NDVI
Bi-weekly normalized difference vegetation index (NDVI) data for the
approx. 8 × 8 km2 pixel incorporating the research
area was extracted from the third generation of the Global Inventory
Modeling and Mapping Studies (GIMMS3g) dataset for the
period July 1981 - 2015. The GIMMS3g-NDVI dataset is
assembled from different sensors of the Advanced Very High-Resolution
Radiometers (AVHRR) satellites. It was tested whether there were
positive linear trends in maximum NDVI (NDVImax) and
mean late summer (July-August) NDVI over time with one-sided Pearson’s
correlation tests. Furthermore, it was tested whether there were
positive relationships between the mean standardized shoot length
chronology / branch initiation on the one hand and
NDVImax / late summer NDVI on the other. In addition,
negative relationships between standardized branch mortality frequencies
in the previous year and NDVImax / late summer NDVI were
hypothesized and tested. Lastly, positive relationships between seasonal
climate variables (mean temperatures and precipitation sums; the same as
in the climate-growth models) were hypothesized and tested for
significance with one-sided Pearson’s correlation tests.