Summer field performance of Quercus petraea (Matt.) Liebl and Quercus pyrenaica Willd seedlings, planted in three sites with contrasting canopy cover

In 2000, one-year-old seedlings of pyrenean oak (Quercus pyrenaica Willd.) and sessile oak (Quercus petraea [Matt.] Liebl) were planted in a thinned and an unthinned plot in a pinewood (Pinus sylvestris), and in a nearby clearing. In summer 2002 and 2003, water relations and gas exchange parameters were measured to address the impact of drought on the seedlings. Chlorophyll a fluorescence was also measured to explore leaf photochemistry and a possible non-stomatal limitation to photosynthesis (A). Reduction in stomatal conductance (g) in response to the decrease of predawn water potential (Psi(pd)) resulted the main cause affecting net carbon uptake. Water potential at midday (Psi(md)) was similar in both species but Quercus petraea was more sensitive to soil water deployment occurred along summer, showing slightly lower Psi(pd) because worse recover of water potential during night. Rate of photosynthesis was higher in Q.pyrenaica probably in relation to its greater leaf mass per area (LMA) and nitrogen content per leaf area (N-a). Mortality was highest in the clearing and lowest in the thinned pinewood. Throughout the summer, soil moisture was higher in the thinned area, possibly because of the reduction in tree transpiring surface and interception of rainfall. Accordingly, Psi(pd) of both species was higher in the thinned site.