Spatial patterns of lost and remaining trees in the Iberian wooded rangelands

Spanish dehesas and Portuguese montados are similar Iberian silvopastoral systems that provide important environmental services and hold a high biodiversity. However, there is a raising concern for the sustainability of these cultural landscapes, as they suffer from tree recruitment failure. Consequently, trees in those systems are lessening due to tree ageing, diseases and other causes. A key to developing successful management strategies in dehesas is to understand the spatiotemporal behaviour of this process. We analysed the spatial pattern of lost and remaining trees in five dehesas in Extremadura (SW Spain) between 1956 and 2009 based on aerial images. We studied the possible spatial dependency (clustering or segregation) of the lost and remaining trees in areas with low or high land use intensity at different spatial distances. Analyses were performed under the random labelling hypothesis and the critical values were obtained by using 199 permutations of a Monte Carlo test. A linear mixed model was used to investigate the possible factors affecting tree loss. The results indicate a clear segregation between remaining and lost trees in areas of high intensity land use, while in areas with low intensity this pattern was less evident. Also, lost trees were more clustered than the remaining ones. Higher tree loss density occurred at lower slopes, the areas initially occupied by high or medium-density shrubs being less prone to undergo tree loss than those that appeared in 1956 like ploughed lands, grasslands or low-density shrubs. These results show a concentration of the risk of tree loss in certain areas where conservation efforts should be focused.     

滇西北亚高山针叶林、硬叶栎类林的树种多样性

在滇西北香格里拉地区,以长苞冷杉(Abiesgeorgei)林、丽江云杉(Picealikiangensis)林作为亚高山针叶林的代表,以川滇高山栎(Quercusaquifolioides)林作为硬叶栎类林的代表,分别选择了6块样地,采用Shannon-Wiener多样性指数(H)、Simpson多样性指数的倒数(D')、Pielou均匀度指数(JH、JD)分别测定其树种多样性,并进行了比较研究。结果表明,三个森林类型的Shannon Wiener指数、Simpson指数的倒数、物种丰富度的顺序均为:长苞冷杉林>川滇高山栎林>丽江云杉林;而前两类森林的Pielou均匀度指数差异不大,但都高于丽江云杉林的均匀度指数值。滇西北的川滇高山栎具有很强的萌生能力。人工种植的亚高山针叶林的树种多样性较原始林低,需要较长时间的恢复过程。亚高山针叶林的存在极大地丰富了云南省的树种多样性,使云南植被的垂直地带性得到了充分体现。     

Spatio-temporal analysis of tree height in a young cork oak plantation

Cork oak is one of the most valuable natural forest genera in the Mediterranean basin. Modelling cork oak growth has been a challenge for foresters in recent years because of strong site and genetic influences, below-ground competition, management regimes and age effects. Because cork productivity is related to forest height, which is, in turn, related directly with site characteristics, an increase in the accuracy of height prediction implies improved productivity estimation. A Bayesian maximum entropy (BME) geostatistical model was applied to characterize the space–time pattern of height of young cork oak in a forest stand from central Sardinia in the years 2000, 2002, 2003, 2006 and 2008. Cork oak height maps were produced for each of the 5 years. The main goals were to analyse and interpret through time (i) the changes in spatial correlation and (ii) the changes in spatial distribution of cork oak height. The plantation was characterized by an increasing spatial dependence through time, whereas the temporal range was 2 years. Cork oak height was significantly correlated with wind speed (reduced by a neighbouring forest) in all the years implying a single trend. The correlations were larger for 2006 and 2008 than for previous years. Three other environmental variables (shade, elevation and slope) were less significant and their influence restricted to 2 years only. This research has several implications for the management of cork oak in the young phase.     

Effects of Disturbance Frequency on Canopy Age Structure and Species Composition in a Managed Midwestern Oak Savanna

In the Midwest, the oak savanna is a unique plant community located between the western prairies and northern forests, but, once the most common plant community in Wisconsin, it is now one of the region's rarest plant communities. To assess the success of varying intensities of management practices, standard dendrochronology and vegetation surveying methods were used to determine the age structure and species composition in oak savanna sites that varied from intensely managed to unmanaged. Size and age distributions revealed that the number of distinct cohorts of oaks increased in less managed sites. Seedlings and saplings were present in high frequencies in all sites; however, management practices such as fire applications that were too frequent apparently decreased oak survivorship, as did the competition in the closed canopies of the unmanaged sites. A fire frequency of 2-3 years eliminates competition, but a frequency of 4-6 years, typical of fire-adapted forest communities, allows for slower-growing species and younger oaks to survive, while still maintaining a low density. In these conditions, where growth rates vary in time, the use of dendrochronology is necessary to obtain an accurate canopy structure for fire-adapted forest communities. Quercus macrocarpa was dominant in the more managed sites, but shade-tolerant species were dominant in the unmanaged site. At present, the canopy composition in the managed sites represented an oak savanna assemblage; however, the unmanaged composition was shifting to a woodland assemblage. Thus, remnant oak savanna can be over- or under-managed, and unmanaged oak savanna can be overlooked as woodlands.     

辽东栎萌生个体的地上生物量及其构件分配与生长参数的关系

选择岷江上游海拔2100-2300m地段的辽东栎萌生株个体69株,测定了每株生长参数（年龄、高度、基径、冠幅）、各器官生物量及地上总生物量,分析了各器官生物量及构件分配与生长参数的相互关系。结果表明,辽东栎萌生株构件分配的大小排序为：茎干〉枝〉皮〉叶：随径级增大。茎干、大枝生物量所占总生物量比例上升,其他器官所占比例减小。辽东栎年龄与高度呈S曲线关系,其他均表现出很好的幂指数函数关系;各构件器官生物量与生长参数大多也呈幂指数函数W=aD^h（D为基径）关系。基径为估测辽东栎萌生株地上各器官生物量及总生物量的最佳因子。     

Hydrogeomorphic control on tree growth responses in the Elton area of the Cheshire Saltfield, UK

Increment cores were sampled from oak (Quercus robur) and ash (Fraxinus excelsior) growing at Elton, an area of the Cheshire Saltfield that has experienced significant subsidence and damage to the natural and built environments in the latter part of the twentieth century. Ring-width measurements for Elton trees permitted the construction of one main site chronology (Elton) and four sub chronologies (Elton A, Elton B, Elton C, Elton ASH). Ring-width difference between these and a control chronology identified periods of sustained growth reduction in oak trees commencing in AD 1859/1861, 1886 and 1934. Growth reductions after 1934 are related to watertable draw down caused by brine pumping from a concentration of nine boreholes at Elton, up to 2 km from tree sampling locations. Growth reductions in 1859/1861 and 1886 are likely to be the result of earlier phases of brine pumping in the Wheelock Valley, up to 5 km to the east of Elton, and these reductions correlate well with historic records of subsidence and pumping activity. Cessation of pumping in 1977 led to a lagged growth recovery in oak trees between 1981 and 1986, indicating that an artificial drought had been imposed on the Elton area for a period in excess off 100-y. This research demonstrates a hydrological separation of surface water and groundwater in an area where salt beds are overlain by till and that ring-width records of Q. robur can be used to reconstruct watertable variability and also the spatial impact of solution mining.     

Holocene changes in precipitation seasonality in the western Mediterranean Basin: a multi‐species approach using δ13C of archaeobotanical remains

Precipitation has been of utmost importance in shaping the evolution of landscapes and human settlements in the Mediterranean. However, information on seasonal precipitation patterns through the Holocene is scarce. This study attempts to quantify the evolution of seasonal precipitation in the East Iberian Peninsula (5000 BC to AD 600) based on the carbon isotope composition (δ¹³C) of archaeobotanical remains. Data on Holm oak, Aleppo pine and small‐grain cereals were combined, and precipitation was inferred from models relating present‐day records to the δ¹³C of modern samples. Subsequently, charred grains were used as a proxy for ancient moisture during April–May, whereas oak and pine charcoals provided complementary rainfall estimates for September–December and January–August, respectively. The results reveal aridity changes throughout the Holocene in the western Mediterranean. Past spring–summer precipitation was consistently higher than at present. In contrast, autumn and early winter precipitation showed stronger fluctuations, particularly during the first millennium BC, and often exhibited values below those of the present. The high contribution of autumn precipitation to the annual water budget, typical of the present Mediterranean climate, was definitively established at the beginning of the current era. This study shows how a combination of species holding complementary environmental signals can contribute to a wider knowledge of local precipitation dynamics. Copyright © 2011 John Wiley & Sons, Ltd.     

木炭遗存揭示藏东南过去2700年人类生存环境及木材利用策略

木材是早期人类在青藏高原高海拔区域生存所需的关键性资源之一, 对木材的利用是人类适应极端环境的策略之一。本研究通过对青藏高原东南部8处考古遗址出土木炭的系统分析, 重建了遗址周边的木材资源, 发现过去2700年以来, 在木材资源丰富的藏东南, 先民采集的木材种类多样, 只在少数遗址显示出对特定木材的选择偏好; 波堆藏布流域人群利用的主要木材为当地的果树光核桃(Prunus mira), 其他区域则以川滇高山栎(Quercus semecarpifolia)和高山松(Pinus densata)为主, 显示出木材利用策略具有明显的空间差异; 研究还发现, 在气候变化影响下, 高山松在先民日常生活中的比重逐渐降低, 而川滇高山栎等灌木逐渐增加。本研究第一次对藏东南区域木材利用历史进行了重建, 为探讨人类对藏东南的行为适应提供了新的视角。

     

甘肃小陇山锐齿栎群落生物量特征分析

对甘肃小陇山锐齿栎群落乔木层生物量进行大量的野外调查和分析,结果表明:1.锐齿栎单一优势种混交群落生物量最大,其次为锐齿栎-鹅耳枥等小乔木混交群落,而锐齿栎单一优势种群落(纯林)生物量相对较小,杂木林群落生物量最小,其生物量比锐齿栎单一优势种混交群落低34.27%;2.锐齿栎群落乔木层的生物量增长规律为在群落发育成熟(80林龄左右)之前生物量在一直增加,到形成稳定的群落结构(成熟林),生物量趋于稳定(200 t/hm2),随群落发育进入末期,生物量有可能稍有降低;3.锐齿栎群落乔木层生物量在阴、阳坡都随海拔梯度先升高,后降低,呈典型的“中间膨胀”模式。     

Rainfall interception by an isolated evergreen oak tree in a Mediterranean savannah

Redistribution of ground‐level rainfall and interception loss by an isolated Quercus ilex tree were measured over 2 years in a Mediterranean oak savannah. Stemflow, meteorological variables and sap flow were also monitored. Rainfall at ground level was measured by a set of rain‐gauges located in a radial layout centred on the tree trunk and extending beyond the crown limits. Interception loss was computed as the difference between the volume of rainwater that would reach the ground in the absence of the tree and the volume of water that actually fell on the ground sampling area (stemflow included). This procedure provided correct interception loss estimates, irrespective of rainfall inclination. Results have shown a clear non‐random spatial distribution of ground‐level rainfall, with rainwater concentrations upwind beneath the crown and rain‐shadows downwind. Interception loss amounted to 22% of gross rainfall, per unit of crown‐projected area. Stand interception loss, per unit of ground area, was only 8% of gross rainfall and 28% of tree evapotranspiration. These values reflect the low crown cover fraction of the stand (0·39) and the specific features of the Mediterranean rainfall regime (predominantly with few large storms). Nevertheless, it still is an important component of the water balance of these Mediterranean ecosystems. Copyright © 2005 John Wiley & Sons, Ltd.