Evapotranspiration of a high-density poplar stand in comparison with a reference grass cover in the Czech–Moravian Highlands

Abstract

This study reports on 3.5 years (2008–2011) of actual evapotranspiration (ET) measurements of two short-rotation poplar coppice (SRC) plantations in comparison with two reference grass covers (clipped turf grass and permanent grassland) in a rain-fed area of the Czech–Moravian Highlands in the Czech Republic. The Bowen ratio/energy balance method (BREB) was used to measure the ET of all the four investigated covers. Although the ET of the SRCs sometimes exceeded that of grass vegetation or the reference evapotranspiration (ET0) on an hourly, daily or monthly time scale, it was always lower over an entire annual cycle. The cumulative annual ET of the SRCs ranged from 344 to 549mmyear−1, whereas that of the turf grass ranged from 561 to 573mmyear−1, and the ET of the grassland reached up to 619mmyear−1. The wide range of the annual ET totals for the SRCs was determined mainly by the different rotation periods and ages of the stands, along with the leaf area index (LAI), the state of the canopy closure, and tree heights. The lowest annual ET (344mm) was observed during the 1st year of the 2nd rotation (following the winter harvest) in 2010. During that year, the SRC regrowth started only after mid-June, and the maximum LAI of the canopy, which was approximately 2m in height and was not completely closed, reached 3.7. In contrast, in the case of a mature stand approximately 12m in height in 2009, i.e., the 8th year of the 1st rotation, the maximum canopy LAI reached 7.5, and the annual ET reached its highest observed value (549mm). The leaf area duration of the SRCs was always lower than that of the grass covers, and this seemed to be the main reason for the significantly lower annual ET totals of the SRCs. The absolute maximum daily ET of 6.3mmday−1 was achieved by the SRC in July 2011 when the stand was in the 2nd year of the 2nd rotation cycle. The highest daily ET of the grass covers (5.9mmday−1) was reached by turf grass in June 2011. The maximum monthly ET occurred in July 2009 when the weather was wet and relatively warm; the ET was identical for the SRCs and the turf grass, reaching 107mmmonth−1, just slightly lower than the ET0 (109mmmonth−1). These results are in the middle range of ET values from 21 recent studies on poplar and willow cultures. The results of this study indicate that conversion of arable land into poplar SRCs instead of grasslands does not result in significant differences in ET of the agricultural landscape.