SHORT-TERM VARIATIONS AND LONG-TERM CHANGES IN OAK PRODUCTIVITY IN NORTHEASTERN FRANCE - THE ROLE OF CLIMATE AND ATMOSPHERIC CO2

A dendroecological study was carried out in 2 forests in northeastern France with the aim of identifying and quantifying possible long-term trends in the radial growth of sessile oak (Quercus petraea (Matt) Liebl) and pedunculate oak (Q robur L). A total of 150 sites were selected to represent the ecological diversity of these forests. An index Cd was used to correct annual ring width in order to compensate for the effect of different competition situations. The data were standardized with reference to the mean curve 'basal area increment vs cambial age'. The growth index curves revealed a strong increase in sessile oak growth (+ 64% during the period 1888 to 1987) as well as in that of peduncutate oak (+40%). The growth increase in the 'young' rings (<60 years) of sessile oak was + 87 %, and that of young rings of pedunculate oak was + 49%. The corresponding increase in the 'old' rings (>65 years) was + 48% and 15% respectively (not significant for the latter). It would thus appear that pedunculate oak has benefited to a lesser extent than sessile oak from the progressive changes in its environment. Years showing a strong growth decrease are more common for pedunculate oak than for sessile oak. These results are consistent with a recent hypothesis about a slow but general retreat of pedunculate oak, including severe episodic declines, in favour of sessile oak in many regions of France. A model was created using a combination of meteorological data (monthly precipitation and temperature) starting in 1881, and increasing atmospheric CO, concentrations. The model explains 78.3% of the variance for sessile oak and 74.3% for pedunculate oak. This includes some monthly parameters of year y (year of ring formation), and also some parameters of the years y-1 to y-4 for sessile oak and y-1 to y-5 for pedunculate oak. The models satisfactorily reproduce the long-term trends and the interannual variation. The climatic variables alone (ie excluding the CO, concentration) were insufficient to explain the trends observed. The possible direct and indirect effects of increasing CO2 concentration on the growth of both species are discussed.