In general, all climate studies analyzing Vegetation over years and merely few decades are very noisy, as the weather and atmospheric conditions vary between years and even decades due to well explainable atmospheric phenomenons (e.g. volcanic activity).
From what I have gleaned, climate scientists are not too happy using these dirty and noisy data generated over just a couple decades, unless they have a huge and well balanced data-set (e.g. vegetation data from different continents and climate zones over a larger time frame).
These data seen alone, never hold up to rigorous analysis (both mathematically and physically) and would discredit research in that field.
Although meta-analyses of all solid studies so far (this may be one of them) point towards climate change being real event, studies like the one cited are just a small piece of a large puzzle.