Forest edges are widespread transition zones that strongly influence forest microclimates and land-atmosphere exchanges. They modify the turbulent transfer of momentum, heat, and trace gases, with potential feedbacks on regional climate. Yet, their flow dynamics under non-neutral thermal stratification remain poorly understood. Using large-eddy simulations that couple canopy-scale biophysical processes with atmospheric boundary-layer (ABL) dynamics, we examine how thermal stratification, from neutral to free convection, modifies the micrometeorology at the transition between a crop and a dense forest over a flat terrain. Under windy but unstable conditions, the internal boundary layer (IBL) developing at the transition keeps the main turbulent edge-flow features identified under neutral conditions. Scalars adjust to the forest more slowly than momentum, due to their accumulation within the stable lower forest layer and their partial upward entrainment by intermittent intrusions of ABL-scale motions. Under free convection, the contrast in buoyancy forces between the crop and the forest generates thermally-driven, ABL-scale circulations, producing a weak near-surface breeze towards the forest and a persistent rising motion above the forest center. Although too weak to form a classical shear-driven IBL, this breeze still induces a sub-canopy jet similar to that in windy conditions. At the top of the forest canopy, thermally-driven turbulence is less effective at coupling with the forest interior than shear-driven turbulence under windy conditions, strengthening the stable layer in the understorey where scalar accumulate towards the forest center, where the breeze converges. Overall, the micrometeorological fields never fully adjust to the forest. These findings, albeit specific to the chosen forest and soil moisture conditions, offers valuable insights for environmental and ecological applications.

Leaf photosynthesis and respiration respond to leaf temperature, which differs from air temperature depending on radiation load, transpiration and heat exchange rates. However, most terrestrial biosphere models (TBMs) do not use leaf temperature to compute photosynthesis and respiration. Instead, they use directly air temperature, or an average surface temperature that incorporate soil and non-green biomass compartments. While these two approaches are computationally efficient, the absence of explicit leaf temperature simulation potentially hinders the representation of extreme events (e.g. heat stress) and their repercussion on carbon and water fluxes. To predict leaf temperature, it is necessary to explicitly account for leaf energy budget, photosynthesis and transpiration feedbacks (E-P-T).Here, we explored the merits of coupling E-P-T processes in TBMs to simulate explicit leaf temperature and its feedback on carbon assimilation. Sensitivity analysis of the coupled E-P-T processes using the big-leaf configuration of the ORCHIDEE v2.2 TBM (used in CMIP6) resulted into leaf-to-air temperature differences varying between 1 and 10 °C in natural conditions. This translated into a change in carbon assimilation ranging from -35 % to +110 % at the leaf level. A comparison of simulated leaf temperature with measured canopy surface temperature at eddy-covariance fluxes sites in various E-P-T configurations showed that adding ecological constraints on photosynthesis and transpiration through the photosynthesis coordination and the least-cost hypothesis (P-model) improved the representation of top canopy temperature compared to a classical fixed parameterization. The improvement in canopy temperature estimates was best for deciduous broadleaved forests with an average reduction of the error by 1.2 ± 0.8 °C (9 sites). More importantly, the improvement in leaf temperature estimates mainly occurs at elevated temperature (> 30°C). Our results argue for the inclusion of an explicit representation of leaf temperature in TBMs to avoid biases in the carbon balance estimates. Fully coupling leaf processes through temperature will also be essential for accurately simulating and disentangling the effects of heat and drought stresses under future conditions. However, such implementation will only be possible if accompanied with space-time concomitant observations of leaf temperature and traits that are currently lacking. The ongoing deployment of digital cameras (e.g. thermal, multispectral, SIF etc.) on existing networks (e.g. ICOS) for tracking canopy temperature and trait variability, combined with punctual field observation campaigns, future remote sensing missions (e.g. TRISHNA), as well as new hybrid modelling methods are all timely and promising ways for improving our understanding and representation of leaf temperature in TBMs.

Forests are aerodynamically rough surfaces above which turbulent exchange is enhanced, generating smaller vertical gradients in windspeed and air temperature than those predicted by the Monin-Obukhov Similarity Theory (MOST). Roughness sublayer (RSL) corrections, based on canopy structure, have been proposed to account for this enhancement in turbulent exchange, but evaluation of these RSL corrections above a range of forest types is still lacking. In this study, we mobilised multiyear datasets of canopy structure, turbulent fluxes and microclimate gradients from the ICOS European network to evaluate two widely-used RSL corrections in a range of climates, forest structures and atmospheric conditions. As expected, observed windspeed gradients above these forest sites were smaller than MOST predictions. The two RSL corrections improved the windspeed gradient predictions with similar accuracy, irrespective of atmospheric stability conditions. The need of RSL corrections for predicting air temperature gradients above forests was more contrasted than for windspeed, and depended on the site and the atmospheric stability conditions. Overall, the RSL corrections at canopy height remained relatively small, around 0.5°C for air temperature and 0.5 m s-1 for windspeed on average, and were most pronounced under stable atmospheric conditions. Based on the evaluation of the simplifying assumptions behind each RSL correction, we built recommendations to implement those corrections in models. We also discussed the potential impact of wind sensor positions to estimate aerodynamic parameters needed to apply MOST and RSL corrections and provide recommendations for improvement of the ICOS network. Our results highlight the difficulty to estimate displacement height and relate it to canopy structure, and question the idea that the drag coefficient does not change with leaf area or clumping.

Photosynthetic (COO)-O-18 fractionation (Delta O-18) has been used to estimate mesophyll conductance (g(m)) in C-3 and C-4 species. However, this requires knowledge of the degree of isotopic equilibration (theta(r)) between leaf water and the CO2 molecules penetrating the leaf interior and re-escaping without being assimilated (F-retro). Here, we re-examine the theory of Delta O-18-g(m) estimation and its sensitivity to changes in the O-18 composition (delta O-18) of CO2 on young and old leaves of six C-3 species. Our results showed that, when full isotopic equilibrium was assumed, g(m) values were sensitive to the delta O-18 of the CO2 sources. Utilizing a new isotopic mass balance model that distinguishes metabolic (R-t) and purely diffusive (F-retro) CO2 fluxes, we found that the gross H2O-equilibrated efflux (theta F-r(retro)+R-t) represented 157 +/- 12 % (mean +/- SE) of net CO2 assimilation, and was positively correlated to stomatal conductance, implying a limitation imposed by stomata. Furthermore, leaf water was not completely in equilibrium with F-retro (theta(r)<1) or the CO2 pool at the site of hydration (theta<1), which led to errors in estimated g(m). Our results have broad implications for the interpretation of Delta O-18 in terms of mesophyll conductance, and highlight the necessity of considering the isotopic disequilibrium of the retro-diffusive CO2 efflux.

Understanding the adaptations of terrestrial plants to water stress is crucial as climate change is already altering precipitation patterns. Mycorrhizal fungi enhance host water status through indirect mechanisms like nutrient uptake or plant osmoregulation. Direct water transport via fungal hyphae has also been demonstrated, but its exact contribution to total plant water uptake is still debated.To demonstrate and quantify the direct transport of water from arbuscular mycorrhizal fungi (AMF) to its host plant, we utilized a plant mesocosm comprised of two compartments, separated by a porous membrane and an air gap. In the ‘plant-hyphae’ compartment, seedlings of microtomatoes were grown and inoculated with Rhizophagus irregularis. Hyphae, rather than plant roots, could cross the physical barrier of the porous membrane and the air gap to enter the ‘hyphae-only’ compartment. After several weeks of plant and hyphal growth, the ‘hyphae-only’ compartment was labelled with deuterated water (2H2O) and the isotopic composition of plant transpiration and soil water of both compartments were determined at different times after irrigation.The presence of deuterated water in the plant transpiration stream confirmed that there was direct water transport via AMF hyphae to the plant. Previous studies have quantified the relative contribution of fungal-transported water by solving an isotope mass balance that includes the leaf transpired water and water extracted from soils of both ‘plant-hyphae’ and ‘hyphae-only’ compartments. This framework assumes that movement of deuterated water from the ‘hyphae-only’ to the ‘plant-hyphae’ compartment occurs only through fungal hyphae. However, we found that there was also diffusion of deuterated water vapour across the air gap separating the two compartments. This contamination led to overestimations of the relative contribution of AMF to total plant water uptake. After accounting for this contamination, the water contributed by AMF hyphae was quantified to 1% to 6% of total plant water uptake. Furthermore, using plant biomass as a weighing factor in the mixing model to account for differences in soil volume exploration by plant roots was critical for an accurate estimate of the contribution.

Anthropogenically emitted CO2 is warming the earth’s climate to temperatures that already exceed pre-industrial levels by more than 1.2 oC. Terrestrial vegetation has slowed the rate of climate change by removing part of this anthropogenic emission. Accurate estimations of the present and future terrestrial carbon sink are still needed for forecasting climate and for informing policies for climate stabilization. This requires precise knowledge of the photosynthetic C uptake over land (gross primary production, GPP), independently of the C released through plant and soil respiration. The gas carbonyl sulfide (COS) has emerged as a promising tracer for GPP. This is because both CO2 and COS are a substrate for carbonic anhydrase (CA), the first enzyme involved in photosynthesis, so that the uptake by foliage of COS and CO2 often covaries. Estimating GPP from COS measurements and atmospheric budgets also requires quantifying ocean and industrial COS sources, which is challenging. Isotopic constrained COS tropospheric mass balances can help quantify the relative contribution of these sources if the isotope discrimination during COS uptake by terrestrial vegetation (the main COS sink) is known. However, little is known about plant-atmosphere COS isotope exchange; measurements are challenging and theory to interpret these measurements is limited. Herein, we present a new comprehensive model for discrimination during COS uptake by plants (∆34S) and use it to revisit existing COS isotope datasets and atmospheric budgets. Our ∆34S model expands Davidson et al. (2022) pioneer framework by accounting for leaf COS production. By analogy with the well-established model for photosynthetic discrimination against 13CO2, Davidson et al. ∆34S model stated that COS discrimination occurs as COS diffuses into the leaf and binds to CA. Leaf COS emission was not considered, although it has been reported in species ranging from bryophytes to wheat and trees. Because it is uncertain where these emissions occur, we tested different leaf-level COS emission scenarios - including zero emissions - in various leaf compartments (cuticle, intercellular space, cytosol), alone or in combination. We used this comprehensive model to generate predictions for ∆34S in C3 and C4 species and discussed implications for determining a global plant uptake fractionation factor. Our mechanistic model provides a framework to interpret vegetation-atmosphere COS isotope exchange that can prove useful to improve COS uptake-based GPP estimates and our understanding of plant function, especially when combined with other isotopes (C, O, H).

Quantitative data are needed to constrain vegetation-hydroclimate in water cycle modelling. Here, we use the triple oxygen isotope composition (δ'18O and 17O-excess) of water compartments to track water transfers and mixing within the soil-plant-atmosphere continuum. At three AMMA-CATCH sites in Benin and Senegal we monitored the δ'18O and 17O-excess of precipitation, groundwater, soil water and plant water, as well as the 17O-excess of phytoliths, an indicator of atmospheric relative humidity. We found that : 1) the 17O-excess in precipitation is very stable over several years; 2) groundwater has δ'18O and 17O-excess values consistent with a multi-year recharge by modern precipitation; 3) the 17O-excess in soil water shows a limited contribution of evaporated water, despite high evaporation conditions, which has important implications for our knowledge of water transfers within soils; 4) extrapolating linear relationships between δ'18O and excess 17O-excess of leaf and stem water allows us to determine the origin of the water absorbed by the roots. At the savanna and dry forest sites, during the rainy season, grasses absorb soil water supplied by precipitation. In contrast, during the dry season, trees reach the perennial groundwater recharge. 5) the 17O-excess of grass and tree leaf water follow the dynamics of relative humidity; 6) the 17O-excess of grass phytoliths records daily relative humidity during the growing season. These results provide a solid basis for using the triple oxygen isotope composition of water and phytoliths to trace present and past water cycles at the soil-plant-atmosphere interface.This study was conducted in the framework of the HUMI-17 and PAST-17 projects supported by the ANR (ANR-17-CE01-0002-01 and ANR-22-CE01-0027-01), JA and CV have benefited from a Marie Sklodowska-Curie grant from the European Union (n°101063961 for JA and 101063961 for CV). TT acknowledge funds from FORMAS (Dnr 2021-00644), and the European Union under the Development Smart Innovation through Research in Agriculture (DeSIRA) Initiative (FOOD/2019/410-169).

Tree seedlings have their leaves very close to the ground and their roots are very shallow. They therefore experience more severe heat and water stress during hot summers than mature trees. As droughts and heatwaves increase in severity and frequency, the growth and survival of tree seedlings thus become more difficult, impairing forest regeneration in many regions. In response, forest managers are increasingly shifting from thinning regimes that promote light availability and seedling growth to regimes that promote seedling survival and the buffering of climate extremes. However, the identification of such thinning regimes is not trivial because the mechanisms underpinning the impact of canopy cover on understory microclimate, although all well understood, can have opposite effects on climate extremes. In particular, wind attenuation and water consumption by the remaining adult trees can sometimes create conditions for seedlings in the understory hotter and drier than in an open field. This has led researchers to hypothesize the existence of thinning thresholds beyond which forest canopies transition from buffering to amplifying climate extremes. Metrics such as leaf area index (LAI), crown aggregation and canopy height have emerged as critical factors, as well as other local factors such as species composition or water availability. Here, we use a physics-based model of forest hydrology, physiology and microclimate (MuSICA), in combination with microclimate observations from a variety of forest types across Europe, to address the following questions. (1) Is the threshold in LAI and/or crown aggregation below which summertime temperature and evaporative demand become amplified in the understory generic, or is this threshold site-specific? (2) How does understory microclimate evolve during heatwaves depending on the structure of the canopy above and the duration of the heatwave? (3) How does this translate in terms of plant water and heat stress for understory species?

Forest canopies can buffer or amplify macroclimate temperature extremes in their understory depending on structural and ecological parameters such as canopy height, canopy openness, and species composition. Forest management practices that alter these parameters have a strong impact on understory temperature extremes, with potential repercussions on forest resilience to climate change. Physics-based microclimate models offer a means to explore the effects of forest canopy structure on understory temperatures. Compared to empirical models, they allow to extrapolate results outside the range of canopy structure and climatic conditions that have been observed so far. Although physics-based models may differ in their representation of soil-vegetation-atmosphere interactions, they all rely on a priori knowledge of meteorological data above the forest (also called climate forcing), including air temperature, relative humidity and wind speed timeseries. This raises challenges when conducting sensitivity analyses on structural parameters because canopy structure influences the microclimate not only inside and below the forest canopy but also above it. Employing the same climate forcing for all scenarios of canopy structure is thus deemed inappropriate and adjustments of climate forcing must be accounted for. In this study, we propose a new physics-based modelling approach to perform sensitivity analyses of canopy structure on understorey microclimate that incorporates the feedback of a change in canopy structure on the climatic conditions above it. This approach relies on existing theories on the similarity of turbulent flux-gradient relationships within the atmospheric surface boundary layer between different scalars (e.g. air temperature and humidity) and wind speed, adapted to rough surfaces such as forest canopies. This approach is tested against datasets collected in various forest ecosystems across Europe and applied to explore the impact of canopy structure, in particular canopy density and clumping, on understory microclimate. Our approach will advance our understanding of the intricate relationships between canopy structure, boundary layer dynamics and microclimate, offering insights for more effective forest management strategies.

Mycorrhizal fungal species are widespread across nearly all ecosystems worldwide and are generally found in symbiotic association with most plant species. In forested ecosystems, mycorrhizal fungi play a crucial role in facilitating plant nutrient acquisition and defending the plant from abiotic and biotic stress events, such as drought or pathogen attack.We have recently shown that different tree species that associate with either arbuscular mycorrhizal (AM) fungi or ectomycorrhizal (EM) fungi exhibit distinct phytochemical differences, that might be linked to the type of fungal symbiont. In this study, we investigated the metabolic diversity of several ectomycorrhizal (EM) fungal species commonly found in forests, with the aim of linking their metabolic toolkits to functional processes important in forest ecosystems, such as soil respiration and enzyme activities.In this presentation, we show that ectomycorrhizal fungi contain a diverse suite of metabolites (> 10000 metabolic features in the 5 species studied) composed largely of lipids and benzenoids with many of these metabolic features serving as reliable predictors that facilitate the distinction of different EM fungal species from one another.We also present the results of a microcosm gas exchange experiment on the 5 EM fungal species grown under controlled temperature and CO₂ concentration conditions to investigate the link between fungal metabolic profiles and primary functions, such as respiration and enzymatic activity.This research aims to deepen our understanding of plant-fungal symbioses in forests and the potential shifts in plant and fungal metabolism and function during interaction with one another and when exposed to changes in climate and atmospheric chemistry.Arette-Hourquet P., Demullier E., Ogée J., Guzman T., Valls-Fonayet J. Petriacq P., Devert N., Ubierna N. Wingate L.

Riparian corridors often act as low-land climate refugia for temperate tree species in their southern distribution range. A plausible mechanism is the buffering of regional climate extremes by local physiographic and biotic factors. We tested this idea using a 3-year-long microclimate dataset collected along the Ciron river, a refugia for European beech ( Fagus sylvatica ) in southwestern France. Across the whole network, canopy gap fraction was the main predictor for spatial microclimatic variations, together with two other landscape features (elevation above the river and woodland fraction within a 300m radius). However, within the riparian forest only (canopy gap fraction < 25%, distance to the river < 150m), variations of up to -4°C and + 15% in summertime daily maximum air temperature and minimum relative humidity, respectively, were still found from the plateau to the cooler, moister river banks, only ~ 5-10m below. Elevation above the river was then identified as the main predictor, and explained the marked variations from the plateau to the banks much better than canopy gap fraction. The microclimate measured near the river is as cool but moister than the macroclimate encountered at 700-1000m asl further east in F. sylvatic a's main distribution range. Indeed, at all locations, we found that air relative humidity was higher than expected from a temperature-only effect, suggesting that extra moisture is brought by the river. Our results explain well why beech trees in this climate refugium are restricted to the river gorges where microtopographic variations are the strongest and canopy gaps are rare.

The 18O enrichment (Δ18O) of cellulose (Δ18OCel) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (pex) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C3 grasses demonstrated that the Δ18O of leaf sucrose (Δ18OSuc, the parent substrate for cellulose synthesis) can be much higher than predicted from daytime Δ18O of leaf water (Δ18OLW), which could alter conclusions on photosynthetic versus post-photosynthetic effects on Δ18OCel via pex. Here, we assessed pex in leaves of perennial ryegrass (Lolium perenne) grown at different atmospheric relative humidity (RH) and CO2 levels, by determinations of Δ18OCel in leaves, Δ18OLGDZW (the Δ18O of water in the leaf growth-and-differentiation zone) and both Δ18OSuc and Δ18OLW (adjusted for εbio, the biosynthetic fractionation between water and carbohydrates) as alternative proxies for the substrate for cellulose synthesis. Δ18OLGDZW was always close to irrigation water, and pex was similar (0.53 ± 0.02 SE) across environments when determinations were based on Δ18OSuc. Conversely, pex was erroneously and variably underestimated (range 0.02-0.44) when based on Δ18OLW. The photosynthetic signal fraction in Δ18OCel is much more constant than hitherto assumed, encouraging leaf physiological reconstructions.

This article is a Commentary on Rao et al. (2024), 243: 2102–2114.

Microclimate within the rural landscape is difficult to predict due to its strong spatial heterogeneity resulting from the juxtaposition landscape elements contrasted in terms of biophysical parameters. Canopy edge is one of the dominant heterogeneities in rural areas, whose influences on microclimate and turbulent structures have mainly been described under neutral thermal stratification.This study focuses on the micrometeorology over and within a crop-forest transition under unstable atmospheric conditions, when shear and buoyancy have a combined action on turbulence motions within the atmospheric boundary layer (ABL). This study is based on both field and numerical experiments. The field experiment was carried out in Lannemezan (South of France) from March 2023 to March 2024 as part of the MOSAI project (Model and Observation for Surface-Atmosphere Interactions, https://mosai.aeris-data.fr/). High-frequency sensors were used to measure wind speed and direction, air temperature and humidity, and CO 2 mixing ratio at varying distances from the forest edge. The numerical experiment was performed using a Large Eddy Simulation (LES) atmospheric model, coupled to a one-dimensional multi-layer soil and canopy energy and gas exchanges model. The novelty of this simulation is to resolve both within- and above-canopy turbulence, allowing the characterization of turbulent exchanges during free convection.We will show how the forest edge flow differs depending on the thermal stability, and the influence of canopy- and ABL-scale motions on the canopy-atmosphere turbulent exchanges. This study is a first step towards simulating micrometeorology over heterogeneous landscapes in unstable conditions. Such detailed simulations should contribute to a better understanding of surface-atmosphere exchanges, and in particular to a better account of surface heterogeneity in meteorological models.

Triple oxygen isotopes (17O-excess) of water are useful to trace evaporation at the soil–plant–atmosphere interface. The 17O-excess of plant silica, i.e., phytoliths, inherited from leaf water, was previously calibrated in growth chambers as a proxy of atmospheric relative humidity (RH). Here, using a model–data approach, we examine the parameters that control the triple oxygen isotope composition of bulk grass leaf water and phytoliths in natura, at the O3HP experimental platform located in the French Mediterranean area. A grass plot was equipped to measure for 1 year, all environmental and plant physiological parameters relevant for modeling the isotope composition of the grass leaf water. In particular, the triple oxygen and hydrogen isotope composition of atmospheric water vapor above the grass was measured continuously using a cavity ring-down spectrometer, and the grass leaf temperature was monitored at plot scale using an infrared (IR) radiometer. Grass leaves were collected in different seasons of the year and over a 24 h period in June. Grass leaf water was extracted by cryogenic vacuum distillation and analyzed by isotope ratio mass spectrometry (IRMS). Phytoliths were analyzed by IR–laser fluorination–IRMS after chemical extraction. We showed that the traditional Craig–Gordon steady-state model modified for grass leaves reliably predicts the triple oxygen isotope composition of leaf water during daytime but is sensitive to uncertainties on the leaf-to-air temperature difference. Deviations from isotope steady state at night are well represented in the triple oxygen isotope system and predictable by a non-steady-state model. The 17O-excess of phytoliths confirms the applicability of the 17O-excessphyto vs. RH equation established in previous growth chamber experiments. Further, it recorded average daytime RH over the growth period rather than daily RH, related to low transpiration and silicification during the night. This model–data approach highlights the utility of the triple oxygen isotope system to improve the understanding of water exchange at the soil–plant–atmosphere interface. The in natura experiment underlines the applicability of 17O-excess of phytoliths as a RH proxy.

The 18 O enrichment (Δ18 O) of leaf water affects the Δ18 O of photosynthetic products such as sucrose, generating an isotopic archive of plant function and past climate. However, uncertainty remains as to whether leaf water compartmentation between photosynthetic and nonphotosynthetic tissue affects the relationship between Δ18 O of bulk leaf water (Δ18 OLW ) and leaf sucrose (Δ18 OSucrose ). We grew Lolium perenne (a C3 grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO2 level (200, 400 or 800 μmol mol-1 ) as factors, and determined Δ18 OLW , Δ18 OSucrose and morphophysiological leaf parameters, including transpiration (Eleaf ), stomatal conductance (gs ) and mesophyll conductance to CO2 (gm ). The Δ18 O of photosynthetic medium water (Δ18 OSSW ) was estimated from Δ18 OSucrose and the equilibrium fractionation between water and carbonyl groups (εbio ). Δ18 OSSW was well predicted by theoretical estimates of leaf water at the evaporative site (Δ18 Oe ) with adjustments that correlated with gas exchange parameters (gs or total conductance to CO2 ). Isotopic mass balance and published work indicated that nonphotosynthetic tissue water was a large fraction (~0.53) of bulk leaf water. Δ18 OLW was a poor proxy for Δ18 OSucrose , mainly due to opposite Δ18 O responses of nonphotosynthetic tissue water (Δ18 Onon-SSW ) relative to Δ18 OSSW , driven by atmospheric conditions.