Hydraulic limitations in dominant trees as a contributing mechanism to the age-related growth decline of boreal forest stands

Hydraulic limitations in dominant trees may play a role in the growth decline that is commonly observed in aging forest stands. We hypothesized that hydraulic limitations affect dominant trees but not non-dominant trees in aging forests, resulting in changes in growth dominance during stand development. To test this hypothesis, we used a 1067-year-long post-fire chronosequence that was established in the eastern Canadian boreal forest. Within each stand, we estimated transpiration of different sized trees using direct (stem sap flow) and indirect (leaf isotope composition) measurements, which were combined with tree and stand growth estimates (growth dominance coefficient, GD). Stem sap flow measurements indicated that transpiration rate per unit leaf area (EL) of dominant trees indeed decreased with increasing stand age and, based upon temporal changes in leaf C and O isotope ratios, was related to an increase in stomatal closure. These results suggest that hydraulic limitation of dominant trees increased with stand age. Also, the slope of the relationship between EL and tree diameter that was calculated for each stand decreased with decreasing GD, implying that hydraulic limitation was responsible for the shift in stand growth dominance that was observed over time. Therefore, hydraulic limitation in dominant trees clearly contributes to reverse growth dominance of old-growth boreal forests, and could be involved in age-related declines in forest productivity.