Modelling the spectral response of the desert tree Prosopis tamarugo to water stress     

《International Journal of Applied Earth Observation and Geoinformation》2013

In this paper, we carried out a laboratory experiment to study changes in canopy reflectance of Tamarugo plants under controlled water stress. Tamarugo (Prosopis tamarugo Phil.) is an endemic and endangered tree species adapted to the hyper-arid conditions of the Atacama Desert, Northern Chile. Observed variation in reflectance during the day (due to leaf movements) as well as changes over the experimental period (due to water stress) were successfully modelled by using the Soil-Leaf-Canopy (SLC) radiative transfer model. Empirical canopy reflectance changes were mostly explained by the parameters leaf area index (LAI), leaf inclination distribution function (LIDF) and equivalent water thickness (EWT) as shown by the SLC simulations. Diurnal leaf movements observed in Tamarugo plants (as adaptation to decrease direct solar irradiation at the hottest time of the day) had an important effect on canopy reflectance and were explained by the LIDF parameter. The results suggest that remote sensing based assessment of this desert tree should consider LAI and canopy water content (CWC) as water stress indicators. Consequently, we tested fifteen different vegetation indices and spectral absorption features proposed in literature for detecting changes of LAI and CWC, considering the effect of LIDF variations. A sensitivity analysis was carried out using SLC simulations with a broad range of LAI, LIDF and EWT values. The Water Index was the most sensitive remote sensing feature for estimating CWC for values less than 0.036 g/cm², while the area under the curve for the spectral range 910–1070 nm was most sensitive for values higher than 0.036 g/cm². The red-edge chlorophyll index (CI_red-edge) performed the best for estimating LAI. Diurnal leaf movements had an effect on all remote sensing features tested, particularly on those for detecting changes in CWC.  相似文献   

The importance of being reliable – Local ecological knowledge and management of forage plants in a dryland pastoral system (Morocco)     

《Journal of Arid Environments》2013

In dryland rangelands with their high environmental variability, local ecological knowledge of forage plants is essential for management decisions. Ecological apparency hypothesis (EAH) predicts plants' availability and visibility to be important criteria for local valuation. However, EAH has mainly been tested in low-variability systems. We ask whether EAH is valid for forage plants in drylands; which other local criteria exist; and how criteria are connected to management decisions.In a Moroccan pastoral system, we applied a novel ethnobotanical method by calculating the Cognitive Salience Index (CSI) for plants' valuation (CSI_antro) and availability (CSI_eco). To evaluate explicit criteria, we correlated palatability and nutritive value to CSI_anthro. ANCOVAs related CSI_anthro to EAH criteria (CSI_eco and lifetime) and to plant occurrence on pasture types. We found EAH criteria to better predict CSI_antro than explicit criteria. Apparent plants from semi-arid pastures were more valued than those from arid pastures (HSD; p < 0.05). We introduce the criterion of reliability into EAH to explain this, and demonstrate how pastoralists adjust management decisions to resource reliability. Linking resource valuation to management decisions can thus improve our understanding of resilience mechanisms. Our study also confirms the validity of EAH for forage species and dryland environments.  相似文献   

Floods,drought, and seed mass of riparian plant species     

《Journal of Arid Environments》2013

Plant communities in dryland riparian ecosystems are influenced by flood disturbance and water availability. As global climate change alters stream flow regimes, there is increasing need to understand which traits allow plant species to persist under increased stress and disturbance. Small seed mass is part of a strategy that adapts for disturbance, but the role of seed mass as an adaptation for drought is less well documented. For dryland riparian plant communities, we asked, does seed mass vary with water availability and flood frequency? We compared community seed mass between sites that vary in flow permanence (longitudinal water gradient) and between hydrogeomorphic surfaces within sites (lateral gradients of moisture and disturbance). Using data from four rivers in Arizona, we contrasted seed mass between plant groups. We found community seed mass to be greater at sites with ephemeral than perennial flow, and to increase laterally from wet, frequently-flooded channel edges to dry, less disturbed terraces. Seed mass varied by moisture group (smallest for hydroriparian species) and by disturbance group (smallest for disturbance species), and showed a trend for being greater in introduced species. We conclude that small seed mass is independently associated with wet and disturbed conditions in dryland riparian ecosystems.  相似文献   

Long-term dynamics of a semiarid grass steppe under stochastic climate and different grazing regimes: A simulation analysis     

J.M. Paruelo  S. Pütz  G. Weber  M. Bertiller  R.A. Golluscio  M.R. Aguiar  T. Wiegand 《Journal of Arid Environments》2008,72(12):2211-2231

We built a grid-based spatial explicit stochastic model that simulates grazing events and basic processes like seedling establishment, growth or mortality of the dominant species in the grass steppes of Patagonia. After evaluating the model with field data, we performed simulation experiments aimed to explore the interaction of precipitation and grazing regimes on vegetation dynamics. Grazing generated a reduction in tussock density which results in a decline in aboveground net primary production (ANPP). Both response variables presented a non-linear behavior including high temporal variability and delay effects, which may prolong for decades. There was a clear threshold in the response of the variables to stock density, though changes become evident only when a highly selective grazing scenario was used. Under high stock density conditions, precipitation use efficiency (PUE) was 82% lower than the values for non-grazed runs. The inter-annual variability of precipitation was more important than the grazing regime in explaining differences in tussock density. Simulation results highlight important issues regarding rangeland management: grazing regime might be as important as stocking density as a degradation agent, temporal lags might obscure degradation processes for decades, the definition of monitoring variables need to consider their response time constants.  相似文献   

干旱区土壤种子库的研究进展   总被引：24，自引：0，他引：24  

李秋艳  赵文智 《地球科学进展》2005,20(3):350-358

土壤种子库在连接过去、现在和将来的植物种群和群落的结构和动态中起着重要的生态和进化作用,对保护和恢复干旱区植被起着重要的作用。从土壤种子库的研究方法、数量特征、分布格局、时空动态、与地上植被关系、年龄结构及影响因素等方面论述了干旱区土壤种子库研究的若干进展,展望了干旱区土壤种子库的研究趋势。  相似文献   

Origin and characteristics of yardangs in the Um Al-Rimam depressions (N Kuwait)   总被引：3，自引：0，他引：3  

Ali. M. Al-Dousari  Maraheb Al-Elaj  Eqbal Al-Enezi  Abdulaziz Al-Shareeda 《Geomorphology》2009,104(3-4):93-104

Although Arabia hosts one of the largest deserts in the world, studies regarding aeolian erosional features in general and yardangs in particular are rare. The principal aim of this study is to delineate and investigate the various forms of yardangs in the Um Al-Rimam depressions in the northern part of Kuwait. The study area consists of two main depressions connected by neck area made up of continental sediments. Both depressions are surrounded by a dissected escarpment and their floors host muddy playas. On lithological bases, three main types of yardang have been identified: 7 yardangs are sculptured in sandstones of Lower Fars Formation (Lower to Middle Miocene), 23 are carved in calcretic rocks of Ghar Formation (Oligocene to Lower Miocene), and 11 yardangs are developed in Quaternary sediments. Their mean orientation NW (297°) coincides with the prevailing direction of the strong local northwesterly wind called the shamal. The average length/width ratio is 1.5:1. The wide geographic distribution of yardangs around the margins of the northern depression is due to sand blasting. This is explained by the annual sand accumulation measured in the study area where the northern depression shows 143 times higher accumulation than the southern depression, the neck area and the platform above the depressions. Also, the field measurements for annual erosional rates show that the sides of the yardangs are more affected by erosion compared to their middle parts. Remarkably slight mineralogical variations were detected within the surface sediments of Um Al-Rimam depressions and the Quaternary muddy yardangs which indicate a common origin. The age of yardangs is unknown but annual erosional rates on a small Quaternary muddy yardang suggest that they range in age between 44 and 1500 years depending on yardang size. It is believed, as indicated from field observation and interpretation of aerial photos, that the majority of yardangs are pre-existing bedrock forms. These forms were developed by paleo-surface hydrological processes which were modified by wind action in latter phase.  相似文献   

High-frequency rock temperature data from hyper-arid desert environments in the Atacama and the Antarctic Dry Valleys and implications for rock weathering   总被引：1，自引：0，他引：1  

Christopher P. McKay  Jamie L. Molaro  Margarita M. Marinova 《Geomorphology》2009,110(3-4):182-187

In desert environments with low water and salt contents, rapid thermal variations may be an important source of rock weathering. We have obtained temperature measurements of the surface of rocks in hyper-arid hot and cold desert environments at a rate of 1/s over several days. The values of temperature change over 1-second intervals were similar in hot and cold deserts despite a 30 °C difference in absolute rock surface temperature. The average percentage of the time dT/dt > 2 °C/min was ~ 8 ± 3%, > 4 °C/min was 1 ± 0.9%, and > 8 °C/min was 0.02 ± 0.03%. The maximum change over a 1-second interval was ~ 10 °C/min. When sampled to simulate data taken over intervals longer than 1 s, we found a reduction in time spent above the 2 °C/min temperature gradient threshold. For 1-minute samples, the time spent above any given threshold was about two orders of magnitude lower than the corresponding value for 1-second sampling. We suggest that a rough measure of efficacy of weathering as a function of frequency is the product of the percentage of time spent above a given threshold value multiplied by the damping depth for the corresponding frequency. This product has a broad maximum for periods between 3 and 10 s.  相似文献   

亚洲中部干旱区气候变化与丝路文明变迁研究:进展与问题     

陈发虎  董广辉  陈建徽  郜永祺  黄伟  王涛  陈圣乾  侯居峙 《地球科学进展》2019,(6):561-572

亚洲中部干旱区是对全球气候变化响应最为敏感的地区之一,也是水文变化剧烈和生态环境脆弱的地区。该地区包括了古丝绸之路的主体,在东西方交流和丝路文明兴衰历史中发挥了关键的作用。科学评估全球增温背景下亚洲中部干旱区人类社会可持续发展面临的风险,是广受关注的重大科学问题。东西方交流和丝路文明发展历史及其与气候环境变化关系的研究,可为认识该地区不同时间尺度人地关系演变的规律提供科学依据。通过总结东西方交流与丝路文明兴衰历史、亚洲中部干旱区全新世气候变化过程、多时间尺度气候一水文变化机制以及人与环境相互作用的过程与规律等领域的研究进展,提出目前亚洲中部干旱区全新世气候环境变化时空格局和丝路文明演化的过程,以及人与环境相互作用机制的研究存在明显不足。破解亚洲中部干旱区气候变化和文化演化研究区域不均衡问题,加强地学与考古学等多学科交叉研究,是推进气候变化与丝路文明变迁研究的有效途径。这对理解该地区人地关系演化规律、应对气候变化带来的挑战、服务国家“一带一路”倡议具有重要的科学价值和现实意义。  相似文献   

Late Permian to Middle Triassic correlations and palaeogeographical reconstructions in south-western European basins: New sedimentological data from Minorca (Balearic Islands, Spain)     

Bastien Linol  Antoine Bercovici  Sylvie Bourquin  Jos Bienvenido Diez  Jos Lpez-Gmez  Jean Broutin  Marc Durand  Uxue Villanueva-Amadoz 《Sedimentary Geology》2009,220(1-2):77-94

  相似文献   

基于敦煌戈壁站观测对几套再分析产品夏季地表感热通量的评估     

王典  周德刚  黄荣辉  范广洲 《气候与环境研究》2017,22(5):601-612

西北干旱区是欧亚大陆夏季感热输送的高值区,此区域感热输送对东亚季风气候系统的变异有重要影响。然而,再分析资料的感热通量在此区域存在很大的不确定性,影响该地区感热变化对东亚区域气候影响的认识。本文基于敦煌戈壁站2001~2014年夏季的观测数据,评估了NCEP/NCAR、NCEP/DOE、ERA-Interim和JRA-55这4套再分析产品的感热通量。敦煌戈壁站的感热通量是根据敦煌戈壁站常规观测数据和Y08方案计算得到,代表敦煌戈壁站的实际感热。结果显示,戈壁站夏季感热多年平均约为85.7 W m-2,但受局地降水的影响存在较大的波动;再分析资料的感热通量之间存在很大的不确定性,与观测相比,ERA-Interim的感热通量在大小和变化上好于其他再分析资料,在没有局地性降水的影响时比较接近观测。进一步分析了再分析感热与观测差异的原因。研究表明,再分析资料中的地表风速和地气温差与粗糙度设置和热力参数化方案相关联。各再分析资料均不同程度地低估了敦煌戈壁站的地气温差(观测值约6.5°C),这主要是由于再分析系统中对戈壁下垫面的粗糙度设置偏高以及热力参数化方案不太适用于戈壁下垫面造成的。相对而言,ERA-Interim的参数化方案在戈壁下垫面优于其他再分析产品的参数化方案,使得ERA-Interim的地气温差相对其他再分析资料更接近实际观测,感热通量较为合理。  相似文献