抄録
The establishment of the technology for a large scale afforestation of arid land for carbon fixation on land is required. The effective water use is very important for afforestation of arid land, because arid land has the problem of water shortage for plant growth. The object of this study is the establishment of a method to estimate water/carbon ratio (RW/C) of tree in an arid land. In the present study, we measured to the nutrients in a tree tissue and sap for estimating RW/C more accurately. Nutrients go into a tree with water from soil, and are fixed in the tree with carbon. Six tree species were used for our experiment. The distribution of nutrients in each part of a tree were measured. Furthermore, dry weight and nutrient content in each part (leaf, branch and trunk) of a tree were measured. Using the above data, the water quantity required to fix unit carbon for a tree (RW/C) was calculated for potassium which was uniformly distributed, while calcium contents in each part and sap of the tree were found to be different at different levels (height). A modified model to calculate the RW/C was constructed by the consideration of calcium distribution to get about 40-300kg-water/kg-carbon, which was almost same as that for potassium as tracer. Finally, the RW/C based on calcium content agreed well with the ratio calculated from measured photosynthetic and transpiration rates. It was also measured that these values (RW/C) were large for Eucalyptus, small for Acacia and middle for Casuarina. These can explain a natural growth of these trees in an experimental arid land in Western Australia.
