Spectral measures and mixed models as valuable tools for investigating controls on land surface phenology in high arctic Greenland

Snow melt and temperature are of major importance for the timing of the maximum growth as well as for the seasonal growth. More than 85% of the variance in timing of the maximum growth is explained by the models and similar for the seasonal growth of mesic and wet vegetation types. We find several non-linear growth responses to the environmental variables.We conclude that the uses of GAMMs are valuable for investigating growth dynamics in the Arctic. Contrary to several other studies in the Arctic we found a significant decreasing trend of the seasonally integrated NDVI-FR (SINDVI) in some vegetation types. This indicates that although greening might occur wide-spread in the Arctic there are variations on the local scale that might influence the regional trends on the longer term.Land surface phenology is a key variables for modelling of the terrestrial ecosystems in a global change perspective and as such as input into circulation models (GCM's) [1]. Recent models agree that changes in vegetation and soil processes will have net positive feedback on future global warming [2]. Further, the way terrestrial ecology is implemented in GCM's will have a strong impact on the ability to predict future climatic changes [3] and knowledge of changes in the vegetation cover and the reasons for these are therefore of major importance. This has led to a number of publications and assessments on the greening and impact of global climate change in northern high latitudes [4-6] based on monitoring studies [7] and experimental studies [8,9]. Several of these studies have investigated the effect of temperature, light and fertilization on photosynthesis [10,11] and found that fertilization and increased temperatures significantly increases the photosynthesis while shading decreases the photosynthesis. Thawing degree-days and time of snow cover melt was found as the dominating controls on the phenology in the Subarctic [12]. Recently, Walker et al. included warming experiments from 11