Increasing productivity in recent European oak trees?

As an attempt to analyze the role of the biosphere in the disturbed carbon cycle, as far as the standing biomass is concerned, a search for growth trends in tree ring series was started. A group of 27 cut oak trees from various locations in Europe and with ages between 25 and 262 years was analyzed in the following way: (1) A two-parameter smoothing function applied to each single tree cancelled out the year-to-year noise of ring widths; (2) this function containing the growth information over the whole life span of a tree was used to compute the (smoothed) width of the 40th and the 80th ring for each tree; (3) each ring width derived from a single tree in this way was plotted vs time, i.e., over the particular year of its formation. A statistical analysis of these plots shows that the variation of ring widths (delta r) in time (t) is best represented by an exponential function of the form delta rn(t) = delta r0n + aebt (n = 40; 80). The pre-industrial value, delta r0n (for t approximately less than 1,800), turns out to be identical with the 2,700 years average of European oak trees, as found by Hollstein (1979) in his dendrochronological work. Several possibilities are discussed what might have caused such growth trend.