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1.
Of all terrestrial ecosystems, the productivity of deserts has been suggested to be the most responsive to increasing atmospheric CO2. The extent to which this prediction holds will depend in part on plant responses to elevated CO2under the highly variable conditions characteristic of arid regions. The photosynthetic responses ofLarrea tridentata , an evergreen shrub, to a step-increase in atmospheric CO2(to 550 μmolmol−1) were examined in the field using Free-Air CO2Enrichment (FACE) under seasonally varying moisture conditions. Elevated CO2substantially increased net assimilation rate (Anet) in Larrea during both moist and dry periods of the potential growing season, while stomatal conductance (gs) did not differ between elevated and ambient CO2treatments. Seasonal and diurnal gas exchange dynamics in elevated CO2mirrored patterns in ambient CO2, indicating that elevated CO2did not extend photosynthetic activity longer into the dry season or during more stressful times of the day. Net assimilation vs. internal CO2(A/Ci) responses showed no evidence of photosynthetic down-regulation during the dry season. In contrast, after significant autumn rains, Amax(the CO2saturated rate of photosynthesis) and CE (carboxylation efficiency) were lower in Larrea under elevated CO2. In situ chlorophyll fluorescence estimation ofLarrea Photosystem II efficiency (Fv/Fm) responded more to water limitation than to elevated CO2. These findings suggest that predictions regarding desert plant responses to elevated CO2should account for seasonal patterns of photosynthetic regulatory responses, which may vary across species and plant functional types.  相似文献   

2.
Africa is a sink of carbon, but there are large gaps in our knowledge regarding the CO2 exchange fluxes for many African ecosystems. Here, we analyse multi-annual eddy covariance data of CO2 exchange fluxes for a grazed Sahelian semi-arid savanna ecosystem in Senegal, West Africa. The aim of the study is to investigate the high CO2 exchange fluxes measured at the peak of the rainy season at the Dahra field site: gross primary productivity and ecosystem respiration peaked at values up to ?48 μmol CO2 m?2 s?1 and 20 μmol CO2 m?2 s?1, respectively. Possible explanations for such high fluxes include a combination of moderately dense herbaceous C4 ground vegetation, high soil nutrient availability and a grazing pressure increasing the fluxes. Even though the peak net CO2 uptake was high, the annual budget of ?229 ± 7 ± 49 g C m?2 y?1 (±random errors ± systematic errors) is comparable to that of other semi-arid savanna sites due the short length of the rainy season. An inter-comparison between the open-path and a closed-path infrared sensor indicated no systematic errors related to the instrumentation. An uncertainty analysis of long-term NEE budgets indicated that corrections for air density fluctuations were the largest error source (11.3% out of 24.3% uncertainty). Soil organic carbon data indicated a substantial increase in the soil organic carbon pool for the uppermost .20 m. These findings have large implications for the perception of the carbon sink/source of Sahelian ecosystems and its response to climate change.  相似文献   

3.
Decomposition of soil organic carbon (SOC) regulates the partitioning between soil C-stock and release of CO2 to the atmosphere and is vital for soil fertility. Agricultural expansion followed by decreasing amounts of SOC and soil fertility is a problem mainly seen in tropical agro-ecosystems where fertilizers are in short supply. This paper focuses on factors influencing temporal trends in soil respiration measured as CO2 effluxes in grass savanna compared with groundnut (Arachis hypogaea L.) fields in the semi-arid part of Senegal in West Africa. Based on laboratory experiments, soil CO2 production has been expressed as a function of temperature and soil water content by fit equations. Field measurements included soil CO2 effluxes, soil temperatures and water contents. Effluxes in grass savanna and groundnut fields during the dry season were negligible, while effluxes during the rainy season were about 3–8 μmol CO2 m?2 s?1, decreasing to less than 1 μmol by the end of the growing season. Annual soil CO2 production was simulated to be in the range of 31–38 mol C m?2. Furthermore, a controlled water addition experiment revealed the importance of rain during the dry season for the overall turnover of soil organic matter.  相似文献   

4.
《Polar Science》2014,8(4):397-412
We used observational and experimental analyses to investigate the photosynthetic activity and water relationships of five lichen species attached to different substrates in a glacier foreland in the High Arctic, Ny-Ålesund, Svalbard (79°N) during the snow-free season in 2009 and 2010. After the rains ceased, lichens and their attached substrates quickly dried, whereas photosynthetic activity in the lichens decreased gradually. The in situ photosynthetic activity was estimated based on the relative electron transportation rate (rETR) in four fruticose lichens: Cetrariella delisei, Flavocetraria nivalis, Cladonia arbuscula ssp. mitis, and Cladonia pleurota. The rETR approached zero around noon, although the crustose lichen Ochrolechia frigida grown on biological soil crust (BSC) could acquire water from the BSC and retain its WC to perform positive photosynthesis. The light-rETR relationship curves of the five well-watered lichens were characterized into two types: shade-adapted with photoinhibition for the fruticose lichens, and light-adapted with no photoinhibition for O. frigida. The maximum rETR was expected to occur when they could acquire water from the surrounding air or from substrates during the desiccation period. Our results suggest that different species of Arctic lichens have different water availabilities due to their substrates and/or morphological characteristics, which affect their photosynthetic active periods during the summer.  相似文献   

5.
In Northern Mexico, long-term grazing has substantially degraded semiarid landscapes. In semiarid systems, ecological and hydrological processes are strongly coupled by patchy plant distribution and biological soil crust (BSC) cover in plant-free interspaces. In this study, we asked: 1) how responsive are BSC cover/composition to a drying/wetting cycle and two-year grazing removal, and 2) what are the implications for soil erosion? We characterized BSC morphotypes and their influence on soil stability under grazed/non-grazed conditions during a dry and wet season. Light- and dark-colored cyanobacteria were dominant at the plant tussock and community level. Cover changes in these two groups differed after a rainy season and in response to grazing removal. Lichens with continuous thalli were more vulnerable to grazing than those with semi-continuous/discontinuous thalli after the dry season. Microsites around tussocks facilitated BSC colonization compared to interspaces. Lichen and cyanobacteria morphotypes differentially enhanced resistance to soil erosion; consequently, surface soil stability depends on the spatial distribution of BSC morphotypes, suggesting soil stability may be as dynamic as changes in the type of BSC cover. Longer-term spatially detailed studies are necessary to elicit spatiotemporal dynamics of BSC communities and their functional role in biotically and abiotically variable environments.  相似文献   

6.
In this paper, we investigate spatial variations in soil CO2 efflux and carbon dynamics across five sites located between 65.5°N and 69.0°N in tundra and boreal forest biomes of Alaska. Growing and winter mean CO2 effluxes for the period 2006–2010 were 261 ± 124 (Coefficients of Variation: 48%) and 71 ± 42 (CV: 59%) gCO2/m2, respectively. This indicates that winter CO2 efflux contributed 24% of the annual CO2 efflux over the period of measurement. In tundra and boreal biomes, tussock is an important source of carbon efflux to the atmosphere, and contributes 3.4 times more than other vegetation types. To ensure that representativeness of soil CO2 efflux was determined, 36 sample points were used at each site during the growing season, so that the experimental mean fell within ±20% of the full sample mean at 80% and 90% confidence levels. We found that soil CO2 efflux was directly proportional to the seasonal mean soil temperature, but inversely proportional to the seasonal mean soil moisture level, rather than to the elevation-corrected July air temperature. This suggests that the seasonal mean soil temperature is the dominant control on the latitudinal distribution of soil CO2 efflux in the high-latitude ecosystems of Alaska.  相似文献   

7.
Soil carbon dioxide content, temperature, and moisture were measured biweekly for one year at Pigeon Mountain, GA. Levels of soil CO2 ranged from 0.04% to 2.4% and were highest during the growing season, lowest during the winter. Soil temperature correlated positively with soil CO2, accounting for 90% of CO2 variation. Soil moisture variations decreased CO2 concentration at times of high soil water content when CO2 was flushed downward, and also at times of low soil moisture content when CO2 production was inhibited. A predictive model of logistic form using 14-day means of daily actual evapotranspiration fit the data well (R2= 0.83). The model also tested well against soil CO2 data acquired in the coastal plain at Rocky Point, NC. If actual evapotranspiration rates are known, the model permits estimation of soil CO2 without preliminary field work, and can be used for studies of karst denudation, which require soil CO2 data for seasonal and regional comparison of solution rates.  相似文献   

8.
Birger Ulf Hansen: Satellite Monitoring of the Biomass Production in Southern Greenland. Geografisk Tidsskrift 88:94–99. Copenhagen, 1988.

Based on satellite data from Southern Greenland the application of Normalized Difference Vegetation Index (NDVI) for monitoring biomass production has been evaluated. Field measurements of spectral reflectance data are quantitatively correlated with clipped samples of total above-ground biomass production. The relation between NDVI measured on the ground and biomass production is discussed. The seasonal and geographical variation in NDVI is correlated with the climate and water balance. The potential production is estimated as the product of mean NDVI and the length of the growing season. The results suggest that operational satellite monitoring of NDVI provides valuable assistance in agricultural management and forward planning of the potential breeding capacity in an arctic/subarctic environment.  相似文献   

9.
The spatial variability and co-variability of two different types of remote sensing derivatives that portray vegetation and geomorphic patterns are analyzed in the context of estimating regional-scale CO2 flux from land surfaces in the arctic tundra. For a study area encompassing the Kuparuk River watershed of the North Slope of Alaska, we compare satellite-derived maps of the normalized difference vegetation index (NDVI) generated at two different spatial resolutions to a map of vegetation types derived by image classification of data from the Landsat multispectral scanner (MSS). Mean values of NDVI for each cover type stratum are unique (with the exception of moist acidic tundra and shrubland types). Based on analysis of semi-variograms generated for SPOT-NDVI data, most of the vegetation cover and landform features of this arctic tundra landscape have spatial dimensions of less than 1 km. Thaw lakes on the coastal plain and glacial depositional landforms, such as moraines in the foothills, seem to be the largest features, with vegetation units having dimensions no larger than 700 m. Frequency distributions of NDVI and vegetation types extracted for sampling transects flown by an aircraft sensing CO2 flux, relative to distributions for the entire Kuparuk River watershed, suggest a slight sampling bias towards greater cover of mesic wet sedge tundra and thaw lakes and associated lower NDVI values. The regional pattern of NDVI for the North Slope of Alaska corresponds primarily to differences between the two major physiographic provinces of this region.  相似文献   

10.
《Polar Science》2014,8(3):218-231
To investigate the dynamics and environmental drivers of CO2 flux through the winter snowpack in a dwarf bamboo ecosystem (Hokkaido, northeast Japan), we constructed an automated sampling system to measured CO2 concentrations at five different levels in the snowpack, from the base to the upper snow surface. Using a gas diffusion approach, we estimated an average apparent soil CO2 flux of 0.26 μmol m−2 s−1 during the snow season (December–April); temporally, the CO2 flux increased until mid-snow season, but showed no clear trend thereafter; late-season snow-melting events resulted in rapid decreases in apparent CO2 flux values. Air temperature and subnivean CO2 flux exhibited a positive linear relationship. After eliminating the effects of wind pumping, we estimated the actual soil CO2 flux (0.41 μmol m−2 s−1) to be 54% larger than the apparent flux. This study provides new constraints on snow-season carbon emissions in a dwarf bamboo ecosystem in northeast Asia.  相似文献   

11.
《Polar Science》2014,8(2):146-155
The Boreal black spruce forest is highly susceptible to wildfire, and postfire changes in soil temperature and substrates have the potential to shift large areas of such an ecosystem from a net sink to a net source of carbon. In this paper, we examine CO2 exchange rates (e.g., NPP and Re) in juniper haircap moss (Polytrichum juniperinum) and microbial respiration in no-vegetation conditions using an automated chamber system in a five-year burned black spruce forest in interior Alaska during the fall season of 2009. Mean ± standard deviation microbial respiration and NEP (net ecosystem productivity) of juniper haircap moss were 0.27 ± 0.13 and 0.28 ± 0.38 gCO2/m2/hr, respectively. CO2 exchange rates and microbial respiration showed temporal variations following fluctuation in air temperature during the fall season, suggesting the temperature sensitivity of juniper haircap moss and soil microbes after fire. During the 45-day fall period, mean NEP of P. juniperinum moss was 0.49 ± 0.28 MgC/ha following the five-year-old forest fire. On the other hand, simulated microbial respiration normalized to a 10 °C temperature might be stimulated by as much as 0.40 ± 0.23 MgC/ha. These findings demonstrate that the fire-pioneer species juniper haircap moss is a net C sink in the burned black spruce forest of interior Alaska.  相似文献   

12.
Abstract

Páramo grasslands are important carbon sinks in the Ecuadorian Andes. Although carbon content of páramo Andisols is correlated with high water retention, the effects of differences in soil moisture under different types of land use on soil carbon processes have not been explicitly tested in the Ecuadorian Andes. This study assessed the relationship between soil moisture and soil CO2 flux among a mature páramo grassland, recently burned páramo grassland, native montane forest, and pine plantation in an Ecuadorian páramo landscape. Soil CO2 flux was greater in the forest sites compared to the grassland sites. Thus, a shift from grassland to forest cover may have significant implications for soil carbon loss via CO2 flux. Our results suggest that although soil moisture plays a significant role in differences of soil CO2 flux rates among land-use types, more investigation into mechanisms for soil carbon loss and how they are driven by land-use change is needed. To our knowledge, these are the first soil CO2 flux rates reported for the Ecuadorian páramo.  相似文献   

13.
Based on the static opaque chamber method,the respiration rates of soil microbial respiration,soil respiration,and ecosystem respiration were measured through continuous in-situ experiments during rapid growth season in semiarid Leymus chinensis steppe in the Xilin River Basin of Inner Mongolia,China. Soil temperature and moisture were the main factor affecting respiration rates. Soil temperature can explain most CO2 efflux variations (R2=0.376-0.655) excluding data of low soil water conditions. Soil moisture can also effectively explain most of the variations of soil and ecosystem respiration (R2=0.314-0.583),but it can not explain much of the variation of microbial respiration (R2=0.063). Low soil water content (≤5%) inhibited CO2 efflux though the soil temperature was high. Rewetting the soil after a long drought resulted in substantial increases in CO2 flux at high temperature. Bi-variable models based on soil temperature at 5 cm depth and soil moisture at 0-10 cm depth can explain about 70% of the variations of CO2 effluxes. The contribution of soil respiration to ecosystem respiration averaged 59.4%,ranging from 47.3% to 72.4%; the contribution of root respiration to soil respiration averaged 20.5%,ranging from 11.7% to 51.7%. The contribution of soil to ecosystem respiration was a little overestimated and root to soil respiration little underestimated because of the increased soil water content that occurred as a result of plant removal.  相似文献   

14.
黄土碳酸盐碳同位素广泛应用于第四纪气候环境变化的研究中,以往研究中多利用钙结核、次生碳酸盐或成壤碳酸盐,认为其反映了C4植物的丰度.黄土高原碳酸盐碳同位素表现为黄土层高,古土壤层中低,即黄土层中C4植物丰度高于古土壤层.然而,这样的结果和黄土有机碳同位素得到的结果矛盾,有机碳同位素的结果表明温度对C4植物的分布起到了决...  相似文献   

15.
对干旱区优势固沙灌木柠条群落不同微生境（灌丛外和灌丛下）的土壤温度（0 cm、5 cm、10 cm和20 cm剖面深度）进行了连续测定,对比分析了晴天、降水日与灌丛对其微生境土壤温度时空变异的影响。结果表明:降水和植被灌丛对土壤温度均具有显著影响。在无雨日,受灌丛遮阴影响,灌丛外土壤温度明显高于灌丛下相同深度的土壤温度;在降水日,土壤温度主要受降水影响,降水使土壤温度明显降低,灌丛的影响作用减弱,灌丛下和灌丛外同一剖面深度的土壤温度差异较小。无雨日土壤温度日变化呈单峰型正弦曲线,随剖面深度增加,土壤温度振幅逐渐减小,峰值出现时间滞后,土壤温度垂直分布变化呈现4种典型变化曲线。夜间（日落至日出）柠条灌丛下地表温度（0 cm土壤温度）比灌丛外地表温度略高。降水日土壤温度日变化随降水过程的持续呈逐渐递减趋势,土壤温度垂直分布变化表现为随深度增加而递增的趋势。  相似文献   

16.
对人工固沙植被区柠条(Caragana korshinskii)群落和油蒿(Caragana korshinskii)群落下不同深度的土壤气体采样,主要研究和讨论了不同类型人工植被区下土壤CO2浓度的变化特征及土壤温度和土壤水分对其的影响。结果表明:柠条和油蒿群落0~80cm处的土壤空气CO2浓度随着土壤深度的增加而增加,并且在0~40cm,油蒿群落下的土壤CO2浓度值大于柠条,而在40cm以下则相反。其平均值分别为1 229.3μmol·mol-1和1 242.7μmol·mol-1,大于同一深度流沙下土壤CO2浓度值978.9μmol·mol-1。土壤水分与二者的土壤CO2浓度变化趋势在年际尺度上具有一致性,浅层40cm内油蒿群落下的土壤CO2浓度和土壤水分含量的相关性明显大于柠条和流沙。而在40cm以下,则表现为柠条油蒿流沙。土壤温度对土壤CO2浓度的影响程度一般为流沙油蒿柠条,特别是流沙,在表层达到了极显著的水平,之后随着土壤深度的增加而降低。而土壤温度对油蒿和柠条样地土壤CO2浓度的影响较为复杂,呈现出先增加后减小的趋势。在年际尺度上,土壤水分含量是不同植被群落下土壤剖面CO2浓度的关键限制因子,而在日尺度上,土壤温度则为主要限制因子。据粗略估计,在0~80cm内,柠条和油蒿根系呼吸所占的比例约为30.7%和33.3%。  相似文献   

17.
生物土壤结皮（BSC）广泛分布在干旱、半干旱区,对荒漠生态系统的稳定、平衡等具有重要意义。初步探讨了降雪量对腾格里沙漠东南缘藻类、地衣和藓类结皮叶绿素荧光参数、光合色素、丙二醛（MDA）及可溶性蛋白含量的影响。结果表明：结皮光合生理特性对降雪量的响应有显著差异。适量降雪激发3种结皮PSⅡ反应中心,引起光合色素、可溶性蛋白合成升高,MDA含量降低;过量降雪导致藻类和地衣结皮光合电子传递受阻,光合色素、可溶性蛋白合成减少,MDA含量升高,产生光抑制现象。适量冬季降雪对结皮具有一定的促进作用,而藓类结皮更能适应极端降雪环境,但相关分子机制还需深入研究。  相似文献   

18.
Land degradation and global warming are currently highly active research topics. Land degradation can both change land cover and surface climate and significantly influence atmospheric circulation. Researches have verified that carbon dioxide (CO2) and methane (CH4) are major greenhouse gases (GHG) in the atmosphere and are directly affected by human activity. However, to date, there is no research on the spatial distribution of GHG concentrations and also no research on how land degradations affect GHG concentrations in arid and semi-arid regions. In this study, we used GHG data from the ENVIronment SATellite (ENVISAT) and the Greenhouse gases Observing Satellite (GOSAT), the Normalized Difference Vegetation Index (NDVI) and Land Surface Temperature (LST) data from the MODerate resolution Imaging Spectroradiometer (MODIS) and precipitation data from ground stations to analyze the way land degradation affects GHG concentrations in northern China and Mongolia, which exhibit the most serious land degradation process in East Asia. Our research revealed that the CO2 and CH4 concentrations (XCO2 and XCH4) increased from 2003 to 2009 and then decreased into 2011. We used geostatistics to predict and simulate the spatial distribution of XCO2 and XCH4 and found that the distribution of XCO2 displays a seasonal trend and is primarily affected by plant photosynthesis, soil respiration and precipitation. As the distribution of XCH4 is mainly affected by the sources' distribution, microbial processes, LST and submarine hydrate, the CH4 concentration presents no obvious seasonal changes and the high XCH4 values are primarily found in northeast and southeast China. Land degradation increases the concentration of GHG: the correlation coefficient between NDVI and XCO2 is R2 = 0.76 (P < 0.01) and the value between NDVI and XCH4 is R2 = 0.75 (P < 0.01).  相似文献   

19.
Soil temperatures at 0, 5, 10 and 20 cm depths were monitored continuously at different microhabitats (beneath shrub canopy (BSC); bare intershrub spaces (BIS)) induced by xerophytic shrub (Caragana korshinskii Kom.) canopy, respectively. We mainly aimed to assess the effects of shrub canopy and precipitation on the spatial-temporal variability of soil temperature. Results indicate that both precipitation and vegetation canopy significantly affect soil temperature. In clear days, soil temperatures within the BSC area were significantly lower than in the BIS at the same soil depth due to shading effects of shrub canopy. Diurnal variations of soil temperature show a unimodal sinusoidal curve. The amplitude of soil temperature tended to decrease and a hysteresis of diurnal maximum soil temperature existed at deeper soil layers. Vertical fluctuations of soil temperature displayed four typical curves. In the nighttime (approximately from sunset to sunrise), surface temperature within the BSC area was higher than in the BIS. In rainy days, however, soil temperatures were affected mainly by precipitation and the shrub canopy had a negligible effect on soil temperature, and little difference in soil temperature at the same soil depth was found between the BSC area and in the BIS. Diurnal variations in soil temperature decreased exclusively as rainfall continued and the vertical fluctuations of soil temperature show an increased tendency with increasing soil depth.  相似文献   

20.
崇明东滩湿地CO2 、CH4和N2O 排放的时空差异   总被引：10，自引：0，他引：10  
汪青  刘敏  侯立军  程书波 《地理研究》2010,29(5):935-946
通过静态暗箱—气相色谱法研究了长江口崇明东滩四类典型湿地(围垦湿地、高潮滩、中潮滩和低潮滩)CO2、CH4和N2O排放特征及影响因素。结果表明,在生长季尺度下,CO2、CH4和N2O均以排放为主;在昼夜尺度下,CO2和CH4在夜间排放量大于白昼排放量,而N2O的排放高峰出现在下午;在潮水退去、潮滩暴露初期,CH4和N2O有大量排放,CO2正好相反。崇明东滩温室气体排放通量自岸向海有明显的梯度变化,总体趋势是越近岸通量值越大。观测与实验表明,温度、潮汐、土壤理化性质、植物和土地利用变化都对温室气体排放通量有明显的影响,其中滨海潮滩湿地特有环境因子潮汐以"淹没—暴露"光滩沉积物的方式控制温室气体的排放。  相似文献   

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