Transpiration and water relations of poplar trees growing close to the water table

Sap flow was measured on five branches of two poplar (Populus trichocarpa T orr. & A. Gray x P. tacamahaca L.) trees from June to September 1994 in the south of England with stem-surface, heat balance gauges, and was scaled up to estimate transpiration from single trees on the basis of leaf area. On six days, stomatal conductance and plant water potential were measured simu ltaneously with a porometer and pressure chamber, respectively. The effects of solar radiation (S), vapor pressure deficit (D) and stomatal conductanc e on transpiration were evaluated. Sap flow per unit leaf area (F-a) was cl osely related to the time course of demand attributable to S and D througho ut the season, and only slightly affected by the water content of the top 1 20 cm of soil. Although F-a increased linearly at low values of D, it showe d a plateau with increases in D above 1.2 kPa. The canopy coupling coeffici ent (1 - Ohm) ranged from 0.48 to 0.78 with a mean of 0.65 +/- 0.01, indica ting that transpiration was controlled more by stomatal conductance than by incident radiation. The seasonal pattern of tree water loss followed poten tial evaporation with a peak in late June or early July. On bright days, da ily transpiration over the projected crown area was 3.6 mm early in the sea son, 3.8 mm in mid-season, and 2.7 mm late in the season. The water balance of the system indicated that poplar trees took 15-60% of water transpired from groundwater, with the proportion increasing as the soil in the unsatur ated zone dried out. Access to the water table resulted in high predawn wat er potentials throughout the season. Estimated hydraulic resistance to wate r flow in the soil-tree system was in the range of 1.5 to 1.93 x 10(6) MPa s m(-3).