Comparing the influence of thinning treatments with low to high residual basal area on red maple transpiration in a temperate mixed forest

Abstract

Under climate change, forests are expected to experience drier conditions that may increase tree mortality. Silvicultural treatments, such as thinning, have been proposed to reduce moisture competition and to improve forest resistance to drought events. Most studies have investigated the effectiveness of thinning under semi-arid conditions, while little information is available regarding temperate forest responses, together with the residual basal area (BA) that is required to reap the benefits of these treatments. This research aims to understand how the residual BA influences transpiration in mixed temperate forest stands that are dominated by red maple (Acer rubrum) in southeastern Canada. We monitored the sap flux density (Fd) with thermal dissipation-type sensors for 18 red maples spread across three sites that were each thinned to obtain a gradient of residual BA (20, 12.5, 6 m2/ha). The study was conducted during the first growing season following treatment. Low residual BA plots (6 m2/ha) incurred drier atmospheric conditions as shown by a greater vapor pressure deficit (VPD) compared to high residual BA plots (20 m2/ha), although we observed considerable variability between sites. At the tree scale, Fd increased with residual BA, with the most pronounced differences under dry atmospheric conditions: when daily VPD exceeded 1.1 kPa, mean Fd in high residual BA plots was respectively 20 % and 75 % greater than in medium (12.5 m2/ha) and low residual BA plots. At the stand level, we simulated total transpiration considering the stand as only made of red maples. The transpiration in medium and low residual BA plots amounted to 41 % and 79 % of transpiration simulated in the high residual BA plot. Overall, this work highlighted broad variation in response to residual BA treatments, emphasizing the need to better model forest water budgets, and partitioning overstory and understory evapotranspiration to make more adequate residual BA prescriptions in temperate forests.