Osmotic adaptation of Quercus robur L. under water stress in stands with different tree density - relation with groundwater table

Solute accumulation in plant leaves during drought through accumulation of organic compounds is known to compensate turgor loss andpromote higher stomatal conductance at lower water potentials. Recent studies have involved experiments on saplings and younger stands, while evidence of osmotic adjustment in adult pedunculate oak under natural stand conditions on different sites is scarce. Pressure volume curves technique was used to define differences in osmotic potential – water stress adaptation – of 120 year oldped unculate oak trees in two managedforest complexes andin virgin forest remnant. Tree response between the managedstandafter thinning andthe standwithout any silvicultural measures within same forest complex was also compared during summer months in two consecutive (dry and favourable) years with groundwater table. Significant differences were observed in adaptation between forest complexes and during dry (2003) and favourable (2004) years. Osmotic component of thinnedforest was the highest, showing most negative values of stress adjustment. Measured values on all plots responded well to drop in groundwater table, especially in pronounced drought conditions. Such response may be in relation with lower stand density and increased individual space for growth in thinned stand. Adult oaks did not loose their ability to water stress adaptation, in spite of their age and progressively decreasing health conditions.