© 2001 Heron Publishing—Victoria, Canada
Tree stem diameter variations and transpiration in Scots pine: an analysis using a dynamic sap flow model
Martti Perämäki (1), Eero Nikinmaa (1), Sanna Sevanto (2), Hannu Ilvesniemi (1), Erkki Siivola (3), Pertti Hari (1) and Timo Vesala (2)
1. Department of Forest Ecology, P.O. Box 24, FIN-00014 University of Helsinki, Finland / 2. Department of Physics, P.O. Box 9, FIN-00014 University of Helsinki, Finland / 3. Laboratory of Applied Electronics, Helsinki University of Technology, Otakaari 5 A, FIN-02150, Espoo, Finland / Received August 18, 2000
Summary
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 water tension in accordance with Hooke's law. The model was tested against field
measurements of the diurnal xylem diameter change at different heights in a 37-year-old Scots pine at Hyytiälä, southern Finland
(61°51′ N, 24°17′ E, 181 m a.s.l.). Shoot transpiration and soil water potential were input data for the model. The biomechanical
and hydraulic properties of wood and fine root hydraulic conductance were estimated from simulated and measured stem diameter
changes during the course of 1 day. The estimated parameters attained values similar to literature values. The ratios of estimated
parameters to literature values ranged from 0.5 to 0.9. The model predictions (stem diameters at several heights) were in
close agreement with the measurements for a period of 6 days. The time lag between changes in transpiration rate and in sap
flow rate at the base of the tree was about half 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.
Keywords:
Pinus sylvestris, water tension, xylem diameter change.
