Evaluation of fast growing tree water use under different soil moisture regimes using wick lysimeters

The cultivation of fast growing trees has high potential for bioenergy production on marginal lands. Information about the water use efficiency in combination with the ecophysiological response to drought stress is still incomplete. For the study of the transpiration-biomass relations satisfactory results can be obtained under controlled conditions by using lysimeters. Nonetheless, several technical problems related to the manufacturing can bias the outcome. Pan lysimeters are economical and relatively easy to assemble, however, saturation at the lower boundary can happen. On the other hand tension lysimeters, where the suction is applied by a vacuum pump, are effective but expensive and need constant maintenance. To avoid stagnancy and reduce production costs we assembled a tension controlled wick lysimeter, where the tension at the bottom layer is actively exerted by a hanging water column through the wick material (rockwool). We aim to assess the performance of the wick lysimeter coupled with an automatic irrigation system for the study of water use-yield relation black locust (Robinia pseudoacacia L.) under different irrigation regimes. The volumetric soil content for each treatment, namely low water (LW), medium water (MW) and high water amount (HW), was maintained at 0.07, 0.14 and 0.2 m3 m-3 respectively. During the vegetation period (161 days) the plants growing at LW reduced their water use by 60%, their leaf area by 40% and the total above ground biomass by 46%. As a result, we found a linear correlation (R2 = 0.98) between the relative biomass produced and the relative water use. The calculated relative water use efficiency (RWUE) for black locust total above ground dry biomass is 0.96. Furthermore, after the harvest we evaluated the effectiveness of the lysimeter and irrigation system: the soil moisture of the treatments was kept up at the predefined values and an unsaturated condition at the boundary layer was maintained throughout the vegetation period. Because of the improved design no roots grew into the drainage extension and no unexpected change of the lysimeters hydraulic property occurred. Considering the relatively low-cost material used and the low maintenance needed, compared to other tension controlled lysimeters, wick lysimeters are effective tools for forestry studies of young trees.