Transpiration at leaf and tree level in a poplar short-rotation coppice culture: seasonal and genotypic differences

Abstract

Poplar (Populus spp.) is one of the most commonly cultivated genera in experimental and commercial short-rotation coppice (SRC) plantations. The genus is among the fastest growing in temperate latitudes, but the success of highly productive poplar SRC plantations strongly depends on soil water availability. We examined the transpiration at the leaf and the individual tree levels of four different poplar genotypes under an SRC regime. Measurements were performed for the entire growing season of 2016, during the seventh growth year of the plantation and before the third coppice on four poplar genotypes (three individual multiple-stem trees per genotype) belonging to different species and from a different genetic background. The experimental site was a commercial scale multi-genotype SRC plantation, established in Flanders (Belgium). Measurements at the leaf level were performed on specific days of the growing season that differed in evaporative demand, temperature and incoming radiation. To determine the transpiration at the stem level, we measured single-stem sap flow using the stem heat balance (SHB) method and daily stem diameter variation measurements. The whole-tree transpiration was estimated by summing the sap flow rates from all stems. Measurements at the stem level were continuously monitored during the entire growing season. Sap flow was tightly connected to the phenological stage of the trees, thus onset of spring (leaf area development) and late autumn (leaf fall) were easily identifiable from sap flow measurements, showing differences among the four genotypes. The dynamics of transpiration at the leaf and tree level were driven by photosynthetic photon flux density (PPFD), but the sap flow intensity was controlled by vapour pressure deficit (VPD). The four poplar genotypes showed different water use strategies, based on determination of transpiration and other plant water status indicators.