Age-related decline in stand water use: sap flow and transpiration in a pine forest chronosequence

We investigated changes in the pattern of stand water use in a chronosequence of four even-aged maritime pine stands, differing in age (10-, 32-, 54- and 91-year old) and growing under similar environmental conditions. Extensive sap flow measurements were carried out during a period of 2 years (2001-2002), using thermal dissipation sensors located 1 m below the live crown and corrected for the radial profile of sap flow. We found significant differences in transpiration among stands. Overstorey transpiration, ET, declined with stand age from 508 mm per year for the 10-year-old stand to 144 mm per year for the 54-year-old stand in 2001. This age-related decline was driven by (i) a decrease in transpiration per unit leaf area (55%), due to a decrease in stomatal conductance in taller trees and (ii) a significant reduction in leaf area index (38%). There was a strong effect of soil moisture deficit on transpiration and an interaction with age. A strong decrease in overstorey transpiration resulted from the prolonged drought in 2002 due to both a lower transpiration rate per unit leaf area and lower leaf area evidenced both by optical and litterfall measurements. The drought effect was greater in the younger stands compared to older stands. In the 32-year-old stand, simultaneous measurements of sap flow and eddy covariance showed that the overstorey transpiration contributed to half of the total ecosystem water loss, E. Moreover, the total ecosystem evaporation estimated for each stand by a water balance approach was not different between stand ages. We suggest, therefore, that the decline in overstorey transpiration with stand age mainly affects partitioning of the components of evaporation between trees, understorey and soil but not the magnitude of total evaporation.