What was done
The authors evaluated the growth rates of two Norway spruce (Picea abies) stands in the Bohemian Massif of Northern Austria over the past four decades by analyzing the stem characteristics (height and diameter) of several trees they felled at two different sites, and by comparing their results with data obtained from control plots of other experiments that have been conducted there over an even longer period of time.

What was learned
Jandl et al. report that "forest productivity is currently about two yield classes higher than it is in the regionally valid yield tables," which were derived from data collected at the end of the 19th century, and that "the height and diameter of dominant stems exceed expectations." Hence, they conclude that "the sites are in a steady process of aggradation and that site productivity is rising."

As for the cause of this phenomenon, the Austrian researchers contend that climate is unlikely to be the main driver of the elevated growth rates of the forest stands they studied "because neither air temperature nor precipitation are strong predictors of the increment rates at our experimental sites." In addition, they say that the "ongoing improvement" is "not the mere consequence of a nitrogen-enriching effect." Hence, we are left with what they call "the enriching effect of increasing CO2 concentrations" and possible changes in management practices (such as the abandonment of forest litter raking) as the only viable alternative explanations for the steady historical increase in Norway spruce productivity at the sites they studied.

What it means
As has been found to be the case with numerous forests scattered over the globe (see Forests (Modern Growth Trends) in our Subject Index), Norway spruce forests of the Bohemian Massif of Northern Austria have been growing ever better over the past four decades; and it appears that the concomitant rise in the air's CO2 content has been one of the major factors responsible for this welcome development.