Study On The Dynamics Of Water Ascent And Loss During Physiological Drying Process For Living Poplar

Wood is a nature and renewable biomaterial. Its properties are seriously related to the moisture content (MC). Wood will have excellent processing and utilization properties when it is subjected to reasonable drying. However, conventional drying technology needs to consume huge energy. And the new and energy conservation drying technologies, i.e. dehumidification drying, solar drying and heat pump drying etc., cannot be conducted in large-scale due to several reasons. Hence, physiological drying, a new pre-drying technology based on the transpiration, is proposed to reducing water in wood, which has the elegant theoretical and practical significance for saving the energy use by wood drying.In this paper, the sapwood of poplar (Populus alba L. var. pyramidalis) trees was sawed off to cut off water sources from roots. Also, the poplar trees were subjected to several different conditions including keeping the canopy intact, cutting-off the canopy, keep the trunk vertical, leaning 45 degrees and horizontal to the ground. Then, the driving force for reducing water, effects on water reduction, water states and pore structure distribution were explored by measuring the changes of MC, transpiration properties, transverse relaxation time (Y2), the water states and pore structures during the water reduction process. Finaly, the dynamics of water ascent and loss during physiological drying based on the transpiration was summarized. The main research contents and conclusions were:(1) The changes of MC and leaf transpiration properties were measured during reducing water by cutting off the water supply and keeping the canopy intact or sawing off the canopy. The results shown that the MC decreased from 60.54% to 41.32% in 9 days with an average MC reduction rate of 2.30% MC per day after the cut-off of water supply. But the MC decreased rapidly in the first 3 to 5 days. Leaf transpiration was the main driving force to reduce water. The transpiration rate and MC loss were in positive correlations. For control poplar trees and trees sawed off the sapwood, the transpiration rate and stomatal conductance increased with increasing the RH in air, and decreased with increasing the vapor pressure deficit (VPD) and air temperature. The water mainly lost when the temperature differences between air and leaf surface were 1℃癈 to 3℃.(2) Three positions for trunk, vertical, leaning 45° and horizontal, were conducted to explore the effects on changes of MC and transpiration properties. The results indicated that the cut-off of water supply resulted in significant decrease on leaf transpiration rate. However, the different leaning positons for trunk had little influence on transpiration rate as well as MC decrease or water loss. The water lost significantly in the first 3 to 5 days. The transpiration rate and water loss rate increased with the decrease of VPD or increase of RH.(3) Low temperature nuclear magnetic resonance (NMR) and room temperature NMR technology was employed to determine the cut-off value (T2cutoff) for distinguishing free water and bound water in poplar wood. Then, the free water and bound water content, free water to bound water ratio (Ω) and the changes of them were calculated according to the T2cutoff. The results implied that the T2cutoff was different between sapwood and heartwood with 91.07±15.07 ms for heartwood and 36.82±7.12 ms for sapwood. There were two main water states, free and bound water, in wood during the conventional drying process. For poplar trees with the cutoff of water supply, the content of free water and bound water was all decrease with decreasing MC before the leaves withering. The free water content decreased to about 5%, the bound water decrease to about 30%. After leaves withering, the bound water content would increase to about 35% due to the partial free water transferred to bound water. The metabolism ability for control trees increased with a high Ω value, but the metabolism ability for those poplar trees cutting off water supply decreased with a low Ω.(4) The transverse surface relaxivity(p2) was used to transfer the T2 distribution to water-related pore structure distribution. The changes of water-related pore structure distribution of poplar sapwood were analyzed during reducing water. The results shown that the water-related pore structure distribution is from nano-scale to micro-scale. The meso-scale pore (2 nm< r2 < 50 nm) was account for 51.88% with the highest proportion. The nano-macro-scale pore (50 nm< r3< 100 nm) was account for 23.99%, the medium micro-macro-scale pore (1 μm< r5< 15 μm) 10.62%, the small micro-macro-scale pore (100 nm< r4< 1 μm) 8.04%, the large micro-macro-scale pore (15μm< r6< 40μm) 4.59%. The micro-scale pore (r1< 2 nm) and the super large micro-macro-scale pore (r7> 40μm) were account for less 1%. The total water-related pore volume for control poplar trees increased during the testing period, but that for trees sawed off sapwood decreased. During reducing water process, the changes on the volume of r2 were similar with control trees. The volume of r2 increased, but other scales pores volume all decreased. Especially, the volume of r2 and r4 decreased significantly.(5) On the basis of the results aoove, the dynamics of water ascent and loss were proposed as:water loss from leaf due to the vapor pressure deficit between the leaf and air. Such the negative pressure or tension derived in the cell wall of petiole xylem. Then, the negative pressure transfer alternately between cell wall pores(r2, r3, the micro-capillary system) and the vessel and/or fiber cell lumen (r5, r6, the macro-capillary system) to complete the water long-distance ascent.