Doktor, D, Bondeau, A, Koslowski, D, Badeck, FW (2009). Influence of heterogeneous landscapes on computed green-up dates based on daily AVHRR NDVI observations. REMOTE SENSING OF ENVIRONMENT, 113(12), 2618-2632.
The main objective of the current paper is to evaluate and explain differences between computed green-up dates of vegetated land surface derived from satellite observations and budburst dates from ground observational networks. Landscapes dominated by deciduous broad-leaved trees in Germany are analysed. While ground observations generally record the onset of bud break, remote sensing refers to a detectable change of surface reflectance, which accounts for the unfolding of the majority of the leaves. The satellite detects, even in a homogeneous stand, two signals: the green-up of the understorey and, shortly after, the green-up of the canopy (overstorey). Results of comparisons indicate an earlier, although not consistently, satellite-derived green-up than bud break derived from ground observations. We hypothesise that this is due to heterogeneous ground cover and a detection of the greening of non-tree vegetation by the satellite. This hypothesis is tested by analysing the difference between satellite-derived green-up dates (d(GU)) and budburst observed on the ground (d(BB)) in function of the proportion of non-deciduous-forest (ndf) land use types in satellite scenes. The satellite data (a daily 1-km resolution AVHRR product) are analysed with progressively more restricted selection criteria regarding the land surface elements. The two sets of observations are compared using Gaussian Mixture Models to evaluate the statistical properties of the probability density functions (pdf) as produced by the two sets rather than comparisons of geographically coincident data. It is shown that a heterogeneous vegetation cover is likely to be the main factor determining the difference between the computed green-up date and date of budburst of the dominating tree species. (C) 2009 Elsevier Inc. All rights reserved.