Summary• We examined the relationships among productivity, water use efficiency ( WUE) and drought tolerance in 29 genotypes of Populus × euramericana ( Populus deltoides × Populus nigra ), and investigated whether some leaf traits could be used as predictors for productivity, WUE and drought tolerance.• At Orléans, France, drought was induced on one field plot by withholding water, while a second plot remained irrigated and was used as a control. Recorded variables included stem traits (e.g. biomass) and leaf structural (e.g. leaf area) and functional traits [e.g. intrinsic water use efficiency ( W i ) and carbon isotope discrimination ( ∆ )].• Productivity and ∆ displayed large genotypic variability and were not correlated. ∆ scaled negatively with W i and positively with stomatal conductance under moderate drought, suggesting that the diversity for ∆ was mainly driven by stomatal conductance.• Most of the productive genotypes displayed a low level of drought tolerance (i.e. a large reduction of biomass), while the less productive genotypes presented a large range of drought tolerance. The ability to increase WUE in response to water deficit was necessary but not sufficient to explain the genotypic diversity of drought tolerance.

Summary• Here we tested whether some leaf traits could be used as predictors for productivity in a range of Populus deltoides × P. nigra clones. These traits were assessed in 3-yr-old rooted cuttings from 29 clones growing in an open field trial, in a five randomized complete block design, under optimal irrigation.• Variables were assigned to four groups describing productivity (above-ground biomass, total leaf area), leaf growth (total number of leaves increment rate), leaf structure (area of the largest leaf, specific leaf area, carbon and nitrogen contents), and carbon isotope discrimination in the leaves ( ∆ ).• High-yielding clones displayed larger total leaf area and individual leaf area, while no correlation could be detected between productivity and either leaf structure or ∆ . By contrast, ∆ was negatively correlated with number of leaves increment rate and leaf N content.• Our study shows that there is a potential to improve water-use efficiency in poplar without necessarily reducing the overall productivity. IntroductionPoplars are among the fastest growing trees under temperate latitudes. Their large productivity is associated with large water requirements and, as a consequence, a tight dependency of productivity upon water availability (Ceulemans et al ., 1988;Tschaplinski & Blake, 1989;Barigah et al ., 1994;Tschaplinski et al ., 1994;Zsuffa et al ., 1996). To sustain the extension of poplar cultivation from flood plains and bottomlands towards uplands where soil water availability is subjected to seasonal changes, more water-use efficient hybrids are required. One functional trait that could be of interest as an index for improved or maintained productivity under reduced water availability is the intrinsic water-use efficiency (Wi), i.e. the ratio between net CO 2 assimilation and stomatal conductance. Wi can be indirectly estimated at leaf level, via the carbon isotope discrimination ( ∆ ), assuming the occurrence of a linear and negative correlation between ∆ and intrinsic water-use efficiency shown both for cereals (Farquhar & Richards, 1984;Farquhar et al ., 1989) and for trees (Ponton et al ., 2001). This trait is easily measurable, highly heritable and has been used as a selection criterion in crop breeding Rebetzke et al ., 2002). Among tree species, it also displays a high heritability (Brendel et al ., 2002) and a large variability at interspecific level (Guehl et al ., 1998;Bonal et al ., 2000;Ponton et al ., 2001) and provenance level (Lauteri et al ., 1997;Roupsard et al ., 1998) as well as at family level (Brendel et al ., 2002). Such a clonal diversity of ∆ has already been shown to occur among poplars (Zhang et al ., 2004;Marron et al ., 2005). The detection of clones combining satisfactory productivity and high

Though the root biomass of tropical rainforest trees is concentrated in the upper soil layers, soil water uptake by deep roots has been shown to contribute to tree transpiration. A precise evaluation of the relationship between tree dimensions and depth of water uptake would be useful in tree-based modelling approaches designed to anticipate the response of tropical rainforest ecosystems to future changes in environmental conditions. We used an innovative dual-isotope labelling approach (deuterium in surface soil and oxygen at 120-cm depth) coupled with a modelling approach to investigate the role of tree dimensions in soil water uptake in a tropical rainforest exposed to seasonal drought. We studied 65 trees of varying diameter and height and with a wide range of predawn leaf water potential (Ψpd) values. We confirmed that about half of the studied trees relied on soil water below 100-cm depth during dry periods. Ψpd was negatively correlated with depth of water extraction and can be taken as a rough proxy of this depth. Some trees showed considerable plasticity in their depth of water uptake, exhibiting an efficient adaptive strategy for water and nutrient resource acquisition. We did not find a strong relationship between tree dimensions and depth of water uptake. While tall trees preferentially extract water from layers below 100-cm depth, shorter trees show broad variations in mean depth of water uptake. This precludes the use of tree dimensions to parameterize functional models.

Hydraulic lift (HL) by tree roots in a young, broad-leaved, mixed temperate European forest was investigated during the 2008 growing season by injecting 18 O-enriched soil water at a depth of 75-90 cm under drought conditions experimentally imposed in a rainexclusion system. Based on sap flow, leaf water potential, 2-D root distribution measurements, soil isotope profiles, and xylem water isotope composition, water acquisition and use by two tree species, beech (Fagus sylvatica) and oak (Quercus petraea) was compared. We showed that, unlike oak, beech experienced a marked decrease in sap flow and predawn leaf water potential with increasing soil drought. This behaviour was logical considering the shallower root system in beech than in oak. Six days after 18 O-labelling, we observed isotopic enrichment in the shallower soil layers. Since the intermediate soil layers did not display any enrichment, our results clearly pointed to hydraulic lift by tree roots. The superficial enrichment that was observed in the vicinity of oak trunks and the increase in the isotopic signature of xylem sap in the oak trees but not in the beech trees confirmed the predominant role of oak in the hydraulic lift at our site. Even though facilitation for water acquisition among species was not observed here, our results suggest a potential positive contribution of species like oak toward maintaining species diversity in mixed forest ecosystems submitted to severe drought events.

-Nitrogen concentration [N] and isotopic composition (δ 15 N) in annual growth rings of 16-year-old beech trees (Fagus sylvatica L.) were measured before and after treatment of wood using organic solvents. The trees, grown under field conditions in Northeastern France, were labelled with 15 N-enriched urea solution at leaf level for three successive years (1993, 1994 and 1995), and were sampled in 2001. δ 15 N values in this experiment ranged from -4 to -7‰ for non-labelled tree. Labelling resulted in markedly increased δ 15 N of whole wood for years 1994, 1995 and 1996. Significant labelling-induced increases, compared to non-labelled rings, in δ 15 N were also found in the rings formed before and after the labelling period, pointing to an important mobility of N across the rings. The extraction removed 36% of total nitrogen and 14% of total carbon.