Does strategy of resource acquisition in tropical woody species vary with life form,leaf texture,and canopy gradient?

In order to test whether the strategy of resource acquisition varies with life form, leaf texture and canopy gradient, we measured light-saturated net photosynthetic capacity (A _max-mass) and leaf nitrogen and phosphorus concentrations (N _mass and P _mass) for 127 woody species of understory (small shrubs and tree seedlings) and 47 woody species of canopy (large shrubs and tree adults) in a tropical montane rain forest of Hainan Island, South China. Photosynthetic nitrogen use efficiency (PNUE) and photosynthetic phosphorus use efficiency (PPUE) were studied by taking into account functional groups, which classified by either life form (FG1) or leaf texture (FG2). For FG1, there were significant (P < 0.05) differences between trees and shrubs in A _mass, N _mass and P _mass, but not in PNUE and PPUE; whereas for FG2, there were significant (P < 0.05) differences between the papery-leaved species and those with leathery leaves in the measured leaf traits inclusive of PNUE and PPUE. Within the same classification systems, significant allometric scaling relationships were found in the leaf trait pairs of A _mass-N _mass and A _mass-P _mass. Considered separately, the slope and y-intercept of the linear regression between A _mass and N _mass did not differ among functional groups nor between understory and canopy, but those for the linear regression between A _mass and P _mass differed significantly (P = 0.001) between understory and canopy species regardless of functional groups. The results of phylogenetic comparative analyses were in accordance with the observed positive scaling relationships. The overall results indicate that there are no fundamentally different nitrogen capture strategies in tropical woody species regardless of their life form, leaf texture and canopy gradient. However, the strategies of phosphorus acquisition of tropical woody species differ with canopy layer increases. This variation may correspond to the special soil conditions in this ecosystem, as phosphorus is an element limiting plant growth in tropical areas.     

Does growth temperature affect the temperature responses of photosynthesis and internal conductance to CO2? A test with Eucalyptus regnans

Internal conductance to CO(2) transfer from intercellular spaces to chloroplasts (g(i)) poses a major limitation to photosynthesis, but only three studies have investigated the temperature dependance of g(i). The aim of this study was to determine whether acclimation to 15 versus 30 degrees C affects the temperature response of photosynthesis and g(i) in seedlings of the evergreen tree species Eucalyptus regnans F. Muell. Six-month-old seedlings were acclimated to 15 or 30 degrees C for 6 weeks before g(i) was estimated by simultaneous measurements of gas exchange and chlorophyll fluorescence (variable J method). There was little evidence for acclimation of photosynthesis to growth temperature. In seedlings acclimated to either 15 or 30 degrees C, the maximum rate of net photosynthesis peaked at around 30 or 35 degrees C. Such lack of temperature acclimation may be related to the constant day and night temperature acclimation regime, which differed from most other studies in which night temperatures were lower than day temperatures. Internal conductance averaged 0.25 mol m(-2) s(-1) at 25 degrees C and increased threefold from 10 to 35 degrees C. There was some evidence that g(i) was greater in seedlings acclimated to 15 than to 30 degrees C, which resulted in seedlings acclimated to 15 degrees C having, if anything, a smaller relative limitation due to g(i) than seedlings acclimated to 30 degrees C. Stomatal limitations were also smaller in seedlings acclimated to 15 degrees C than in seedlings acclimated to 30 degrees C. Based on chloroplast CO(2) concentration, neither maximum rates of carboxylation nor RuBP-limited rate of electron transport peaked between 10 and 35 degrees C. Both were described well by an Arrhenius function and had similar activation energies (57-70 kJ mol(-1)). These findings confirm previous studies showing g(i) to be positively related to measurement temperature.     

Why does leaf nitrogen decline within tree canopies less rapidly than light? An explanation from optimization subject to a lower bound on leaf mass per area

A long-established theoretical result states that, for a given total canopy nitrogen (N) content, canopy photosynthesis is maximized when the within-canopy gradient in leaf N per unit area (N(a)) is equal to the light gradient. However, it is widely observed that N(a) declines less rapidly than light in real plant canopies. Here we show that this general observation can be explained by optimal leaf acclimation to light subject to a lower-bound constraint on the leaf mass per area (m(a)). Using a simple model of the carbon-nitrogen (C-N) balance of trees with a steady-state canopy, we implement this constraint within the framework of the MAXX optimization hypothesis that maximizes net canopy C export. Virtually all canopy traits predicted by MAXX (leaf N gradient, leaf N concentration, leaf photosynthetic capacity, canopy N content, leaf-area index) are in close agreement with the values observed in a mature stand of Norway spruce trees (Picea abies L. Karst.). An alternative upper-bound constraint on leaf photosynthetic capacity (A(sat)) does not reproduce the canopy traits of this stand. MAXX subject to a lower bound on m(a) is also qualitatively consistent with co-variations in leaf N gradient, m(a) and A(sat) observed across a range of temperate and tropical tree species. Our study highlights the key role of constraints in optimization models of plant function.     

How nutrient availability influences acclimation to shade of two (pioneer and late-successional) Mediterranean tree species?

Examining ecological limits to shade acclimation at whole-plant level is determinant for evaluating the success of sapling establishment in low-light environments. We studied nutritional effects on whole-plant development in response to shade in two Mediterranean forest tree species with different successional status: the early-successional Pinus halepensis Mill. and the late-successional Quercus pubescens Wild. Through a nursery-based factorial experimental design approach, we measured height increment along 2 years and final leaf, stem and root biomass in both species saplings subjected to two lights and two soil nutrient availability treatments. The shade avoidance response was exclusive to P. halepensis, appeared as timely dependent, and persisted longer in saplings exposed to higher nutrient availability. Q. pubescens benefited from the higher nutrient availability by lowering the light-driven plastic response in aerial support investment and belowground carbon allocation, whereas P. halepensis heightened its light-driven plastic response. These contrasted responses are thus clearly related to the shade acclimation strategy of each species: the shade-intolerant P. halepensis enhances shade avoidance when non-nutrient-limited, whereas the shade-tolerant Q. pubescens assumes a conservative strategy by limiting phenotypic plasticity-induced costs. Maintaining greater shade avoidance in non-nutrient-limited soil conditions might be an adaptive advantage for P. halepensis seedlings growing in the understory, in response to gap formation in the overstory. In contrast, the more conservative and less costly shade responsiveness of Q. pubescens may confer it a better adaptive advantage in long-term light-limited environments.     

Does ontogeny modulate irradiance-elicited plasticity of leaf traits in saplings of rain-forest tree species? A test with Dicorynia guianensis and Tachigali melinonii (Fabaceae, Caesalpinioideae)

? Irradiance elicits a large plasticity in leaf traits, but little is known about the modulation of this plasticity by ontogeny. Interactive effects of relative irradiance and ontogeny were assessed on leaf traits for two tropical rainforest tree species: Dicorynia guianensis Amshoff and Tachigali melinonii (Harms) Barneby (Fabaceae, Caesalpinioideae).

? Eleven morphological and physiological leaf traits, relative to photosynthetic performance, were measured on saplings at three different architectural development stages (ASD 1, 2 and 3) and used to derive composite traits like photosynthetic N-use efficiency. Measurements were made along a natural irradiance gradient.

? The effect of ASD was very visible and differed between the two species. For Dicorynia guianensis, only leaf mass-per-area (LMA) significantly increased with ASDs whereas for Tachigali melinonii, almost all traits were affected by ASD: LMA, leaf N content and photosynthetic capacity increased from ASD 1 to ASD 3. Photosynthetic N-use-efficiency was not affected by ASD in any species.

? Leaf traits were severely modulated by irradiance, whereas the degree of plasticity was very similar among ASDs. Only few interactions were detected between irradiance and ASD, for leaf thickness, carbon content, and the ratio Chl/N in T. melinonii and for photosynthetic capacity in D. guianensis.

? We conclude that ontogenic development and irradiance-elicited plasticity modulated leaf traits, with almost no interaction, i.e., the degree of irradiance-elicited plasticity was stable across development stages and independent of ontogeny in these two species, at least in the early stages of development assessed here.

     

Correction to: How does light availability affect the aboveground biomass allocation and leaf morphology of saplings in temperate mixed deciduous forests?

New Forests - The original version of this article was inadvertently published without the acknowledgements section.     

Unraveling the tripartite interactions among the woolly poplar aphid,its host tree,and their environment: a lead to improve the management of a major tree plantation pest?

Key message

For an optimal deployment of poplar resistance to the gall-inducing aphid Phloeomyzus passerinii , a laboratory susceptibility assay has been developed. The nature of aphid–tree interactions during compatible and incompatible situations has been studied in detail. This should help at identifying specific resistance markers and at testing effects of site conditions on tree–pest interactions.

Context

P. passerinii is a major pest of poplar plantations in Europe, and the plantation of resistant poplar genotypes is regarded as the best long-term management strategy for this pest. This requires a sound knowledge of the interactions among the pest, its host and their environment.

Aims

Here, we review the recent advances aiming at developing an optimal deployment of host resistance versus P. passerinii.

Results

Investigations on aphid-host interactions demonstrated that P. passerinii induces pseudogalls within the bark of susceptible hosts. This results in a reduction of starch bark content during aphid outbreaks, which could be involved in tree death. The constitutive level of starch in the bark could be related to the tolerance level of trees. A susceptibility test has been designed for poplar genotypes, discriminating three categories of susceptibility depending on tree’s ability to totally or partially inhibit pseudogall induction. The test still has several limitations however. It neither takes into account the large level of individual genetic diversity of the aphid in France, nor the environmental modulation of tree resistance and tolerance, while water deficit and fertilization could potentially affect these parameters.

Conclusion

The hypotheses drawn regarding the processes leading to tree death or resistance should help at identifying resistance markers, and at testing effects of site conditions on tree–pest interactions.

     

Pseudomonas associated with Bursaphelenchus xylophilus,its insect vector and the host tree: A role in pine wilt disease?

In this study, we characterized the diversity of Pseudomonas associated with Bursaphelenchus xylophilus, its insect vector (Monochamus galloprovincialis) and its host (Pinus pinaster), by a culture‐independent approach using rpoD clone libraries. Clone libraries of Pseudomonas rpoD were obtained from B. xylophilus, M. galloprovincialis and infected P. pinaster. Most M. galloprovincialis and B. xylophilus sequences grouped together in the P. fluorescens group. Genes related to xenobiotics degradation and phenylacetate synthesis were present in the genomes of the type strains closely related to sequences retrieved from the nematode libraries. Results demonstrated that the nematode, during its life stages inside the tree, maintains a diverse Pseudomonas community that is closely related to the one associated with the insect vector. These bacteria might contribute to degradation of xenobiotics and tree weakening during the nematode tree infection.     

Intra-specific patterns of δ13C,growth and wood density variation at sites of contrasting precipitation with implications for modelling carbon sequestration of tropical tree species

Agroforestry Systems - Tropical agroforestry practices play a crucial role in mitigating global climate change by absorbing CO2 from the atmosphere through photosynthesis and storing carbon in...