Transpiration and water use strategies of a young and a full-grown short rotation coppice differing in canopy cover and leaf area

Abstract

In order to evaluate the influence of canopy cover and leaf area on the water use of short rotation coppices (SRC), we measured evapotranspiration and transpiration of a young and a full-grown mature poplar SRC throughout one growing season, using the Bowen-ratio energy balance method and sap flux technique, respectively. The young SRC at Fuhrberg had a sparse though developing canopy reaching a maximum leaf area index (LAI) of 3.8m2m−2 in August, while the mature SRC at Großfahner earlier reached maximum LAI at 7.4m2m−2. Despite contrasting canopy densities, growing season total evapotranspiration (Fuhrberg: 380mm, Großfahner: 445mm) and transpiration ratios did not differ substantially, because understorey transpiration and soil evaporation probably compensated low tree transpiration in the first half of the growing season at LAI<3m2m−2. Mid-season mean daily transpiration on rainless days was 2.34±0.13mmd−1 at Fuhrberg and 3.16±0.81 at Großfahner. The values for the full-grown SRC at Großfahner were in the middle range of reported values for poplar SRCs of comparable LAI and canopy density, and came from efficient stomatal regulation of transpiration, in which poplar clones might differ. Bulk canopy conductance (gc), calculated by inverting the Penman–Monteith equation and related to vapor pressure deficit (D), revealed stomatal control of transpiration at the mature plantation, and according to a simple hydraulic model, was sufficient to maintain a minimum leaf water potential at high atmospheric demand. This indicated isohydric behavior and marks a conservative water use strategy, which avoids water stress by limiting transpiration rates at high D and might be typical for the investigated poplar hybrid (J-105). The young plantation exhibited a similar water use strategy, when LAI was above 3m2m−2. Before canopy closure, the ratio of stomatal sensitivity (m) and reference conductance (gcref, i.e. gc @ D=1kPa), which are parameters of a logarithmical response curve of gc to D, was significantly lower than the theoretical ratio for isohydric responses. This indicates poor stomatal control of water loss and reflected the contribution of understorey-transpiration and soil evaporation to total stand evapotranspiration, which might increase severely at high evaporative demand in sparse poplar stands.