A method to determine warm and cool steppe biomes from pollen data; application to the Mediterranean and Kazakhstan regions

An objective method for the assignment of pollen spectra to appropriate biomes has been published recently. The aim of this paper is to improve the distinction between warm and cool steppes, thus refining vegetation and climate reconstruction, particularly during the Last Glacial Maximum. A set of modern pollen spectra from the Mediterranean and Kazakhstan regions, dominated today by open vegetation types, has been analysed statistically in order to relate pollen taxa abundances to warm and cool grass/shrub plant functional types (PFTs). A statistical test using modern pollen data shows that the method is able to distinguish between cool and warm steppe biomes with a high degree of confidence. The method has been applied to two fossil pollen records. The results of this exercise showed that cool steppe dominated in central Greece between 18 000 and 13 000 yr BP, while in western Iran the vegetation was at the boundary between cool and warm steppes. These vegetation types were replaced by warm mixed forest in Greece and warm steppe in Iran after that time span. © 1998 John Wiley & Sons, Ltd.     

喀斯特山区石漠化耕地遥感精准提取与分析——以贵州省北盘江镇与花江镇为例

选取北盘江镇与花江镇作为研究区,利用谷歌高精度遥感影像,结合分区分层分级思想,基于深度学习与传统约束方法对研究区耕地进行精准提取。结果表明：1）在数量精度上,以视觉形态差异对研究区进行分区并选取不同模型训练获得的精细地块,面积约为9 867 hm ²,与实际数量基本一致,F-Measure主要分布在[0.82, 0.98]之间,受到地形和岩石裸露率的影响,石漠化严重地区耕地提取精度较低。2）在形态精度上,预测耕地与实际耕地的GIOU主要分布在[0.7, 1]之间,分割正确率>0.85,表明预测耕地边界与实际地块边界吻合度高,提取结果符合研究区实际情况。3）利用地貌坡度等约束条件对耕地进行划分发现,研究区以喀斯特耕地为主,占比74%,并且石漠化程度较严重,其中轻度石漠化耕地与中度石漠化耕地占总耕地的32%。在石漠化地区,耕地的狭长程度、破碎程度受人类影响较大;距离居民地越近、可达性越高的地区,其土地利用率越高,斑块越破碎。文章所提出方法可用于耕地破碎、地形复杂地区的耕地提取,能够为地区的发展、耕地治理与研究、环境保护与决策等提供精准数据支持。     

呼伦贝尔草原沙漠化土地的综合整治区划

呼伦贝尔草原的沙漠化土地现在多处于由级向级发展的阶段,有很大的发展潜力和危险性,必须对其进行综合整治。其主要成因是过度放牧,过度放牧不仅直接使砂质草原发生沙漠化,而且还严重影响天然樟子松的天然更新和扩展,因此必须对放牧进行科学的规划和管理。作者遵循地带性和非地带性相结合原则、发生学原则、主导性原则、与已有的相关区划相统一原则,把呼伦贝尔草原划分为2个综合整治亚区和5个综合整治小区,并提出了相应的综合整治对策。     

Influencing factors and partitioning of respiration in a Leymus chinensis steppe in Xilin River Basin, Inner Mongolia, China

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. Bivariable 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.     

锡林郭勒草原不同生态地理区气候干燥度周期变化的小波分析

以锡林郭勒盟及其周边地区17个气象站点1960年以来的月气温和降水量数据为基础,利用气象学与生态学知识,按照荒漠草原区、典型草原区、草甸草原区和农牧交错区4个生态地理区对锡林郭勒盟及其周边地区的气象站点进行分区归并,统计各区的De Martonne干燥度指数,在Matlab等软件的支持下,用小波分析的方法,研究各区年平均气温的周期性变化规律及各区之间的异同点。研究结果表明,在25-32a时间尺度上,随着时间的推移,锡林郭勒盟4个生态地理区年干燥度均呈现明显的周期变化,形成正负相间的震荡中心;草甸草原区和农牧交错区表现出干燥程度增加的趋势。总体上推测,2007年以后的15a左右时间,锡林郭勒盟的4个生态地理区的气候均将呈现比较干燥的状态。     

草甸草原降水特征与土壤水分对降水脉动响应——以呼伦贝尔草原额尔古纳市为例

利用额尔古纳牧业气象试验站降水量与土壤水分数据,通过降水与土壤水分动态变化及转化过程分析,确定土壤水分响应的降水临界值与不同降水级别引起土壤水分响应的概率,构建了降水过程量与土壤水分增量函数关系。结果表明：（1） 研究区降水量呈“先降后升”变化趋势,年内降水量呈单峰型分布。（2） 研究区以无降水天气为主,降水又以小降水事件占主导,大降水事件发生频次低、过程降水量大,小降水事件则相反。（3） 可以引起研究区0~50 cm各层土壤水分响应的降水临界值分别为8.1 mm、10.1 mm、19.0 mm、27.9 mm和31.6 mm,小雨仅能引起0~10 cm土壤水分响应的概率为28.6%,中雨不能引起40~50 cm土壤水分的响应。（4） 降水量与0~10 cm和10~20 cm土壤水分达到最大值时的滞后时间呈现出极显著负相关关系,与20~30 cm呈显著负相关关系,0~30 cm各层土壤水分达到最大值时的滞后时间与降水量符合幂函数关系。（5） 降水量和0~50 cm土壤水分增量均呈现出极显著正相关关系,降水量与0~10 cm和10~20 cm土壤水分增量符合线性关系,与20~30 cm、30~40 cm和40~50 cm土壤水分增量符合多项式关系。检验结果表明,构建的函数模型可以较好地模拟研究区0~30 cm各层水分增量。研究结果为地方政府抗旱减灾提供了科学依据。     

A SYNOPTIC CLIMATOLOGY OF MOISTURE STRESS GRADIENTS IN THE WESTERN GREAT LAKES REGION

Surface pressure distributions and 500 millibar flow patterns are identified for summer season days with a moisture stress gradient across the western Great Lakes region that may be related to the location of the prairie-forest ecotone. Results indicate that days with moisture stress gradients are a characteristic feature of the summer climate in the area and that these days are associated with unique, nonrandom groupings of mid-tropospheric flow patterns. SW-NE and W-E stress gradients, those that appear to contribute most to defining and reinforcing the location of the ecotone, occur most frequently, and the modal 500-mb flow direction for these transect groups, an azimuth of 290°, is the same as the mode for the entire population of summer season flow directions. We conclude that the modal mid-tropospheric circulation of the summer season plays a significant role in determining the surface weather conditions that regulate the most common moisture stress patterns within the western Great Lakes region.     

Topography-Controlled Soil Water Content and the Coexistence of Forest and Steppe in Northern China

The semi-arid forest-steppe ecotone in China is characterized by a patchy pattern of forest and steppe, with forest patches restricted to shady slopes. To address the effect of topography on forest distribution through regulation of available water, we calculated evaporation as a function of slope aspect and inclination. Field vegetation records from randomly selected sites with minimum slope inclination were used to test the simulated forest distribution. Seasonal and diurnal changes of surface soil temperature and moisture of shady and sunny slopes were recorded. Soil water content was measured during two growing seasons on both sunny and shady slopes with the same forest type at three sites located along the mean annual precipitation (MAP) gradient. Evaporation decreases with slope inclination on shady slopes, but increases with inclination on sunny slopes. The shady slope received 35% of the annual direct solar radiation received by the sunny slope when the slope inclination was 25°, and the contrast in annual direct solar radiation between the shady and sunny slopes further widens as slope inclination increases. Steeper shady slopes can support forests in dryer climates, with log-linear regression revealing a minimum slope inclination for forest distribution along the MAP gradient. The simulated minimum slope inclination for forest growth was larger than the observed minimum inclination, and the difference was greater in wetter conditions. A larger forest area fraction was considered to lead to a reduction in soil temperature and evaporation, as verified by soil temperature and moisture records and soil water content measurements. The slope-specific forest distribution in the semi-arid region of China can be explained by a topography-controlled soil water supply. Lower evaporation, resulting from lower direct solar radiation on shady slopes, allows shady slopes to retain a water supply sufficient for sustaining forests, and the existence of forests on shady slopes further reduces evaporation. Different tree species coexist at the xeric timberline due to regulation by slope inclination and aspect.     

Aboveground total and green biomass of dryland shrub derived from terrestrial laser scanning

Sagebrush (Artemisia tridentata), a dominant shrub species in the sagebrush-steppe ecosystem of the western US, is declining from its historical distribution due to feedbacks between climate and land use change, fire, and invasive species. Quantifying aboveground biomass of sagebrush is important for assessing carbon storage and monitoring the presence and distribution of this rapidly changing dryland ecosystem. Models of shrub canopy volume, derived from terrestrial laser scanning (TLS) point clouds, were used to accurately estimate aboveground sagebrush biomass. Ninety-one sagebrush plants were scanned and sampled across three study sites in the Great Basin, USA. Half of the plants were scanned and destructively sampled in the spring (n = 46), while the other half were scanned again in the fall before destructive sampling (n = 45). The latter set of sagebrush plants was scanned during both spring and fall to further test the ability of the TLS to quantify seasonal changes in green biomass. Sagebrush biomass was estimated using both a voxel and a 3-D convex hull approach applied to TLS point cloud data. The 3-D convex hull model estimated total and green biomass more accurately (R² = 0.92 and R² = 0.83, respectively) than the voxel-based method (R² = 0.86 and R² = 0.73, respectively). Seasonal differences in TLS-predicted green biomass were detected at two of the sites (p < 0.001 and p = 0.029), elucidating the amount of ephemeral leaf loss in the face of summer drought. The methods presented herein are directly transferable to other dryland shrubs, and implementation of the convex hull model with similar sagebrush species is straightforward.     

Impacts of reduced wind speed on physiology and ecosystem carbon flux of a semi-arid steppe ecosystem

Decreasing wind speed is one aspect of global climate change as well as global warming, and has become a new research orientation in recent decades. The decrease is especially evident in places with frequent perennially high wind speeds. We simulated decreased wind speed by using a steel-sheet wind shield in a temperate grassland in Inner Mongolia to examine the changes in physical environmental variables, as well as their impacts on the photosynthesis of grass leaves and net ecosystem exchange （NEE）. We then used models to calculate the variation of boundary layer conductance （BLC） and its impact on leaf photosynthesis, and this allowed us to separate the direct effects of wind speed reduction on leaf photo- synthesis （BLC） from the indirect ones （via soil moisture balance）. The results showed that reduced wind speed primarily resulted in higher moisture and temperature in soil, and indirectly affected net assimilation and water use efficiency of the prevalent bunch grass Stipa krylovii. Moreover, the wind-sheltered plant community had a stronger ability to sequester carbon than did the wind-exposed community during the growing season.