青藏高原隆起和高山栎组（壳斗科）分布的关系

高山栎是一类硬叶常绿的栎属植物,集中分布在横断山区,是这一地区群落中的优势种和建群种。高山栎组分布区的形成和喜马拉雅的隆起、横断山的形成有密切关系。讨论了这种相互关系。先清理了高山栎分类学的混乱,将原来的12种高山栎归并为 9种,在此基础上讨论了高山栎的现代分布和系统位置,总结分析了高山栎的地史资料。认为高山栎组植物出现的时间不晚于中新世,在中新世高山栎组已经有了较广的分布,分布在当时的亚热带常绿阔叶林。在上新世,随着喜马拉雅山脉的隆起和抬升,横断山地区的环境发生了巨大的变化,环境变冷、变旱。高山栎组植物由于自身所具有的抗寒耐旱的形态特征,如被较厚的毛被,单位面积气孔数目较少等使之在生物竞争中成为优胜者,而逐步成为横断山地区群落中的优势种和建群种。并以已有地质学、古植物学、形态解剖学、植物地理学和分子生物学的证据论证了这一假说。     

What limits evaporation from Mediterranean oak woodlands – The supply of moisture in the soil,physiological control by plants or the demand by the atmosphere?

The prediction of evaporation from Mediterranean woodland ecosystems is complicated by an array of climate, soil and plant factors. To provide a mechanistic and process-oriented understanding, we evaluate theoretical and experimental information on water loss of Mediterranean oaks at three scales, the leaf, tree and woodland. We use this knowledge to address: what limits evaporation from Mediterranean oak woodlands – the supply of moisture in the soil, physiological control by plants or the demand by the atmosphere?     

Summer precipitation for the England and Wales region, 1201–2000 ce,from stable oxygen isotopes in oak tree rings

Oxygen isotope ratios from oak tree rings are used to extend the May–August precipitation totals of the England and Wales precipitation series back to 1201 ce. The agreement between instrumental and reconstructed values is unusually strong, with more than half of the variance explained and standard verification tests passed. The stability of this relationship is confirmed using split-period calibration and verification. This allows the reconstruction to be variance-scaled to the full length of the instrumental series back to 1766. Direct comparison with historical reports of very wet and dry summers show good agreement. Near-constant replication, with a minimum of 10 timbers sourced from historic buildings across central southern England ensures signal strength does not change over time. Summers during the late 20th century appear anomalously dry and those of the 21st century very close to the pre-20th century average with no evidence in the record of prolonged ‘megadroughts’ across England and Wales.     

Tree ring dating using oxygen isotopes: a master chronology for central England

Ring‐width dendrochronology, based on matching patterns of ring width variability, works best when trees are growing under significant environmental (climatic) stress. In the UK, and elsewhere in the temperate mid‐latitudes, trees generally experience low stress, so dating is more difficult and often fails. Oxygen isotopes in tree rings passively record changes in the isotopic ratios of summer precipitation, so they carry a strong common signal, which offers potential for cross‐dating. A master chronology covering the period 1200–2000 ce was constructed using the oxygen isotope ratios of the latewood cellulose of oak samples from central England. Two independent chronologies, developed to verify the isotopic signal, were combined (n = 10 trees) and the method was evaluated by dating timbers of known age and historical timbers that could not be dated by ring‐width dendrochronology, from both within and beyond the central England region. The agreement between samples and the master chronology is exceptionally strong, allowing the dating of timbers with far fewer rings than is normally the case for ring‐width dendrochronology. Tree‐ring oxygen isotope values are more suited to correlation analysis than tree‐ring widths, so it is possible to provide t‐values that conform to Student's t‐distribution and can be converted into probabilities of error. A protocol for assigning dates using ‘stable‐isotope dendrochronology’ is proposed, which has the potential to revolutionize the dating of wooden structures and artefacts, allowing the dating of short and invariant ring sequences from young, fast‐grown trees. Such samples are commonplace throughout the historical building and archaeological records and were, until now, considered impossible to date. © 2019 The Authors. Journal of Quaternary Science Published by John Wiley & Sons Ltd     

TDR measurement of stem and soil water content in two Mediterranean oak species

Abstract

Since 1990s, time domain reflectometry (TDR) has been applied to estimate the stem water content of living trees. Here, new calibration equations relating the apparent dielectric constant (K_a ) to the volumetric water content (θ) were developed for two Mediterranean oak species. Our calibration equations differ from those previously calculated for other species, suggesting that stem water contents could be monitored more accurately using species-specific curves. The stem water content in the trees of these species and the surrounding soil were monitored with TDR to examine the feasibility of this technology for recording changes in trunk water storage. The average stem water contents of the oaks reflect the soil water contents, and the temporal differences observed (17%) point to the importance of trunk water for coping with soil water deficit. Although it would be very useful to obtain a single function to estimate the stem water content of trees, it remains necessary to obtain the results in more species.     

Estimation of tree water stress from stem and soil water monitoring with time‐domain reflectometry in two small forested basins in Spain

Soil‐tree water relationships were studied using time domain reflectometry (TDR) in two small forested basins in Spain. The stem water content of two Mediterranean Quercus species (Quercus pyrenaica and Quercus rotundifolia) was measured using previously constructed species‐specific equations. To monitor soil moisture, a TDR station network was used in both cases. Sixteen Q. pyrenaica and six Q. rotundifolia individuals were selected to install two TDR probes in their trunks (at 20 and 120 cm above the ground) to monitor stem water content. Stem and soil water contents were measured fortnightly. The stem water content of both species showed a similar temporal trend for the period studied. A spring maximum (0·654 cm³ cm^?3 for Q. pyrenaica and 0·568 cm³ cm^?3 for Q. rotundifolia) was found to be associated with high transpiration and no soil moisture deficit, and a late‐summer minimum (0·520 cm³ cm^?3 for Q. pyrenaica and 0·426 cm³ cm^?3 for Q. rotundifolia) was associated with the end of the dry season. This drop in stem water content occurs when the available water in the soil decreases. This seasonal difference presumably reflects water withdrawn from stem storage to support the transpirational demands of the tree. Since plant water stress results in reduced stem water content and since this drop can be measured by TDR, it may be concluded that this technology offers a suitable tool for detecting plant water stress. Copyright © 2007 John Wiley & Sons, Ltd.     

Oxygen and hydrogen isotope ratios in tree rings: how well do models predict observed values?

We have measured annual oxygen and hydrogen isotope ratios in the α-cellulose of the latewood of oak (Quercus robur L.) growing on well-drained ground in Norfolk, UK. We compare the observed values of isotope ratios with those calculated using equations that allow for isotopic fractionation during the transfer of oxygen and hydrogen from source water taken by the tree to cellulose laid down in the cambium. The equations constitute a model in which isotopic fractionation occurs during evaporative enrichment within the leaf and during isotopic change between carbohydrates and water in the trunk during cellulose synthesis. From the relationship between isotope ratios in precipitation and α-cellulose, we deduce that the source water used by the tree comprises a constant mixture of groundwater and precipitation, chiefly from the months of May, June and July of the growth year. By selection of isotopic fractionation factors and the degree of isotope exchange within the trunk, we are able to model the observed annual values of oxygen isotope ratios of α-cellulose to a significant level (r=0.77, P<0.01). When we apply the same model to hydrogen isotope ratios, however, we find that, although we can predict the average value over the time series, we can no longer predict the year-to-year variation. We suggest that this loss of environmental signal in the hydrogen isotopes is caused by differences in the kinetic isotope effects of the biochemical reactions involved in the fixation of hydrogen in different positions of the glucose molecule. Owing to these effects, the hydrogen isotope ratios of cellulose can vary in a way not anticipated in current models and hence may induce non-climatic ‘noise’ in the hydrogen isotope time series.     

GEOMORPHOLOGICAL AND DENDROCHRONOLOGICAL ANALYSES OF A COMPLEX LANDSLIDE IN THE SOUTHERN APENNINES

Complex landslides, capable of reactivation, are typical slope movements in high relief areas. Due to their distribution, size and kinematics, these landforms represent a major hazard, posing a high risk to populations, settlements and infrastructures. This paper integrates geomorphological analyses, instrumental measurements and dendrochronological approaches in assessing a large, reactivated landslide system on the southern piedmont of Monte Sirino (southern Italy). The landslide system is associated with weak geological structures, earthquake activity, and rapid recent incision of the mid-Pleistocene Noce lake deposits. Potential reactivation triggers include a higher regional annual rainfall, one of the highest in southern Italy, and more frequent heavy snowfalls in recent decades. Reactivation of the Sirino landslide system has important implications for the motorway connecting Salerno and Reggio Calabria, which crosses it. The results of our study show that the slide is reactivated with an almost decadal frequency and that major reactivations are correlated to prolonged snowfall, which occurs with increasing frequency in the southern Apennines. The last observation suggests the need for similar studies on the behaviour of other landslide systems in the southern Apennines, performing integrated approaches such as geotechnical and dendrogeomorphological analysis.     

岩溶区青冈栎树干液流特征及其与环境因子关系

为阐明岩溶区植物液流变化特征,应用Granier 热消散探针法,连续测定了桂林岩溶区植物青冈栎1年整的树干液流密度变化,并用ICT-2000TE环境气象站同步监测林地环境因子。结果表明: 不同季节青冈栎树干液流密度差异较大,液流密度峰值分别为: 1月份36. 17g H2O /m2· s, 4月份62. 01g H2O /m2· s, 7月份63. 41g H2O /m2· s, 10月份39. 03gH2O /m2· s。土壤含水量在1月份、4月份、7月份与树干液流呈极显著相关;空气湿度和水汽压亏缺在1月份、10月份与树干液流呈极显著相关;空气温度在4月份、10月份与树干液流呈极显著相关,而光合有效辐射在所有的观测月份均与树干液流呈极显著相关。对各时期树干液流密度与环境因子的多元线性回归方程进行F 值检验,均达到极显著水平。