Laurent Misson

Frequency responses of radial growth series after different thinning intensities in Norway spruce (Picea abies (L.) Karst.) stands

A thinning intensity experiment started in 1971 on two experimental 22-year-old stands of Norway spruce in the Belgian Ardenne. The stands had strongly contrasting water availability conditions. Heavy thinning resulted in up to 77% removal of the basal area. In 1995, tree ring-area series were measured from dendrochronological cores sampled in three different thinning intensity treatments and on a control plot. Radial growth variations at different frequencies (low, medium and high) were isolated by standardisation with polynomials and differencing. Results showed that long, medium and short-term radial growth variations are widely altered by heavy thinning. At low-frequency, growth tended to increase during 15 years, until the stand basal area reached 26-29 m(2) ha(-1) again. At medium-term, the positive growth lasted for 10 years: after this period radial growth was limited by an undetermined factor. Medium-frequency growth variation showed an inverse cycling behaviour between treatments during the entire period studied (25 years). At short-term, positive growth variation lasted 4 years. We showed empirical evidences that stand density alters the classical climate-growth relationship. At individual tree level, thinning could be a method to increase tree resistance to drought stress. This effect is limited when site conditions,are limiting

Predicting transpiration from forest stands in Belgium for the 21st century

Canopy transpiration is a major element of the hydrological cycle of temperate forests. Levels of water stress during the 21st century will be largely controlled by the response of canopy transpiration to changing environmental conditions. One year of transpiration measurement in two stands (Quercus robur L. and Fagus sylvatica L.) was used to calibrate the ASPECTS model on a(1) and D-0, two parameters of a modified version of Leunings equation of stomatal conductance. A second year of data was used to validate the model. The results indicate a higher sensitivity of g(sc), to vapour pressure deficit (DS) in oak than in beech (D-0 (oak) < D-0 (beech)). To simulate future forest transpiration, site specific weather data sets were constructed from GCM outputs, spatially and temporally downscaled with local climatic data. Temperature increase between the end of the 20th and 21st centuries was predicted to be 2.8 degreesC in the beech stand and 3.1 degreesC in the oak stand. Based solely on temperature change, ASPECTS predicted an increase in transpiration of 17% in the beech and 6% in the oak stand, the difference being due to variation in local climate and the sensitivity of both species to D-s. Based solely on increased atmospheric CO2 (355 ppm in 1990 to 700 ppm in 2100), ASPECTS predicted that transpiration would decrease by 22% in beech and 19% in oak. With the combined scenarios of climatic change and increased atmospheric CO2, ASPECTS showed a decrease of 7% in transpired water in the oak stand and only 4% in the beech stand, which are not significant differences from zero. Consequently, water stress should not increase in either stand during the 21st century