Abstract Forests significantly influence regional and global water cycles through transpiration, which is affected by meteorological variables, soil water availability, and stand and site characteristics. Variable retention harvesting (VRH) is a forest management practice in which varying densities of trees, such as 55% and 33%, are retained after thinning or harvesting. These trees can be grouped together or evenly distributed. VRH aims to enhance forest growth, improve biodiversity, preserve ecosystem functions, and generate economic revenue from harvested timber. Application of VRH treatment in forest ecosystems can potentially impact the response of forest transpiration to environmental controls. This study analyzed the impacts of four different VRH treatments on sap flow velocity (SV) in an 83‐year‐old red pine ( Pinus resinosa Ait .) plantation forest in the Great Lakes region in Canada. These VRH treatments included 55% aggregated (55A), 55% dispersed (55D), 33% aggregated (33A), and 33% dispersed (33D) basal area retention, and an unharvested control (CN) plot, 1 ha each. Analysis of counterclockwise hysteresis loops between SV and meteorological variables showed larger hysteresis areas between SV and photosynthetically active radiation (PAR) than vapor pressure deficit (VPD) and air temperature ( T air ), particularly in clear sky and warm temperatures in the summer. It demonstrated that PAR was the primary control on SV across VRH treatments, followed by VPD and T air . Larger hysteresis loop areas and higher SV values were observed in the CN and 55D treatments, with lower values found in the 55A, 33D, and 33A plots. This suggests that maintaining dispersed retention of 55% basal area (55D) is the optimal forest management practice that can be utilized to enhance transpiration and forest growth. These findings will assist forest managers and other stakeholders to adopt sustainable forest management practices, thereby enhancing forest growth, water use efficiency, and resilience to climate change. Additionally, these practices will contribute to nature‐based climate solutions.
Plain Language Summary Forests play a crucial role in the regional water cycle and are influenced by various factors, including weather conditions, soil water status, forest characteristics, and management practices. One effective forest management technique is variable retention harvesting (VRH), which involves retaining a portion of trees during logging operations. This method aims to enhance forest growth, biodiversity, ecosystem health, and economic benefits from timber. This study investigated the impact of different VRH methods on stem water transport or transpiration in an 83‐year‐old red pine forest in Canada. The VRH methods examined included different levels of tree retention—55% and 33%—with trees either grouped together or evenly distributed, alongside an unharvested control plot. The findings showed that light (photosynthetically active radiation) was the primary factor affecting water movement, followed by humidity and temperature. The best results for water movement and forest growth were seen in the unharvested and 55% dispersed retention plots. This suggests that retaining 55% of trees evenly spread across the plot is the most effective practice for promoting forest growth and water use. These insights can assist forest managers in choosing the best methods to enhance forest health and resilience to climate change.
Key Points Application of variable retention harvesting treatment in a conifer forest impacted the response of transpiration to environmental controls Hysteresis loops were found between sap flow and meteorological variables in all forest treatments Dispersed retention of 55% basal area was the most optimal forest management practice to enhance transpiration and forest growth