Canopy Gradient Research Articles

Are shade tolerance and canopy gradient affecting twig-leaf trait scaling relationships in temperate forests?

Abstract Scaling relationships among twig size, leaf size and leafing intensity is pivotal in understanding plant resource allocation and carbon investment strategies. However, it remained unclear how these relationships might maintain stability across genetic traits (shade tolerance) and canopy gradients (microclimates). We investigated eight different shade-tolerant tree species within five mixed broad-leaved-Korean pine (Pinus koraiensis) forests in Northeast China. Employing linear mixed-effects models and phylogenetically independent contrasts, we examined the scaling relationships between twig-leaf size and leafing intensity. Shade tolerance altered the scaling relationships between twig and leaf size, as well as leafing intensity. We discovered that the scaling relationships between twig cross-sectional area and individual leaf area, leafing intensity and between individual leaf mass and leafing intensity were allometric (slope ≠ −1 or 1). However, the relationship between individual leaf area and individual leaf mass was isometric (slope = 1). Moreover, these scaling relationships exhibited consistent trends across canopy gradients, with shade tolerance playing a critical role in the coordinated evolution of twigs and leaves across these gradients. These results emphasized the significant role of shade tolerance in coordinating the covariation patterns between plant leaves and twigs, adopting conservative strategies in heterogeneous microclimates.

The role of sheltered habitats in biodiversity conservation of species sensitive to drought: a case study using ground beetles (Coleoptera, Carabidae) in the Gorongosa National Park

In the aftermath of 2019’ tropical cyclones Idai and Kenneth, we assessed ground beetle communities of the Gorongosa National Park (GNP) in Mozambique. The influence of habitat shelters, namely the buffering role of closed microhabitat conditions on Alpha diversity and community trait values during a long period of drought, was evaluated across the main habitat types of the park: miombo forests, mixed forests, transitional forests, and grasslands (open savannas). These habitat types comprised a distance gradient in relation to lake Urema, in the center of GNP. Miombo forests were farther from the lake while grasslands and transitional forests were in the floodplain area. Ground beetle communities were sampled using pitfall traps set up at 25 sites of each habitat type along an environmental gradient of tree canopy cover during the last twenty days of the dry season. Higher species richness of ground beetles was found in closed habitat shelters along the distance gradient to lake Urema. A higher functional diversity was also found along the canopy gradient, with larger sized and wingless species being more abundant in closed habitats than in open areas. This result highlights the crucial role of habitat closedness in the protection of sensitive ground beetles. In particular, the buffer effect of tree canopy and the protection of the understory plants and the litter layer was critical for the survival of hygrophilous species and will be key in conservation strategies to face climate aridification and habitat fragmentation.

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 (Tleaf), 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.

Microclimatic gradients cause phenological variations within temperate tree canopies in autumn but not in spring

Tree crowns experience strong vertical microclimatic gradients, particularly in light availability. Surprisingly, little is known about whether these gradients cause within-crown variation in leaf phenology and whether such variations represent different light-use strategies of sun- and shade leaves.In a temperate mixed forest at the Swiss Canopy Crane II site in Switzerland, we measured over three years the annual leaf phenology in the upper and lower crowns of mature trees from six broadleaved and three conifer species. We further recorded the microclimate (temperature, humidity, light) continuously in various positions within the canopy.We found microclimatic canopy gradients to be strongest during summer, but negligible in winter and spring, indicating that any phenological gradients in autumn, but not in spring, might be driven by microclimatic differences. Budbreak timing did not differ within the crowns of any of the broadleaved trees. However, in the three species with deeper crowns, leaf unfolding was up to 10 days faster in the lower crown but showed no difference in the more shallow-crowned species. Surprisingly, only the evergreen conifers Abies alba and Picea abies showed earlier bud break in the lower crowns. In autumn, senescence in all broadleaved species progressed from the upper crown downwards, resulting in up to two weeks longer vegetation seasons in the lower crown.With this first broad assessment of within-tree phenology, we show how microclimatic gradients and different light-use strategies lead to a considerable variability of within-crown phenological gradients. We interpret the faster leaf unfolding in the lower crown of three broadleaved species as a shade avoidance strategy, allowing shade branches to improve their C balance early in the season. In contrast, longer retention of lower leaves in autumn is unlikely to significantly improve the C balance, and more likely caused by higher summer temperature and irradiance in the upper leaves.

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, photosynthesisand 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.

Variation in White spruce needle respiration at the species range limits: A potential impediment to Northern expansion.

White spruce (Picea glauca) spans a massive range, yet the variability in respiratory physiology and related implications for tree carbon balance at the extremes of this distribution remain as enigmas. Working at both the most northern and southern extents of the distribution range more than 5000 km apart, we measured the short-term temperature response of dark respiration (R/T) at upper and lower canopy positions. R/T curves were fit to both polynomial and thermodynamic models so that model parameters could be compared among locations, canopy positions, and with previously published data. Respiration measured at 25°C (R25 ) was 68% lower at the southern location than at the northern location, resulting in a significantly lower intercept in R/T response in temperate trees. Only at the southern location did upper canopy leaves have a steeper temperature response than lower canopy leaves, likely reflecting canopy gradients in light. At the northern range limit respiration is nearly twice that of the average R25 reported in a global leaf respiration database. We predict that without significant thermal acclimation, respiration will increase with projected end-of-the-century warming and will likely constrain the future range limits of this important boreal species.

Vertical stratification of insect abundance and species richness in an Amazonian tropical forest

Tropical forests are among the most biodiverse biomes on the planet. Nevertheless, quantifying the abundance and species richness within megadiverse groups is a significant challenge. We designed a study to address this challenge by documenting the variability of the insect fauna across a vertical canopy gradient in a Central Amazonian tropical forest. Insects were sampled over two weeks using 6-m Gressitt-style Malaise traps set at five heights (0 m–32 m–8 m intervals) on a metal tower in a tropical forest north of Manaus, Brazil. The traps contained 37,778 specimens of 18 orders of insects. Using simulation approaches and nonparametric analyses, we interpreted the abundance and richness of insects along this gradient. Diptera, Hymenoptera, and Coleoptera had their greatest abundance at the ground level, whereas Lepidoptera and Hemiptera were more abundant in the upper levels of the canopy. We identified species of 38 of the 56 families of Diptera, finding that 527 out of 856 species (61.6%) were not sampled at the ground level. Mycetophilidae, Tipulidae, and Phoridae were significantly more diverse and/or abundant at the ground level, while Tachinidae, Dolichopodidae, and Lauxaniidae were more diverse or abundant at upper levels. Our study suggests the need for a careful discussion of strategies of tropical forest conservation based on a much more complete understanding of the three-dimensional distribution of its insect diversity.

Does the functional leaf anatomy of Justicia calycina (Acanthaceae) reflect variation across a canopy gradient in the Southern Brazilian Amazon?

Justicia calycina is a subshrub well distributed in northern South America. Despite its widespread occurrence, little is known about its anatomy and plasticity in different environments. Our study aimed to evaluate how leaf anatomical traits of J. calycina adjust in forest areas with variations in canopy openness in the Southern Amazon, Mato Grosso State, Brazil. Forest areas were: Open and Dense Ombrophilous Forest, and Seasonal Deciduous Forest (rocky outcrop). We performed anatomical measurements in leaves sampled on 10 individuals in each forest site, with the help of usual anatomy techniques. Foliar anatomy of J. calycina showed similar pattern among the studied forests, with significant difference (p<0.05) in four leaf traits among 22 studied: vascular bundle diameter, palisade parenchyma thickness, stomatal density and trichome density in adaxial epidermis. These leaf traits showed high plasticity index, with higher values in Dense Forest and in Deciduous Forest. Even though J. calycina showed overall little anatomical variation among forest types, the few attributes that differed are fundamental in the photosynthesis process. The adjustment in leaf anatomical traits under different luminosity conditions demonstrates a degree of phenotypic plasticity in the species, which contributes to its distribution in different forest phytophysiognomies in Brazilian Amazonia.

Identifying the Branch of Kiwifruit Based on Unmanned Aerial Vehicle (UAV) Images Using Deep Learning Method.

It is important to obtain accurate information about kiwifruit vines to monitoring their physiological states and undertake precise orchard operations. However, because vines are small and cling to trellises, and have branches laying on the ground, numerous challenges exist in the acquisition of accurate data for kiwifruit vines. In this paper, a kiwifruit canopy distribution prediction model is proposed on the basis of low-altitude unmanned aerial vehicle (UAV) images and deep learning techniques. First, the location of the kiwifruit plants and vine distribution are extracted from high-precision images collected by UAV. The canopy gradient distribution maps with different noise reduction and distribution effects are generated by modifying the threshold and sampling size using the resampling normalization method. The results showed that the accuracies of the vine segmentation using PSPnet, support vector machine, and random forest classification were 71.2%, 85.8%, and 75.26%, respectively. However, the segmentation image obtained using depth semantic segmentation had a higher signal-to-noise ratio and was closer to the real situation. The average intersection over union of the deep semantic segmentation was more than or equal to 80% in distribution maps, whereas, in traditional machine learning, the average intersection was between 20% and 60%. This indicates the proposed model can quickly extract the vine distribution and plant position, and is thus able to perform dynamic monitoring of orchards to provide real-time operation guidance.

Disentangling the multiple effects of stream drying and riparian canopy cover on the trophic ecology of a highly threatened fish

Abstract Understanding risks to aquatic systems posed by changing drought regimes is particularly important for the conservation of already threatened taxa. However, little is known about how local environmental conditions, especially those in heavily human‐influenced situations, interact with regional shifts such as droughts to alter realised impacts on aquatic communities, including threatened top predators. Here, we investigated the combined effects of stream drying intensity and riparian canopy cover on the trophic interactions of critically endangered kōwaro, or Canterbury mudfish (Neochanna burrowsius) in an agricultural area of New Zealand. Fish populations and their potential prey, both terrestrial and aquatic, as well as environmental variables, including riparian canopy cover and drying measured with stage loggers, were sampled over eight visits to 24 sites spanning orthogonal drying and canopy gradients. Stable isotope ratios, 13C/12C and 15N/14N, were used to investigate trophic links between mudfish and their terrestrial and aquatic prey across these gradients. When non‐native willows (predominantly Salix fragilis) dominated the riparian canopy, increased tree cover led to elevated drying intensity, probably driven by their relatively high water demands compared to other trees. However, in the absence of willows, canopy cover had no effect on drying intensity. Although this was the only direct link between these two environmental factors, they had opposing effects on kōwaro populations, which will be important for management under drought. Increased drying intensity contributed to elevated abundance of microcrustacea and aquatic Diptera larvae, and an increase in the relative abundance of kōwaro juveniles. However, drying‐affected kōwaro populations also had fewer large reproductive adults and elevated δ15N values, probably driven by physiological limitations and an increase in kōwaro cannibalism, respectively. By comparison, increased canopy cover enhanced input of terrestrial invertebrates, a food resource for larger kōwaro, leading to elevated kōwaro δ13C values, no effects on δ15N values, and higher relative abundance of large kōwaro in shaded streams compared to unshaded streams. Thus, the riparian canopy cover was able to offset some of the effects of drying. Overall, we found no interactions between drying intensity and canopy cover affecting kōwaro. However, their opposing effect highlights the important role local conditions such as riparian canopies play on aquatic communities and their potential role as a restoration tool to mitigate the effects of large‐scale shifts such as drought.

Impact of Horizontal Edge-Interior and Vertical Canopy-Understory Gradients on the Abundance and Diversity of Bark and Woodboring Beetles in Survey Traps.

Simple SummaryTraps baited with sex attractants and plant odors are used by regulatory agencies to survey for alien invasive forest insects that may arrive via importation of goods from overseas. The performance of these surveys is affected not only by the type of traps and attractants used, but also by where the traps are placed at survey sites. We tested the effect of trap position along horizontal (relative to the forest edge) and vertical (canopy-understory) forest gradients on the diversity and abundance of species of bark and wood boring beetles detected. Both horizontal and vertical trap position affected trap performance, but trends differed among taxa and were context-dependent. For example, jewel beetles were detected mainly in canopy traps regardless of horizontal position, whereas bark and ambrosia beetles were detected mainly in understory traps placed along the forest edge. For optimal early detection of potentially invasive bark and wood boring beetles, surveys should place traps at multiple locations along horizontal and vertical gradients.Semiochemical-baited intercept traps are important tools used to collect information about the presence/absence and population dynamics of forest insects. The performance of these tools is influenced by trap location along both horizontal edge–interior and vertical understory–canopy gradients. Consequently, the development of survey and detection programs requires both the development of effective traps and semiochemical lures but also deployment protocols to guide their use. We used field trapping experiments to examine the impact of both horizontal edge–interior and vertical understory–canopy gradients and their interactions with the species richness and abundance of Buprestidae, Cerambycidae and Curculionidae. Both gradients had significant effects on the diversity and abundance of all three families collected in traps and the pattern of gradient effects differed between the two experiments. In the first experiment, traps were deployed along transects involving large (>100 m) forest gaps and in the second experiment traps transected small (ca. 15 m) forest gaps. These results were consistent with the idea that gradient effects on the abundance and diversity of these three families of forest Coleoptera are context dependent. The results of this study suggest that monitoring programs for bark and woodboring beetles should deploy traps at multiple locations along both vertical understory–canopy and horizontal edge–interior gradients.

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

While foliar photosynthetic relationships with light, nitrogen, and water availability have been well described, environmental factors driving vertical gradients of foliar traits within forest canopies are still not well understood. We, therefore, examined how light availability and vapour pressure deficit (VPD) co-determine vertical gradients (between 12 and 42m and in the understorey) of foliar photosynthetic capacity (Amax), 13C fractionation (∆), specific leaf area (SLA), chlorophyll (Chl), and nitrogen (N) concentrations in canopies of Fagus sylvatica and Abies alba growing in a mixed forest in Switzerland in spring and summer 2017. Both species showed lower Chl/N and lower SLA with higher light availability and VPD at the top canopy. Despite these biochemical and morphological acclimations, Amax during summer remained relatively constant and the photosynthetic N-use efficiency (PNUE) decreased with higher light availability for both species, suggesting suboptimal N allocation within the canopy. ∆ of both species were lower at the canopy top compared to the bottom, indicating high water-use efficiency (WUE). VPD gradients strongly co-determined the vertical distribution of Chl, N, and PNUE in F. sylvatica, suggesting stomatal limitation of photosynthesis in the top canopy, whereas these traits were only related to light availability in A. alba. Lower PNUE in F. sylvatica with higher WUE clearly indicated a trade-off in water vs. N use, limiting foliar acclimation to high light and VPD at the top canopy. Species-specific trade-offs in foliar acclimation to environmental canopy gradients may thus be considered for scaling photosynthesis from leaf to canopy to landscape levels.

The effect of canopy openness of European beech (Fagus sylvatica) forests on ground‐dwelling spider communities

Abstract The composition of spider communities in forests is affected by habitat characteristics, such as canopy openness, vegetation cover or tree species diversity; therefore, we assume that the effect of canopy will differ among tree species. As research in European beech forests has not been conducted, we studied the effect of understory environmental characteristics associated with a gradient of overstory canopy openness in beech forest on ground‐dwelling spider richness, abundance and guild composition in the Western Carpathians, Slovakia. Canopy openness, the cover of understory vegetation and litter, litter thickness and variability and soil reaction were recorded. In total, 17 332 individuals of 174 spider species were trapped during 518 days. We used Generalised Linear Models to explain the effect of environmental characteristics on the assemblages of ground‐dwelling spiders; the unique and shared contributions of the predictors were calculated using variation partitioning. The association of canopy openness with spider species richness was hump‐shaped and displayed positive linear relationship with abundance. Abundance was also affected by soil pH. The composition of spider communities was significantly affected by canopy openness and understory characteristics; a major part of this variation was shared between all variables. The guild composition of the assemblages interacted with canopy openness, as sheet‐web weavers were dominant under the most closed canopy, that below 20% of canopy openness, while ground hunters dominated in the rest of the canopy gradient. The gradient of beech forest canopy openness affected spider species richness, abundance and guild composition and supported distinct assemblages.

Photosynthetic parameters of Juglans nigra trees are linked to cumulative water stress

The influence of water deficits and drought on tree physiological processes, growth, and survival has been the focus of substantial research efforts and debate over the past decades, but there is still a need to quantitatively link finer scale mechanistic explanations of the influence of water status with the physiological responses of trees, particularly for those past the sapling stage. Hence, the objective of this study was to link accumulated water stress during the growing season to leaf physiological response mechanisms of Juglans nigra L. trees. Results showed that trees subjected to higher cumulative water stress had lower maximum light-saturated photosynthesis (Amax), initiated net photosynthesis at higher light levels (Ic), and displayed reduced effectiveness of CO2 fixation per photons absorbed (Qe) at the bottom and upper positions along the vertical canopy gradient. Results suggest that water stress integral (Sψ), a variable that takes into account accumulated water deficits, would be useful to help future research efforts aimed at investigating responses to drought in trees past the sapling stage.

Strong influence of landscape structure on hair lichens in boreal forest canopies

This study examines how island size, isolation, and orientation influence epiphytic hair lichens in old-growth boreal spruce forests within a naturally heterogeneous landscape with approximately 1000 forest islands distributed in open wetland matrix. Forest structure, length of Alectoria sarmentosa (Ach.) Ach., Bryoria spp., and Usnea spp., and mass of Alectoria in the lower canopy (0–5 m) of Picea abies (L.) Karst. were quantified in 30 islands (0.11–10.9 ha). Length and mass of Alectoria were also studied in 25 edges with different orientation and fetch (wind exposure). Island area had a strong positive effect on length of Alectoria but a minor effect on Bryoria and Usnea. Edge orientation influenced length and mass of Alectoria, with the strongest reduction in wind-exposed western edges, whereas fetch size had no effect. Edge influence on microclimate drives hair lichen response to landscape configuration. The gradient from Bryoria in small islands to Alectoria in large islands is caused by the same mechanisms that influence vertical canopy gradients in large homogeneous stands, with Bryoria in the upper canopy and Alectoria in the lower canopy. Genus-specific, sun-screening pigments contribute to this niche differentiation, but thallus fragmentation by wind and water storage are also important. Our findings imply that lichen conservation must consider the spatial structure of the landscape.

Vertical canopy gradient shaping the stratification of leaf-chewer-parasitoid interactions in a temperate forest.

Knowledge about herbivores and their parasitoids in forest canopies remains limited, despite their diversity and ecological importance. Thus, it is important to understand the factors that shape the herbivore–parasitoid community structure, particularly the effect of vertical gradient. We investigated a quantitative community dataset of exposed and semiconcealed leaf‐chewing larvae and their parasitoids along a vertical canopy gradient in a temperate forest. We sampled target insects using an elevated work platform in a 0.2 ha broadleaf deciduous forest plot in the Czech Republic. We analyzed the effect of vertical position among three canopy levels (first [lowest], second [middle], and third [highest]) and tree species on community descriptors (density, diversity, and parasitism rate) and food web structure. We also analyzed vertical patterns in density and parasitism rate between exposed and semiconcealed hosts, and the vertical preference of the most abundant parasitoid taxa in relation to their host specificity. Tree species was an important determinant of all community descriptors and food web structure. Insect density and diversity varied with the vertical gradient, but was only significant for hosts. Both host guilds were most abundant in the second level, but only the density of exposed hosts declined in the third level. Parasitism rate decreased from the first to third level. The overall parasitism rate did not differ between guilds, but semiconcealed hosts suffered lower parasitism in the third level. Less host‐specific taxa (Ichneumonidae, Braconidae) operated more frequently lower in the canopy, whereas more host‐specific Tachinidae followed their host distribution. The most host‐specific Chalcidoidea preferred the third level. Vertical stratification of insect density, diversity, and parasitism rate was most pronounced in the tallest tree species. Therefore, our study contradicts the general paradigm of weak arthropod stratification in temperate forest canopies. However, in the network structure, vertical variation might be superseded by variation among tree species.

Identifying critical thresholds to guide management practices in agro-ecosystems: Insights from bird community response to an open grassland-to-forest gradient

Landscapes are showing increased fragmentation and habitat loss due to land-use conversion and intensification, leading to species-poor and homogeneous communities. The identification of ecological thresholds above which major changes in community composition take place, may prevent the critical downfall of biodiversity while improving the effectiveness of conservation, resource management and restoration practices. In this study, we provide a new insight on how species distribute along a highly variegated agro-ecosystem in the Mediterranean region. We aim to define the thresholds of occurrence of a bird community inhabiting a tree canopy gradient, and determine the patterns of community change. We fit Huisman-Olff-Fresco models to bird occurrence data (assuming non-linear responses) to identify species-specific responses to the gradient, species richness, and turnover patterns. The tree canopy gradient is responsible for major changes in bird community likely related to the variation of the tree stratum and canopy enclosure which reflect different niche segregation opportunities. Maximum species richness was reached at 10% canopy cover while total turnover rate was higher than expected from a null model up to 10% canopy cover. Ecological thresholds can be used as indicators of specific resource limits responsible for changes in community composition and species occurrence, identifying where populations may be more sensitive. Choosing a single management scheme will invariably result in winners and losers, but optimal levels of management can be explored in order to maximize species diversity across Mediterranean agro-ecosystems.

Temperate and Tropical Forest Canopies are Already Functioning beyond Their Thermal Thresholds for Photosynthesis

Tropical tree species have evolved under very narrow temperature ranges compared to temperate forest species. Studies suggest that tropical trees may be more vulnerable to continued warming compared to temperate species, as tropical trees have shown declines in growth and photosynthesis at elevated temperatures. However, regional and global vegetation models lack the data needed to accurately represent such physiological responses to increased temperatures, especially for tropical forests. To address this need, we compared instantaneous photosynthetic temperature responses of mature canopy foliage, leaf temperatures, and air temperatures across vertical canopy gradients in three forest types: tropical wet, tropical moist, and temperate deciduous. Temperatures at which maximum photosynthesis occurred were greater in the tropical forests canopies than the temperate canopy (30 ± 0.3 °C vs. 27 ± 0.4 °C). However, contrary to expectations that tropical species would be functioning closer to threshold temperatures, photosynthetic temperature optima was exceeded by maximum daily leaf temperatures, resulting in sub-optimal rates of carbon assimilation for much of the day, especially in upper canopy foliage (&gt;10 m). If trees are unable to thermally acclimate to projected elevated temperatures, these forests may shift from net carbon sinks to sources, with potentially dire implications to climate feedbacks and forest community composition.

Canopy gradients in leaf functional traits for species that differ in growth strategies and shade tolerance

In temperate deciduous forests, vertical gradients in leaf mass per area (LMA) and area-based leaf nitrogen (Narea) are strongly controlled by gradients in light availability. While there is evidence that hydrostatic constraints on leaf development may diminish LMA and Narea responses to light, inherent differences among tree species may also influence leaf developmental and morphological response to light. We investigated vertical gradients in LMA, Narea and leaf carbon isotope composition (δ13C) for three temperate deciduous species (Carpinus caroliniana Walter, Fagus grandifolia Ehrh., Liriodendron tulipifera L.) that differed in growth strategy (e.g., indeterminate and determinate growth), shade tolerance and leaf area to sapwood ratio (Al:As). Leaves were sampled across a broad range of light conditions within three vertical layers of tree crowns to maximize variation in light availability at each height and to minimize collinearity between light and height. All species displayed similar responses to light with respect to Narea and δ13C, but not for LMA. Light was more important for gradients in LMA for the shade-tolerant (C. caroliniana) and -intolerant (L. tulipifera) species with indeterminate growth, and height (e.g., hydrostatic gradients) and light were equally important for the shade-tolerant (F. grandifolia) species with determinate growth. Fagus grandifolia had a higher morphological plasticity in response to light, which may offer a competitive advantage in occupying a broader range of light conditions throughout the canopy. Differences in responses to light and height for the taller tree species, L. tulipifera and F. grandifolia, may be attributed to differences in growth strategy or Al:As, which may alter morphological and functional responses to light availability. While height was important in F. grandifolia, height was no more robust in predicting LMA than light in any of the species, confirming the strong role of light availability in determining LMA for temperate deciduous species.

Leaf water 18 O and 2 H enrichment along vertical canopy profiles in a broadleaved and a conifer forest tree.

Distinguishing meteorological and plant-mediated drivers of leaf water isotopic enrichment is prerequisite for ecological interpretations of stable hydrogen and oxygen isotopes in plant tissue. We measured input and leaf water δ2 H and δ18 O as well as micrometeorological and leaf morpho-physiological variables along a vertical gradient in a mature angiosperm (European beech) and gymnosperm (Douglas fir) tree. We used these variables and different enrichment models to quantify the influence of Péclet and non-steady state effects and of the biophysical drivers on leaf water enrichment. The two-pool model accurately described the diurnal variation of leaf water enrichment. The estimated unenriched water fraction was linked to leaf dry matter content across the canopy heights. Non-steady state effects and reduced stomatal conductance caused a higher enrichment of Douglas fir compared to beech leaf water. A dynamic effect analyses revealed that the light-induced vertical gradients of stomatal conductance and leaf temperature outbalanced each other in their effects on evaporative enrichment. We conclude that neither vertical canopy gradients nor the Péclet effect is important for estimates and interpretation of isotopic leaf water enrichment in hypostomatous trees. Contrarily, species-specific non-steady state effects and leaf temperatures as well as the water vapour isotope composition need careful consideration.