Higher Leaf Mass Per Area Research Articles

Palaeo leaf economics reveal a shift in ecosystem function associated with the end-Triassic mass extinction event.

Climate change is likely to have altered the ecological functioning of past ecosystems, and is likely to alter functioning in the future; however, the magnitude and direction of such changes are difficult to predict. Here we use a deep-time case study to evaluate the impact of a well-constrained CO2-induced global warming event on the ecological functioning of dominant plant communities. We use leaf mass per area (LMA), a widely used trait in modern plant ecology, to infer the palaeoecological strategy of fossil plant taxa. We show that palaeo-LMA can be inferred from fossil leaf cuticles based on a tight relationship between LMA and cuticle thickness observed among extant gymnosperms. Application of this new palaeo-LMA proxy to fossil gymnosperms from East Greenland reveals significant shifts in the dominant ecological strategies of vegetation found across the Triassic-Jurassic transition. Late Triassic forests, dominated by low-LMA taxa with inferred high transpiration rates and short leaf lifespans, were replaced in the Early Jurassic by forests dominated by high-LMA taxa that were likely to have slower metabolic rates. We suggest that extreme CO2-induced global warming selected for taxa with high LMA associated with a stress-tolerant strategy and that adaptive plasticity in leaf functional traits such as LMA contributed to post-warming ecological success.

Effects of structural complexity on within-canopy light environments and leaf traits in a northern mixed deciduous forest.

Canopy structure influences forest productivity through its effects on the distribution of radiation and the light-induced changes in leaf physiological traits. Due to the difficulty of accessing and measuring forest canopies, few field-based studies have quantitatively linked these divergent scales of canopy functioning. The objective of our study was to investigate how canopy structure affects light profiles within a forest canopy and whether leaves of mature trees adjust morphologically and biochemically to the light environments characteristic of canopies with different structural complexity. We used a combination of light detection and ranging (LiDAR) data and hemispherical photographs to quantify canopy structure and light environments, respectively, and a telescoping pole to sample leaves. Leaf mass per area (LMA), nitrogen on an area basis (Narea) and chlorophyll on a mass basis (Chlmass) were measured in red maple (Acer rubrum), american beech (Fagus grandifolia), white pine (Pinus strobus), and northern red oak (Quercus rubra) at different heights in plots with similar leaf area index but contrasting canopy complexity (rugosity). We found that more complex canopies had greater porosity and reduced light variability in the midcanopy while total light interception was unchanged relative to less complex canopies. Leaf phenotypes of F. grandifolia, Q. rubra and P. strobus were more sun-acclimated in the midstory of structurally complex canopies while leaf phenotypes of A. rubrum were more shade-acclimated (lower LMA) in the upper canopy of more complex stands, despite no differences in total light interception. Broadleaf species showed further differences in acclimation with increased Narea and reduced Chlmass in leaves with higher LMA, while P. strobus showed no change in Narea and Chlmass with higher LMA. Our results provide new insight on how light distribution and leaf acclimation in mature trees might be altered when natural and anthropogenic disturbances cause structural changes in the canopy.

Cell-level anatomical characteristics explain high mesophyll conductance and photosynthetic capacity in sclerophyllous Mediterranean oaks.

Leaf mass per area (LMA) has been suggested to negatively affect the mesophyll conductance to CO2 (gm ), which is the most limiting factor for area-based photosynthesis (AN ) in many Mediterranean sclerophyll species. However, despite their high LMA, these species have similar AN to plants from other biomes. Variations in other leaf anatomical traits, such as mesophyll and chloroplast surface area exposed to intercellular air space (Sm /S and Sc /S), may offset the restrictions imposed by high LMA in gm and AN in these species. Seven sclerophyllous Mediterranean oaks from Europe/North Africa and North America with contrasting LMA were compared in terms of morphological, anatomical and photosynthetic traits. Mediterranean oaks showed specific differences in AN that go beyond the common morphological leaf traits reported for these species (reduced leaf area and thick leaves). These variations resulted mainly from the differences in gm , the most limiting factor for carbon assimilation in these species. Species with higher AN showed increased Sc /S, which implies increased gm without changes in stomatal conductance. The occurrence of this anatomical adaptation at the cell level allowed evergreen oaks to reach AN values comparable to congeneric deciduous species despite their higher LMA.

Convergence in leaf phenology traits of two understorey ferns in the northwestern Iberian Peninsula

Plants control leaf phenology to maximize annual photosynthetic product. Although ferns play an important ecological role in many habitats, especially forests, their phenology traits have been poorly studied. Here, we investigate the leaf phenology of two ferns of the forest understorey and analyse the relationship between the timing of leaf emergence and spore dispersal and the effect of between-year climatic variation. We compared the leafing and sporing phenologies of two ferns with very large (>2 m), overwintering leaves: Culcita macrocarpa and Woodwardia radicans. We regularly monitored individuals of six populations in the northwestern Iberian Peninsula during a 3-year study. We studied eight phenology variables: leafing start date, leafing end date, leaf expansion time, number of new leaves per individual, between-individual synchrony, within-individual synchrony, percentage of fertile leaves and spore release date. We also determined leaf mass per area (LMA) and gathered data on air temperature and humidity. Both C. macrocarpa and W. radicans produce few leaves (~2 leaves individual⁻¹ year⁻¹), which expand simultaneously for a very long period (from midwinter to early summer), are retained for more than 1 year (37 and 19 months, respectively) and have relatively high LMAs. Such traits, together with large leaf size, have also been found in seed plants from the forest understorey and represent adaptations to this light-limited environment. Spores of both study ferns are simultaneously released in late winter, with little between-year variation caused by differences in air humidity. This remarkable similarity between species suggests that the convergence in timing of leaf emergence favours the convergence in timing of spore dispersal.

Precipitation, not air temperature, drives functional responses of trees in semi‐arid ecosystems

Summary Model scenarios of climate change predict that warming and drought will occur simultaneously in the future in many regions. The capacity of woody species to modify their physiology and morphology in response to environmental conditions is widely recognized, but little is known about the responses of trees to reduced precipitation and increased temperature acting simultaneously. In a semi‐arid woodland, we assessed the responses in physiological (needle emergence, maximum photosynthesis, stomatal conductance, water use efficiency (WUE) and shoot elongation) and morphological (needle length and thickness, and leaf mass per area (LMA)) foliar traits of piñon pine (Pinus edulis) in response to three years of a 45% reduction in precipitation, a 4.8 °C increase in air temperature and their simultaneous effects. A strong change in physiological and morphological traits in response to reduced precipitation was observed. Precipitation reduction delayed needle emergence, decreased photosynthesis and stomatal conductance, increased WUE, decreased shoot elongation and induced shorter needles with a higher LMA. Trees subjected to simultaneous reductions in precipitation and warming demonstrated a similar response. However, atmospheric warming did not induce a response in any of the measured traits. Physiological and morphological traits of trees in this semi‐arid climate were more responsive to changes in soil moisture than air temperature. Long‐term exposure to seasonal drought stress in arid sites may have resulted in strong plastic responses to this first stressor. However, atmospheric warming probably was not experienced as a stress for trees in this warm and dry climate. Overall, our results indicate that in semi‐arid ecosystems where tree functioning is already highly limited by soil water availability, atmospheric warming as anticipated with climate change may have less impact on foliar trait responses than previously thought.

Leaf density explains variation in leaf mass per area in rice between cultivars and nitrogen treatments.

Leaf mass per area (LMA) is an important leaf trait; however, correlations between LMA and leaf anatomical features and photosynthesis have not been fully investigated, especially in cereal crops. The objectives of this study were (a) to investigate the correlations between LMA and leaf anatomical traits; and (b) to clarify the response of LMA to nitrogen supply and its effect on photosynthetic nitrogen use efficiency (PNUE). In the present study, 11 rice varieties were pot grown under sufficient nitrogen (SN) conditions, and four selected rice cultivars were grown under low nitrogen (LN) conditions. Leaf anatomical traits, gas exchange and leaf N content were measured. There was large variation in LMA across selected rice varieties. Regression analysis showed that the variation in LMA was more closely related to leaf density (LD) than to leaf thickness (LT). LMA was positively related to the percentage of mesophyll tissue area (%mesophyll), negatively related to the percentage of epidermis tissue area (%epidermis) and unrelated to the percentage of vascular tissue area (%vascular). The response of LMA to N supplementation was dependent on the variety and was also mainly determined by the response of LD to N. Compared with SN, photosynthesis was significantly decreased under LN, while PNUE was increased. The increase in PNUE was more critical in rice cultivars with a higher LMA under SN supply. Leaf density is the major cause of the variation in LMA across rice varieties and N treatments, and an increase in LMA under high N conditions would aggravate the decrease in PNUE.

Leaf Mass per Area (LMA) and Its Relationship with Leaf Structure and Anatomy in 34 Mediterranean Woody Species along a Water Availability Gradient.

Leaf mass per area (LMA) is a morphological trait widely used as a good indicator of plant functioning (i.e. photosynthetic and respiratory rates, chemical composition, resistance to herbivory, etc.). The LMA can be broken down into the leaf density (LD) and leaf volume to area ratio (LVA or thickness), which in turn are determined by anatomical tissues and chemical composition. The aim of this study is to understand the anatomical and chemical characteristics related to LMA variation in species growing in the field along a water availability gradient. We determined LMA and its components (LD, LVA and anatomical tissues) for 34 Mediterranean (20 evergreen and 14 deciduous) woody species. Variation in LMA was due to variation in both LD and LVA. For both deciduous and evergreen species LVA variation was strongly and positively related with mesophyll volume per area (VA or thickness), but for evergreen species positive relationships of LVA with the VA of epidermis, vascular plus sclerenchyma tissues and air spaces were found as well. The leaf carbon concentration was positively related with mesophyll VA in deciduous species, and with VA of vascular plus sclerenchymatic tissues in evergreens. Species occurring at the sites with lower water availability were generally characterised by a high LMA and LD.

Variations in nitrogen use efficiency reflect the biochemical subtype while variations in water use efficiency reflect the evolutionary lineage of C4 grasses at inter-glacial CO2.

C4 photosynthesis evolved multiple times in diverse lineages. Most physiological studies comparing C4 plants were not conducted at the low atmospheric CO2 prevailing during their evolution. Here, 24 C4 grasses belonging to three biochemical subtypes [nicotinamide adenine dinucleotide malic enzyme (NAD-ME), phosphoenolpyruvate carboxykinase (PCK) and nicotinamide adenine dinucleotide phosphate malic enzyme (NADP-ME)] and six major evolutionary lineages were grown under ambient (400 μL L(-1) ) and inter-glacial (280 μL L(-1) ) CO2 . We hypothesized that nitrogen-related and water-related physiological traits are associated with subtypes and lineages, respectively. Photosynthetic rate and stomatal conductance were constrained by the shared lineage, while variation in leaf mass per area (LMA), leaf N per area, plant dry mass and plant water use efficiency were influenced by the subtype. Subtype and lineage were equally important for explaining variations in photosynthetic nitrogen use efficiency (PNUE) and photosynthetic water use efficiency (PWUE). CO2 treatment impacted most parameters. Overall, higher LMA and leaf N distinguished the Chloridoideae/NAD-ME group, while NADP-ME and PCK grasses were distinguished by higher PNUE regardless of lineage. Plants were characterized by high photosynthesis and PWUE when grown at ambient CO2 and by high conductance at inter-glacial CO2 . In conclusion, the evolutionary and biochemical diversity among C4 grasses was aligned with discernible leaf physiology, but it remains unknown whether these traits represent ecophysiological adaptation.

Can atmospheric composition influence plant fossil preservation potential via changes in leaf mass per area? A new hypothesis based on simulated palaeoatmosphere experiments.

Atmospheric composition, particularly levels of CO2 and O2, impacts all aspects of life but its role in relation to plant preservation in the fossil record is largely unconsidered. Plants, angiosperms in particular, have been widely shown to increase leaf mass per area (LMA) under high CO2 conditions and decrease LMA in low CO2 conditions. Leaf thickness has long been known to be a contributory factor in preservation potential in the plant fossil record, with thicker leaves considered to have a greater recalcitrance than thinner ones. Therefore, any change in leaf density/thickness, through changes to LMA, could lead to an increased or decreased preservation potential of fossil leaves at times of elevated or decreased CO2, respectively. Additionally, the impact of changes to atmospheric O2 and to the atmospheric CO2:O2 ratio on LMA has not been previously considered in detail. This investigation examines the effect of simulated Mesozoic atmospheres, times of high CO2 and low O2, on LMA in a suite of gymnosperms that act as nearest living equivalents for common elements of Mesozoic floras. Exposure to high CO2 (~1500ppm) led to a statistically significant (p<0.001) increase in LMA in four out of 6 species, and exposure to combined high CO2 and low O2 (~13%) induced a statistically significant (p<0.001) increase in LMA in all six species. The investigation also examined the effects of atmospheric composition on %N, a key plant trait known to co-vary with LMA under modern atmospheric compositions that provides information on plant function and relates to photosynthetic efficiency. Most species showed decreased %N in treatments with increased LMA in agreement with modern ecological studies and supporting the co-varying nature of LMA and %N regardless of CO2:O2 ratio. These findings suggest that atmospheric composition has a pronounced impact on LMA. Based on these results, we propose the hypothesis that atmospheric composition is an important taphonomic filter of the fossil leaf record. Further research is now required to test the significance of atmospheric composition versus other well-known taphonomic filters.

Leaf anatomy is associated with the type of growth form in Neotropical savanna plants

Scleromorphic leaf structures are associated with plants growing under drought-prone conditions, high irradiances, and nutrient-poor soils. Sclerophylly can also be a valuable deterrent against herbivores. However, comprehensive studies of leaf anatomical traits encompassing different growth forms are lacking. The savannas of central Brazil (Cerrado) are characterized by high species diversity and a variety of growth forms. We performed a field study to characterize leaf anatomical traits and leaf mass per area (LMA) in 57 co-occurring Cerrado species of distinct plant families (located at different positions in the angiosperm phylogeny) and categorized according to the following growth forms: trees, palms, shrubs, subshrubs, vines, grasses and herbs. Plant growth form and leaf structure were interrelated. Four groups with distinct leaf anatomical syndromes were identified by NMDS analysis: grasses, palms, herbaceous (herbs, vines, and most subshrubs), and woody (trees and shrubs) plants. Trees and shrubs had scleromorphic dorsiventral leaves, with high tissue thickness and LMA. Herbaceous plants had thin, mesomorphic leaves. While most herbaceous plants had dorsiventral type of mesophyll, grasses and palms had homogenous mesophyll with sclerenchymatous bundle sheath extensions. Palms differed from grasses by having thicker leaves and sclerenchymatous hypodermis. In conclusion, Neotropical savannas cannot be exclusively described as scleromorphic vegetation.

Manipulation of the hypocotyl sink activity by reciprocal grafting of two Raphanus sativus varieties: its effects on morphological and physiological traits of source leaves and whole-plant growth.

To reveal whether hypocotyl sink activities are regulated by the aboveground parts, and whether physiology and morphology of source leaves are affected by the hypocotyl sink activities, we conducted grafting experiments using two Raphanus sativus varieties with different hypocotyl sink activities. Comet (C) and Leafy (L) varieties with high and low hypocotyl sink activities were reciprocally grafted and resultant plants were called by their scion and stock such as CC, LC, CL and LL. Growth, leaf mass per area (LMA), total non-structural carbohydrates (TNCs) and photosynthetic characteristics were compared among them. Comet hypocotyls in CC and LC grew well regardless of the scions, whereas Leafy hypocotyls in CL and LL did not. Relative growth rate was highest in LL and lowest in CC. Photosynthetic capacity was correlated with Rubisco (ribulose 1·5-bisphosphate carboxylase/oxygenase) content but unaffected by TNC. High C/N ratio and accumulation of TNC led to high LMA and structural LMA. These results showed that the hypocotyl sink activity was autonomously regulated by hypocotyl and that the down-regulation of photosynthesis was not induced by TNC. We conclude that the change in the sink activity alters whole-plant growth through the changes in both biomass allocation and leaf morphological characteristics in R. sativus.

Leaf life span and the leaf economic spectrum in the context of whole plant architecture

Summary The leaf economics spectrum (LES) has been an organizing framework of plant functional ecology for the past decade. The LES describes a set of trade‐offs among traits related to plant carbon balance. Species with a long leaf life span (LLS) invest additional material for leaf protection and structural support and consequently tend to have a lower leaf photosynthetic rate per unit mass than species with a shorter LLS. While the LES is most apparent in comparing species with extreme differences in their traits, it has nonetheless been adopted as a general explanation of leaf trait variation at all scales and in all plants. It highlights the ‘trait‐based’ approach to plant ecology, which has generally used a small set of traits to predict whole organism and even whole ecosystem attributes. Few studies have investigated the relationships between LES traits and organismal attributes not directly related to carbon economy. We explored the LES in 32 deciduous woody species of Viburnum (Adoxaceae). We found no evidence for any mass‐based LES trade‐offs. Rather, on an area basis, photosynthetic rates were positively correlated with leaf mass per area (LMA); higher LMA was associated with greater investment in photosynthetic tissue, with most of the variation due to changes in the thickness of photosynthetic mesophyll. Species’ mean LLS varied between 19 and 26 weeks and was not correlated with other LES traits. Instead, LLS was strongly associated with the diverse set of whole‐plant branching patterns in Viburnum. In the most common growth pattern, LLS was significantly correlated with flowering time, because branches end in terminal inflorescences, and all leaves and inflorescences are pre‐formed in overwintering buds. Synthesis. Plants may recover the cost of their leaves early in the growing season, allowing LLS to vary independently of the plant carbon budget. In deciduous species, LLS may be strongly influenced by whole plant architecture, which, in Viburnum, is evolutionarily conserved. In general, positive area‐based LES trait relationships will limit the relevance of LLS to this spectrum and allow LLS to vary for reasons that are not directly related to carbon economy.

Photosynthetic responses and acclimation of two castor bean cultivars to repeated drying–wetting cycles

To investigate the responses of castor bean to repeated drying–wetting cycles (RDWC), morpho-physiological parameters of two cultivars (Jiaxiang 2 and Hangbi 8) were determined by a pot experiment under well-watered control and RDWC. RDWC inhibited plant growth and leaf development, decreased water loss rate (WLR), and enhanced leaf mass per area (LMA) and chlorophyll content as indicated by spectral reflectance indices for both cultivars. Photosynthesis was inhibited by progressive drought stress but quickly recovered after rewatering for each cycle. Both cultivars exhibit a similar pattern of acclimation to RDWC: (1) higher LMA and lower WLR, (2) increased photosynthetic capacity under drought stress with increasing cycle numbers, (3) quick recovery and over-compensation for photosynthesis after rewatering, and (4) increased chlorophyll content. Jiaxiang 2 shows a high capacity for water preservation under drought stress and an over-compensation for photosynthesis after rewatering compared with Hangbi 8.

Effects of microhabitat on leaf traits in Digitalis grandiflora L. (Veronicaceae) growing at forest edge and interior

The morphological, anatomical and biochemical traits of the leaves of yellow foxglove (Digitalis grandiflora Mill.) from two microhabitats, forest interior (full shade under oak canopy) and forest edge (half shade near shrubs), were studied. The microhabitats differed in the mean levels of available light, but did not differ in soil moisture. The mean level of light in the forest edge microhabitat was significantly higher than in the forest interior. Multivariate ANOVA was used to test the effects of microhabitat. Comparison of the available light with soil moisture revealed that both factors significantly influenced the morphological and anatomical variables of D. grandiflora. Leaf area, mass, leaf mass per area (LMA), surface area per unit dry mass (SLA), density and thickness varied greatly between leaves exposed to different light regimes. Leaves that developed in the shade were larger and thinner and had a greater SLA than those that developed in the half shade. In contrast, at higher light irradiances, at the forest edge, leaves tended to be thicker, with higher LMA and density. Stomatal density was higher in the half-shade leaves than in the full-shade ones. LMA was correlated with leaf area and mass and to a lesser extent with thickness and density in the forest edge microsite. The considerable variations in leaf density and thickness recorded here confirm the very high variation in cell size and amounts of structural tissue within species. The leaf plasticity index (PI) was the highest for the morphological leaf traits as compared to the anatomical and biochemical ones. The nitrogen content was higher in the ?half-shade leaves? than in the ?shade leaves?. Denser leaves corresponded to lower nitrogen (N) contents. The leaves of plants from the forest edge had more potassium (K) than leaves of plants from the forest interior on an area basis but not on a dry mass basis; the reverse was true for phosphorus.

Exploring variation in leaf mass per area (LMA) from leaf to cell: An anatomical analysis of 26 woody species

Plant species differ widely in the leaf biomass invested per unit area (LMA). LMA can be explained by variation in leaf thickness and/or density, both of which are influenced by anatomical tissue composition. The aim of this study is to quantify the anatomical characteristics that underlie variation in LMA in a range of woody species. • Twenty-six woody species, forming 13 species pairs with a deciduous and evergreen species from the same genus or family, were grown in a glasshouse. The youngest full-grown leaves were analyzed for LMA and morpho-anatomical characteristics at leaf, tissue, and cell level. • Considered over all species studied, leaf thickness and density were equally important to explain the variation in LMA, but the class difference between deciduous and evergreen species was mainly determined by thickness, whereas variation within each group was largely due to density. Evergreens had thicker leaves, predominantly caused by a larger volume of mesophyll and air spaces, whereas the higher leaf density within each group was due to a lower proportion of epidermis and air spaces, and overall smaller cells. • The anatomical basis for variation in LMA in woody species depends on the contrast made. Higher LMA in evergreens is mainly due to a greater leaf thickness, caused by a larger volume of mesophyll and air spaces. Within deciduous species and evergreens, higher LMA is caused by a higher density, due to higher volumetric fractions of mesophyll and lower fractions of air spaces and epidermis.

Global Leaf Trait Relationships: Mass, Area, and the Leaf Economics Spectrum

The leaf economics spectrum (LES) describes multivariate correlations that constrain leaf traits of plant species primarily to a single axis of variation if data are normalized by leaf mass. We show that these traits are approximately distributed proportional to leaf area instead of mass, as expected for a light- and carbon dioxide-collecting organ. Much of the structure in the mass-normalized LES results from normalizing area-proportional traits by mass. Mass normalization induces strong correlations among area-proportional traits because of large variation among species in leaf mass per area (LMA). The high LMA variance likely reflects its functional relationship with leaf life span. A LES that is independent of mass- or area-normalization and LMA reveals physiological relationships that are inconsistent with those in global vegetation models designed to address climate change.

Leaf allocation patterns and 13C and 15N natural abundances of tropical lianas (Passiflora sp.) as dependent on external climbing support

The transformation from self-supporting lianas to host-supported climbing lianas is related to re-allocation of biomass and nutrients among plant organs. Therefore, first, variations in leaf mass per area (LMA), leaf carbon and nitrogen allocation and 13C and 15N natural abundances were analysed among three tropical Passiflora species (P. edulis, P. ligularis, and P. tripartita) in a greenhouse study. Second, the influence of a climbing support was considered for each species and parameter. P. ligularis leaves were most enriched in 13C in both treatments when compared with the other two species. This enrichment was caused by a high LMA, which is related to a high internal resistance to CO2 diffusion. For P. edulis and P. tripartita, δ13C was additionally increasing with nitrogen content per area. Generally, there were no differences when considering carbon and nitrogen allocation to leaves of host-supported and self-supporting lianas. The only hints towards increased investment into leaves after the transition from self-supporting to host-supported stages could be seen by a trend to increased leaf areas and masses. δ13C values of supported P. edulis or P. tripartita plants were significantly increasing faster than those of non-supported plants once the interactions of leaf mass or nitrogen content per area were accounted for. Hence, the offer of a climbing support had only a minor impact on δ13C or δ15N values in vitro, but this could be different with increasing age of lianas in vivo.

Within-species leaf trait variation and ecological flexibility in resprouting tropical trees

Plants have an inherent flexibility to respond to different environmental conditions. One axis of plant ecophysiological strategy is seen in the spectrum of leaf functional traits. Flexibility in these traits would be suggestive of plants’ phenotypic plasticity in response to environmental changes. This research seeks to identify differences between leaves of sprout and non-sprout shoots of a broad ecological range of neotropical tree species. Using a functional-trait approach, this study assesses a large pool of species for within-species physiological flexibility. Leaf mass per area (LMA) and leaf area were measured for plants of sprout and non-sprout origin for 26 tree species grown in a reforestation plantation in Panama. Sprouts had a consistently lower LMA than non-sprouts, but there was no consistent pattern for leaf area. These trends show that sprouts are more like pioneer species than conspecific saplings, a finding in general agreement with fast sprout growth seen in previous studies. Further, later-successional (high LMA) species showed a greater reduction of LMA in sprouts. These results show that tropical tree species adjust physiologically to changing ecological roles and suggest that certain species may be more resilient than realized to changing climate and disturbance patterns.

Functional traits and their plasticity predict tropical trees regeneration niche even among species with intermediate light requirements

Summary Niche differentiation is a key issue in the current debate on community assembly mechanisms. In highly diverse moist tropical forests, tree species sensitivity to canopy openness is thought to be a major axis in niche differentiation. In the past, the syndrome of traits driving the demographic trade‐off involved in the niche‐based theory of coexistence has always been established among species situated at the two extremities of the shade‐tolerance gradient, even though most tropical tree species have intermediate light requirements. In addition, trait plasticity has seldom been linked to tropical tree species distribution along environmental gradients. This article examines covariations between leaf traits, whole‐plant traits and niche parameters among 14 tree species with intermediate light requirements in French Guiana and across a range of canopy openness. Each functional trait measured under field conditions was characterized by a median value and a degree of plasticity expressed under contrasting light regimes. Niche differentiation was characterized in terms of spatial light gradient. We first examined covariations between functional traits then explored to what degree the median value and plasticity in functional traits could predict light niche characteristics at the sapling stage and the ontogenetic change in light availability estimated by adult stature. Leaf mass per area (LMA) was positively correlated with leaf life span (LLS); species with higher LMA and higher LLS displayed lower diameter growth rates (GRs) and lower responsiveness to canopy gap at both whole‐plant and population levels. This proved that the relationships previously established over a broader range of species held true within the narrow range of the light requirements covered. Height GR plasticity accounted for 49% of the variation in light niche optimum. LMA plasticity, unlike LLS plasticity, was significantly correlated with light niche breadth and adult stature. Synthesis. This study demonstrates the relevance of considering the phenotypic plasticity in functional traits in community ecology, particularly for quantifying breadth of species distribution over environmental gradients. Our findings did not support Hubbell's hypothesis of functional equivalence and suggest that even a rather subtle variation in forest canopy disturbance promotes the coexistence of tropical tree species.

Variations in Leaf Photosynthetic and Morphological Traits with Tree Height in Various Tree Species in a Cambodian Tropical Dry Evergreen Forest

In this study, we demonstrate changes in leaf morphological and physiological traits with tree height from dark understory to bright canopy conditions in various tree species in the Cambodian tropical dry evergreen forest. The vegetation mainly consisted of Dipterocarpaceae and Myristicaceae and the canopy trees usually reached 30-40 m in height. We investigated 25 individuals of 18 tree species ranging from 0.8 to 33 m in height. We measured the leaf photosynthetic rate, stomatal conductance and respiration rate for 3 to 5 leaves per sampling position in the early dry season. All leaves were then divided into two parts: one for measuring dry weight, nitrogen content and δ13C; the other for observation of leaf morphology. The leaf morphological traits, such as leaf mass per area (LMA), cuticle thickness, palisade layer thickness, leaf hardness and stomatal density increased linearly with tree height. The leaf nitrogen content per unit leaf area (Narea) peaked at 10 m from the ground, though the nitrogen content per unit dry leaf mass (Nmass) decreased linearly with tree height. Higher LMA, cuticle thickness and hard leaves in canopy condition may contribute to high drought tolerance and physical strength. The leaf-area-based photosynthetic rate (Amax-area) peaked at an intermediate tree height of approximately 10 m, and then decreased toward the upper canopy. In contrast, the leaf-mass-based photosynthetic rate (Amax-mass) decreased linearly with tree height. Reduction of leaf nitrogen content and stomatal conductance mainly limit photosynthetic capacities with tree height. Overall, many leaf morphological traits could be summarized in a simple and significant relation with tree height, though increasing tree height, which is related to the micro-climatic gradient, leads to both nitrogen and stomatal constraints of leaf photosynthetic capacities, even when considering many different tree species.