EFFECTS OF THINNING ON SOIL AND TREE WATER RELATIONS, TRANSPIRATION AND GROWTH IN AN OAK FOREST (QUERCUS-PETRAEA (MATT) LIEBL)

To quantify the effects of crown thinning on the water balance and gro wth of the stand and to analyze the ecophysiological modifications ind uced by canopy opening on individual tree water relations, we conducte d a thinning experiment in a 43-year-old Quercus petraea stand by remo ving trees from the upper canopy level. Soil water content, rainfall i nterception, sap flow leaf water potential and stomatal conductance we re monitored for two seasons following thinning. Seasonal time courses of leaf area index (LAI) and girth increment were also measured. Pred awn leaf water potential was significantly higher in trees in the thin ned stand than in the closed stand, as a consequence of higher relativ e extractable water in the soil. The improvement in water availability in the thinned stand resulted from decreases in both interception and transpiration. From Year 1 to Year 2, an increase in transpiration wa s observed in the thinned stand without any modification in LAI, where as changes in transpiration in the closed stand were accompanied by va riations in LAI. The different behaviors of the closed and open canopi es were interpreted in terms of coupling to the atmosphere. Thinning i ncreased inter-tree variability in sap flow density, which was closely related to a leaf area competition index. Stomatal conductance varied little inside the crown and differences in stomatal conductance betwe en the treatments appeared only during a water shortage and affected m ainly the closed stand. Thinning enhanced tree growth as a result of a longer growing period due to the absence of summer drought and higher rates of growth. Suppressed and dominant trees benefited more from th inning than trees in the codominant classes.