Canopy transpiration in a chronosequence of Central Siberian pine forests

Tree transpiration was measured in 28, 67, 204 and 383-y-old uniform stands and in a multicohort stand (140-430 y) of Pinus sylvestris ssp. sibirica Lebed. in Central Siberia during August 1995. In addition transpiration of three codominant trees was monitored for two years in a 130-y-old stand. All stands established after fire. Leaf area index (LAI) ranged between 0.6 (28-y-old stand) and 1.6 for stands older than 67-y. Stand xylem area at 1.3 m height increased from 4 cm(2) m(-2) (28-y) to 11.5 cm(2) m(-2) (67-y) and decreased again to 7 cm(2) m(-2) in old stands. Above-ground living biomass increased from 1.5 kg dry weight m(-2) (28-y) to 14 kg dry weight m(-2) (383-y). Day-to-day variation of tree transpiration in summer was dependent on net radiation, vapour pressure deficit, and soil water stress. Tree-to-tree variation of xylem flux was small and increased with heterogeneity in canopy structure. Maximum rates of xylem flux density followed the course of net radiation from mid April when a constant level of maximum rates was reached until mid September when low temperatures and light strongly reduced flux density. Maximum sap flux density (60 g m(-2) s(-1)) and canopy transpiration (1.5 mm d(-1)) were reached in the 67-y stand. Average canopy transpiration of all age classes was 0.72 +/- 0.3 mm d(-1.) Canopy transpiration (E) was not correlated with LAI but related to stand sapwood area SA (E = - 0.02 + 1.15SA R-2) which was determined by stand density and tree sapwood area.