There's no place like home: seedling mortality contributes to the habitat specialisation of tree species across Amazonia

Understanding the mechanisms generating species distributions remains a challenge, especially in hyperdiverse tropical forests. We evaluated the role of rainfall variation, soil gradients and herbivory on seedling mortality, and how variation in seedling performance along these gradients contributes to habitat specialisation. In a 4-year experiment, replicated at the two extremes of the Amazon basin, we reciprocally transplanted 4638 tree seedlings of 41 habitat-specialist species from seven phylogenetic lineages among the three most important forest habitats of lowland Amazonia. Rainfall variation, flooding and soil gradients strongly influenced seedling mortality, whereas herbivory had negligible impact. Seedling mortality varied strongly among habitats, consistent with predictions for habitat specialists in most lineages. This suggests that seedling performance is a primary determinant of the habitat associations of adult trees across Amazonia. It further suggests that tree diversity, currently mostly harboured in terra firme forests, may be strongly impacted by the predicted climate changes in Amazonia

Linking functional leaf and root traits with endophytic communities in seedlings in a seasonally flooded tropical forest.

An important unanswered question in the understanding of plant strategies is the extent to which leaf and root microbial communities contribute to tropical tree species functioning. Leaves and roots of tropical tree species harbor taxonomically diverse bacterial and fungi assemblages which enhance plant growth and performance by increasing nutrient and water supply and resistance to stress. Water excess or shortage, found in seasonally flooded (SF) forests in the Amazon basin, has been overlooked as being an important plant stressor in these particular habitats. Few studies have linked the ecophysiological functioning with the microbial composition of SF tropical tree seedlings. Our objective is to answer how functional traits are linked to the diversity and composition of leaf and root associated microbial communities in SF tree seedlings. We investigated 13 leaf and root functional traits, in 70 individuals belonging to seven tropical 1-year old SF tree seedlings. Seedlings were sampled exclusively in the seasonally flooded forests of French Guiana. The investigation of the diversity and composition of leaf and root associated microbial communities was conducted using ITS2 and 16S high-throughput sequencing. This study will confer robust knowledge of SF tropical forests through the lens of their microbial composition and functional traits syndrome

Linking functional leaf and root traits with endophytic communities in seedlings in a seasonally flooded tropical forest.

An important unanswered question in the understanding of plant strategies is the extent to which leaf and root microbial communities contribute to tropical tree species functioning. Leaves and roots of tropical tree species harbor taxonomically diverse bacterial and fungi assemblages which enhance plant growth and performance by increasing nutrient and water supply and resistance to stress. Water excess or shortage, found in seasonally flooded (SF) forests in the Amazon basin, has been overlooked as being an important plant stressor in these particular habitats. Few studies have linked the ecophysiological functioning with the microbial composition of SF tropical tree seedlings. Our objective is to answer how functional traits are linked to the diversity and composition of leaf and root associated microbial communities in SF tree seedlings. We investigated 13 leaf and root functional traits, in 70 individuals belonging to seven tropical 1-year old SF tree seedlings. Seedlings were sampled exclusively in the seasonally flooded forests of French Guiana. The investigation of the diversity and composition of leaf and root associated microbial communities was conducted using ITS2 and 16S high-throughput sequencing. This study will confer robust knowledge of SF tropical forests through the lens of their microbial composition and functional traits syndrome

The effect of drought on carbon storage capacity in a tropical rainforest of French Guiana

In a perfect-deficit approach (Yi, 2012), the daily maximum carbon storage capacity (CPC) of a given ecosystem for each year defines the perfect annual CPC curve. Deficits are the differences in the daily observational data for a given year against a perfect curve. The area between this curve and instantaneous canopy photosynthetic rates represents the potential productivity. Using an 11-year (2004 – 2014) eddy covariance flux and meteorological data, this perfect-deficit approach was used to examine the relationship between potential productivity and droughts occurring in 2005 and 2010 in a tropical rainforest of French Guiana, South America. Surprisingly, CPC deficits were only reduced by 24% (2005) and 19% (2010) from their respective perfect CPCs, indicating a subtle effect of drought to ecosystem productivity. Highest precipitation occurred in 2010 but precipitation deficits dropped drastically during this year (71% reduction from its perfect value) which reflects a drought condition. However, its deficits showed no correlation annually and seasonally. Soil water content (SWC) appeared to be the single driver for CPC deficits during long dry periods but is weakly correlated (r = 0.30; P <0.01 and r = 0.31; P <0.01, in 2005 and 2010, respectively). These results indicate that soil water is not a major limiting factor for productivity of this ecosystem during drought periods. In contrast, global radiation (Rg) corresponds to the peak of CPC deficits in 2010, but only 19% had been reduced from its ideal value. Yet, highest gross primary production (GPP) of 4106±231 gCm−2yr−1 occurred this year (mean 3753±231 gCm−2yr−1). Therefore, smaller deficits in Rg coupled with sufficient water may have induced higher productivity in 2010. Nevertheless, weaker correlations between potential productivity and climatic drivers may imply that other controlling aspects such as biological constraints may also have an effect to the dynamics of potential productivity during drought events, hence, must also be considered

Tree stem and soil methane and nitrous oxide fluxes, but not carbon dioxide fluxes, switch sign along a topographic gradient in a tropical forest

International audiencePurposeTropical forests exchange large amounts of greenhouse gases (GHGs: carbon dioxide, CO2; methane, CH4; and nitrous oxide, N2O) with the atmosphere. Forest soils and stems can be either sources or sinks for CH4 and N2O, but little is known about what determines the sign and magnitude of these fluxes. Here, we aimed to study how stem and soil GHG fluxes vary along a topographic gradient in a tropical forest.MethodsFluxes of GHG from 56 individual tree stems and adjacent soils were measured with manual static chambers. The topographic gradient was characterized by a soil moisture gradient, with one end in a wetland area ("seasonally flooded"; SF), the other end in an upland area ("terra firme"; TF) and in between a transitional area on the slope (SL).ResultsTree stems and soils were always sources of CO2 with higher fluxes in SF compared to TF and SL. Fluxes of CH4 and N2O were more variable, even within one habitat. Results showed that, in TF, soils acted as sinks for N2O whereas, in SF and SL, they acted as sources. In contrast, tree stems which were predominantly sources of N2O in SF and TF, were sinks in SL. In the soil, N2O fluxes were significantly influenced by both temperature and soil water content, whereas CH4 fluxes were only significantly correlated with soil water content.ConclusionSF areas were major sources of the three gases, whereas SL and TF soils and tree stems acted as either sources or sinks for CH4 and N2O. Our results indicate that tree stems represent overlooked sources of CH4 and N2O in tropical forests that need to be further studied to refine GHG budgets

There's no place like home: seedling mortality contributes to the habitat specialisation of tree species across Amazonia

Understanding the mechanisms generating species distributions remains a challenge, especially in hyperdiverse tropical forests. We evaluated the role of rainfall variation, soil gradients and herbivory on seedling mortality, and how variation in seedling performance along these gradients contributes to habitat specialisation. In a 4-year experiment, replicated at the two extremes of the Amazon basin, we reciprocally transplanted 4638 tree seedlings of 41 habitat-specialist species from seven phylogenetic lineages among the three most important forest habitats of lowland Amazonia. Rainfall variation, flooding and soil gradients strongly influenced seedling mortality, whereas herbivory had negligible impact. Seedling mortality varied strongly among habitats, consistent with predictions for habitat specialists in most lineages. This suggests that seedling performance is a primary determinant of the habitat associations of adult trees across Amazonia. It further suggests that tree diversity, currently mostly harboured in terra firme forests, may be strongly impacted by the predicted climate changes in Amazonia

Plant mutations: slaying beautiful hypotheses by surprising evidence

The Weismann theory (1) states that hereditary traits are transmitted exclusively from the germline. The theory is valid in most animals (2) where germline cells are set aside early in development (1). In plants, germline segregation is generally assumed to occur late in development (3-5), which leads to several predictions on the fate of somatic mutations occurring in plant tissues: mutations have generally low frequency in plant tissues (6); mutations at high frequency have a higher chance of intergenerational transmission; branching topology of the tree dictates mutation distribution (7); and, exposure to UV radiation increases mutagenesis (8). We produced a unique plant dataset of 60 high-coverage whole-genome sequences of two tropical tree species and identified 18,274 de novo somatic mutations, almost all at low frequency in tissues. We demonstrate that: 1) low-frequency mutations are transmitted to the next generation; 2) mutation phylogenies deviate from the branching topology of the tree; and 3) mutation rates and mutation spectra are not demonstrably affected by differences in UV exposure. Altogether, our results suggest far more complex links between plant growth, ageing, UV exposure, and mutation rates than commonly thought

Low-frequency somatic mutations are heritable in tropical trees Dicorynia guianensis and Sextonia rubra

Significance The origin and fate of new mutations have received less attention in plants than in animals. Similarly to animals, plant mutations are expected to accumulate with growth and time and under exposure to ultraviolet light. However, contrary to animals, plant reproductive organs form late in an individual's development, allowing the transmission to the progeny of mutations accumulated along growth. Here, we resequenced DNA from different branches differentially exposed to sunlight of two tropical tree species. We showed that new mutations are generally rare in plant tissues and do not mimic branching patterns but can nevertheless be transmitted to the progeny. Our findings provide a perspective on heritable plant mutation and its pivotal role as the engine of evolution

Large hydraulic safety margins protect Neotropical canopy rainforest tree species against hydraulic failure during drought

AbstractKey messageAbundant Neotropical canopy-tree species are more resistant to drought-induced branch embolism than what is currently admitted. Large hydraulic safety margins protect them from hydraulic failure under actual drought conditions.ContextXylem vulnerability to embolism, which is associated to survival under extreme drought conditions, is being increasingly studied in the tropics, but data on the risk of hydraulic failure for lowland Neotropical rainforest canopy-tree species, thought to be highly vulnerable, are lacking.AimsThe purpose of this study was to gain more knowledge on species drought-resistance characteristics in branches and leaves and the risk of hydraulic failure of abundant rainforest canopy-tree species during the dry season.MethodsWe first assessed the range of branch xylem vulnerability to embolism using the flow-centrifuge technique on 1-m-long sun-exposed branches and evaluated hydraulic safety margins with leaf turgor loss point and midday water potential during normal- and severe-intensity dry seasons for a large set of Amazonian rainforest canopy-tree species.ResultsTree species exhibited a broad range of embolism resistance, with the pressure threshold inducing 50% loss of branch hydraulic conductivity varying from − 1.86 to − 7.63 MPa. Conversely, we found low variability in leaf turgor loss point and dry season midday leaf water potential, and mostly large, positive hydraulic safety margins.ConclusionsRainforest canopy-tree species growing under elevated mean annual precipitation can have high resistance to embolism and are more resistant than what was previously thought. Thanks to early leaf turgor loss and high embolism resistance, most species have a low risk of hydraulic failure and are well able to withstand normal and even severe dry seasons