南方亚热带季风区将乐县森林植被动态变化及其对气候变化的响应

在全球气候变化背景下,植被动态变化以及植被对气候变化的响应方式已经成为生态学和地理学领域的热点。本文对比分析了南方亚热带季风区将乐县不同类型森林植被对不同时间尺度的干旱响应的差别。基于2000-2017年MODIS-EVI数据及气象站点数据,用最大值合成法、趋势分析法以及相关分析法,分析了森林植被及气象因子的动态变化特征,并对比不同森林植被对气候变化响应的差别。研究表明：① 2000-2017年,研究区植被覆盖度、EVI和降水均显著增加,区域内湿度增加,森林长势渐趋良好;② EVI在生长季初期和末期与同期的降水、温度均显著正相关（P<0.1）,初期森林受降水因子的影响更大,末期受温度因子的影响大;③ 1-3月和周年的气候变化对森林的生长至关重要,长时间尺度的湿度增加对森林生长具有显著的促进作用,SPEI的时间尺度越长与EVI的相关性也越大;④ 针阔混交林与同期温度、降水的相关系数最高,并且与不同时间尺度的SPEI相关性均比较高,属于气候敏感型林型,在生产经营中要谨慎预防气候变化对该林型带来的伤害;⑤ 森林覆盖度变化与降水和SPEI_24的相关性极显著,长时间尺度的降水变化是影响森林植被覆盖率变化的重要因素之一。     

Trophic significance of the kelp Laminaria digitata (Lamour.) for the associated food web: a between-sites comparison

This study aimed at establishing the trophic significance of the kelp Laminaria digitata for consumers inhabiting two rocky shores of Northern Brittany (France), displaying contrasted ecological conditions. The general trophic structure did not vary between these two sites, with a wide diversity of filter-feeders and predators, and only 14% of the species sampled belonging to the grazers' trophic group. The diversity of food sources fueling the food web appeared also similar. The food webs comprised four trophic levels and the prevalence of omnivory appeared relatively low compared to previous studies in the same area. Conversely, to the food web structure, which did not differ, the biochemical composition of L. digitata differed between the two sites, and was correlated to a larger diversity of grazers feeding on this kelp in sheltered conditions. This indicated that the spatial variability occurring in the nutritive value of L. digitata is likely to deeply affect the functioning of kelp-associated food webs. The contribution of L. digitata-derived organic matter to the diet of filter-feeders inhabiting these two environments was assessed using the mixing model Isosource, which showed the higher contribution of kelp matter in sheltered conditions. These results highlight the spatial variability that may occur in the functioning of kelp-associated food webs. Moreover, this suggests that hydrodynamics is likely to control the availability of kelp-derived organic matter to local filter-feeders, probably through an increase of detritus export in exposed areas.     

西安高陵人工林土壤干层与含水量季节变化研究

通过野外调查和室内测定,利用烘干称重法对高陵地区丰水年前后不同人工林下0~6 m土壤含水量及土壤水分的季节变化进行研究。结果表明,2002年高陵田家村中国梧桐林和杨树林下160~400 cm范围内均已发育了土壤干层。经过2003年丰水年充沛的降水补给,2004年高陵团庄槐树林、杏树林0~6 m土层均未出现土壤干层,说明水分在丰水年得到很好恢复。丰水年后梨、杏、槐三种人工林160~400、410~600 cm层位土壤含水量均显示春季最高,夏季次之,秋季降到最低或略微上升。     

ICESat-2植被冠层高度和地表高程数据产品用于森林高度提取的效果评价

由美国宇航局(NASA)研制的新一代冰、云和陆地高程卫星(ICESat-2)于2018-09-15发射成功,其搭载的先进地形激光测高系统(ATLAS)采用微脉冲多波束光子计数激光雷达技术,可用于全球高程线采样数据的获取.目前公开发布了9种数据产品,其中包括植被冠层高度和地表高程数据产品(ATL08),为全球森林结构参数...     

A simple model to estimate monthly forest evapotranspiration in Japan from monthly temperature

The estimation of evapotranspiration (E) in forested areas is required for various practical purposes (e.g. evaluation of drought risks) in Japan. This study developed a model that estimates monthly forest E in Japan with the input of monthly temperature (T). The model is based on the assumptions that E equals the equilibrium evaporation rate (E_eq) and that E_eq is approximated by a function of T. The model formulates E as E (mm month⁻¹) = 3·48 T ( °C) + 32·3. The accuracy of the model was examined using monthly E data derived using short‐term water balance (WB) and micrometeorological (M) methods for 15 forest sites in Japan. The model estimated monthly E more accurately than did the Thornthwaite and Hamon equations according to regression analysis of the estimated E and E derived using the WB and M methods. Although the model tended to overestimate monthly E, the overestimation could be reduced by considering the effect of precipitation on E. As T data are commonly available all over Japan, the model would be a useful tool to estimate forest E in Japan. Copyright © 2010 John Wiley & Sons, Ltd.     

Analysis of the contributions of topographic,soil, and vegetation features on the spatial distributions of surface soil moisture in a steep natural forested headwater catchment

Surface soil moisture has been extensively studied for various land uses and landforms. Although many studies have reported potential factors that control surface soil moisture over space or time, the findings have not always been consistent, indicating a need for identification of the main factors. This study focused on the static controls of topographic, soil, and vegetation features on surface soil moisture in a steep natural forested headwater catchment consisting of three hillslope units of a gully area, side slope, and valley‐head slope. Using a simple correlation analysis to investigate the effects of the static factors on surface soil moisture at depths of 0–20 cm at 470 points in 13 surveys, we addressed the characteristics of surface soil moisture and its main controlling factors. The results indicated that the mean of surface soil moisture was in the decreasing order of gully area > valley‐head slope > side slope. The relationship between the mean and standard deviation of surface soil moisture showed a convex‐upward shape in the headwater catchment, a negative curvilinear shape in the gully area, and positive curvilinear shapes at the side and valley‐head slopes. At the headwater catchment and valley‐head slope, positive contributions of soil porosity and negative contributions of slope gradient and saturated hydraulic conductivity were the main controlling factors of surface soil moisture under wetter conditions, whereas positive contributions of topographic wetness index and negative contributions of vegetation density were the main controlling factors of surface soil moisture under drier conditions. At the side slope underlain by fractured bedrocks, only saturated hydraulic conductivity and vegetation density were observed to be the controlling factors. Surface soil moisture in the gully area was mainly affected by runoff rather than were static features. Thus, using hillslope units is effective for approximately estimating the hydrological behaviours of surface moisture on a larger scale, whereas dependency between the main static factors and moisture conditions is helpful for estimating the spatial distributions of surface moisture on a smaller scale.     

Suspended sediment transport regime in a debris‐flow gully on Vancouver Island,British Columbia

In debris‐flow‐prone channels, normal fluvial sediment transport occurs (nearly exclusively in suspended mode) between episodic debris‐flow events. Observations of suspended sediment transport through a winter season in a steepland gully in logged terrain revealed two event types. When flows exceeded a threshold of 270 l s⁻¹, events yielded significant quantities of sediment and suspended sediment concentration increased with flow. Smaller events were strongly ‘supply limited’; sediment concentration decreased as flow increased. Overall, there is no consistent correlation between runoff and sediment yield. Within the season, three subseasons were identified (demarcated by periods of freezing weather) within which a pattern of fine sediment replenishment and evacuation occurred. Finally, a signature of fine sediment mobilization and exhaustion was observed within individual events. Fine sediment transport occurred in discrete pulses within storm periods, most of the yield occurring within 5 to 15% of storm runoff duration, so that it is unlikely that scheduled sampling programs would identify significant transport. Significant events are, however, generally forecastable on the basis of regional heavy rainfall warnings, providing a basis for targeted observations. Radiative snowmelt events and rain‐on‐snow remain difficult to forecast, since the projection of temperatures from the nearest regular weather station yields variable results. Copyright © 2004 John Wiley & Sons, Ltd.     

Effects of commercial timber harvesting on streamflow regimes in the Plynlimon catchments,mid‐Wales

This paper presents the first large‐scale British study of the impacts of commercial forest cutting on stream‐flow regimes. The 70% forested headwaters of the River Severn are part of the intensively instrumented long‐term Plynlimon catchment study into the impact of land use on stream flow. The forest area, comprising predominantly Sitka spruce (Picea sitchensis), was planted mainly in the 1930s and 1940s. Harvesting commenced in the mid‐1980s and over the study period about half the forest has been felled. Changes in annual water yield and extreme flows were studied in four nested catchments ranging in area from about 1 to 10 km² and compared with an adjacent benchmark grassland catchment. As expected from earlier process studies the cutting of the forest increased total annual flows. Less expected was the clear evidence that the felling augmented low flows. This informs a long‐standing debate whether upland forestry increases or reduces baseflows. A particularly notable result was the lack of impact of the harvesting on storm peak flows. This may result from the application of forest management guidelines designed to reduce soil damage and erosion during the harvesting, and indicates that the forest itself has a limited impact on flooding. These findings are timely because British forest expansion peaked in the 30 years following the Second World War, and large areas of these woodlands are now approaching economic maturity and will be harvested in the next two decades. Copyright © 2004 John Wiley & Sons, Ltd.     

The interaction of two asymmetric quasi-geostrophic vortex patches

ABSTRACT

Herein we study the general interaction of two vortex patches in a single-layer quasi-geostrophic shallow-water flow. Steadily-rotating equilibrium states are found over a wide parameter space spanning the Rossby deformation length, vortex area ratio, potential vorticity ratio, and gap between their innermost edges. A linear stability analysis is then used to identify the critical gap separating stable and unstable solutions, over the entire range of area and potential vorticity ratios, and for selected values of the Rossby deformation length. A representative set of marginally unstable equilibrium states are then slightly perturbed and evolved by an accurate contour dynamics numerical algorithm to understand the long-term fate of the instabilities. Not all instabilities lead to vortex merger; many in fact are characterised by weak filamentation and a small adjustment of the vortex shapes, without merger. Stronger instabilities lead to material being torn from one vortex and either wrapped around the other or reduced to ever thinning filamentary debris. A portion of the vortex may survive, or it may be completely strained out by the other.