SummaryThe pulse of the tree (diurnal cycle of stem radius fluctuations) has been widely studied as a way of analyzing tree responses to the environment, including the phenotypic plasticity of tree–water relationships in particular. However, the genetic basis of this daily phenotype and its interplay with the environment remain largely unexplored. We characterized the genetic and environmental determinants of this response, by monitoring daily stem radius fluctuation (dSRF) on 210 trees from a Eucalyptus urophylla × E. grandis full‐sib family over 2 years. The dSRF signal was broken down into hydraulic capacitance, assessed as the daily amplitude of shrinkage (DA), and net growth, estimated as the change in maximum radius between two consecutive days (ΔR). The environmental determinants of these two traits were clearly different: DA was positively correlated with atmospheric variables relating to water demand, while ΔR was associated with soil water content. The heritability for these two traits ranged from low to moderate over time, revealing a time‐dependent or environment‐dependent complex genetic determinism. We identified 686 and 384 daily quantitative trait loci (QTL) representing 32 and 31 QTL regions for DA and ΔR, respectively. The identification of gene networks underlying the 27 major genomics regions for both traits generated additional hypotheses concerning the biological mechanisms involved in response to water demand and supply. This study highlights that environmentally induced changes in daily stem radius fluctuation are genetically controlled in trees and suggests that these daily responses integrated over time shape the genetic architecture of mature traits.

Summary In the context of climate change, the water‐use efficiency (WUE) of highly productive tree varieties, such as eucalypts, has become a major issue for breeding programmes. This study set out to dissect the genetic architecture of carbon isotope composition (δ13C), a proxy of WUE, across several environments. A family of Eucalyptus urophylla × E. grandis was planted in three trials and phenotyped for δ13C and growth traits. High‐resolution genetic maps enabled us to target genomic regions underlying δ13C quantitative trait loci (QTLs) on the E. grandis genome. Of the 15 QTLs identified for δ13C, nine were stable across the environments and three displayed significant QTL‐by‐environment interaction, suggesting medium to high genetic determinism for this trait. Only one colocalization was found between growth and δ13C. Gene ontology (GO) term enrichment analysis suggested candidate genes related to foliar δ13C, including two involved in the regulation of stomatal movements. This study provides the first report of the genetic architecture of δ13C and its relation to growth in Eucalyptus. The low correlations found between the two traits at phenotypic and genetic levels suggest the possibility of improving the WUE of Eucalyptus varieties without having an impact on breeding for growth.

Genetic maps are key tools in genetic research as they constitute the framework for many applications, such as quantitative trait locus analysis, and support the assembly of genome sequences. The resequencing of the two parents of a cross between Eucalyptus urophylla and Eucalyptusgrandis was used to design a single nucleotide polymorphism (SNP) array of 6000 markers evenly distributed along the E.grandis genome. The genotyping of 1025 offspring enabled the construction of two high-resolution genetic maps containing 1832 and 1773 markers with an average marker interval of 0.45 and 0.5cM for E.grandis and E.urophylla, respectively. The comparison between genetic maps and the reference genome highlighted 85% of collinear regions. A total of 43 noncollinear regions and 13 nonsynthetic regions were detected and corrected in the new genome assembly. This improved version contains 4943 scaffolds totalling 691.3Mb of which 88.6% were captured by the 11 chromosomes. The mapping data were also used to investigate the effect of population size and number of markers on linkage mapping accuracy. This study provides the most reliable linkage maps for Eucalyptus and version 2.0 of the E.grandis genome.

The association of N-2-fixing species (NFS) could be an attractive option for achieving a sustainable increase of Eucalyptus plantations (EP) productivity through a positive balance between facilitative effects and competition between species. A randomised block design was replicated at four sites (Cenibra, USP, Suzano and IP) in Southern Brazil and at one site in Congo. The development of mono-specific stands of Acacia mangium (100A) and Eucalyptus grandis or urophylla x grandis (100E), was compared with N fertilisation treatment (100E + N) and with mixed-species plantations in a 1:1 ratio (50A:50E), and in an additive series with varying densities of acacia for the same density of eucalypt (25A:100E, 50A:100E, 100A:100E). The objectives were to assess the effect of mixtures on tree growth and stand production, and the behaviour of the two species in contrasting soil and climatic conditions. Tree growth was monitored over stand rotation and the biomass of aboveground tree components estimated at mid-rotation and at harvesting age. Eucalyptus height was 13% higher in Brazil than in Congo. Favourable ecological conditions in Congo and Cenibra led to 50% higher Acacia tree height than at the other sites. A depressive effect of Eucalyptus neighbour trees on Acacia height and circumference growth, lower in Congo than in Brazil, was observed in the mixtures from age 1-2 years onwards. Depressive effects of acacia on eucalypt height and circumference growth were low in USP, Suzano and IP, high in Cenibra, and not observed in Congo, in 50A:50E and 25A:100E. A positive though insignificant response to N fertilisation was only found in USP and Congo. Complementarity for light and soil resource capture between Eucalyptus and Acacia trees resulted in mean annual increments in total stand stemwood biomass (MAI) that were 7-15%, 6-12%, and 40% higher in the additive series than for 100E in Cenibra, LISP and Congo, respectively at mid-rotation. Whilst lasting complementarity and facilitation in Congo led to 17-34% higher MAI in mixtures than for 100E at harvesting age, MAIs were not significantly higher in mixtures than for 100E in Brazil. Mixed-species plantations of Eucalyptus and A. mangium might enhance aboveground stand production on poor nutrient soils in warm and humid tropical climates with low water limitations. (c) 2012 Elsevier B.V. All rights reserved.

In the context of climate change, water availability will be the main limiting factor affecting biomass production, in particular in intensively managed forest tree plantations. The complexity of drought stress response calls for integrative approaches combining physiological, anatomical and molecular investigations at different scales ranging from particular cells to the whole plant. Such a strategy was developed for two eucalyptus clones used in industrial plantations in the Congo Republic and known for their different water-use efficiency. These two genetic units were submitted to two watering modes in open field conditions and followed during 18 months. We will describe and compare the molecular plasticity of these two genotypes at the transcriptomic (qPCR on candidate genes and profiling without a priori based on 454 sequencing) and proteomic (2DE combined with tandem MS) levels. We will interpret the results based on the analysis of reaction norms at the phenotypic level. By doing so, this study intend to identify genes of adaptive significance to environmental constraints. (Texte integral)

Specific leaf area (SLA; m2leafkg−1leaf) is a key ecophysiological parameter influencing leaf physiology, photosynthesis, and whole plant carbon gain. Both individual tree-based models and other forest process-based models are generally highly sensitive to this parameter, but information on its temporal or within-stand variability is still scarce. In a 2–4-year-old Eucalyptus plantation in Congo, prone to seasonal drought, the within-stand and seasonal variability in SLA were investigated by means of destructive sampling carried out at 2-month intervals, over a 2-year period. Within-crown vertical gradients of SLA were small. Highly significant relationships were found between tree-average SLA (SLAt) and tree size (tree height, Ht, or diameter at breast height, DBH): SLAt ranged from about 9m2kg−1 for dominant trees to about 14–15m2kg−1 for the smallest trees. The decrease in SLAt with increasing tree size was accurately predicted from DBH using power functions. Stand-average SLA varied by about 20% during the year, with lowest values at the end of the 5-month dry season, and highest values about 2–3 months after the onset of the wet season. Variability in leaf water status according to tree size and season is discussed as a possible determinant of both the within-stand and seasonal variations in SLA.

• Sustainability of Eucalyptus plantations is often questioned in resource-limited environments, especially in areas characterized by soils with poor nutrient and water holding capacities. Yet, field-based observations of fine root dynamics in relation with the seasonality of rainfall are lacking.

Eucalyptus plantations have been introduced since 1978 on savannah soils of the coastal plains of Congo, but there is still little information on the effect of silvicultural practices on soil organic carbon dynamics after afforestation on these savannahs. The objectives of this study were to assess the effects of two experimental site preparation treatments on soil CO2 efflux, tree growth and soil carbon balance during the first year following plantation establishment. One treatment involved mechanical soil disturbance with disk harrowing (D), whereas in the second treatment (H), savannah grasses were killed by herbicide application before planting, without mechanical soil disturbance. Soil respiration and soil water content were monitored for 1 year following treatment application, at 2-week intervals. We hypothesized that mechanical soil disturbance would increase soil CO2 efflux, but the results did not support this hypothesis. The cumulated soil CO2 efflux over 1 year was not significantly different in the two treatments and averaged 658gCm−2. In contrast, tree growth was significantly increased by disk harrowing, maybe as a result of decreased soil penetration resistance. Carbon inputs to the soil from savannah residues (428gCm−2) were outweighed by the annual carbon outputs through heterotrophic respiration (505 and 456gCm−2 in the H and D treatments, respectively) leading to a slightly negative soil carbon budget in both treatments 1 year after afforestation.

Since October 2000, CO2 and water fluxes are measured within a young eucalypt stand in Congo. In this experiment, our main objective is to derive the net carbon ecosystem exchange (carbon sequestration) from continuous eddy flux measurements, and to compare these estimates to those obtained from 1) measurements of Net Primary Productivity (NPP) and soil heterotrophic respiration, and 2) measurements of carbon stocks (soil and biomass) and their variations over a chronosequence. Carbon stocks and soil respiration measurements were obtained over a chronosequence that includes 6 stands from 6 months up to 10 years within a 43000 ha massif of clonal eucalypt plantations established around Pointe Noire (4°S 12°E, Republic of Congo) over poor, sandy, and highly desaturated soils previously covered by littoral savannas. The Eddy correlation measurements were obtained from the top of a tower erected within the 3 year-old stand of the chronosequence. Soil CO2 efflux (monitored with a respiration chamber connected to a portable Li6200 infrared gas analyzer) exhibited strong seasonal variations, reflecting the seasonal changes in soil water content. Maximum values were obtained during the wet season, while minimum values were obtained in September-October at the end of the dry season (that lasts from June to October). Annual soil CO2 efflux was 11.8 t C/ha at Eddy correlation site, and 16.7 t C/ha at the 10 year old stand. At each site, good relationships were obtained between soil volumetric water content (measured from 0 to 6 cm from the surface) and soil respiration. The soil moisture effect on soil respiration was easily described by a 3 parameters equation. Rhizospheric and heterotrophic contributions to total soil CO2 efflux were estimated from comparison of soil CO2 efflux measured over trenched plots and soil CO2 fluxes measured over non-trenched plots: at the eddy correlation site (3 year-old stand), root respiration contributed to 26% of total annual CO2 efflux. First results obtained at other stands indicate that root contribution to total soil C02 efflux increases with stand age (e.g. around 57% at the 6 year-old stand), probably as a result of root biomass increase with stand age. Similar to soil CO2 efflux, NEE measured by eddy correlation exhibited strong seasonal variations with lowest NEE values (highest CO2 uptake) obtained during the wet season, when LAI and REW (relative extractable water) were maximum. At this time, minimum diurnal peaks of NEE were typically about -25 micromol m-2 s-1. By contrast, minimum peaks obtained at the end of the dry season were about - 12 micromol m-2 s-1. Monthly NEP ranged from -85.3 g C m-2 up to 29.8 g C m-2. This positive value (net carbon emission) has been observed at the transition between the dry and the wet season, and resulted from a faster increase of ecosystem respiration (estimated from nocturnal NEE measurements) than photosynthesis, after the first rains. Daily gross primary production (GPP) was found to be strongly correlated with daily actual evapo-transpiration (AET), resulting in water use efficiencies (WUE-GPP) of about 5.2 g DM/kg H2O. For the two-year period covered by eddy correlation measurements (October 2000 to October 2002), mean annual net C exchange (NEP) was -370 g C m-2/year. This corresponds to a net carbon uptake by the stands, but represents a small fraction of GPP (-1990 g C m-2/year), due to important carbon loss by ecosystem respiration (1620 g C m-2/year). Aboveground respiration, root respiration, and heterotrophic soil respiration represented 27% (438 g C m-2/year), 19% (307 g C m-2/year) and 54% (874 g C m-2/year) of ecosystem respiration, respectively. Over the same period NPP was -1203 g C m-2/year (-676 g C m-2/year for total tree biomass increment, and -527 g C m-2/year for fine root turnover and litter production). Summing NPP and heterotrophic respiration provides another estimates of NEP (-330 g C m-2/year), slightly lower to the one obtain