In a context of climate change and forest decline, a better understanding of the sources of tree flexibility involved in phenotypic plasticity and adaptation is needed. These last years, the role of ...epigenetics in the response to environmental variations has been established in several model plants at the genotype level but little is known at the level of natural populations grown in pedoclimatic sites. Here, we focused on three French natural populations of black poplar, a key pioneer tree from watersheds, planted in common garden and subjected to controlled variations of water availability. We estimated common genetic parameters such as narrow-sense heritability (h2), phenotypic differentiation index (PST), and the overall genetic differentiation index (FST) from genome-wide SNPs to evaluate the extent of epigenetic variations. Indeed, global DNA methylation levels from individuals exposed to drought or irrigated in a common garden were used. We found that the three populations were not distinguished by their levels of DNA methylation. However, a moderate drought was associated to a significant decrease in DNA methylation in the populations. Narrow-sense heritability and PST estimates of DNA methylation were similar to those found for biomass productivity. Heritability and PST were higher when trees were subjected to drought than in control condition. Negative genetic correlations between global DNA methylation and height or biomass were detected in drought condition only. Altogether, our data highlight that global DNA methylation acts as a genetic marker of natural population differentiation under drought stress in a pedoclimatic context.
We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides x Populus nigra genotypes grown under two ...contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Psi(50)) varied from -1.60 to -2.40 MPa. Drought-acclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Psi(50) being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Psi(50) was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water-use efficiency. At the whole-plant level, increased safety was associated with higher shoot growth potential under well-irrigated regime only. We conclude that common trade-offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales.
Short‐rotation coppice (SRC) has great potential for supplying biomass‐based heat and energy, but little is known about SRC's ecological footprint, particularly its impact on the water cycle. To this ...end, we quantified the water use of a commercial scale poplar (Populus) SRC plantation in East Flanders (Belgium) at tree and stand level, focusing primarily on the transpiration component. First, we used the AquaCrop model and eddy covariance flux data to analyse the different components of the stand‐level water balance for one entire growing season. Transpiration represented 59% of evapotranspiration (ET) at stand scale over the whole year. Measured ET and modelled ET were lower as compared to the ET of reference grassland, suggesting that the SRC only used a limited amount of water. Secondly, we compared leaf area scaled and sapwood area scaled sap flow (Fs) measurements on individual plants vs. stand scale eddy covariance flux data during a 39‐day intensive field campaign in late summer 2011. Daily stem diameter variation (∆D) was monitored simultaneously with Fs to understand water use strategies for three poplar genotypes. Canopy transpiration based on sapwood area or leaf area scaling was 43.5 and 50.3 mm, respectively, and accounted for 74%, respectively, 86%, of total ecosystem ET measured during the intensive field campaign. Besides differences in growth, the significant intergenotypic differences in daily ∆D (due to stem shrinkage and swelling) suggested different water use strategies among the three genotypes which were confirmed by the sap flow measurements. Future studies on the prediction of SRC water use, or efforts to enhance the biomass yield of SRC genotypes, should consider intergenotypic differences in transpiration water losses at tree level as well as the SRC water balance at stand level.
In this study the seasonal variation in carbon, water and energy fluxes as well as in net primary productivity (NPP) of different tree components is presented for a 2‐year‐old poplar (Populus spp.) ...plantation. A thorough ecophysiological study was performed at ecosystem scale, at tree and at leaf level, in this high‐density bioenergy plantation. Seasonal variation in NPP and fluxes was analysed in relation to meteorological parameters at the field site. The growing season length in terms of carbon uptake was controlled by leaf area development until the maximum leaf area index (LAImax) was reached. Afterwards, a shift to belowground carbon allocation was observed. A dry period in spring caused a reduced leaf area production as well as a decrease in net ecosystem exchange and gross primary production (GPP) due to stomatal closure. Water use efficiency and fine root growth increased in response to limiting soil water availability in the root zone. When soil water availability was not limiting, GPP was controlled by a decrease in solar radiation and air temperature. The results of this study indicate that the productivity of recently established bioenergy plantations with fast‐growing trees is very sensitive to drought. The interaction between soil water availability and factors controlling ecosystem GPP is crucial in assessing the CO2 mitigation potential under future climate conditions.
Vulnerability to drought‐induced cavitation is a key trait of plant water relations. Here, we summarize the available literature on vulnerability to drought‐induced cavitation in poplars (Populus ...spp.), a genus of agronomic, ecological and scientific importance. Vulnerability curves and vulnerability parameters (including the water potential inducing 50% loss in hydraulic conductivity, P50) were collected from 37 studies published between 1991 and 2014, covering a range of 10 species and 12 interspecific hybrid crosses. Results of our meta‐analysis confirm that poplars are among the most vulnerable woody species to drought‐induced cavitation (mean P50 = −1.44 and −1.55 MPa across pure species and hybrids, respectively). Yet, significant variation occurs among species (P50 range: 1.43 MPa) and among hybrid crosses (P50 range: 1.12 MPa), within species and hybrid crosses (max. P50 range reported: 0.8 MPa) as well as in response to environmental factors including nitrogen fertilization, irradiance, temperature and drought (max. P50 range reported: 0.75 MPa). Potential implications and gaps in knowledge are discussed in the context of poplar cultivation, species adaptation and climate modifications. We suggest that poplars represent a valuable model for studies on drought‐induced cavitation, especially to elucidate the genetic and molecular basis of cavitation resistance in Angiosperms.
Over the last 25 years our knowledge with regard to the anatomical, physiological and ecological aspects of vulnerability to cavitation has grown considerably. It is now clear that vulnerability to drought‐induced cavitation is a key trait of plant water‐relations. As poplars (Populus spp.) are among the fastest growing temperate hardwood trees and high water consumers, they are particularly sensitive to water limitations and to drought induced cavitation. The Populus genus has therefore been studied quite intensively. In our review contribution we review and synthesize all the data published in the literature since 1991 on drought induced cavitation in poplar. We discuss and analyse the literature data in terms of (i) genetic variations among and within pure species or hybrids, and (ii) acclimation in response to environmental factors (drought, nutrients, light…). We also link the observations from this comprehensive review with other anatomical and ecophysiological traits. Practical implications, actual gaps in knowledge and future research opportunities offered by poplar as a model tree are also presented.
The ongoing global change could be an additional threat to the establishment and the long-term survival of Populus nigra L., an emblematic European riparian species. With the general aim of gaining ...insights into the adaptive potential of this species, we (i) quantified variations within and among three French P. nigra populations for key physiological attributes, i.e., water-use efficiency (assessed from bulk leaf carbon isotope discrimination, Δ13C), growth performance and related leaf traits, (ii) examined genotype and population by environment interactions, and (iii) explored the relationship between Δ13C and growth. Thirty genotypes were sampled in each of three naturally established populations and grown in two different sites, Orléans (ORL) and Guémené-Penfao (GMN). In ORL, two similar plots were established and different watering regimes were applied in order to test for the drought response. Significant variations were observed for all traits within and among populations irrespective of site and watering. Trait variation was larger within than among populations. The effect of drought was neither genotype- nor population-dependent, contrary to the effect of site. The population ranking was maintained in all sites and watering regimes for the two most complex traits: Δ13C and growth. Moreover, these two traits were unrelated, which indicates that (i) water-use efficiency and growth are largely uncoupled in this species, and (ii) the environmental factors driving genetic structuration for Δ13C and growth act independently. The large variations found within populations combined with the consistent differences among populations suggest a large adaptive potential for P. nigra.
It is hoped that lignocellulosic sources will provide energy security, offset carbon dioxide enrichment of the atmosphere, and stimulate the development of new economic sectors. However, little is ...known about the productivity and sustainability of plant cell-wall energy industries. In this study, we used 16 global circulation models to project the global distribution of relative water availability in the coming decades and summarized the available data on the water-use efficiency of tree- and grass-based bioenergy systems. The data on bioenergy water use were extremely limited. Productivity was strongly correlated with water-use efficiency, with C4 grasses having a distinct advantage in this regard. Our analysis of agro climatic drivers of bioenergy productivity suggests that relative water availability will be one of the most important climatic changes to consider in the design of bioenergy systems.
Poplar coppice plantations for biomass production can be conducted under either short rotation coppice (SRC) or short rotation forestry (SRF) systems, depending on planting density and rotation ...length. It is likely that differences in planting density affect tree physiology through competition for resource acquisition, including light, water and nutrients. In this paper, we hypothesized that the effects of planting density on growth and water-use efficiency (assessed through bulk leaf carbon isotope discrimination, Delta C-13) in poplar depend on site characteristics in terms of soil fertility and water availability. To test this hypothesis, 56 Populus deltoides x P. nigra genotypes were planted under both SRC and SRF and replicated at two sites differing for pedoclimatic conditions. At the most favorable site for growth, trees grown at the higher density (SRC) displayed higher stem height, lower stem circumference, higher specific leaf area, higher mass-based leaf nitrogen contents and higher Delta C-13, indicating that increased tree density mainly accentuated competition for light. Under less favorable conditions, trees grown under SRC still displayed lower stem circumference, higher specific leaf area and higher mass-based leaf nitrogen contents. However, stem height remained unaffected by increasing planting density while Delta C-13 was lower, likely because of increased competition for water availability. Genotypic rankings across planting densities were overall conserved while they were significantly modified across sites, suggesting that rankings for genotypic performances were much less affected by planting density than by site. Realized growth measured after 2 years (height and circumference) was weakly correlated with Delta C-13, but a negative relationship between Delta C-13 and growing season leaf increment rate was observed in most cases. The absence of trade-off between growth and water-use efficiency combined with the large genotypic variations observed for these traits confirms the potential for selecting genotypes with high water-use efficiency without counter-selecting on biomass production in P. deltoides x P. nigra.
To buffer against the high spatial and temporal heterogeneity of the riparian habitat, riparian tree species, such as black poplar ( Populus nigra L.), may display a high level of genetic variation ...and phenotypic plasticity for functional traits. Using a multisite common garden experiment, we estimated the relative contribution of genetic and environmental effects on the phenotypic variation expressed for individual leaf area, leaf shape, leaf structure and leaf carbon isotope discrimination (Delta C-13) in natural populations of black poplar. Twenty-four to 62 genotypes were sampled in nine metapopulations covering a latitudinal range from 48A degrees N to 42A degrees N in France and in Italy and grown in two common gardens at Orl,ans (ORL) and at Savigliano (SAV). In the two common gardens, substantial genetic variation was expressed for leaf traits within all metapopulations, but its expression was modulated by the environment, as attested by the genotype x environment ( G x E) interaction variance being comparable to or even greater than genetic effects. For LA, G x E interactions were explained by both changes in genotype ranking between common gardens and increased variation in SAV, while these interactions were mainly attributed to changes in genotype ranking for Delta C-13. The nine P. nigra metapopulations were highly differentiated for LA, as attested by the high coefficient of genetic differentiation ( Q(ST) = 0.50 at ORL and 0.51 at SAV), and the pattern of metapopulation differentiation was highly conserved between the two common gardens. In contrast, they were moderately differentiated for Delta C-13 ( Q(ST) = 0.24 at ORL and 0.25 at SAV) and the metapopulation clustering changed significantly between common gardens. Our results evidenced that the nine P. nigra metapopulations present substantial genetic variation and phenotypic plasticity for leaf traits, which both represent potentially significant determinants of populations' capacities to respond, on a short-term basis and over generations, to environmental variations.
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