Assessing the environmental controls on Scots pine transpiration and the implications for water partitioning in a boreal headwater catchment

Climate change projections indicate reduced summer precipitation and increased air temperature for the northern high latitudes. These climate changes are likely to alter forest water budgets of which plant transpiration (T) forms a significant component. Plant transpiration is regulated by stomata behavior of particular species, which is constrained by ambient air and soil conditions. Here, we measured sap flow in a Scots pine (Pinus Sylvestris) plantation in a low energy Scottish headwater catchment during the main summer growth period. Effects of rainfall on forest transpiration, as well as the response of T to four environmental variables were investigated at a daily scale. In this boreal environment, transpiration was mainly restricted by radiation and vapor pressure deficit. Air temperature was the least important controlling factor. Soil water became an important factor when rainfall was limited. Frequent but small rain events dictated that precipitation met short-term transpiration demand most of the time. The trees needed supplementary water from antecedent soil water stores when weekly rainfall was below ∼8 mm, but such periods were rare. Water exchange mainly occurred in the canopy or upper 10 cm of the soil, with 47% of rainfall transpired, 45% intercepted and <8% evaporated from the soil surface. Understanding interactions between forests and their hydroclimate, as well as the role of forests in water partitioning is crucial to assist a sustainable land and water management in a changing climate. Whilst such studies are common in semi-arid regions, they are limited in boreal zones, therefore, our findings are a valuable contribution to understanding plant-water relations in a changing environment.