Light and VPD gradients drive foliar nitrogen partitioning and photosynthesis in the canopy of European beech and silver fir

Vertical gradient of needle ozone uptake within the canopy of Cryptomeria japonica

Leaf ozone uptake through the stomata is an important index for the ozone risk assessments on trees. Stomatal conductance (gs) and ozone concentration ([O3]), determinants of the leaf ozone uptake, are known to show vertical gradients within a tree canopy. However, less is known about the within-canopy vertical gradient of leaf ozone uptake. This study was aimed to elucidate how the vertical gradient of [O3] and gs affect needle ozone uptake within a canopy of mature Cryptomeria japonica trees in a suburban forest at Tokyo, Japan. For this purpose, a multilayer gas exchange model was applied to estimate the vertical gradient of needle gs and the accumulated ozone uptake during the study period (POD1, Phytotoxic Ozone Dose above a threshold of 1 nmol m-2 s-1). In addition, we also tested several scenarios of vertical gradient of [O3] within the canopy for sensitivity analysis. The POD1 was declined from the top to the bottom of the canopy. This tendency strongly depended on the vertical gradient of gs and was hardly affected by the changes in simulated vertical reductions of the [O3]. We further assessed the photosynthesis of sunlit needles (needles absorbing both direct and diffuse light) and shaded needles (needles only absorbing diffuse light). The photosynthesis of shaded needles in the upper half of the canopy made a great contribution to the entire canopy photosynthesis. In addition, given that their POD1 was lower than that of sunlit needles, ozone may affect sunlit and shaded needles differently. We concluded that these considerations should be incorporated into modeling of the calculation of ozone uptake for mature trees to make accurate predictions of the ozone effects on trees at the canopy scale.

Response of beech and fir to different light intensities along the Carpathian and Dinaric Mountains

Predicting global change mitigations based on environmental variables, like temperature and water availability, although yielding insightful hypothesis still lacks the integration of environmental responses. Physiological limits should be assessed to obtain a complete representation of a species' fundamental niche. Detailed ecophysiological studies on the response of trees along the latitudinal gradient are rare. They could shed light on the behaviour under different light intensities and other studied traits. The forests of the Dinaric Mountains and the Carpathians represent the largest contiguous forest complexes in south-eastern Europe. In uneven-aged Carpathian (8 plots) and Dinaric Mountain (11 plots) forests, net assimilation (Amax) and maximum quantum yield (Φ) were measured for beech and fir in three predefined light intensity categories according to the indirect site factor (ISF%) obtained by the analysis of hemispherical photographs in managed and old growth forests, all located above 800 m a.s.l. The measurements were carried out under fixed environmental conditions in each light category per plot for three consecutive years. Data from the last 50-year average period from the CRU TS 4.01 dataset were used for the comparison between Amax, Φ, and climate. The highest Φ for beech were observed in the central part of the Dinaric Mountains and in the south westernmost and northwesternmost part of the Carpathians for both beech and fir, while they were highest for fir in the Dinaric Mountains in the northwesternmost part of the study area. The Φ-value of beech decreased in both complexes with increasing mean annual temperature and was highest in the open landscape. For fir in the Carpathians, Φ decreased with increasing mean annual temperature, while in the Dinaric Mountains it increased with higher temperature and showed a more scattered response compared to the Carpathians. Short-term ecophysiological responses of beech and fir were consistent to long-term radial growth observations observed on same locations. The results may provide a basis and an indication of the future response of two tree species in their biogeographical range to climate change in terms of competitiveness, existence and consequently forest management decisions.

Intraspecific plasticity and co-variation of leaf traits facilitate Ficus tinctoria to acclimate hemiepiphytic and terrestrial habitats

Despite intensive studies on plant functional traits, the intraspecific variation and their co-variation at the multi-scale remains poorly studied, which holds the potential to unveil plant responses to changing environmental conditions. In this study, intraspecific variations of 16 leaf functional traits of a common fig species, Ficus tinctoria G. Frost., were investigated in relation to different scales: habitat types (hemiepiphytic and terrestrial), growth stages (small, medium and large) and tree crown positions (upper, middle and lower) in Xishuangbanna, Southwest China. Remarkable intraspecific variation was observed in leaf functional traits, which was mainly influenced by tree crown position, growth stage and their interaction. Stable nitrogen isotope (δ15N) and leaf area (LA) showed large variations, while stable carbon isotope (δ13C), stomata width and leaf water content showed relatively small variations, suggesting that light- and nitrogen-use strategies of F. tinctoria were plastic, while the water-use strategies have relatively low plasticity. The crown layers are formed with the growth of figs, and leaves in the lower crown increase their chlorophyll concentration and LA to improve the light energy conversion efficiency and the ability to capture weak light. Meanwhile, leaves in the upper crown increase the water-use efficiency to maintain their carbon assimilation. Moreover, hemiepiphytic medium (transitional stage) and large (free-standing stage) figs exhibited more significant trait differentiation (chlorophyll concentration, δ13C, stomata density, etc.) within the crown positions, and stronger trait co-variation compared with their terrestrial counterparts. This pattern demonstrates their acclimation to the changing microhabitats formed by their hemiepiphytic life history. Our study emphasizes the importance of multi-scaled intraspecific variation and co-variation in trait-based strategies of hemiepiphyte and terrestrial F. tinctoria, which facilitate them to cope with different environmental conditions.

Bark wounding triggers gradual embolism spreading in two diffuse-porous tree species

Xylem transport is essential for the growth, development and survival of vascular plants. Bark wounding may increase the risk of xylem transport failure by tension-driven embolism. However, the consequences of bark wounding for xylem transport are poorly understood. Here, we examined the impacts of the bark wounding on embolism formation, leaf water potential and gas exchange in the terminal branches of two diffuse-porous tree species (Acer platanoides L. and Prunus avium L.). The effects of bark removal were examined on field-grown mature trees exposed to increased evaporative demands on a short-term and longer-term basis (6 h vs 6 days after bark wounding). Bark removal of 30% of branch circumference had a limited effect on the xylem hydraulic conductivity when embolized vessels were typically restricted to the last annual ring near the bark wound. Over the 6-day exposure, the non-conductive xylem area had significantly increased in the xylem tissue underneath the bark wound (from 22-29% to 51-52% of the last annual ring area in the bark wound zone), pointing to gradual yet relatively limited embolism spreading to deeper xylem layers over time. In both species, the bark removal tended to result in a small but non-significant increase in the percent loss of hydraulic conductivity compared with control intact branches 6 days after bark wounding (from 6 to 8-10% in both species). The bark wounding had no significant effects on midday leaf water potential, CO2 assimilation rates, stomatal conductance and water-use efficiency of the leaves of the current-year shoot, possibly due to limited impacts on xylem transport. The results of this study demonstrate that bark wounding induces limited but gradual embolism spreading. However, the impacts of bark wounding may not significantly limit water delivery to distal organs and leaf gas exchange at the scale of several days.

Variation in leaf photosynthetic capacity within plant canopies: optimization, structural, and physiological constraints and inefficiencies

Leaf photosynthetic capacity (light-saturated net assimilation rate, AA) increases from bottom to top of plant canopies as the most prominent acclimation response to the conspicuous within-canopy gradients in light availability. Light-dependent variation in AA through plant canopies is associated with changes in key leaf structural (leaf dry mass per unit leaf area), chemical (nitrogen (N) content per area and dry mass, N partitioning between components of photosynthetic machinery), and physiological (stomatal and mesophyll conductance) traits, whereas the contribution of different traits to within-canopy AA gradients varies across sites, species, and plant functional types. Optimality models maximizing canopy carbon gain for a given total canopy N content predict that AA should be proportionally related to canopy light availability. However, comparison of model expectations with experimental data of within-canopy photosynthetic trait variations in representative plant functional types indicates that such proportionality is not observed in real canopies, and AA vs. canopy light relationships are curvilinear. The factors responsible for deviations from full optimality include stronger stomatal and mesophyll diffusion limitations at higher light, reflecting greater water limitations and more robust foliage in higher light. In addition, limits on efficient packing of photosynthetic machinery within leaf structural scaffolding, high costs of N redistribution among leaves, and limited plasticity of N partitioning among components of photosynthesis machinery constrain AA plasticity. Overall, this review highlights that the variation of AA through plant canopies reflects a complex interplay between adjustments of leaf structure and function to multiple environmental drivers, and that AA plasticity is limited by inherent constraints on and trade-offs between structural, chemical, and physiological traits. I conclude that models trying to simulate photosynthesis gradients in plant canopies should consider co-variations among environmental drivers, and the limitation of functional trait variation by physical constraints and include the key trade-offs between structural, chemical, and physiological leaf characteristics.

Trade-off of leaf-scale resource-use efficiencies along the vertical canopy of the subtropical forest

Leaf resource-use efficiencies are key indicators of plant adaptability to climate change, as they depend on both photosynthetic carbon assimilation and available resources. However, accurately quantifying the response of the coupled carbon and water cycles is challenging due to the canopy vertical variability in resource-use efficiencies, which introduces greater uncertainty into the calculations. Here we experimented to ascertain the vertical variations of leaf resource-use efficiencies along three canopy gradients of coniferous (Pinus elliottii Engelmann.) and broad-leaved (Schima Superba Gardn & Champ.) forests over one year in the subtropical region of China. The efficiency of water (WUE), and nitrogen (NUE) showed higher values in the top canopy level for the two species. The maximum efficiency of light (LUE) occurred in the bottom canopy level for both species. The impact of photosynthetic photon flux density (PPFD), leaf temperature (T_leaf), and vapor pressure deficit (VPD) on leaf resource-use efficiencies varied with canopy gradients in slash pine and schima superba. We also observed a trade-off between NUE and LUE for slash pine and between NUE and WUE for schima superba. Moreover, the variation in the correlation between LUE and WUE indicated a change in resource-use strategies for slash pine. These results emphasize the significance of vertical variations in resource-use efficiencies to enhance the prediction of future carbon-water dynamics in the subtropical forest.

Late-season biosynthesis of leaf fatty acids and n-alkanes of a mature beech (Fagus sylvatica) tree traced via 13CO2 pulse-chase labelling and compound-specific isotope analysis

Leaf cuticular waxes play an important role in reducing evapotranspiration via diffusion. However, the ability of mature trees to regulate the biosynthesis of waxes to changing conditions (e.g., drought, light exposition) remain an open question, especially during the late growing season. This holds also true for one of the most widely distributed trees in Central Europe, the European beech tree (Fagus sylvatica L.). In order to investigate the ongoing formation of wax constituents like alkanes and fatty acids, we conducted a ¹³CO₂ pulse-chase labelling experiment on sun-exposed and shaded branches of a mature beech tree during the late summer 2018. The ¹³C-label was traced via compound-specific δ¹³C isotope analysis of n-alkanes and fatty acids to determine the de-novo biosynthesis within these compound classes. We did not observe a significant change in lipid concentrations during the late growing season, but we found higher n-alkane concentrations in sun-exposed compared to shaded leaves in August and September. The n-alkane and fatty acid composition showed ongoing modifications during the late growing season. Together with the uptake and following subsequent decrease of the ¹³C-label, this suggests ongoing de-novo biosynthesis, especially of fatty acids in European beech leaves. Moreover, there is a high variability in the ¹³C-label among individual branches and between sun-exposed and shaded leaves. At the same time, sun-exposed leaves invest more of the assimilated C into secondary metabolites such as lipids than shaded leaves. This indicates that the investigated mature beech tree could adjust its lipid production and composition in order to acclimate to changes in microclimates within the tree crown and during the investigated period.

Thermal sensitivity across forest vertical profiles: patterns, mechanisms, and ecological implications

Rising temperatures are influencing forests on many scales, with potentially strong variation vertically across forest strata. Using published research and new analyses, we evaluate how microclimate and leaf temperatures, traits, and gas exchange vary vertically in forests, shaping tree, and ecosystem ecology. In closed-canopy forests, upper canopy leaves are exposed to the highest solar radiation and evaporative demand, which can elevate leaf temperature (T_leaf ), particularly when transpirational cooling is curtailed by limited stomatal conductance. However, foliar traits also vary across height or light gradients, partially mitigating and protecting against the elevation of upper canopy T_leaf . Leaf metabolism generally increases with height across the vertical gradient, yet differences in thermal sensitivity across the gradient appear modest. Scaling from leaves to trees, canopy trees have higher absolute metabolic capacity and growth, yet are more vulnerable to drought and damaging T_leaf than their smaller counterparts, particularly under climate change. By contrast, understory trees experience fewer extreme high T_leaf 's but have fewer cooling mechanisms and thus may be strongly impacted by warming under some conditions, particularly when exposed to a harsher microenvironment through canopy disturbance. As the climate changes, integrating the patterns and mechanisms reviewed here into models will be critical to forecasting forest-climate feedback.

Vertical gradients in photosynthetic physiology diverge at the latitudinal range extremes of white spruce

Light availability drives vertical canopy gradients in photosynthetic functioning and carbon (C) balance, yet patterns of variability in these gradients remain unclear. We measured light availability, photosynthetic CO₂  and light response curves, foliar C, nitrogen (N) and pigment concentrations, and the photochemical reflectance index (PRI) on upper and lower canopy needles of white spruce trees (Picea glauca) at the species' northern and southern range extremes. We combined our photosynthetic data with previously published respiratory data to compare and contrast canopy C balance between latitudinal extremes. We found steep canopy gradients in irradiance, photosynthesis and leaf traits at the southern range limit, but a lack of variation across canopy positions at the northern range limit. Thus, unlike many tree species from tropical to mid-latitude forests, high latitude trees may not require vertical gradients of metabolic activity to optimize photosynthetic C gain. Consequently, accounting for self-shading is less critical for predicting gross primary productivity at northern relative to southern latitudes. Northern trees also had a significantly smaller net positive leaf C balance than southern trees suggesting that, regardless of canopy position, low photosynthetic rates coupled with high respiratory costs may ultimately constrain the northern range limit of this widely distributed boreal species.

Root water uptake depth determines the hydraulic vulnerability of temperate European tree species during the extreme 2018 drought

We took advantage of the European 2018 drought and assessed the mechanisms causing differences in drought vulnerability among mature individuals of nine co-occurring tree species at the Swiss Canopy Crane II site in Switzerland. Throughout the drought we monitored leaf water status and determined native embolism formation in the canopy of the trees as indicators of drought vulnerability. We also determined hydraulic vulnerability thresholds (Ψ₁₂ -, Ψ₅₀ - and Ψ₈₈ -values), corresponding hydraulic safety margins (HSMs) and carbohydrate reserves for all species as well as total average leaf area per tree, and used stable isotopes to assess differences in root water uptake depth among the nine species as variables predicting differences in drought vulnerability among species. Marked differences in drought vulnerability were observed among the nine tree species. Six species maintained their water potentials above hydraulic thresholds, while three species, Fagus sylvatica, Carpinus betulus and Picea abies, were pushed beyond their hydraulic thresholds and showed loss of hydraulic conductivity in their canopies at the end of the drought. Embolism resistance thresholds and associated HSMs did not explain why the co-existing species differed in their drought vulnerability, neither did their degree of isohydry, nor their regulation of carbohydrate reserves. Instead, differences in structural-morphological traits, in particular root water uptake depth, were associated with the risk of reaching hydraulic vulnerability thresholds and embolism formation among the nine species. Our study shows that structural-morphological traits, such as root water uptake depth, determine how quickly different species approach hydraulic vulnerability thresholds during a drought event and can thus explain species differences in drought vulnerability among mature field-grown trees.

Photosynthetic carbon and nitrogen metabolism of Camellia oleifera Abel during acclimation to low light conditions

Tea-oil tree (Camellia oleifera Abel) is an important woody oil crop with high economic value. However, it has low photosynthetic production considering the low light intensity of its growth environment. To understand the acclimation mechanism of tea-oil trees to low light conditions, three light intensity treatments were conducted: high light (450-500 μmol. m^-2. s^-1), medium light (180-200 μmol. m^-2. s^-1), and low light (45-50 μmol. m^-2. s^-1). The carbon (C) and nitrogen (N) metabolism network were constructed by investigating the leaf anatomy, photosynthetic characteristics, N partitioning, transcriptome and metabolome. Results demonstrated that a larger proportion light energy was used for photochemical reactions in an environment with lower light intensity, which resulted in an increase in photosystem II photochemical efficiency and instantaneous light use efficiency (LUE) at the leaf level. As the light intensity increased, decreased electron transfer and carboxylation efficiencies, photorespiration and dark respiration rates, LUE at plant level, and N use efficiency (P_NUE) were observed. Leaves trended to harvest more light using higher expression levels of light-harvesting protein genes, higher chlorophyll content, more granum and more tightly stacked granum lamella under lower light intensity. At transcriptional and metabolic levels, the TCA cycle, and the synthesis of starch and saccharides were weakened as light intensity decreased, while the Calvin cycle did not show the regularity between different treatments. Less N was distributed in Rubisco, respiration, and cell wall proteins as light decreased. Storage N was prominently accumulated in forms of amino acids (especially L-arginine) and amino acid derivatives as under medium and low light environments, to make up for C deficiency. Therefore, tea-oil trees actively improve light-harvesting capacity and enlarges the storage N pool to adapt to a low light environment, at the cost of a decrease of photosynthetic C assimilation and P_NUE.

Distribution of biomass dynamics in relation to tree size in forests across the world

Tree size shapes forest carbon dynamics and determines how trees interact with their environment, including a changing climate. Here, we conduct the first global analysis of among-site differences in how aboveground biomass stocks and fluxes are distributed with tree size. We analyzed repeat tree censuses from 25 large-scale (4-52 ha) forest plots spanning a broad climatic range over five continents to characterize how aboveground biomass, woody productivity, and woody mortality vary with tree diameter. We examined how the median, dispersion, and skewness of these size-related distributions vary with mean annual temperature and precipitation. In warmer forests, aboveground biomass, woody productivity, and woody mortality were more broadly distributed with respect to tree size. In warmer and wetter forests, aboveground biomass and woody productivity were more right skewed, with a long tail towards large trees. Small trees (1-10 cm diameter) contributed more to productivity and mortality than to biomass, highlighting the importance of including these trees in analyses of forest dynamics. Our findings provide an improved characterization of climate-driven forest differences in the size structure of aboveground biomass and dynamics of that biomass, as well as refined benchmarks for capturing climate influences in vegetation demographic models.

Environmental Factors at Different Canopy Heights Had Significant Effects on Leaf Water-Use Efficiency in Cold-Temperate Larch Forest

It is of great significance to study short-term water-use efficiency (WUEs) at different canopy heights for accurately evaluating the adaptability of cold-temperate larch (Larix gmelinii) forest to climate change. The stable isotope method combining data of gradient meteorology, photosynthetic properties and leaf structure were used to assess the influence of different canopy heights on short-term water-use efficiency (WUEs) in larch forests in the northern Da Hinggan Mountains. The results show that: (1) The rank of leaf WUEs at different canopy heights was upper canopy > middle canopy > lower canopy. The leaf WUEs in upper canopy was significantly higher than those in the middle and lower canopy (p < 0.01), and no significant difference was found between the middle and lower canopy (p > 0.05). (2) The environmental factors, the photosynthetic characteristics, the specific leaf weight (LMA) and stomatal density (SD) had significant impact (p < 0.05) on leaf WUEs at different canopy heights of larch forest. (3) The results of the weighted random forest analysis show that the main factor affecting WUEs in larch forests at different canopy heights was vapor pressure deficit (VPD), followed by relative humidity (RH) and net photosynthetic rate (Pn), while LMA and SD made relatively small contributions. This indicates that the variation of leaf WUEs at different canopy heights is mainly due to environmental factors. Our results highlight that the difference of environmental factors at different canopy heights should be considered in the future study of leaf WUE. Our results contribute to a better understanding of water utilization strategies and carbohydrate relations in the boreal forest ecosystems, which is of great significance for improving the sustainable management measures and strategies of boreal forest resources.

Photosynthetic acclimation and sensitivity to short- and long-term environmental changes in a drought-prone forest

Future climate will be characterized by an increase in frequency and duration of drought and warming that exacerbates atmospheric evaporative demand. How trees acclimate to long-term soil moisture changes and whether these long-term changes alter trees' sensitivity to short-term (day to months) variations of vapor pressure deficit (VPD) and soil moisture is largely unknown. Leaf gas exchange measurements were performed within a long-term (17 years) irrigation experiment in a drought-prone Scots pine-dominated forest in one of Switzerland's driest areas on trees in naturally dry (control), irrigated, and 'irrigation-stop' (after 11 years of irrigation) conditions. Seventeen years of irrigation increased photosynthesis (A) and stomatal conductance (gs) and reduced gs sensitivity to increasing VPD and soil drying. Following irrigation-stop, gas exchange decreased only after 3 years. After 5 years, maximum carboxylation (Vcmax) and electron transport (Jmax) rates in irrigation-stop recovered to similar levels as to before the irrigation-stop. These results suggest that long-term release from soil drought reduces the sensitivity to VPD and that atmospheric constraints may play an increasingly important role in combination with soil drought. Moreover, our study indicates that structural adjustments lead to an attenuation of initially strong leaf-level acclimation to strong multiple-year drought.

Intra-specific leaf trait variability of F. sylvatica, Q. petraea and P. abies in response to inter-specific competition and implications for forest functioning

Variability in functional traits (FT) is increasingly used to understand the mechanisms behind tree species interactions and ecosystem functioning. In order to explore how FT differ due to interactions between tree species and its influence on stand productivity and other ecological processes, we examined the effects of tree species composition on the intra-specific variability of four widely measured FT: specific leaf area, leaf nitrogen content, leaf angle and stomatal conductance response to vapor pressure deficit. This study focused on three major central European tree species: European beech (Fagus sylvatica L.), Sessile oak (Quercus petraea Liebl.) and Norway spruce (Picea abies [L.] H. Karst.). Each species was examined in monoculture and two-species mixtures in the 13-year-old tree biodiversity experiment BIOTREE-Kaltenborn. Trait distributions and linear mixed models were used to analyze the effect of species mixing, tree size and stand variables on the intra-specific FT variability. A significant effect of branch height on most traits and species indicated a vertical gradient of foliar trait frequently related to light availability. Beech and oak showed a high overall trait variability and sensitivity to species mixing and stand basal area, while the trait variability of spruce was limited. Greater shifts in trait distributions due to mixing were found in specific leaf area for oak and leaf nitrogen content for beech. Thus intra-specific variability of key leaf traits was already influenced at this young development stage by inter-specific interactions. Finally, we used the 3-PG (Physiological Processes Predicting Growth) process-based forest growth model to show that the measured intra-specific variability on single FT values could influence stand productivity, light absorption and transpiration, although the net effect depends on the considered trait and the species composition of the mixture. The results of this study will aid better understanding of the effects of inter-specific competition on intra-specific FT variability, which has implications for the parameterization of process-based forest growth models and our understanding of ecosystem functioning.

The Effect of Low Irradiance on Leaf Nitrogen Allocation and Mesophyll Conductance to CO2 in Seedlings of Four Tree Species in Subtropical China

Low light intensity can lead to a decrease in photosynthetic capacity. However, could N-fixing species with higher leaf N contents mitigate the effects of low light? Here, we exposed seedlings of Dalbergia odorifera and Erythrophleum fordii (N-fixing trees), and Castanopsis hystrix and Betula alnoides (non-N-fixing trees) to three irradiance treatments (100%, 40%, and 10% sunlight) to investigate the effects of low irradiance on leaf structure, leaf N allocation strategy, and photosynthetic physiological parameters in the seedlings. Low irradiance decreased the leaf mass per unit area, leaf N content per unit area (N_area), maximum carboxylation rate (V_cmax), maximum electron transport rate (J_max), light compensation point, and light saturation point, and increased the N allocation proportion of light-harvesting components in all species. The studied tree seedlings changed their leaf structures, leaf N allocation strategy, and photosynthetic physiological parameters to adapt to low-light environments. N-fixing plants had a higher photosynthesis rate, N_area, V_cmax, and J_max than non-N-fixing species under low irradiance and had a greater advantage in maintaining their photosynthetic rate under low-radiation conditions, such as under an understory canopy, in a forest gap, or when mixed with other species.

Influence of Microclimate Factors on Halyomorpha halys Dehydration

Simple Summary

The brown marmorated stink bug Halyomorpha halys has become a serious invasive species in Northern America and Europe, where it causes major damage to a wide range of crops. Understanding the ecology and behaviour of this pest is key to identifying the most effective strategies to contain its spread. Here we demonstrate that microclimate conditions affect H. halys water loss and that transpiration is influenced by feeding regime and sex. In the overwintering generation, transpiration does not seem influenced by population density and the first nutritional need of individuals exiting diapause is represented by hydration, likely due to water loss during the diapause. Our data suggest that hot and dry climates are not favourable for H. halys and may limit its geographical range. Similarly, microclimatic conditions within crops may have a significant impact on the distribution of H. halys and insect activity may be affected by crop management practices (e.g., pruning and irrigation).

Abstract

Understanding the interaction between insects and microclimate can be essential in order to plan informed and efficient treatments against agricultural pests. Microclimatic factors such as humidity and temperature can influence the population dynamics of the invasive agricultural pest Halyomorpha halys, the brown marmorated stink bug. The aim of this work was to evaluate the level of transpiration of H. halys in dry, normal and humid microclimates according to the sex, physiological conditions and developmental stage of individuals. Water loss during diapause and the effect of population density on insects’ transpiration were also assessed, as were the nutritional preferences of adults upon exiting diapause. Our data demonstrate that microclimatic conditions significantly influence the transpiration of this pest species. The effect of sex and feeding status on insects’ water loss is marked, while population density does not influence water loss in diapausing individuals. The first nutritional need of the overwintering generations is represented by hydration, likely due to the water loss during diapause.

Distributions of photosynthetic traits, shoot growth, and anti-herbivory defence within a canopy of Quercus serrata in different soil nutrient conditions

The hypothesis of the present study is that not only distributions of leaf photosynthetic traits and shoot growth along light gradient within a canopy of forest trees, but also that of leaf anti-herbivory defence capacities are influenced by soil nutrient condition. To test this hypothesis, we investigated the distributions of photosynthetic traits, shoot growth, anti-herbivory defence and leaf herbivory rate throughout the canopy of Quercus serrata grown in two sites with different soil nutrient conditions. In both sites, photosynthetic traits, shoot growth, and anti-herbivory defence were greater in the upper canopy. The overall defence and herbivory rate in the lower nutrient condition were higher and lower than those in the higher nutrient condition, respectively. Although differences in leaf traits between upper and lower canopies in the higher nutrient condition were smaller than those in the lower nutrient condition, no difference was found for anti-herbivory defence. These results suggest that soil nutrient condition does not affect the distributions of leaf anti herbivory defence along light gradient within a canopy of Q. serrata.

Stratigraphy of stable isotope ratios and leaf structure within an African rainforest canopy with implications for primate isotope ecology

The canopy effect describes vertical variation in the isotope ratios of carbon (δ¹³C), oxygen (δ¹⁸O) and partially nitrogen (δ¹⁵N) within plants throughout a closed canopy forest, and may facilitate the study of canopy feeding niches in arboreal primates. However, the nuanced relationship between leaf height, sunlight exposure and the resulting variation in isotope ratios and leaf mass per area (LMA) has not been documented for an African rainforest. Here, we present δ¹³C, δ¹⁸O and δ¹⁵N values of leaves (n = 321) systematically collected from 58 primate food plants throughout the canopy (0.3 to 42 m) in Côte d'Ivoire, West Africa. Besides leaf sample height and light availability, we measured leaf nitrogen and carbon content (%N, %C), as well as LMA (n = 214) to address the plants' vertical resource allocations. We found significant variation in δ¹³C, δ¹⁸O and δ¹⁵N, as well as LMA in response to height in combination with light availability and tree species, with low canopy leaves depleted in ¹³C, ¹⁸O and ¹⁵N and slightly higher in %N compared to higher canopy strata. While this vertical isotopic variation was not well reflected in the δ¹³C and δ¹⁵N of arboreal primates from this forest, it did correspond well to primate δ¹⁸O values.

Optimal carbon partitioning helps reconcile the apparent divergence between optimal and observed canopy profiles of photosynthetic capacity

Photosynthetic capacity per unit irradiance is greater, and the marginal carbon revenue of water (∂A/∂E) is smaller, in shaded leaves than sunlit leaves, apparently contradicting optimization theory. I tested the hypothesis that these patterns arise from optimal carbon partitioning subject to biophysical constraints on leaf water potential. In a whole plant model with two canopy modules, I adjusted carbon partitioning, nitrogen partitioning and leaf water potential to maximize carbon profit or canopy photosynthesis, and recorded how gas exchange parameters compared between shaded and sunlit modules in the optimum. The model predicted that photosynthetic capacity per unit irradiance should be larger, and ∂A/∂E smaller, in shaded modules compared to sunlit modules. This was attributable partly to radiation-driven differences in evaporative demand, and partly to differences in hydraulic conductance arising from the need to balance marginal returns on stem carbon investment between modules. The model verified, however, that invariance in the marginal carbon revenue of N (∂A/∂N) is in fact optimal. The Cowan-Farquhar optimality solution (invariance of ∂A/∂E) does not apply to spatial variation within a canopy. The resulting variation in carbon-water economy explains differences in capacity per unit irradiance, reconciling optimization theory with observations.

Leaf traits and canopy structure together explain canopy functional diversity: an airborne remote sensing approach

Plant functional diversity is strongly connected to photosynthetic carbon assimilation in terrestrial ecosystems. However, many of the plant functional traits that regulate photosynthetic capacity, including foliar nitrogen concentration and leaf mass per area, vary significantly between and within plant functional types and vertically through forest canopies, resulting in considerable landscape-scale heterogeneity in three dimensions. Hyperspectral imagery has been used extensively to quantify functional traits across a range of ecosystems but is generally limited to providing information for top of canopy leaves only. On the other hand, lidar data can be used to retrieve the vertical structure of forest canopies. Because these data are rarely collected at the same time, there are unanswered questions about the effect of forest structure on the three -dimensional spatial patterns of functional traits across ecosystems. In the United States, the National Ecological Observatory Network's Airborne Observation Platform (NEON AOP) provides an opportunity to address this structure-function relationship by collecting lidar and hyperspectral data together across a variety of ecoregions. With a fusion of hyperspectral and lidar data from the NEON AOP and field-collected foliar trait data, we assessed the impacts of forest structure on spatial patterns of N. In addition, we examine the influence of abiotic gradients and management regimes on top-of-canopy percent N and total canopy N (i.e., the total amount of N [g/m² ] within a forest canopy) at a NEON site consisting of a mosaic of open longleaf pine and dense broadleaf deciduous forests. Our resulting maps suggest that, in contrast to top of canopy values, total canopy N variation is dampened across this landscape resulting in relatively homogeneous spatial patterns. At the same time, we found that leaf functional diversity and canopy structural diversity showed distinct dendritic patterns related to the spatial distribution of plant functional types.

Vertical distribution of gas exchanges and their integration throughout the entire canopy in a maize field

Fluxes of carbon and water along a vertical profile within a canopy, particularly the associations between canopy and ecosystem levels, are not well studied. In this study, gas exchange along the vertical profile in a maize canopy was examined. The relationships between leaf- and ecosystem-level carbon and water fluxes were compared. The results from research conducted over two growing seasons showed that during vegetative growth, the top and middle leaf layers in the canopy contribute most to the carbon and water fluxes of the entire canopy. During the grain-filling stage, gas exchange processes were performed mostly in the middle leaves with and near the ears. Significant relationships were observed between the net ecosystem CO₂ exchange rate (NEE) plus soil respiration and the assumed canopy levels (A_canopy) and between evapotranspiration rates at the ecosystem (ET) and assumed canopy levels (E_canopy). This highlights the close associations between these parameters by integrating the leaf gas exchange rates measured in a conventional leaf cuvette and those at the ecosystem level via the eddy covariance technique. These results improve our understanding of how carbon assimilation varies vertically within a canopy, highlighting the critical role of ear leaves.

Contrasting Leaf Trait Responses of Conifer and Broadleaved Seedlings to Altered Resource Availability Are Linked to Resource Strategies

(1) Understanding tree seedling responses to water, nutrient, and light availability is crucial to precisely predict potential shifts in composition and structure of forest communities under future climatic conditions. (2) We exposed seedlings of widespread Central European tree species with contrasting leaf habit, deciduous broadleaves (DB) and evergreen conifers (EC), to factorial combinations of manipulated precipitation (100% and 50% of ambient), shade (40% and 60% of full sunlight), and nutrient availability (low and high NPK), and measured specific leaf area, C/N ratio, soluble sugars, starch and non-structural carbohydrate concentration, and δ¹³C of the leaves. (3) We found contrasting effects of water and nutrient availability on foliar traits of the two species groups: EC exhibited higher tolerance to low resource availability but also less plasticity in foliar traits, which is congruent with a “slow” resource strategy. In contrast, foliage of DB reacted particularly to altered nutrient availability, corresponding to a “fast” resource strategy with high foliar plasticity and rapid adjustments to resource fluctuations, commonly adopted by species with high growth rates. (4) We conclude that DB will respond to environmental change with foliar acclimation, while EC will either tolerate, to some extent, or shift their distribution range in response to environmental change.

Remote sensing of forest gas exchange: Considerations derived from a tomographic perspective

The global exchange of gas (CO2 , H2 O) and energy (sensible and latent heat) between forest ecosystems and the atmosphere is often assessed using remote sensing (RS) products. Although these products are essential in quantifying the spatial variability of forest-atmosphere exchanges, large uncertainties remain from a measurement bias towards top of canopy fluxes since optical RS data are not sensitive for the vertically integrated forest canopy. We hypothesize that a tomographic perspective opens new pathways to advance upscaling gas exchange processes from leaf to forest stands and larger scales. We suggest a 3D modelling environment comprising principles of ecohydrology and radiative transfer modelling with measurements of micrometeorological variables, leaf optical properties and forest structure, and assess 3D fields of net CO2 assimilation (An ) and transpiration (T) in a Swiss temperate forest canopy. 3D simulations were used to quantify uncertainties in gas exchange estimates inherent to RS approaches and model assumptions (i.e. a big-leaf approximation in modelling approaches). Our results reveal substantial 3D heterogeneity of forest gas exchange with top of canopy An and T being reduced by up to 98% at the bottom of the canopy. We show that a simplified use of RS causes uncertainties in estimated vertical gas exchange of up to 300% and that the spatial variation of gas exchange in the footprint of flux towers can exceed diurnal dynamics. We also demonstrate that big-leaf assumptions can cause uncertainties up to a factor of 10 for estimates of An and T. Concluding, we acknowledge the large potential of 3D assessments of gas exchange to unravelling the role of vertical variability and canopy structure in regulating forest-atmosphere gas and energy exchange. Such information allows to systematically link canopy with global scale controls on forest functioning and eventually enables advanced understanding of forest responses to environmental change.