but the corresponding change in soil water potential was probably not sufficient to limit microbial activity. Rather, the warming response of this soil may be constrained by depletion of labile carbon substrates resulting from combustion and elevated soil temperatures in the years after the 1999 fire. We conclude that positive feedbacks between warming and the microbial release of soil carbon are weak in boreal ecosystems lacking permafrost. Since permafrost-free soils underlie 45-60% of the boreal zone, our results should be useful for modeling the warming response during recovery from fire in a large fraction of the boreal forest.     

中国不同生态系统土壤硅的研究进展

硅是土壤和岩石的一种基本成分,具有促进植物的生长、增强植物抗性、参与生物地球化学循环过程、调节全球碳循环和缓解全球气候变暖趋势等方面的作用.本文在全面介绍土壤硅的形态、有效性及生物循环特征基础上,分析了我国不同生态系统中土壤硅及植硅体含量状况,阐明了影响土壤有效硅及植硅体的因素,重点阐述了近年来有关稻田土壤有效硅与水稻...     

气候变暖对陆地生态系统的影响

人类活动引起的温室效应导致全球气候变暖,气候变暖对全球生态环境的影响越来越受到人们的关注.作为人类赖以生存的环境主体,陆地生态系统对气候变暖将做出何种响应,更是人们关注的重点.植物物候的变化可以直观地反映某些气候变化,尤其是气候变暖.气候变暖影响植物的生长节律,进而引起植物与环境关系的改变及生态系统物质循环(如水和碳的循环)的改变.不同种类植物对气候变化的差异响应,会使植物间和动植物间的竞争与依赖关系发生深刻的变化,如北半球中高纬度地区植被生长季延长、植物提早开花、昆虫提早出现、鸟类提早产蛋以及冰川退缩、永冻土带融化、江河湖泊结冰推迟而融化提早等.本文主要从陆地生态系统的分布和演替两方面着眼,以植物和动物作为考察对象,系统论述了森林、草原、荒漠、湿地及农田等陆地生态系统在气候变暖背景下产生的变化,并从微观和宏观尺度上提出陆地生态系统变化的生态学机制,最后在技术和政策层面给出若干对策.     

Vulnerability of freshwater resources to climate change in the tropical pacific region

El Nino events and associated droughts adversely affect freshwater resources on islands in the tropical Pacific region. Particularly vulnerable are low-lying atolls because rainwater collection is the main freshwater source on such islands. During El Nino-related droughts, water can be drawn only from the limited freshwater lenses beneath the islands. If drought conditions such as these intensify, the depletion of freshwater resources could affect the habitability of atolls. Average climate change inthe Pacific region from increased anthropogenic carbon dioxide in a global coupled climate model resembles present-day El Nino conditions as well as the decadal time-scale sea surface temperature and precipitation anomalies observed during the 1980s and early 1990s. These anomalies are a consequence of greater warming of sea surface temperatures in the eastern equatorial Pacific than over the western Pacific warm pool with increased carbon dioxide in the climate model. Attendant increases in precipitation in the central equatorial Pacific are also accompanied by precipitation decreases in the northern and southern tropical Pacific (roughly 5 °N to 15°N and 5°S to 15°S), as well as in the Australasian and eastern Indian Ocean regions. Associated effects in the midlatitude North Pacific also resemble El Nino conditions and the decadal time-scale signals from the 1980s. Future possible increases of drought conditions in certain tropical Pacific regions, as indicated by the climate model results, could limit the sustainability of atoll populations in those regions, causing migration and increased urbanization, with all the attendant problems, on larger high islands with more stable water supplies.     

Responses of soil micronutrient availability to experimental warming in two contrasting forest ecosystems in the Eastern Tibetan Plateau,China

Purpose

As micronutrients are essential for all living organisms, their availability in forest soils is important to the forest ecosystem. Studying the effect of global warming on the availability of mineral elements is more significant for forest management, but the scarcity of these elements is a concern. This study aims to investigate the responses of soil micronutrient availability to experimental warming in two contrasting forest ecosystems in the Eastern Tibetan Plateau of China.

Materials and methods

Using the open-top chamber (OTC) method simulates the global warming and chemical extractants extract soil micronutrients (CaCl₂ and Mehlich-3 extractant for Fe, Mn, Cu, Zn, and with boiling hot water for extracting B) to study the availability of these micronutrients in two contrasting forest ecosystems (a dragon spruce plantation and a natural forest) under experimental warming.

Results and discussion

The results showed that soil temperature in the OTCs was increased by 0.56 and by 0.55 °C in the plantation and the natural forest, respectively. The total and Mehlich-3-extractable Cu, Fe, Mn, and Zn were increased by warming in the plantation (except Mehlich-3-extractable Fe, which decreased slightly) but decreased by warming in the natural forest. The CaCl₂-extractable micronutrients were not significantly affected by warming. The retained total B in both the plantation and natural forest was decreased by experimental warming. Either the effect of warming or forest type on these micronutrients varied due to their different associations with soil properties. What is more, the relative impact of forest type was stronger than warming on the soil properties with exception of the soil pH and total B concentration.

Conclusions

Reforestation would generate greater influences on soil environment although it is an important effective action to remain ecologic balance usually. The responses of the total soil micronutrients and their availability to warming depended on the forest type, as their concentration was significantly correlated with the soil water and pH. It was implied that the soil pH and water content are important to the availability of micronutrients in soil and provide managers with important information to better manipulate their forests for tree growth and as wildlife habitats.     

Water availability and abundance of microbial groups are key determinants of greenhouse gas fluxes in a dryland forest ecosystem

Forests are considered key biomes that could contribute to minimising global warming as they sequester carbon (C) and contribute to mitigate emissions of the potent greenhouse gases (GHG) including nitrous oxide (N₂O), methane (CH₄) and carbon dioxide (CO₂). Management practices are prevalent in forestry, particularly in dryland ecosystems, known to be water and nitrogen (N) limited. Irrigation and fertilisation are thus routinely applied to increase the yield of forest products. However, the contribution of forest management practices to current GHG budgets and consequently to soil net global warming potential (GWP) is still largely unaccounted for, particularly in dryland ecosystems. We quantified the long-term effect (six years) of irrigation and fertilisation and the impact of land-use change, from grassland to a Eucalyptus plantation on N₂O, CH₄ and CO₂ emissions and soil net GWP, within a dryland ecosystem. To identify biotic and abiotic drivers of GHG emissions, we explored the relationship of N₂O, CH₄ and CO₂ fluxes with soil abiotic characteristics and abundance of ammonia-oxidizers, N₂O-reducing bacteria, methanotrophs and total soil bacteria. Our results show that GHG emissions, particularly N₂O and CO₂ are constrained by water availability and both N₂O and CH₄ are constrained by N availability in the soil. We also provide evidence of functional microbial groups being key players in driving GHG emissions. Our findings illustrate that GHG emission budgets can be affected by forest management practices and provide a better mechanistic understanding for future mitigation options.     

Snow removal alters soil microbial biomass and enzyme activity in a Tibetan alpine forest

Projected future decreases in snow cover associated with global warming in alpine ecosystems could affect soil biochemical cycling. To address the objectives how an altered snow removal could affect soil microbial biomass and enzyme activity related to soil carbon and nitrogen cycling and pools, plastic film coverage and returning of melt snow water were applied to simulate the absence of snow cover in a Tibetan alpine forest of western China. Soil temperature and moisture, nutrient availability, microbial biomass and enzyme activity were measured at different periods (before snow cover, early snow cover, deep snow cover, snow cover melting and early growing season) over the entire 2009/2010 winter. Snow removal increased the daily variation of soil temperature, frequency of freeze–thaw cycle, soil frost depth, and advanced the dates of soil freezing and melting, and the peak release of inorganic N. Snow removal significantly decreased soil gravimetric water, ammonium and inorganic N, and activity of soil invertase and urease, but increased soil nitrate, dissolve organic C (DOC) and N (DON), and soil microbial biomass C (MBC) and N (MBN). Our results suggest that a decreased snow cover associated with global warming may advance the timing of soil freezing and thawing as well as the peak of releases of nutrients, leading to an enhanced nutrient leaching before plant become active. These results demonstrate that an absence of snow cover under global warming scenarios will alter soil microbial activities and hence element biogeochemical cycling in alpine forest ecosystems.     

How do persistent organic pollutants be coupled with biogeochemical cycles of carbon and nutrients in terrestrial ecosystems under global climate change?

Purpose  

Global climate change (GCC), especially global warming, has affected the material cycling (e.g., carbon, nutrients, and organic chemicals) and the energy flows of terrestrial ecosystems. Persistent organic pollutants (POPs) were regarded as anthropogenic organic carbon (OC) source, and be coupled with the natural carbon (C) and nutrient biogeochemical cycling in ecosystems. The objective of this work was to review the current literature and explore potential coupling processes and mechanisms between POPs and biogeochemical cycles of C and nutrients in terrestrial ecosystems induced by global warming.     

Home-field advantage of litter decomposition and nitrogen release in forest ecosystems

Litter decomposition is a major fundamental ecological process that regulates nutrient cycling, thereby affecting net ecosystem carbon (C) storage as well as primary productivity in forest ecosystems. Litter decomposes in its home environment faster than in any other environment. However, evidence for this phenomenon, which is called the home-field advantage (HFA), has not been universal. We provide the first HFA quantification of litter decomposition and nutrient release through meta-analysis of published data in global forest ecosystems. Litter mass loss was 4.2 % faster on average, whereas nitrogen (N) release was 1.7 % lower at the home environment than in another environment. However, no HFA of phosphorus (P) release was observed. Broadleaf litter (4.4 %) had a higher litter mass loss HFA than coniferous litter (1.0 %). The positive HFA of N release was found in the coniferous litter. Mass loss HFA was significantly and negatively correlated with the initial lignin:N litter ratio. The litter decomposition and N release HFAs were obtained when mesh size ranged from 0.15 mm to 2.0 mm. The HFA of litter decomposition increased with decomposition duration during the early decomposition stage. The HFA of N release was well correlated with mass loss, and the greatest HFA was at mass loss less than 20 %. Our results suggest that the litter decomposition and N release HFAs are widespread in forest ecosystems. Furthermore, soil mesofauna is significantly involved in the HFA of litter decomposition.     

Effects of warming and nitrogen addition on nutrient resorption efficiency in an alpine meadow on the northern Tibetan Plateau

The increase in two controversial global environmental issues, climate warming and nitrogen (N) deposition, may have distinct effects on the processes and functioning of terrestrial ecosystems. Nutrient resorption is an important determinant of plant community nutrient dynamics, especially in nutrient-limited ecosystems, but information about N and phosphorus (P) resorption in alpine ecosystems is still lacking. A long-term simulated warming and exogenous N addition experiment initiated in July 2010 was conducted in an alpine meadow in Damxung County in northern Tibet. The experiment consisted of conditions of warming and no warming crossed with three N addition levels: 0 (CK), 20 (N20), and 40 (N40) kg N · ha^?1 · year^?1. With increasing N addition levels, the N content and the N/P ratio in plant leaves gradually increased, while the P limitation of plant growth was aggravated by N addition. The moderate N addition level (N20) increased plant N resorption efficiency (NRE), while the high N addition level (N40) had no effect on the NRE of Kobresia pygmaea or Anaphalis xylorhiza. N addition significantly increased the P resorption efficiency (PRE) in Stipa capillacea leaves. However, N addition did not change the community NRE or the community PRE. The soil N content decreased under the warming treatment. At the community level, warming significantly increased the NRE by 12% and 16%, and the PRE by 26% and 24% under the CK and N40 treatments, respectively. The NRE and PRE were higher in S. capillacea than in K. pygmaea and A. xylorhiza, especially at the high N addition level (38% and 45% higher NRE and 36% and 15% higher PRE compared to K. pygmaea and A. xylorhiza, respectively). Correlation analysis showed that the NRE and PRE in plant leaves were mainly mediated by soil inorganic N availability, and tended to decrease with increase of soil N availability, suggesting that N loss due to warming could induce changes in nutrient resorption in alpine ecosystems. The species-specific responses to N addition and the stronger competitive advantage of S. capillacea may change the community structure and subsequently affect the decomposition process in this alpine meadow under future global climate change scenarios.     

短期增温及减少降雨对杉木人工林土壤DOM的数量及其光谱学特征的影响

本文选取我国中亚热带杉木人工林土壤进行短期增温以及减少50%降雨试验,利用光谱技术研究增温及减少降雨对土壤可溶性有机质(DOM)数量和结构的影响。试验设对照(CT)、增温(W,土壤温度增高5℃)、减少降雨(P,自然降雨量减少50%)、增温与减少降雨交互作用(WP)4种处理。结果表明:1)增温增加了土壤可溶性有机碳(DOC)数量,使DOM的芳香性指数和腐殖化指数减小,结构变得简单易于分解;0~10 cm土层的土壤DOM含有较多的烷烃,酯类物质较少;10~20 cm土层的DOM则含有较多的碳水化合物。2)减少降雨使土壤水分相对减少,土壤DOC的数量降低。0~10 cm土层土壤DOM的芳香性指数和腐殖化程度降低,DOM含有大量的烷烃;而10~20 cm土层土壤DOM的芳香性指数和腐殖化指数升高,碳水化合物少。减少降雨处理使土壤可溶性有机氮(DON)数量增加。3)增温和减少降雨的交互作用增加了DOC和DON的数量,降低了DOM的芳香化程度和腐殖化程度;使0~10 cm土层的DOM含有较多的碳水化合物,而10~20 cm土层的DOM碳水化合物较少。4)对于0~10 cm土壤,增温对土壤DOM的数量及结构的作用最强;随着土壤深度增加到10~20 cm,减少降雨的作用逐渐明显,其对DOM结构的影响也达到显著水平。温度及降水对DOM的数量及化学结构的变化具有重要意义,该研究结果可以为阐释全球气候变化背景下土壤DOM的动态周转及预测未来森林土壤碳氮的变化趋势提供科学依据。     

Impact of fire on soil gross nitrogen transformations in forest ecosystems

Purpose

Forests play a key role in the global carbon (C) and nitrogen (N) cycling. Fire is a global phenomenon occurring in many forest ecosystems, which has several environmental and ecological effects. The objective of this review was to improve our understanding of the effect of fire on soil gross N transformations in forest ecosystems.

Methods and results

We have reviewed the published studies using ¹⁵N pool dilution technique with analytical data analysis method to study the effect of fires on gross N transformations in forest ecosystems. Wildfires increased gross N mineralization rates in the short term and the effect disappeared from 3 years after the fire, while the effect of prescribed fires disappeared from 2 years after the burning. Both wildfires and prescribed fires reduced gross nitrification in the short term, while their effects varied from 6 months following the burning.

Conclusions

The different responses of gross N transformations to the fires in forest ecosystems depended on many factors including forest types, the intensity and frequency of fires, the time elapsed between the fires and sampling events, incubation conditions (field or laboratory incubation), climatic conditions and so on. In view of many factors influencing the effect of fires on gross N transformations, more comprehensive studies with physical, chemical, microbial and ecological characterization are needed to improve our knowledge about the effect of fires on soil gross N transformations and then N cycling in forest ecosystems.

     

The isotopic composition of soil organic carbon on a north–south transect in western Canada

The minor isotopes of carbon (¹³C and ¹⁴C) are widely used as tracers in studies of the global carbon cycle. We present carbon‐isotope data for the 0–5 cm layer of soil on a transect from 49.6°N to 68°N, from mature forest and tundra ecosystems in the boreal‐arctic zone of interior western Canada. Soil organic carbon in the < 2000 μm fraction of the soil decreases from 3.14 kg m^?2 in the south to 1.31 kg m^?2 in the north. The ¹⁴C activity of the organic carbon decreases as latitude increases from 118.9 to 100.7 per cent modern carbon (pMC). In addition, the ¹⁴C activities of organic carbon in the particle‐size fractions of each sample decrease as particle size decreases. These results suggest that organic carbon in the 0–5 cm layer of these soils transfers from standing biomass into the coarsest size fractions of the soil and is then degraded over time, with the residue progressively transferred into the more resistant finer particle sizes. We calculate residence times for the coarsest size fractions of 21 years in the south to 71 years in the north. Residence times for the fine size fractions (< 63 μm) are considerably longer, ranging from 90 years in the south to 960 years in the north. The δ¹³C of the organic carbon decreases from ?26.8 ± 0.3‰ in soil under forest in the south to ?26.2 ± 0.1‰ for tundra sites in the north. At all sites there is an increase in δ¹³C with decreasing particle size of 0.7–1.6‰. These changes in δ¹³C are due to the presence of ‘old’ carbon in equilibrium with an atmosphere richer in ¹³C, and to the effects of microbial degradation.     

Comprehensive conservation planning to protect biodiversity and ecosystem services in Canadian boreal regions under a warming climate and increasing exploitation

Boreal regions contain more than half of the carbon in forested regions of the world and over 60% of the world’s surface freshwater. Carbon storage and the flood control and water filtration provided by freshwaters and wetlands have recently been identified as the most important ecosystem services provided by boreal regions, with a value many times greater than current resource exploitation. Ecosystem services and sensitive ways of detecting their impairment have so far not been fully included in boreal conservation planning. Climate warming, via its effect on permafrost melting, insect damage, and forest fire, threatens to trigger large positive carbon feedbacks that may enhance the concentrations of greenhouse gases in the atmosphere, further amplifying climate warming. In a water-scarce world, there is increasing pressure to divert and exploit boreal freshwaters, and devising conservation plans to protect boreal freshwaters and their catchments is urgent. We propose a catchment-based approach that includes water and chemical mass-balances as a sensitive means of detecting early degradation of many ecosystem services in both catchments and freshwaters, and give some examples of where this has been advantageous in the past. The necessary modifications to current conservation planning are simple ones, and the advantages are great.     

Variations in forest aboveground biomass in Miyun Reservoir of Beijing over the past two decades

Purpose

In recent years, climate change, particularly rising carbon dioxide (CO₂) concentration and global warming, has attracted much attention around the world. Forest ecosystems still play a crucial role in global carbon (C) fixation. Quantifying forest aboveground biomass (AGB) and its temporal variations is essential for understanding external impacts (e.g., urbanization, environmental change) as well as assessing the potential of forest ecosystems assimilating the atmospheric CO₂.

Materials and methods

In this study, we established regression models for AGB estimation in the Miyun Reservoir region, based on relationships between Landsat-derived variables and ground truth AGB values, which were obtained from both plot measurements and estimations using Light Detection and Ranging (LiDAR) dataset. The models were applied to calibrated Landsat images acquired in 1990, 2000, and 2010 to track the forest AGB temporal variations and the corresponding spatial distributions for each period. The AGB estimations using LiDAR showed high consistency with values based on the plot measurements, while the established models presented an acceptable accuracy.

Results and discussion

The AGB density in the Miyun Reservoir experienced an overall increase since 1990 and was averaged at 52.20 and 32.12 t ha^?1, for stand forest and shrub in 2010, respectively. Total AGB in 2010 was estimated to be 4.5 × 10⁷ t, which increased by 8% when compared with the level in 1990. Our results are in the similar range of AGB density reported by other studies carried out in Northern China. Ecological programs including Three-North Shelter Forest Project and Returning Farming to Forest promoted the forest expansion and development during this period, while local farming activities exerted certain negative effects on the surrounding forest systems.

Conclusions

Absolute AGB density values indicated that regions with less external interventions present more consistent biomass accumulation. However, the coarse spatial resolution and 10-year interval of the datasets limited detailed analysis of impacts from urbanization of Beijing City. Future studies incorporating sophisticated ecosystem research methods are expected to uncover the mechanisms and key drivers for the observed variations in the AGB in the Miyun Reservoir region of Beijing, China.