Estimating air temperature of an alpine meadow on the Northern Tibetan Plateau using MODIS land surface temperature

Meteorological data are scarce due to lack of meteorology stations in the Qinghai–Tibet Plateau. This often results in an imprecise estimation of air temperature. A linear estimation of air temperature of an alpine meadow on Northern Tibetan Plateau at heights of 1.5 m–2.1 m by using MODIS land surface temperature (LST) data was conducted in this study. The results showed that linear estimation of daily maximum and daytime mean air temperatures from MODIS LST data were not accurate enough (P > 0.01, R² < 0.10) during the growing season. In contrast, the linear relationships between daily maximum and daytime mean air temperature and MODIS LST during the non-growing season were both significant (P < 0.01, R² > 0.40). MODIS LST data were accurate enough to linearly estimate daily minimum and nighttime mean air temperatures (P < 0.01, R² > 0.55). Moreover, derived LST from MODIS/Terra platform (MOD11A2) had higher accuracies than derived LST from MODIS/Aqua platform (MYD11A2) in linearly estimating air temperatures mentioned above.     

Soil respiration in Tibetan alpine grasslands: belowground biomass and soil moisture, but not soil temperature, best explain the large-scale patterns

The Tibetan Plateau is an essential area to study the potential feedback effects of soils to climate change due to the rapid rise in its air temperature in the past several decades and the large amounts of soil organic carbon (SOC) stocks, particularly in the permafrost. Yet it is one of the most under-investigated regions in soil respiration (Rs) studies. Here, Rs rates were measured at 42 sites in alpine grasslands (including alpine steppes and meadows) along a transect across the Tibetan Plateau during the peak growing season of 2006 and 2007 in order to test whether: (1) belowground biomass (BGB) is most closely related to spatial variation in Rs due to high root biomass density, and (2) soil temperature significantly influences spatial pattern of Rs owing to metabolic limitation from the low temperature in cold, high-altitude ecosystems. The average daily mean Rs of the alpine grasslands at peak growing season was 3.92 μmol CO(2) m(-2) s(-1), ranging from 0.39 to 12.88 μmol CO(2) m(-2) s(-1), with average daily mean Rs of 2.01 and 5.49 μmol CO(2) m(-2) s(-1) for steppes and meadows, respectively. By regression tree analysis, BGB, aboveground biomass (AGB), SOC, soil moisture (SM), and vegetation type were selected out of 15 variables examined, as the factors influencing large-scale variation in Rs. With a structural equation modelling approach, we found only BGB and SM had direct effects on Rs, while other factors indirectly affecting Rs through BGB or SM. Most (80%) of the variation in Rs could be attributed to the difference in BGB among sites. BGB and SM together accounted for the majority (82%) of spatial patterns of Rs. Our results only support the first hypothesis, suggesting that models incorporating BGB and SM can improve Rs estimation at regional scale.     

Estimating air temperature of an alpine meadow on the Northern Tibetan Plateau using MODIS land surface temperature

Meteorological data are scarce due to lack of meteorology stations in the Qinghai–Tibet Plateau. This often results in an imprecise estimation of air temperature. A linear estimation of air temperature of an alpine meadow on Northern Tibetan Plateau at heights of 1.5 m–2.1 m by using MODIS land surface temperature (LST) data was conducted in this study. The results showed that linear estimation of daily maximum and daytime mean air temperatures from MODIS LST data were not accurate enough (P > 0.01, R² < 0.10) during the growing season. In contrast, the linear relationships between daily maximum and daytime mean air temperature and MODIS LST during the non-growing season were both significant (P < 0.01, R² > 0.40). MODIS LST data were accurate enough to linearly estimate daily minimum and nighttime mean air temperatures (P < 0.01, R² > 0.55). Moreover, derived LST from MODIS/Terra platform (MOD11A2) had higher accuracies than derived LST from MODIS/Aqua platform (MYD11A2) in linearly estimating air temperatures mentioned above.     

基于用户兴趣点数据与Landsat遥感影像的城市热场空间格局研究

地图用户兴趣点(POI)数据能够反映微观尺度上城市系统中的人类活动。利用2015年夏季Landsat 8遥感影像提取了南京市地表温度和主要土地覆盖类型,利用空间与非空间多元回归模型在2、5、10 km 3个尺度上研究了地表温度与同期POI密度及植被和水体盖度的相关性,并利用方差分解技术定量区分人类活动因子(POI密度)及生态基础设施(植被和水体盖度)对城市热场的相对重要性。结果表明,在3个观测尺度上,POI密度与地表温度均存在极显著的正相关(P0.001),且相关性随观测尺度的增大而升高。植被和水体均具有显著的降温效应,水体盖度与地表温度的相关性仅在2 km尺度上显著,在5 km和10km尺度上其降温效应不再显著。方差分解结果表明,人类活动因子和生态基础设施对地表温度的独立解释率为1.6%—15%,而二者共同解释率达到了40%—70%。研究表明POI作为城市功能节点可以综合反映城市中人类活动的热源强度,在城市热场空间格局研究中是一种可与遥感数据互补的有用数据源。     

青藏高原高寒草地土壤酶活性海拔地带性特征

土壤酶是土壤生态系统中具有催化功能的一类蛋白质,由植物根系分泌物、微生物和动植物残体释放到土壤中,参与了土壤中所有的生物化学过程,其活性的大小可以灵敏地反应土壤中生化反应的方向和强度。青藏高原因海拔高和气候寒冷,被认为是气候变化的敏感区和脆弱区。全球气候变化使土壤酶活性受到强烈的影响,这些影响可能使土壤的质量发生改变,进而对青藏高原高寒草地植被生产力产生一定影响。土壤酶在生态系统物质循环和能量流动中起着关键作用,其格局、功能和转换过程已被广泛的研究,但高寒草地生态系统土壤酶活性的海拔地带性特征还需深入探讨。因此,以青藏高原高寒草地为研究对象,将土壤酶活性海拔梯度研究从站点尺度拓展到样带尺度,分析了与土壤碳循环密切相关的土壤β-1,4-葡萄糖苷酶(βG)、与土壤氮循环密切相关的土壤N-乙酰-β氨基葡萄糖苷酶(NAG)、土壤亮氨酸肌肽酶(LAP)以及与土壤磷循环密切相关的土壤碱性磷酸酶(ALP)的海拔地带性,进一步明晰其主要驱动因子。结果表明：(1)ALP和βG活性在海拔梯度上,展现出显著分异,在约为3546 m和3364 m出现拐点,且低海拔显著高于高海拔(P<0.01)。而NAG...     

Modeling gross primary productivity of alpine meadow in the northern Tibet Plateau by using MODIS images and climate data

The Vegetation Photosynthesis Model (VPM) was used to simulate the gross primary productivities (GPP) of the alpine meadow ecosystem in the northern Tibet Plateau at three different spatial resolutions of 0.5 km, 1.5 km and 2.5 km, respectively. The linear relationships between enhanced vegetation indices (EVI) and GPP, with higher correlative coefficients, were better than those between normalized difference vegetation indices (NDVI) and GPP at the three resolutions. VPM could well simulate the seasonal changes and inter-annual variations of GPP, with similar trends at the three resolutions. There were significant differences (P < 0.0001) among the three modeled GPP with the three resolutions. Therefore, the modeled GPP at high resolution could not be directly extrapolated to low resolution, and vice versa. The contribution levels of different model parameters, including photosynthetically active radiation (PAR), air temperature (Ta), NDVI, EVI and land surface water indices (LSWI), to modeled GPP could vary with spatial resolution based on multiple stepwise linear regression analysis. This indicated that it was important to choose parameters properly and consider their effects on modeled GPP.     

Modeling gross primary productivity of alpine meadow in the northern Tibet Plateau by using MODIS images and climate data

The Vegetation Photosynthesis Model (VPM) was used to simulate the gross primary productivities (GPP) of the alpine meadow ecosystem in the northern Tibet Plateau at three different spatial resolutions of 0.5 km, 1.5 km and 2.5 km, respectively. The linear relationships between enhanced vegetation indices (EVI) and GPP, with higher correlative coefficients, were better than those between normalized difference vegetation indices (NDVI) and GPP at the three resolutions. VPM could well simulate the seasonal changes and inter-annual variations of GPP, with similar trends at the three resolutions. There were significant differences (P < 0.0001) among the three modeled GPP with the three resolutions. Therefore, the modeled GPP at high resolution could not be directly extrapolated to low resolution, and vice versa. The contribution levels of different model parameters, including photosynthetically active radiation (PAR), air temperature (Ta), NDVI, EVI and land surface water indices (LSWI), to modeled GPP could vary with spatial resolution based on multiple stepwise linear regression analysis. This indicated that it was important to choose parameters properly and consider their effects on modeled GPP.     

草地土壤呼吸研究进展

土壤呼吸是生态系统物质循环和能量流动的重要过程,具有多方面的生态意义。在综合比较了土壤呼吸的各种测定方法,概括了草地土壤呼吸的速率和年排放量,并就影响草地土壤呼吸的一些重要因素进行了阐述,文章最后讨论了草地土壤呼吸今后的研究方向。     

西藏高寒草地物种多样性和生产力的环境驱动机制

全球变暖影响物种多样性和生产力及其关系。关于全球和区域的物种多样性与生产力关系已有许多研究,但气候变暖背景下西藏高寒草地物种多样性与生产力的关系及其环境驱动机制研究仍然很少。基于西藏高寒草地实测的35个样点调查数据,利用回归分析、Pearson相关性分析、方差分解和结构方程模型等方法,探究了物种多样性和生产力的关系及其影响机制。研究发现:(1)高寒草地的物种丰富度指数和香农-威纳指数与地上植被净初级生产力(ANPP)呈显著正相关关系,且ANPP对物种丰富度指数的变化更为敏感;(2)物种丰富度指数与经度、土壤有效氮、土壤有效磷、年降水量呈显著正相关;Shannon-Wiener指数与海拔和纬度呈显著负相关,与年均温度呈显著正相关;ANPP与经度、土壤有效氮、年降水量和年均温度呈显著正相关,与海拔、纬度和土壤有效钾呈显著负相关;(3)地理因子、土壤养分和气候因子的交互作用对物种丰富度指数和ANPP的贡献率最大,分别为10.99%和32.91%,地理因子和气候因子的交互作用对Shannon-Wiener指数的贡献率最大,为13.61%;(4)地理因子通过调控土壤养分和气候因子间接影响物种多样性和ANPP,土壤养分和气候因子均直接影响物种多样性和ANPP。研究结果揭示了环境因子对物种多样性和生产力的综合调控机制,为西藏高寒草地生态系统科学应对气候变化提供了依据。     

Microbial community responses reduce soil carbon loss in Tibetan alpine grasslands under short‐term warming

Changes in labile carbon (LC) pools and microbial communities are the primary factors controlling soil heterotrophic respiration (R_h) in warming experiments. Warming is expected to initially increase R_h but studies show this increase may not be continuous or sustained. Specifically, LC and soil microbiome have been shown to contribute to the effect of extended warming on R_h. However, their relative contribution is unclear and this gap in knowledge causes considerable uncertainty in the prediction of carbon cycle feedbacks to climate change. In this study, we used a two‐step incubation approach to reveal the relative contribution of LC limitation and soil microbial community responses in attenuating the effect that extended warming has on R_h. Soil samples from three Tibetan ecosystems—an alpine meadow (AM), alpine steppe (AS), and desert steppe (DS)—were exposed to a temperature gradient of 5–25°C. After an initial incubation period, soils were processed in one of two methods: (a) soils were sterilized then inoculated with parent soil microbes to assess the LC limitation effects, while controlling for microbial community responses; or (b) soil microbes from the incubations were used to inoculate sterilized parent soils to assess the microbial community effects, while controlling for LC limitation. We found both LC limitation and microbial community responses led to significant declines in R_h by 37% and 30%, respectively, but their relative contributions were ecosystem specific. LC limitation alone caused a greater R_h decrease for DS soils than AMs or ASs. Our study demonstrates that soil carbon loss due to R_h in Tibetan alpine soils—especially in copiotrophic soils—will be weakened by microbial community responses under short‐term warming.     

氮添加对高寒草甸土壤微生物呼吸及其温度敏感性的影响

土壤氮素的可利用性是控制土壤微生物呼吸的重要因素之一,大量研究已经表明增加土壤活性氮的含量可以降低微生物呼吸,但是土壤氮输入对土壤微生物呼吸温度敏感性的影响还不清楚。以青藏高原高寒草甸为研究对象,通过野外施氮试验和室内控制试验相结合的方式,在5℃、15℃和25℃条件下对3种施氮水平的土壤(对照,0g N m~(-2)a~(-1);低氮,5g N m~(-2)a~(-1);高氮,15g N m~(-2)a~(-1))进行培养,探讨土壤微生物呼吸及其温度敏感性对不同氮添加水平的响应情况。结果表明:(1)3个温度培养下的土壤微生物呼吸速率和累积碳释放量均随施氮量的增加而显著降低(P0.05);(2)氮添加对5℃和15℃培养条件下的微生物呼吸温度敏感性没有显著影响,但显著地增加了15℃和25℃培养条件下的微生物呼吸温度敏感性(P0.05);(3)线性相关分析表明,土壤累积碳释放量与土壤有机碳的难降解性显著负相关(P0.05),而15℃和25℃培养条件下的微生物呼吸温度敏感性与土壤有机碳的难降解性显著正相关(P0.05)。结果表明,在全球气候变暖的背景下,土壤氮输入将增加预测青藏高原高寒草甸地区土壤碳排放的不确定性。     

Plant clipping may cause overestimation of soil respiration in a Tibetan alpine meadow,southwest China

The chamber-based method is currently the most popular approach used for measuring soil respiration of various ecosystems. When this method is applied, aboveground plant tissues within the chamber need to be clipped some time (usually 24 h) before measuring soil respiration. However, plant clipping may change soil temperature and hence soil respiration because soil respiration is highly temperature-dependent, particularly in cold regions. To determine to what extent soil respiration may be changed by the clipping, we measured soil temperature and respiration of an alpine meadow of southwest China using a chamber-based method over an annual cycle. Based on the measurements, an exponential equation was built to describe the relationship between soil respiration and temperature. Concurrently we measured the soil temperature in clipped and unclipped plots on sunny days of the study months in another independent experiment; subsequently soil respiration was estimated for these plots using the exponential equation. Though daily mean soil temperature was insignificantly different between the clipped and unclipped plots, the clipping increased soil temperature at 5 cm depth by up to 4.3°C at daytime but decreased by up to 1.4°C at nighttime during the growing season. The changes were 2.2 and 1.5°C at daytime and nighttime, respectively, in the non-growing season. It was calculated that the clipping manipulation caused an overestimation of soil respiration by 28.6 and 21.2% for the growing and non-growing seasons, respectively; nevertheless, this calculated overestimation should differ from the actual one because the data were collected on sunny days only.     

Soil Mapping in the Pantanal of Mato Grosso, Brazil, using Multitemporal Landsat TM data

Keywords Multitemporal data sets from the Landsat Thematic Mapper (TM) were used to evaluate their applicability for exploratory soil mapping in the floodplain of the Northern Pantanal of Mato Grosso, Brazil. Fifty-four soil profiles were classified into 21 soil units according to the FAO–UNESCO system. Information layers of vegetation types and dynamics of flooding were elaborated by applying supervised hierarchical classification rules. Geomorphologic units were mapped by visual image interpretation. Multinomial logistic regression was applied to test relations between thematic layers and soil units as well as aggregated soil clusters, developing a statistical mapping model. Northern Pantanal floodplain soils show a high variability as a function of age and granulemetry of underlying sediments, as well as soil moisture and flooding regimes. GIS layers of nine vegetation formations, three geomorphologic units and three multi-temporal moisture types were elaborated. Cross-tabulations and multinomial logistic regression models indicate significant relations between FAO–UNESCO soil units and GIS layers. As soil sampling density had been low, a final predictive model was developed for the mapping of six aggregated soil clusters, obtaining a high significance level (p<0.05) for prediction. Applied methodology was found to be appropriate to develop models on soil–landscape relationships and improve information on spatial distribution of soil groupings in the Northern Pantanal.     

Stochastic processes regulate belowground community assembly in alpine grasslands on the Tibetan Plateau

Understanding biogeographical patterns and underlying processes of belowground community assembly is crucial for predicting soil functions and their responses to global environmental change. However, little is known about potential differences of belowground community assembly among bacteria, fungi, protists and soil animals, particularly for alpine ecosystems. Based on the combination of large-scale field sampling, high-throughput marker-gene sequencing and multiple statistical analyses, we explored patterns and drivers of belowground community assembly in alpine grasslands on the Tibetan Plateau. Our results revealed that the distance–decay rates varied among trophic levels, with organisms of higher trophic level having weaker distance–decay pattern. The spatial and environmental variables explained limited variations of belowground communities. By contrast, the stochastic processes, mainly consisting of dispersal limitation and drift, played a primary role in regulating belowground community assembly. Moreover, the relative importance of stochastic processes varied among trophic levels, with the role of dispersal limitation weakening whereas that of drift enhancing in the order of bacteria, fungi, protists and soil animals. These findings advance our understanding of patterns and mechanisms driving belowground community assembly in alpine ecosystems and provide a reference basis for predicting the dynamics of ecosystem functions under changing environment.     

Litter and its interaction with standing vegetation affect seedling recruitment in Tibetan alpine grasslands

Background: Seedling recruitment is widely recognised to be important in the maintenance of diversity and coexistence of species. It is not clear how local abiotic factors mediated by litter and biotic interactions influence plant species recruitment in alpine grasslands.

Aims: To determine how litter and standing vegetation affected seedling recruitment in plant communities in Tibetan alpine grasslands.

Methods: Seedling recruitment was quantified in response to experimental treatments: (1) removal of standing vegetation and litter; (2) removal of standing vegetation with litter intact; (3) intact standing vegetation and removal of litter and (4) control: intact standing vegetation and litter.

Results: Litter removal increased seedling numbers, while standing vegetation removal had no effect. An interaction between litter and standing vegetation removal marginally increased seedling number. Species richness of seedlings was not related to either litter removal or standing vegetation removal. Species composition of seedling differed significantly between removal of both litter and standing vegetation and control.

Conclusions: Litter and its interaction with standing vegetation has an important role in affecting plant species recruitment in this alpine plant community. In contrast, biotic interactions, such as competition or facilitation from standing vegetation, appear to have only minor effects on recruitment.     

基于TM卫星影像数据的北京市植被变化及其原因分析

植被覆盖变化是全球变化研究的重要内容之一,由于NDVI与植被的分布密度呈线性相关,是指示大尺度植被覆盖的良好指标,因此在宏观植被盖度的估算中常被应用.利用1987年9月26日和2009年9月22日的Landsat TM卫星影像,以NDVI为桥梁,分别计算了北京市域的植被盖度和大于0.1的NDVI差值指数,北京市域与不同生态区域两个尺度对其植被变化情况进行了量化分析,结果表明,2009年与1987年相比,北京市极低覆盖度、中覆盖度和高覆盖度植被的面积均有所减少,其所占全市土地面积的比例从1987年到2009年分别降低了5.15％和0.54％和0.03％;而低覆盖度和极高覆盖度植被的面积比例则分别增加了5.71％和0.01％.大于0.1的植被差值指数统计结果显示,全市域植被质量以改善为主,全市植被发生改善变化的土地面积共919302.3 hm2,其中发生轻微改善的比例为28.31％,中度改善的为41.33％,极度改善的面积为30.36％;全市植被发生退化变化的面积326931.12 hm2,其中发生中度退化、轻微退化和极度退化的面积分别占到了退化变化土地面积的41.98％、43.20％和14.82％.从不同区域的植被差值指数看,植被发生退化变化最明显的区域为燕山山区北部、五环以内和五至六环间区域,这几个区域退化变化的植被面积占相应区域的面积比例分别达到了30.25％、58.17％和47.38％,而且均以严重退化与中度退化为主,两者合计的面积比例分别为15.79％、44.72％和34.19％.而发生退化变化面积比例最小的区域为太行山区和延庆盆地,其退化面积占该区域植被面积的比例分别为13.35％和17.02％,且退化程度均以轻微退化和中度退化为主,其面积比例介于5％-8％之间.从植被变化的驱动力看,目前还看不出北京这种植被变化结果与气候变化之间的直接关联.北京市植被变化的驱动力主要还是人为因素.这包括了区域性的大环境绿化生态工程建设(包括山区与平原区),城市绿化市政工程建设、平原区农业结构调整、新农村生态环境建设,以及由于降水而导致的山区河岸带变化等.其中河流水面变化对河岸带植被变化的影响范围在多年平均水面线外0-150 m范围内,0-100 m范围为受影响较大的区域.     

Response of soil bacterial communities to moisture and grazing in the Tibetan alpine steppes on a small spatial scale

As the Third Pole of the world, the Tibetan Plateau provides a typical alpine grassland environment for soil bacteria with its unique frigid and arid climate. Owing to clear changes in spatial moisture and increased grazing intensity, moisture and livestock grazing have become key factors influencing the microbial communities. Accordingly, we investigated the diversity and composition of soil bacteria in a selected alpine grassland within the dual gradients of moisture and grazing using high-throughput sequencing. Our results showed that grazing changed the soil bacterial diversity and composition, whereas moisture only influenced the relative abundance of the segmental community at the small spatial scale. Species richness was found to be increased by moderate grazing compared with that by high or low-grazing intensity. The relative abundance of dominant species and β-diversity of soil bacteria both showed differences with heavy, moderate, and low grazing. Some dominant bacteria were altered with the moisture content. However, there were no significant differences according to the moisture gradient in terms of the overall bacterial β diversity and composition. These results might be taken account into the small spatial scale as well as the compensation of grazing to moisture on this scale. This work provides new insights into the soil bacterial response to moisture gradients and grazing intensity in alpine steppe habitat.