Maximum transpiration rate and water tension during drought

Transpiration often exceeds rainfall even in the humid boreal forests. Although the soil water pool filled up during autumn and winter is large, it may be depleted during long spells of high-pressure weather and drought is developed. As the soil water tension (PS) becomes increasingly negative during a drought, water transport in the soil-plant-atmosphere system becomes increasingly difficult. In a simple one-dimensional representation of flow, water flow from the bulk soil to the leaf is proportional to the conductance of the pathway, and the tension difference between soil and leaf.