陕北黄土高原植被净初级生产力的估算

基于MODIS和地面气象数据,利用改进的CASA模型,模拟分析了2005年陕北黄土高原地区的植被净初级生产力(NPP)及其时空分布.结果表明:1)根据生态生理过程模型针对不同土地覆被类型选择不同的月平均最大光能利用率,比传统CASA模型中使用固定的全球月平均最大光能利用率进行NPP估算,更符合陕北黄土高原地区的实际情况;在估算植被参数时引入植被覆盖分类,以及利用陕北黄土高原2005年时序NDVI进行土地覆被分类的同时,结合1:100万中国植被图和实地调查情况对分类结果进行修正,可提高分类的精度,从而提高模型估算的精度.2)通过不同模型之间和与陕北部分地区实际调查数据进行比较,显示改进后的CASA模型对区域陆地植被NPP的模拟效果较好,可应用于陕北黄土高原乃至周边地区NPP的计算中.3)2005年陕北黄土高原植被净第一性生产量估计值为4.76×10~(13) g C,约占全国总NPP的1.5%,植被平均NPP为447.3 g C·m~(-2)·a~(-1),高于1992-2000年全国陆地NPP平均值323.8 g C·m~(-2)·a~(-1).4)在NPP的空间分布上,总体趋势是由东南向西北递减,其中最高值出现在东南部的黄龙山次生林区(1087g C·m~(-2)·a~(-1));西北部的荒漠植被覆盖度极低,平均NPP仅为205.0 g C·m~(-2)·a~(-1).5)陕北黄土高原NPP的季节变化明显,其中4月中旬至10月中旬6个月生长季时间里的NPP可占到全年的91.5%,而7月中旬至8月中旬间该区的净初级生产力达到年内的极大值,可占全年的37.8%.

Abstract：

Based on the data from MODIS (Moderate-resolution Imaging Speetroradiometer) and meteorological observatories, and by using improved CASA (Carnegie-Ames-Stanford Approach) model, the vegetation net primary productivity (NPP) and its spatiotemporal distribution on the North Shaanxi Loess Plateau in 2005 were simulated and analyzed. Comparing with the traditional CASA model which only uses a universal mean annual maximum light use efficiency (LUE), the estimated regional NPP by the improved CASA model was more precise, because this improved model used the LUE parameters of different vegetation covers. The detailed land cover classifica-tion also contributed to the increase of the precision via introducing the time-series Normalized Different Vegetation Index (NDVI) and ground survey data to modify and adjust the original clas-sification system based on vegetation map (1: 1000000). The testing of the simulation results from different models with the ground survey data in North Shaanxi showed that the estimation by the modified CASA model was much closer to the real survey data, implying the potential practi-cal significance of this model in estimating the vegetation NPP in North Shaanxi Loess Plateau and its adjacent areas. In 2005, the NPP in North Shaanxi was estimated as 4. 76×1013 g C, ac-counting for about 1.5% of China' s terrestrial total NPP, and the mean NPP was 447.3 g C·m~(-2)·a~(-1), being much higher than that of China' s terrestrial vegetation (323.8 g C·m~(-2)·a~(-1)) in 1982-2000. The spatial distribution pattern of the vegetation NPP showed an apparently declining trend from the southeast to the northwest, with the highest value of 1087 g C·m~(-2)a~(-1) occurred in the broadleaved-and conifer-mixed forests of Huanglong Mountain in southeast part of the region. The mean NPP of desert vegetation in the whole region was the lowest, only about 205.0 g C·m~(-2) ·a~(-1). An obvious seasonal variation of the NPP was observed. The NPP in growth season (from April to October) took about 91.5% of the total in the year, and the peak occurred in mid-July to mid-August, amounting to 37.8% of the total.     

Net ecosystem production of a Douglas-fir stand for 3 years following clearcut harvesting

To investigate the variations in annual and seasonal net ecosystem production (F_NEP) during the development of a young forest, 3 years of continuous eddy covariance measurements of carbon dioxide (CO₂) fluxes were collected following clearcut harvesting and replanting of a coastal Douglas‐fir stand on the east coast of Vancouver Island, BC, Canada. The impact of changing weather and stand structure on F_NEP was examined by developing relationships between F_NEP and variables such as light, temperature, soil moisture, and leaf area index (LAI). In all 3 years, the stand was a large source of CO₂ (620, 520, and 600 g C m^?2 yr^?1 in the first, second, and third years, respectively). During this period, the growth of pioneer and understory species resulted in an increase in maximum growing season LAI from 0.2 in the year the seedlings were planted to 2.5 in the third year. The associated increase in annual gross ecosystem production (P=F_NEP?R_e, where R_e is ecosystem respiration) from 220 g C m^?2 yr^?1 in the first year to 640 g C m^?2 yr^?1 in the third year was exceeded by an increase in annual R_e from 840 to 1240 g C m^?2 yr^?1. Seasonal and interannual variations in daytime F_NEP and P were well described by variations in photosynthetically active radiation, temperature, and changes in LAI. Night‐time measurements of R_e exponentially increased with 2 cm soil temperature with an average Q₁₀ of 2 (relative increase in R_e for a 10°C increase in temperature) and R₁₀ (R_e at 10°C) that increased from 2.1 in the first year to 2.5 in the second year to 3.2 μmol m^?2 s^?1 in the third year. Although the re‐establishment of vegetation in this stand had a major impact on both P and R_e, interannual variations in weather also affected annual F_NEP. Drought, in the summer of the third year, resulted in early senescence and reduced both P and R_e. This resulted in more C being lost from the stand in the third year after harvesting than in the second year.     

Increased leaf area dominates carbon flux response to elevated CO2 in stands of Populus deltoides (Bartr.)

We examined the effects of atmospheric vapor pressure deficit (VPD) and soil moisture stress (SMS) on leaf‐ and stand‐level CO₂ exchange in model 3‐year‐old coppiced cottonwood (Populus deltoides Bartr.) plantations using the large‐scale, controlled environments of the Biosphere 2 Laboratory. A short‐term experiment was imposed on top of continuing, long‐term CO₂ treatments (43 and 120 Pa), at the end of the growing season. For the experiment, the plantations were exposed for 6–14 days to low and high VPD (0.6 and 2.5 kPa) at low and high volumetric soil moisture contents (25–39%). When system gross CO₂ assimilation was corrected for leaf area, system net CO₂ exchange (SNCE), integrated daily SNCE, and system respiration increased in response to elevated CO₂. The increases were mainly as a result of the larger leaf area developed during growth at high CO₂, before the short‐term experiment; the observed decline in responses to SMS and high VPD treatments was partly because of leaf area reduction. Elevated CO₂ ameliorated the gas exchange consequences of water stress at the stand level, in all treatments. The initial slope of light response curves of stand photosynthesis (efficiency of light use by the stand) increased in response to elevated CO₂ under all treatments. Leaf‐level net CO₂ assimilation rate and apparent quantum efficiency were consistently higher, and stomatal conductance and transpiration were significantly lower, under high CO₂ in all soil moisture and VPD combinations (except for conductance and transpiration in high soil moisture, low VPD). Comparisons of leaf‐ and stand‐level gross CO₂ exchange indicated that the limitation of assimilation because of canopy light environment (in well‐irrigated stands; ratio of leaf : stand=3.2–3.5) switched to a predominantly individual leaf limitation (because of stomatal closure) in response to water stress (leaf : stand=0.8–1.3). These observations enabled a good prediction of whole stand assimilation from leaf‐level data under water‐stressed conditions; the predictive ability was less under well‐watered conditions. The data also demonstrated the need for a better understanding of the relationship between leaf water potential, leaf abscission, and stand LAI.     

Regional Patterns in Carbon Cycling Across the Great Plains of North America

The large organic carbon (C) pools found in noncultivated grassland soils suggest that historically these ecosystems have had high rates of C sequestration. Changes in the soil C pool over time are a function of alterations in C input and output rates. Across the Great Plains and at individual sites through time, inputs of C (via aboveground production) are correlated with precipitation; however, regional trends in C outputs and the sensitivity of these C fluxes to annual variability in precipitation are less well known. To address the role of precipitation in controlling grassland C fluxes, and thereby soil C sequestration rates, we measured aboveground and belowground net primary production (ANPP-C and BNPP-C), soil respiration (SR-C), and litter decomposition rates for 2 years, a relatively dry year followed by a year of average precipitation, at five sites spanning a precipitation gradient in the Great Plains. ANPP-C, SR-C, and litter decomposition increased from shortgrass steppe (36, 454, and 24 g C m^–2 y^–1) to tallgrass prairie (180, 1221, and 208 g C m^–2 y^–1 for ANPP-C, SR-C, and litter decomposition, respectively). No significant regional trend in BNPP-C was found. Increasing precipitation between years increased rates of ANPP-C, BNPP-C, SR-C, and litter decomposition at most sites. However, regional patterns of the sensitivity of ANPP-C, BNPP-C, SR-C, and litter decomposition to between-year differences in precipitation varied. BNPP-C was more sensitive to between-year differences in precipitation than were the other C fluxes, and shortgrass steppe was more responsive than were mixed grass and tallgrass prairie.     

Variation of nitric oxide emission potential in plants: a possible link to leaf N content and net photosynthetic activity

Aims Ecological systems, especially soils, have been recently recognized as an important source of atmospheric nitric oxide (NO). However, the study on the contribution of plants to atmospheric NO budget is significantly lagged. The specific objectives of this study are to reveal the phylogenetic variation in NO emission potential existing in various plant species and find out the possible leaf traits affecting NO emission potential.Methods We measured NO emission potential, leaf N and C content, C:N ratio, specific leaf area, net photosynthetic rate (P n) and estimated photosynthetic N use efficiency (PNUE) of 88 plant species. Further investigation of the relationships between NO emission potential and leaf traits were performed by simple linear regression analysis and pair-wise correlation coefficients analysis.Important findings Major results are as follows: (1) NO emission from plant species exhibited large variations, ranging from 0 to 41.7 nmol m ?2 h^-1, and the species frequency distributions of NO emission potential could be fitted to a log-normal curve. (2) Among 88 species, NO emission potential was the highest in Podocarpus macrophyllus, but lowest in Zanthoxylum nitidum and Vernicia montana. (3) NO emission potential has strong correlation to leaf N content, P n and PNUE. The variations in NO emission potential among diverse plant species may be closely related to leaf N level and net photosynthetic ability.     

Can intensification reduce emission intensity of biofuel through optimized fertilizer use? Theory and the case of oil palm in Indonesia

Closing yield gaps through higher fertilizer use increases direct greenhouse gas emissions but shares the burden over a larger production volume. Net greenhouse gas (GHG) footprints per unit product under agricultural intensification vary depending on the context, scale and accounting method. Life cycle analysis of footprints includes attributable emissions due to (i) land conversion (‘fixed cost’); (ii) external inputs used (‘variable cost’); (iii) crop production (‘agronomic efficiency’); and (iv) postharvest transport and processing (‘proportional’ cost). The interplay between fixed and variable costs results in a nuanced opportunity for intermediate levels of intensification to minimize footprints. The fertilizer level that minimizes the footprint may differ from the economic optimum. The optimization problem can be solved algebraically for quadratic crop fertilizer response equations. We applied this theory to data of palm oil production and fertilizer use from 23 plantations across the Indonesian production range. The current EU threshold requiring at least 35% emission saving for biofuel use can never be achieved by palm oil if produced: (i) on peat soils, or (ii) on mineral soils where the C debt due to conversion is larger than 20 Mg C ha^?1, if the footprint is calculated using an emission ratio of N₂O–N/N fertilizer of 4%. At current fertilizer price levels in Indonesia, the economically optimized N fertilizer rate is 344–394 kg N ha^?1, while the reported mean N fertilizer rate is 141 kg N ha^?1 yr^?1 and rates of 74–277 kg N ha^?1 would minimize footprints, for a N₂O–N/N fertilizer ratio of 4–1%, respectively. At a C debt of 30 Mg C ha^?1, these values are 200–310 kg N ha^?1. Sustainable weighting of ecology and economics would require a higher fertilizer/yield price ratio, depending on C debt. Increasing production by higher fertilizer use from current 67% to 80% of attainable yields would not decrease footprints in current production conditions.     

The role of a cytosolic superoxide dismutase in barley–pathogen interactions

Reactive oxygen species (ROS), including superoxide ( / ) and hydrogen peroxide (H₂O₂), are differentially produced during resistance responses to biotrophic pathogens and during susceptible responses to necrotrophic and hemi‐biotrophic pathogens. Superoxide dismutase (SOD) is responsible for the catalysis of the dismutation of / to H₂O₂, regulating the redox status of plant cells. Increased SOD activity has been correlated previously with resistance in barley to the hemi‐biotrophic pathogen Pyrenophora teres f. teres (Ptt, the causal agent of the net form of net blotch disease), but the role of individual isoforms of SOD has not been studied. A cytosolic CuZnSOD, HvCSD1, was isolated from barley and characterized as being expressed in tissue from different developmental stages. HvCSD1 was up‐regulated during the interaction with Ptt and to a greater extent during the resistance response. Net blotch disease symptoms and fungal growth were not as pronounced in transgenic HvCSD1 knockdown lines in a susceptible background (cv. Golden Promise), when compared with wild‐type plants, suggesting that cytosolic / contributes to the signalling required to induce a defence response to Ptt. There was no effect of HvCSD1 knockdown on infection by the hemi‐biotrophic rice blast pathogen Magnaporthe oryzae or the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei, but HvCSD1 also played a role in the regulation of lesion development by methyl viologen. Together, these results suggest that HvCSD1 could be important in the maintenance of the cytosolic redox status and in the differential regulation of responses to pathogens with different lifestyles.     

毛乌素沙地海流兔河流域植被净初级生产力估算

采用CASA(Carnegie-Ames-Stanford Approach)模型,结合TERRA MODIS卫星数据和气象数据,对毛乌素沙地海流兔河流域2015年各月的植被净初级生产力(NPP)进行估算,并对植被NPP月平均值的时空分布规律及其与气象因子和地下水位埋深的关系进行了分析.结果表明:毛乌素沙地海流兔河流域2015年植被NPP总量为2.88×1011 g,生长季(4月份至10月份)的植被NPP总量达2.81×1011 g,占全年植被NPP总量的97.57%.随着时间推移,植被NPP月平均值和归一化差分植被指数(NDVI)月平均值呈"缓慢增加—急剧增加—急剧下降"的变化趋势.植被NPP月平均值季节变化明显,春季、夏季、秋季和冬季植被NPP月平均值之和分别为20.55、69.39、20.46和0.48 g·m-2.从空间分布上看,中部河谷和滩地的植被NPP月平均值总体上高于东南部、西部和西北部等沙丘荒漠区.月平均气温对植被NPP月平均值变化的影响最大,其次为平均实际日蒸散发量和地表月太阳辐射.植被NPP月平均值随着地下水位埋深的增加而减小,最大值出现在地下水位埋深1~2 m之间.上述研究结果显示:采用CASA模型可以较好地估算毛乌素沙地海流兔河流域植被NPP值,月平均气温和地下水位埋深对该流域植被NPP值的影响较大.     

CO2 exchange and evapotranspiration across dryland ecosystems of southwestern North America

Global‐scale studies suggest that dryland ecosystems dominate an increasing trend in the magnitude and interannual variability of the land CO₂ sink. However, such analyses are poorly constrained by measured CO₂ exchange in drylands. Here we address this observation gap with eddy covariance data from 25 sites in the water‐limited Southwest region of North America with observed ranges in annual precipitation of 100–1000 mm, annual temperatures of 2–25°C, and records of 3–10 years (150 site‐years in total). Annual fluxes were integrated using site‐specific ecohydrologic years to group precipitation with resulting ecosystem exchanges. We found a wide range of carbon sink/source function, with mean annual net ecosystem production (NEP) varying from ‐350 to +330 gCm^?2 across sites with diverse vegetation types, contrasting with the more constant sink typically measured in mesic ecosystems. In this region, only forest‐dominated sites were consistent carbon sinks. Interannual variability of NEP, gross ecosystem production (GEP), and ecosystem respiration (R_eco) was larger than for mesic regions, and half the sites switched between functioning as C sinks/C sources in wet/dry years. The sites demonstrated coherent responses of GEP and NEP to anomalies in annual evapotranspiration (ET), used here as a proxy for annually available water after hydrologic losses. Notably, GEP and R_eco were negatively related to temperature, both interannually within site and spatially across sites, in contrast to positive temperature effects commonly reported for mesic ecosystems. Models based on MODIS satellite observations matched the cross‐site spatial pattern in mean annual GEP but consistently underestimated mean annual ET by ~50%. Importantly, the MODIS‐based models captured only 20–30% of interannual variation magnitude. These results suggest the contribution of this dryland region to variability of regional to global CO₂ exchange may be up to 3–5 times larger than current estimates.