Leaf Life Span Research Articles

Atlantic forest and leaf traits: an overview

Leaf traits in Atlantic forest are modulated by environmental filters and biotic interactions. More information about leaf traits and nutrient are needed. Categorizing tree species according to their relationships with ecosystem processes in the Atlantic forest (AF) biome is fundamental to better understand their functionality in this high biodiversity hotspot. For categorizing vegetation, leaf traits are extremely useful and this is the first compilation of leaf traits focusing on photosynthesis, water relations, nutrients, and functional diversity across major AF vegetation types: (1) rainforest (ombrophilous dense and mixed ombrophilous Araucaria forest), (2) seasonal semi-deciduous forest, (3) deciduous forests, and (4) coastal plain forests (restingas). Species showed high plasticity of photosynthetic traits from early developmental stages, including adjustments in Rubisco carboxylation capacity, maximum photosynthetic capacity, photochemical and non-photochemical quenching, specific leaf area (SLA), and chlorophyll and spongy parenchyma thickness. Montane rainforest tree species showed lower SLA and longer leaf lifespan in comparison to lowland rainforest species. Tree communities are structured by environmental filtering leading to trait convergence in early successional stages or in stressful ecosystems such as restingas, and by biotic interactions leading to trait divergence in later stages. Altitudinal gradients in litter production and below/aboveground biomass stocks in AF show contrasting patterns in comparison to other tropical forests, with important consequences for responding to climate change. However, the identity of limiting elements to growth still represents a critical gap in leaf trait data for AF species. Other gaps in knowledge of AF leaf traits are highlighted for consideration in future studies.

Leaf mechanical strength and photosynthetic capacity vary independently across 57 subtropical forest species with contrasting light requirements

Leaf mechanical strength and photosynthetic capacity are critical plant life-history traits associated with tolerance and growth under various biotic and abiotic stresses. In principle, higher mechanical resistance achieved via higher relative allocation to cell walls should slow photosynthetic rates. However, interspecific relationships among these two leaf functions have not been reported. We measured leaf traits of 57 dominant woody species in a subtropical evergreen forest in China, focusing especially on photosynthetic rates, mechanical properties, and leaf lifespan (LLS). These species were assigned to two ecological strategy groups: shade-tolerant species and light-demanding species. On average, shade-tolerant species had longer LLS, higher leaf mechanical strength but lower photosynthetic rates, and exhibited longer LLS for a given leaf mass per area (LMA) or mechanical strength than light-demanding species. Depending on the traits and the basis of expression (per area or per mass), leaf mechanical resistance and photosynthetic capacity were either deemed unrelated, or only weakly negatively correlated. We found only weak support for the proposed trade-off between leaf biomechanics and photosynthesis among co-occurring woody species. This suggests there is considerable flexibility in these properties, and the observed relationships may result more so from trait coordination than any physically or physiologically enforced trade-off.

A through year behavior of 137Cs in a Japanese flowering cherry tree in relation to that of potassium

To understand the transfer of radiocesium (137Cs) in inside of deciduous trees, changes in 137Cs activity concentrations, primarily derived from the Fukushima accident in March 2011, were observed in the upper parts of a Japanese flowering cherry tree (Prunus x yedoensis cv. Somei-Yoshino) between 2015 and 2018. The sampling of the foliar parts occurred over the entire leaf life span from winter bud to litterfall and those of the branches were distinguished based on emergence years (2017, 2016, 2015, 2014–2011, and 2010/before). First, every tissue demonstrated a clear seasonal variation in 137Cs activity concentration. Second, a synchrony of seasonal variations in 137Cs activity concentration with those in the biological analogue of K concentration was observed in foliar parts during their growth season, but not in branches nor during the other seasons. With respect to the timing of changes in each tissue with tree phenology, it is possible that K and 137Cs alternate between leaves and branches via the same translocation mechanisms. The resorption efficiencies (i.e., 1 − [the concentrations in the last litterfall]/[the maximum concentrations in green leaves]) of K and 137Cs were 76% and 46% in average, respectively. In addition, both leaf buds and branches played an important role as reservoirs during dormancy. The buds storage ratio before and after bud burst (i.e., [the inventories in buds at the end of defoliation]/[those before and after bud burst]) for K were 0.57 and 0.10 in median, respectively, and those for and 137Cs were 1.14 and 0.14 in median, respectively. Consequently, the transfer of 137Cs in inside of trees was still visible seven years after deposition, even though the annual reduction in 137Cs activity concentration was apparent in each tissue.

The contribution of leaf life span to sexual dimorphism in deciduous and evergreen Pistacia species under Mediterranean conditions

Gender differentiation may be quite different between species with different strategies to cope with environmental stresses. In this study, we tested the hypothesis that sexual dimorphism is stronger in plant species under summer drought stresses and long leaf longevity. For that, we selected two Mediterranean dioecious species: the winter-deciduous Pistacia atlantica Desf. from Iran and the evergreen-sclerophyllous Pistacia lentiscus L. from Portugal. Our results indicated that there were no morphological differences between genders except for leaf mass-per-area (LMA) in P. lentiscus, where females displayed higher values than males. Leaves from female P. lentiscus presented a higher C/N ratio and lower carbon isotope composition (δ13C), which is related with higher stomatal conductance during photosynthesis; whereas males had higher nitrogen concentration and more enriched leaf δ13C values. Pistacia atlantica, which experiences a shorter growing season, did not show any sexual dimorphism. Accordingly to our initial hypothesis, environmental conditions with the particular stressful conditions and leaf life strategy play a crucial role in gender dimorphism, both in physiological and morphological responses. Thus, a long longevity of P. lentiscus leaves was crucial for the observation of differences in gender-related physiological differences.

CARACTERÍSTICAS MORFOGÊNICAS E ESTRUTURAIS DO SORGO FORRAGEIRO SUBMETIDO A LOTAÇÃO CONTÍNUA COM NOVILHOS DE CORTE SUPLEMENTADOS

Resumo A morfogênese é uma valiosa ferramenta para a compreensão da dinâmica do pasto, porém os estudos morfogênicos referentes à cultura do sorgo ainda são escassos. O objetivo do presente estudo foi avaliar os efeitos de diferentes níveis de suplementação sobre as características morfogênicas do sorgo forrageiro. O experimento foi conduzido na Universidade Federal de Santa Maria entre janeiro e abril de 2013. O delineamento experimental foi inteiramente casualizado, com medidas repetidas no tempo, com três tratamentos e duas repetições. Os tratamentos consistiram em S 0,8 = pastagem de sorgo + 0,8 % PV de suplementação, S 1,0 = pastagem de sorgo + 1,0 % PV de suplementação e S 1,2 = pastagem de sorgo + 1,2 % PV de suplementação. Ao aumentar o nível de suplementação ofertado, observou-se menor valor para Folhas Vivas (2,77, 3,34 e 3,55) e Taxa de Elongação Foliar (0,06, 0,09 e 0,08). Não foram influenciadas (P<0,05) pelos diferentes níveis de suplementação as variáveis Taxa de Aparecimento Foliar (0,0041, 0,0043 e 0,039), Taxa de Senescência Foliar (0,05, 0,06 e 0,11), Filocrono (283,96, 265,21 e 278,62), Duração de Vida da Folha (1081,1, 986,25 e 788,79) e Duração de Elongação Foliar (350,58, 312,83 e 326,36).

Investigation of complex canopies with a functional-structural plant model as exemplified by leaf inclination effect on the functioning of pure and mixed stands of wheat during grain filling.

Because functional-structural plant models (FSPMs) take plant architecture explicitly into consideration, they constitute a promising approach for unravelling plant-plant interactions in complex canopies. However, existing FSPMs mainly address competition for light. The aim of the present work was to develop a comprehensive FSPM accounting for the interactions between plant architecture, environmental factors and the metabolism of carbon (C) and nitrogen (N). We developed an original FSPM by coupling models of (1) 3-D wheat architecture, (2) light distribution within canopies and (3) C and N metabolism. Model behaviour was evaluated by simulating the functioning of theoretical canopies consisting of wheat plants of contrasting leaf inclination, arranged in pure and mixed stands and considering four culm densities and three sky conditions. As an emergent property of the detailed description of metabolism, the model predicted a linear relationship between absorbed light and C assimilation, and a curvilinear relationship between grain mass and C assimilation, applying to both pure stands and each component of mixtures. Over the whole post-anthesis period, planophile plants tended to absorb more light than erectophile plants, resulting in a slightly higher grain mass. This difference was enhanced at low plant density and in mixtures, where the erectophile behaviour resulted in a loss of competitiveness. The present work demonstrates that FSPMs provide a framework allowing the analysis of complex canopies such as studying the impact of particular plant traits, which would hardly be feasible experimentally. The present FSPM can help in interpreting complex interactions by providing access to critical variables such as resource acquisition and allocation, internal metabolic concentrations, leaf life span and grain filling. Simulations were based on canopies identically initialized at flowering; extending the model to the whole cycle is thus required so that all consequences of a trait can be evaluated.

Morphogenic, structural, productive and bromatological characteristics of Braquiária in silvopastoral system under nitrogen doses

his study aimed to evaluate the effect of different doses of nitrogen fertilization on the morphogenic, structural, productive and chemical composition of Urochloa decumbens (Stapf) R. D. Webster cv. Basilisk in a silvopastoral system with eucalyptus. The experimental treatments consisted of pastures of Basilisk cultivar, fertilized with 0, 100, 200, 300 and 400 kg ha-1 year-1 of N, and intercropped with eucalyptus, clone GG 100 (Eucalyptus grandis x E. urophylla). The experimental arrangement followed a randomized block design with four replicates. The evaluated characteristics were: foliar appearance rate, phyllochron, leaf length, leaf elongation rate, leaf life span, foliar senescence rate, number of green leaves per tiller, number of senescent leaves per tiller, daily dry matter accumulation, dry matter production, crude protein, and neutral detergent fiber. In the conditions of the established silvopastoral system, nitrogen fertilization did not influence (p > 0.05) the morphogenic and structural characteristics of Basilisk cultivar, except for height (p < 0.01) and leaf: stem ratio (p = 0.02). The nitrogen fertilization of the Brachiaria grass with eucalyptus in the proposed structural arrangement does not modify (p > 0.05) the morphogenic characteristics. However, in ration to dry matter production, doses of up to 400 kg of N ha-1 are recommended.

How Does Water-Stressed Corn Respond to Potassium Nutrition? A Shoot-Root Scale Approach Study under Controlled Conditions

Potassium (K) is generally considered as being closely linked to plant water dynamics. Consequently, reinforcing K nutrition, which theoretically favors root growth and specific surface, extends leaf lifespan, and regulates stomatal functioning, is often used to tackle water stress. We designed a greenhouse pot-scale device to test these interactions on corn (Zea mays L.), and to analyze their links to plant transpiration. Three levels of K nutrition were combined with two water-supply treatments. Shoot and root development and growth were continuously measured during a 60-day-long experiment. Individual plant transpiration was measured by weighing pots and by calculating water mass balances. The results showed that, although K deficiency symptoms resembled those caused by water shortage, there was no advantage to over-fertilizing water-stressed plants. K failed to decrease either the transpiration per unit leaf surface or to improve water use efficiency. The link between K nutrition and plant transpiration appears solely attributable to the effect of K on leaf area. We conclude that K over-fertilization could ultimately jeopardize crops by enhancing early-stage water transpiration to the detriment of later developmental stages.

Tracking chlorophyll fluorescence as an indicator of drought and rewatering across the entire leaf lifespan in a maize field

Plant growth and photosynthesis in response to water status have been extensively investigated. However, elucidating the photosynthetic process and its indicators under a drought episode and rewatering across the entire leaf lifespan is often neglected. In this experiment, three water treatments were set during two growth seasons: a control treatment, moderate persistent drought (T1), and severe persistent drought (T2). Maize leaf chlorophyll fluorescence emission was analyzed to determine the regulative responses of the photosynthetic potentials and photosystem II (PSII) photochemistry process to drought and rewatering in situ. A severe drought episode during the peak vegetative growth stage resulted in decreases in chlorophyll content, the maximal efficiency of PSII photochemistry (Fv/Fm), and photochemical quenching, but increases in non-photochemical quenching and the yield for dissipation by downregulation. Rewatering only restored partial PSII functions in plants that had undergone historical drought episodes. An analysis of non-photochemical pathways of thermal dissipation indicates that regulative photoprotection of the photosystem apparatus may occur through heat dissipation when an effect of severe drought episode appeared on a young leaf; however, rewatering did not enhance photoprotection with leaf aging. Compared to the control treatment, the yield of T1 and T2 decreased by 25.1% and 27.1% in 2015, and 26.4% and 54.3% in 2016, respectively. The chlorophyll content was significantly and closely correlated with Fv/Fm (R = 0.65, P < 0.001) and the maximum versus minimum fluorescence yield in the dark-adapted state (Fm/Fo) (R = 0.72, P < 0.001). Additionally, the two parameters can be suggested to feasibly track chlorophyll content changes and the degree of leaf senescence in responses to a drought episode and its interaction with leaf aging: Fm/Fo and the relative limitation to photosynthesis (RLP). The current results may provide a profound insight into better understanding the underlying mechanism of photosynthetic potentials and photochemistry efficiency and photoprotection in response to drought episodes and rewatering over the entire leaf lifespan.

Density and biomechanical properties of fossil fronds. A case study of Neuropteris ovata (seed fern, Late Pennsylvanian, Canada)

A theoretical biomechanical model is proposed for the largest known 650 mm-long frond segment Neuropteris ovata var. simonii (Medullosales, Pennsylvanian, Sydney Coalfield, Canada). The study procedure includes, amongst others, a novel methodology for calculating density of fossil pinnules and rachides. This, in conjunction with the trait relationships and mathematical models that linked density and material/structural properties established for modern plant-leaf tissues, permits the estimation of tensile strength, tensile modulus of elasticity, flexural stiffness, and leaf mass per area. Results suggest that, theoretically, (a) the living frond invested a considerable amount of resources for the construction of metabolically expensive, hard, tough, and resistant pinnule and rachial tissues, which were “made to last”, and (b) were used as part of a combination of strategies that increased the plant's biomechanical stability. This translated in resistance to damage during the application of external loads such as those exerted by strong winds or tropical storms. High leaf mass per area (dry-mass investment) of pinnules and rachial tissues points to a slow photosynthetic return that was sustained for long periods of time (long leaf lifespan). The latter possibly indicates a plant adaptation to wet, levee-top habitats, which were characterized by nutrient-deficient soils exposed to intensive solar irradiation. The 3D, chemistry-based model of the N. ovata specimen provided new insights into the hitherto poorly-known biomechanical and related traits of this iconic medullosalean plant that inhabited Pennsylvanian wetlands. Other implications of using our chemistry-based methodology include the estimation of (i) frond size and shape (plant reconstructions), (ii) plant ecophysiological adaptations to different wetland environments, and (iii) plant phylogenetic and ontogenetic adaptations.

An invasive population of Solidago canadensis is less sensitive to warming and nitrogen-addition than its native population in an invaded range

Native and invasive populations might behave differentially due to contrasting genetics and histories of environmental pressures. Here we attempted to understand how climate warming and atmospheric nitrogen (N) deposition influence native and invasive populations, and thus conducted an experiment to address their effects on the nine invasiveness-related traits (leaf dry matter content, specific leaf area, leaf lifespan, ramet height, ramet number, the first inflorescence buds, the first flowering, the first seed-setting, and the first dieback) of Solidago canadensis populations from the USA and China. Solidago canadensis from the USA had shorter leaf lifespan and ramet height, smaller ramet number, and earlier phenology than that from China. Warming and N addition extended leaf lifespan but failed to influence leaf dry matter content and specific leaf area; warming decreased ramet height and delayed the onset of inflorescence and flowering but N addition increased ramet height and advanced the onset of inflorescence and flowering. Six traits were more sensitive to warming or N addition in the native population than in the invasive population, and the other traits were similar; warming and N addition together had stronger effects on six traits of the native population than those of the invasive population, and the opposite was true for ramet height. Our results suggest that climate warming and N deposition might promote S. canadensis invasion, as indicated by enhanced growth, and could influence its native populations to a greater extent than its invasive populations.

Multiple Consequences Induced by Epidermally-Located Anthocyanins in Young, Mature and Senescent Leaves of Prunus.

Anthocyanic morphs are generally less efficient in terms of carbon gain, but, in turn, are more photoprotected than anthocyanin-less ones. To date, mature leaves of different morphs or leaves at different developmental stages within the same species have generally been compared, whereas there is a lack of knowledge regarding different stages of development of red vs. green leaves. Leaves (1-, 7-, and 13-week-old) of red- (RLP) and green-leafed (GLP) Prunus in terms of photosynthetic rate, carbon metabolism and photoprotective mechanisms were compared to test whether anthocyanin-equipped leaves perform better than anthocyanin-less leaves and whether photoprotection is the primary role of epidermally-located anthocyanins, using for the first time a recently-developed parameter of chlorophyll fluorescence (qPd). GLP leaves had a higher photosynthetic rate in 1- and 7-week-old leaves, but RLP leaves performed better at an early stage of senescence and had a longer leaf lifespan. Anthocyanins contributed to leaf photoprotection throughout the leaf development, but were tightly coordinated with carotenoids. Besides photoprotecting, we propose that epidermal anthocyanins may be principally synthetized to maintain an efficient carbon-sink strength in young and senescent leaves, thus extending the RLP leaf lifespan.

ORESARA15, a PLATZ transcription factor, mediates leaf growth and senescence in Arabidopsis.

Plant leaves undergo a series of developmental changes from leaf primordium initiation through growth and maturation to senescence throughout their life span. Although the mechanisms underlying leaf senescence have been intensively elucidated, our knowledge of the interrelationship between early leaf development and senescence is still fragmentary. We isolated the oresara15-1Dominant (ore15-1D) mutant, which had an extended leaf longevity and an enlarged leaf size, from activation-tagged lines of Arabidopsis. Plasmid rescue identified that ORE15 encodes a PLANT A/T-RICH SEQUENCE- AND ZINC-BINDING PROTEIN family transcription factor. Phenotypes of ore15-1D and ore15-2, a loss-of-function mutant, were evaluated through physiological and anatomical analyses. Microarray, quantitative reverse transcription polymerase chain reaction, and chromatin immunoprecipitation as well as genetic analysis were employed to reveal the molecular mechanism of ORE15 in the regulation of leaf growth and senescence. ORE15 enhanced leaf growth by promoting the rate and duration of cell proliferation in the earlier stage and suppressed leaf senescence in the later stage by modulating the GROWTH-REGULATING FACTOR (GRF)/GRF-INTERACTING FACTOR regulatory pathway. Our study highlighted a molecular conjunction through ORE15 between growth and senescence, which are two temporally separate developmental processes during leaf life span.

Investment in reproduction for 14 iteroparous perennials is large and associated with other life‐history and functional traits

AbstractWhile theoretical models predict reproductive allocation (RA) should approach 100% of available energy as a plant ages, available empirical data suggest much lower RA values in perennial plants. In this study, we have two aims. First, we assess whether the discrepancy between theory and data arises from methodological differences in how growth and RA are calculated. Specifically, we hypothesize RA in older plants is large when compared to growth in leaf area, that is, after excluding turnover of stem and leaf tissues. Second, we hypothesize that species with cheap tissues or those that are shorter reach RA = 0.5 at a younger age.We measured investment in leaf, stem and reproduction on individuals from 14 co‐occurring woody perennial iteroparous species. A fire chronosequence allowed us to use a space‐for‐time substitution to estimate RA schedules for each species, simultaneously measuring reproductive and vegetative production on individuals differing in age.For most (11 of 14) species, we found RA eventually reached 100% of available energy, with another two species reaching at least 80%. Increases in RA were associated with a decline in growth of leaf area. Comparing species, we found that species with cheap leaves reached RA = 0.5 sooner (they could be called fast‐living), whereas delayed maturation and slower increases in RA were associated with greater maximum height.Synthesis. Explicitly accounting for the cost of leaf replacement leads to the high estimates of reproductive allocation in perennial plants predicted by theoretical models, limiting or even halting leaf area expansion. For some species, so much energy is allocated to reproduction that leaf area declines year‐upon‐year for multiple growing seasons preceding death. Connecting lifetime reproductive allocation schedules with leaf area expansion, leaf life span, and plant maximum height demonstrates how reproductive allocation schedules synthesize a plant’s life‐history strategy, making them a valuable tool for connecting plant traits and demography.

Evolution of nutrient resorption across the herbaceous genus Helianthus

Foliar nutrient resorption is a key modulator of plant nutrient use. However, evolutionary patterns for nutrient resorption remain unclear, especially in herbs. We measured nitrogen and phosphorus resorption on preselected leaves across the Helianthus (sunflower) genus in a common garden in Athens, GA. We analyzed our data with published leaf traits and native habitat environmental data. Using phylogenetically controlled analyses, we tested if (1) nutrient resorption correlates with leaf economic, vasculature, and defense traits through evolutionary time, and (2) native habitat environment predicts nutrient resorption evolution. For Helianthus, nutrient resorption capacity is greater in resource-conservative species, as previously defined for Helianthus with a principle components analysis of leaf economic spectrum traits (photosynthetic rate, respiration rate, leaf lifespan, leaf mass per area, and green-leaf N and P concentrations). Nutrient resorption capacity also evolutionarily correlates with individual leaf economic traits, though not always as expected based on broad species surveys. Greater nutrient resorption is also positively associated with leaf chemical defenses, but not leaf vasculature or senescence rate. Finally, nitrogen resorption evolution increases with native habitat precipitation, but native habitat soil fertility does not predict nitrogen or phosphorus resorption. Our results suggest nutrient resorption evolution is more closely tied to resource economic strategy than native habitat.

Ontogenetic changes in anti‐herbivore defensive traits in leaves of four Mediterranean co‐occurring Quercus species

AbstractInvestment in anti‐herbivore defence in tree species has been one of the priority research topics in plant terrestrial ecology during the last decades. However, despite considerable experimental effort, interspecific differences in the ontogenetic trends in the investment in defence are still a matter of debate, as to date experimental evidence is contradictory. In the present work, insect herbivory levels were measured in seedlings and mature trees of four co‐occurring Mediterranean Quercus species with differing leaf life spans, as well as several leaf characteristics that can determine herbivore preference. The measured leaf traits included nitrogen (N), fibre (cellulose, hemicellulose and lignin), total phenolic contents, leaf mass per unit area (LMA) and leaf thickness. The leaves of seedlings had a lower LMA and leaf thickness and lower concentrations of N and cellulose, but higher concentrations of lignin and phenols than those of mature trees. However, the loss of leaf area tended to be more severe for seedlings than for mature trees, although the differences were only significant for deciduous species. This constitutes a confirmation of the strong effects of physical traits on herbivore preferences. The greater resource limitations for defensive mechanisms in seedlings with respect to mature trees would explain that at intraspecific level we do observe a compromise between chemical and physical defences. As a result, seedlings rely on chemical rather than on physical defences.

Does the same morphology mean the same physiology? Morphophysiological adjustments of Philodendron hederaceum (Jacq.) Schott, an isomorphic aroid, to ground-canopy transition

Lianescent aroids grow horizontally while on the ground (shaded, humid habitats), but switch to vertical growth when they reach a host (exposed, desiccant habitats). There are three morphotypes with respect to leaf adjustment to canopy conditions: isomorphic, allomorphic, and heteromorphic. Allomorphs and heteromorphs exhibit marked leaf modifications (e.g., leaf area) that have a strategic effect on the plant’s carbon balance, since they increase its light foraging capacity in the canopy. Isomorphs do not show external leaf morphology modifications. Therefore, we hypothesized that leaf anatomy and physiology change instead, allowing their light foraging capacity to improve while they grow towards the canopy. We evaluated leaf area, lamina-specific mass, mesophyll anatomy, venation density and thickness, stomatal density, chlorophyll concentration and fluorescence in leaves of the isomorph Philodendron hederaceum, grown to three heights (terrestrial, 1.5 and 3 m). Stomatal density, chlorophyll a content, and electron transport rate were significantly higher in the canopy leaves (60, 54, and 50%, respectively). Thus, P. hederaceum exhibits leaf adjustments that enhance its light foraging capacity. However, these changes are relatively less marked than those exhibited by sympatric heteromorphs and allomorphs. Although the canopy leaves of P. hederaceum exhibited a limited improvement in light foraging capacity, the species’ construction and maintenance costs are expected to be lower than those of its sympatric heteromorphs and allomorphs. Future studies that consider leaf lifespan will add another perspective for understanding the costs and benefits of these growth habits in aroid vines.

Lignin composition is related to xylem embolism resistance and leaf life span in trees in a tropical semiarid climate.

Wood properties influence the leaf life span (LL) of tree crowns. As lignin is an important component of wood and the water transport system, we investigated its relationship with embolism resistance and the LL of several tree species in a seasonally dry tropical ecosystem. We determined total lignin and the monomer contents of guaiacyl (G) and syringyl (S) and related them to wood traits and xylem vulnerability to embolism (Ψ50 ) for the most common species of the Brazilian semiarid, locally known as Caatinga. Leaf life span was negatively related to Ψ50 and positively related to S:G, which was negatively related to Ψ50 . This means that greater S:G increases LL by reducing Ψ50 . Lignin content was not correlated with any variable. We found two apparently unrelated axes of drought resistance. One axis, associated with lignin monomeric composition, increases LL in the dry season as a result of lower xylem embolism vulnerability. The other, associated with wood density and stem water content, helps leafless trees to withstand drought and allows them to resprout at the end of the dry season. The monomeric composition of lignin (S:G) is therefore an important functional wood attribute affecting several key functional aspects of tropical tree species in a semiarid climate.

Divergent drivers of leaf trait variation within species, among species, and among functional groups

Understanding variation in leaf functional traits-including rates of photosynthesis and respiration and concentrations of nitrogen and phosphorus-is a fundamental challenge in plant ecophysiology. When expressed per unit leaf area, these traits typically increase with leaf mass per area (LMA) within species but are roughly independent of LMA across the global flora. LMA is determined by mass components with different biological functions, including photosynthetic mass that largely determines metabolic rates and contains most nitrogen and phosphorus, and structural mass that affects toughness and leaf lifespan (LL). A possible explanation for the contrasting trait relationships is that most LMA variation within species is associated with variation in photosynthetic mass, whereas most LMA variation across the global flora is associated with variation in structural mass. This hypothesis leads to the predictions that (i) gas exchange rates and nutrient concentrations per unit leaf area should increase strongly with LMA across species assemblages with low LL variance but should increase weakly with LMA across species assemblages with high LL variance and that (ii) controlling for LL variation should increase the strength of the above LMA relationships. We present analyses of intra- and interspecific trait variation from three tropical forest sites and interspecific analyses within functional groups in a global dataset that are consistent with the above predictions. Our analysis suggests that the qualitatively different trait relationships exhibited by different leaf assemblages can be understood by considering the degree to which photosynthetic and structural mass components contribute to LMA variation in a given assemblage.

Nutrient remobilization in tree foliage as affected by soil nutrients and leaf life span

AbstractNutrient remobilization is a key process in nutrient conservation in plants and in nutrient cycling in ecosystems. To predict the productivity of terrestrial ecosystems, we thus need to improve our understanding of the factors that control remobilization. We studied the remobilization rates of several major nutrients (N, P, S, K, Ca, and Mg) in 102 forest ecosystems representing large environmental gradients at the country scale (France). Total amounts or availability of nutrients in soils were correlated with nutrient remobilization: the larger the soil nutrient pool, the lower the remobilization rate (e.g., P remobilization decreased with increasing total or extractable inorganic P in soils). Soil type and soil parent material influenced nutrient remobilization indirectly through their effect on soil nutrients. Nutrient remobilization was also affected by the quality of soil organic matter (C:N and C:P ratios) and K‐Ca‐Mg antagonisms. In addition to soil properties, plant‐related parameters (nutrient concentrations in foliage and leaf life span) and climate variables (e.g., precipitation and actual evapotranspiration) were also correlated with nutrient remobilization. Using multivariate analysis, we found that soil nutrient richness and the life span of the leaf were generally the two most important factors controlling nutrient remobilization. As a whole, the nutrient remobilization rate is regulated by soil nutrients through negative feedback. This general ecological pattern is modulated by ecophysiological constraints of plants, mainly leaf life span or the capability of plants to move Ca through the phloem sap.