Predicting the decline in daily maximum transpiration rate of two pine stands during drought based on constant minimum leaf water potential and plant hydraulic conductance.

The effect of drought on forest water use is often estimated with models, but comprehensive models require many parameters, and simple models may not be sufficiently flexible. Many tree species, Pinus species in particular, have been shown to maintain a constant minimum leaf water potential above the critical threshold for xylem embolism during drought. In such cases, prediction of the relative decline in daily maximum transpiration rate with decreasing soil water content is relatively straightforward. We constructed a soil-plant water flow model assuming constant plant conductance and daily minimum leaf water potential, but variable conductance from soil to root. We tested this model against independent data from two sites: automatic shoot chamber data and sap flow measurements from a boreal Scots pine (Pinus sylvestris L.) stand; and sap flow measurements from a maritime pine (Pinus pinaster Ait.) stand. To focus on soil limitations to water uptake, we expressed daily maximum transpiration rate relative to the rate that would be obtained in wet soil with similar environmental variables. The comparison was successful, although the maritime pine stand showed carry-over effects of the drought that we could not explain. For the boreal Scots pine stand, daily maximum transpiration was best predicted by water content of soil deeper than 5 cm. A sensitivity analysis revealed that model predictions were relatively insensitive to the minimum leaf water potential, which can be accounted for by the importance of soil resistance of drying soil. We conclude that a model with constant plant conductance and minimum leaf water potential can accurately predict the decline in daily maximum transpiration rate during drought for these two pine stands, and that including further detail about plant compartments would add little predictive power, except in predicting recovery from severe drought.

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.

Recovery of galactoglucomannan from wood hydrolysate using regenerated cellulose ultrafiltration membranes.

Hemicelluloses show promise as a renewable source of raw material for various industrial processes. In this study, galactoglucomannan was recovered from pressurized hot water extract of spruce-sawdust in two steps using hydrophilic regenerated cellulose ultrafiltration membranes having different molecular weight cut-off values. The first step was concentration of galactoglucomannan (GGM) by ultrafiltration using a flat sheet unit and the second step was purification of the retained galactoglucomannan by diafiltration using reverse osmosis filtered water. The highest GGM retention (88%), purity (63%) and recovery (70%) were achieved with the UC005 membrane (cut-off value 5-kDa) at a volume reduction (VR%) of 86%. The UC010 and UC030 membranes (cut-off values 10- and 30-kDa, respectively) partly separated xylan from GGM. Generally, diafiltration did not improve the purity of the GGM due to overlapping of the GGM and lignin molar mass distributions and the fact that most of free low molar mass lignin had already been removed in the concentration filtration step. However, by diafiltration, partial removal of xylan and complete removal of monosaccharides from the GGM rich concentrate was achieved.