Department of Geosciences and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, Copenhagen, DK-1350, Denmark.
Glob Chang Biol. 2015 Jan;21(1):250-64. doi: 10.1111/gcb.12734. Epub 2014 Oct 18.
The Dahra field site in Senegal, West Africa, was established in 2002 to monitor ecosystem properties of semiarid savanna grassland and their responses to climatic and environmental change. This article describes the environment and the ecosystem properties of the site using a unique set of in situ data. The studied variables include hydroclimatic variables, species composition, albedo, normalized difference vegetation index (NDVI), hyperspectral characteristics (350-1800 nm), surface reflectance anisotropy, brightness temperature, fraction of absorbed photosynthetic active radiation (FAPAR), biomass, vegetation water content, and land-atmosphere exchanges of carbon (NEE) and energy. The Dahra field site experiences a typical Sahelian climate and is covered by coexisting trees (~3% canopy cover) and grass species, characterizing large parts of the Sahel. This makes the site suitable for investigating relationships between ecosystem properties and hydroclimatic variables for semiarid savanna ecosystems of the region. There were strong interannual, seasonal and diurnal dynamics in NEE, with high values of ~-7.5 g C m(-2) day(-1) during the peak of the growing season. We found neither browning nor greening NDVI trends from 2002 to 2012. Interannual variation in species composition was strongly related to rainfall distribution. NDVI and FAPAR were strongly related to species composition, especially for years dominated by the species Zornia glochidiata. This influence was not observed in interannual variation in biomass and vegetation productivity, thus challenging dryland productivity models based on remote sensing. Surface reflectance anisotropy (350-1800 nm) at the peak of the growing season varied strongly depending on wavelength and viewing angle thereby having implications for the design of remotely sensed spectral vegetation indices covering different wavelength regions. The presented time series of in situ data have great potential for dryland dynamics studies, global climate change related research and evaluation and parameterization of remote sensing products and dynamic vegetation models.
达赫拉野外站位于西非塞内加尔，于 2002 年建立，旨在监测半干旱稀树草原生态系统的生态特性及其对气候和环境变化的响应。本文利用一组独特的现场数据描述了该站点的环境和生态系统特性。研究的变量包括水文气候变量、物种组成、反照率、归一化差异植被指数 (NDVI)、高光谱特征（350-1800nm）、表面反射各向异性、亮温、吸收的光合有效辐射分数（FAPAR）、生物量、植被含水量以及碳（NEE）和能量的地气交换。达赫拉野外站经历了典型的萨赫勒气候，覆盖着共存的树木（~3%的树冠覆盖率）和草种，这是萨赫勒地区的大部分特征。这使得该站点适合研究该地区半干旱稀树草原生态系统的生态系统特性与水文气候变量之间的关系。NEE 具有强烈的年际、季节和日变化动态，在生长季节高峰期，其值高达约-7.5 g C m(-2) day(-1)。我们发现，2002 年至 2012 年间，NDVI 没有出现变棕或变绿的趋势。物种组成的年际变化与降雨量分布密切相关。NDVI 和 FAPAR 与物种组成密切相关，尤其是在以 Zornia glochidiata 为主导的年份。这种影响在生物量和植被生产力的年际变化中并未观察到，这对基于遥感的旱地生产力模型提出了挑战。生长季节高峰期的表面反射各向异性（350-1800nm）强烈依赖于波长和观测角，因此对涵盖不同波长区域的遥感光谱植被指数的设计具有影响。现场数据的时间序列具有很大的潜力，可以用于旱地动态研究、与全球气候变化相关的研究以及遥感产品和动态植被模型的评估和参数化。
