Tree stem diameter variations and transpiration in Scots pine: an analysisusing a dynamic sap flow model

A dynamic model for simulating water flow in a Scots pine (Pinus sylvestris L.) tree was developed. The model is based on the cohesion theory and the assumption that fluctuating water tension driven by transpiration, together with the elasticity of wood tissue, causes variations in the diameter of a tree stem and branches. The change in xylem diameter can be linked to wate r tension in accordance with Hooke's law. The model was tested against fiel d measurements of the diurnal xylem diameter change at different heights in a 37-year-old Scots pine at Hyytiala, southern Finland (61 degrees 51 'N, 24 degrees 17 'E, 181 in a.s.l.). Shoot transpiration and soil water potent ial were input data for the model. The biomechanical and hydraulic properti es of wood and fine root hydraulic conductance were estimated from simulate d and measured stem diameter changes during the course of day. The estimate d parameters attained values similar to literature values. The ratios of es timated parameters to literature values ranged from 0.5 to 0.9. The model p redictions (stem diameters at several heights) were in close agreement with the measurements for a period of 6 days. The time lag between changes in t ranspiration rate and in sap flow rate at the base of the tree was about ha lf an hour. The analysis showed that 40% of the resistance between the soil and the top of the tree was located in the rhizosphere. Modeling the water tension gradient and consequent woody diameter changes offer a convenient means of studying the link between wood hydraulic conductivity and control of transpiration.