Abstract:

The seasonal photosynthesis and dark respiration (R_d) in the upper, middle and lower portions of the canopy of Larix gmelinii and Betula platyphylla from 34-year-old larch-dominated plantation in northeastern of China were measured. We examined the gas exchange pattern of larch plantation in temporal and spatial variation in canopy-level during the growth season. The results indicated that the maximum net photosynthesis (P_nmax) among upper, middle and lower layers differed significantly for current-year branch needles of L. gmelinii in August and September,respectively, and had highest in August, ranged as 13.09,8.63,9.07 μmolCO₂·m^-2s^-1from upper layer to lower layer. There was no significant difference in P_nmax of previous-year branch-needles of larch among different months during the growth season. The mean of P_nmax was 8.22 μmolCO₂·m^-2s^-1. P_nmax of the birch leaves among the three canopy layers differed significantly throughout the growth season, ranged as 17.56, 13.01 and 9.58 μmolCO₂·m^-2s^-1from upper layer to lower layer. There were changes in R_d differences between the two species during the growth season, but no significant was found among the three layers of the canopy in the two species. During the whole growth season the R_d of birch, current-year and previous-year branch-needles of larch were 0.47～1.40, 0.83～1.46 and 0.68～1.63 μmolCO₂·m^-2s^-1, respectively. Our objective was to examine temporal and spatial variation in canopy-level gas exchange patterns during the growing season, notable positive correlation of R_d with temperature but no obvious correlation with illumination in the two species were found. P_nmax had no significant correlation with leave temperature during the growth season excepted the upper layer leave of larch. The gas exchange of the canopy of the larch and birch was variable depending on growth season with species-specific.

Key words: Larix gmelinii, Betula platyphylla, gas exchange, canopy, temperature, PAR