| Hydraulic and stomatal resistances were related to growth and water use in hybrid poplar clones. Poplar trees are valuable for wind protection if they can grow under moisture limitation by maintaining their ability to take up moisture while effectively controlling water loss. Hydraulic resistance is therefore as important as the resistance provided by the stomata. Canopy conductance increased rapidly in the early mornings and then decreased throughout the day, implying a limitation on water supply to the leaves. However, since leaf water potential remained constant (isohydric) for most of the day, stomatal conductance did not control sap flow in the stem, except by regulating the leaf water potential. This meant that the tree's hydraulic resistance, which controlled leaf water supply and leaf water potential was important for combined drought tolerance and growth. Diurnal decrease in root water potential and an increase in hydraulic resistance were inferred from sap velocity patterns. However, irrigation resulted in decreased hydraulic resistance over several weeks in drought-stressed trees, which was attributable to biotic factors. These results implied that leaf water potential and control of root hydraulic resistance were important factors in drought tolerance of poplars. Leaf water potential was less negative in Northwest poplar, which performed better in a dry year than Walker poplar, suffered less shoot dieback and had more sapwood. This suggested that Walker poplar was more likely to sustain xylem cavitation in terminal shoots and in the stem. The clonal differences implied that clone-by-site interactions should be expected, with Walker poplar performing better on wet sites and Northwest poplar performing better on dry sites. |
