Biomedical subjects

E Nikinmaa

Publications and source records attributed to E Nikinmaa.

3 recordsLinked to original sources

PubMed2005

Field measurements of ultrasonic acoustic emissions and stem diameter variations. New insight into the relationship between xylem tensions and embolism.

We examined interrelated xylem water tensions and embolism dynamics under field conditions by simultaneously monitoring ultra-acoustic emissions and changes in stem xylem diameter. Variation in stem xylem diameter was measured with linear displacement transducers to estimate variation in sap tension. Measured ultrasonic acoustic emissions coincided well with changes in xylem diameter, indicating that individual peaks in embolism occurred simultaneously with peaks in water tension. The good time resolution between measurements makes this method especially suitable for observing embolism dynamics on a short timescale. Longer lasting measurements can also be made to monitor inter-daily patterns in water tension and embolism because the techniques are non-destructive. Ultra-acoustic emissions occurred mainly during periods of decreasing stem xylem diameter, i.e., increasing water tension, when the water tension was high enough. Embolism also occurred during periods of increasing xylem diameter, i.e., decreasing water tension, but the number of embolizing conduits under these conditions was small.

Pinus sylvestris↗

PubMed2002

Relationships between embolism, stem water tension, and diameter changes.

A model for embolism in the sapflow process was developed, in which embolism is described as a physical process linked to real physical properties of the conduits and the thermodynamic state of water. Different mechanisms leading to embolism and their effect on water relations and especially diurnal diameter changes in a tree were examined. The mechanisms of heterogeneous nucleation, air-seeding, and bubble growth have been considered. The significance of embolism has been revealed here by examining diameter changes, which is an easily measurable quantity under field conditions. The most fundamental effects of embolism on sapflow are decrease in permeability and release of water from embolizing conduits to the transpiration stream. These can be indirectly detected by observing diameter changes. If possible changes in elasticity are not accounted for, embolism generally tends to enhance the amplitude of the diurnal diameter changes due to reduced permeability and increased tensions. In the case of reduced elasticity, embolism gives rise to smaller amplitudes of diameter changes.

Circadian Rhythm↗

PubMed2001

Tree stem diameter variations and transpiration in Scots pine: an analysis using 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 water tension in accordance with Hookeâ 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 degrees 51' N, 24 degrees 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.

Models, Biological↗