叶面积指数(Leaf area index,LAI)是陆地生态系统的一个十分重要的结构参数。随着空间精细化模型的发展和基于过程的分布式模拟技术的应用,对LAI的区域估算显得越来越重要,但目前尚缺乏有效的估算手段。该项研究以青海云杉(Picea crassifolia)林为研究对象,利用LAI-2000冠层分析仪、鱼眼镜头法和经验公式法对林冠层LAI进行了测定,观测值分别为1.03～3.70、0.48～2.26和2.27～8.20,显然,仪器测定值偏低。针对针叶的集聚效应导致仪器测定值偏低的现象,利用跟踪辐射与冠层结构测量仪(TRAC)测定的青海云杉林聚集系数计算调整系数,对鱼眼镜头法获取的LAI值进行订正。根据高分辨率的遥感数据反演青海云杉林的植被指数与LAI的关系,最后获得了较合理的该地区林冠层LAI的空间分布图。
Riparian vegetation in the lower reaches of Heihe River serves important ecological functions. However, the riparian ecosystems have been constantly deteriorating in the past 30 years simply due to water interception for oasis agricultural irrigation in the middle reaches of the river. This study pays a particular attention to Populus euphratica Oliv. forest because it is a dominant component of the riparian ecosystem in the lower reaches of Heihe River where the depth of groundwater table is the controlling factor in sustaining riparian ecosystems. To reveal leaf-related physiological responses of Populus euphratica Oliv. forest to groundwater table variations, we analyzed the relationships between the depth of groundwater table (DG) and three leaf-related parameters, i.e. leaf stomatal density (SD), specific leaf area (SLA), and stable carbon isotopic composition (δ 13 C). Our results show that the relationship between DG and leaf SD is a bi-mode one shaped by both salt stress and water stress. That is, salt stress appeared in shallow groundwater conditions and water stress happened in deep groundwater conditions, and the thin layer around 2.7 m of DG is a stress-free layer. Leaf SD fluctuated according to the DG variation, first decreased with increasing DG, then increased at depths ranging 2.7-3.7 m, and after a relatively stable plateau of SD at depths ranging 3.7-5.2 m, decreased again with increasing DG. Our results also show that SLA decreased exponentially with increasing DG and foliar δ 13 C values are also strongly dependent on DG, further demonstrating that these two parameters are sensitive indicators of water stress. The exponential curve suggests that SLA is more sensitive to DG when groundwater table is shallow and 3 m seems to be a threshold beyond which SLA becomes less sensitive to DG. Foliar δ 13 C becomes more sensitive when the groundwater table is deep and 7 m seems to be a threshold below which the δ 13 C signature becomes more sensitive to DG. These findings should be helpful in monit
Yang ZHAOChuanYan ZHAOZhongLin XUYiYue LIUYao WANGChao WANGHuanHua PENGXiangLin ZHENG
