Short-term effect of thinning on red maple transpiration in a temperate mixed forest

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 nine experimental plots that were thinned to obtain a gradient of residual BA (20, 12.5, 6 m2 ha-1). The study was conducted during the first growing season following treatment. Low residual BA plots (6 m2 ha-1) incurred drier atmospheric conditions as shown by a greater vapor pressure deficit (VPD) compared to high residual BA plots (20 m2 ha-1). 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-1) 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.

在气候变化背景下，森林预计将面临更为干旱的环境，这可能提升树木死亡率。营林措施如疏伐（thinning）已被提出，用于降低林木间的水分竞争，提升森林对干旱事件的抗性。现有多数研究聚焦于半干旱环境下疏伐的成效，而关于温带森林对疏伐的响应，以及获取该措施收益所需的剩余断面积（residual basal area, BA）的相关信息仍较为匮乏。本研究旨在探究加拿大东南部以红枫（Acer rubrum）为优势种的温带混交林分中，剩余断面积对林木蒸腾作用的影响。我们采用热扩散式传感器对分布于9个经疏伐处理的试验样地内的18株红枫进行树干液流密度（sap flux density, Fd）监测，这些样地的剩余断面积梯度设置为20、12.5、6 m² ha⁻¹。本试验于疏伐后的首个生长季开展。与剩余断面积较高的样地（20 m² ha⁻¹）相比，剩余断面积较低的样地（6 m² ha⁻¹）的大气环境更为干旱，表现为更高的水汽压亏缺（vapor pressure deficit, VPD）。在单木尺度上，树干液流密度随剩余断面积升高而增加，该差异在干旱大气环境下最为显著：当日均水汽压亏缺超过1.1 kPa时，高剩余断面积样地的平均树干液流密度分别比中（12.5 m² ha⁻¹）、低剩余断面积样地高出20%和75%。在林分尺度上，我们假设林分仅由红枫组成，以此模拟总蒸腾量。中、低剩余断面积样地的蒸腾量分别仅为高剩余断面积样地模拟蒸腾量的41%和79%。总体而言，本研究揭示了林分对剩余断面积调控措施的响应存在显著差异，强调需进一步优化森林水量平衡模型，并通过区分林冠层与林下层蒸散量，为温带森林制定更为合理的剩余断面积经营方案。