Water table depth affects Populus alba fine root growth and whole plant biomass

To increase understanding of the water table's influence on both fine-root growth and whole-plant growth of tree species in arid and semi-arid regions, we exposed Populus alba L. cuttings to contrasting soil water conditions via various water table gradients. One-year-old rooted cuttings were grown individually in pots containing sandy soil in a greenhouse for 90 days in four water-table depth treatments: constant depth at 45 cm from the soil surface, constant depth at 30 cm, constant depth at 15 cm, and fluctuating depths between 45 and 30 cm. Growth responses, biomass partitioning, and structure and morphology of leaves and roots were determined for harvested trees every 30 days. Fine (< 1-mm diameter) root growth was affected by water-table levels and water profiles, even in the fluctuating water-table depth treatment. Fine root proliferation was inhibited below the water table and was stimulated in the layers just above the water table and near the soil surface. At the whole-plant level, with deeper water tables, P. alba allocated more biomass to roots and root morphology changed, but total root length did not increase, suggesting that trees faced with soil water deficits are not likely to increase root surface area to obtain more water. Total biomass and root length in each treatment were positively correlated and the relationships were similar among the treatments, although the growth responses varied under the various water-table conditions. These results suggest that variation in root length may have an important effect on the biomass of the rooted cuttings across a gradient of water-table depths.