Stand microclimate and physiological activity of tree leaves in an oak-hornbeam forest

Abstract

The photosynthetic activity of the Turkey oak (Quercus cerris L., height 22 m) and common hornbeam (Carpinus betulus L., height 17 m) in a deciduous forest stand was analysed. Trees studied grew in an uneven-aged forest (80 years old on average, main canopy surface at a height of 18m) forming the former IBP forest research site at Bab, SW Slovakia (Czechoslovakia). The average photosynthetic rate of oak foliage was higher than that of hornbeam. Net photosynthetic rate at saturating photon flux rate (PNmax) of hornbeam amounted to only 60% of that of oak for the leaves of the upper canopy layer (UCL) and to 67% for the leaves of the lower canopy layer (LCL). In the summer months the main photosynthetic activity of this deciduous stand was focused upon the UCL leaves and oak species. The relationship between PN and photon flux rate, as well as the diurnal course of PN and stomatal conductance (gs), was calculated using a mathematical model. The diurnal course of PN and gs were similar for both tree species and both types of leaf. Maximal gs values were observed at noon. The lower values of compensation photon flux rate (Γ1) and photosynthetic efficiency (α) but higher values of the maintenance respiration rate (RM), confirmed the higher shade tolerance of the hornbeam. The dark respiration rate (RD) of the UCL leaves was higher than that (RD) of the LCL leaves. Various photosynthetic features and production capacity of the above-mentioned types of leaf expressed the adaptation pressures to radiation conditions. In the stand studied, the primary production of the greater part of the crown depended on the vertical foliage distribution and on light penetration during the midday hours.