Decametric-scale buffering of climate extremes in forest understory within a riparian microrefugia: the key role of microtopography.

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.

Using Carbon Stable Isotopes to Study C and C Photosynthesis: Models and Calculations.

Stable carbon isotopes are a powerful tool to study photosynthesis. Initial applications consisted of determining isotope ratios of plant biomass using mass spectrometry. Subsequently, theoretical models relating C isotope values to gas exchange characteristics were introduced and tested against instantaneous online measurements of C photosynthetic discrimination.

Half of the O enrichment of leaf sucrose is conserved in leaf cellulose of a C grass across atmospheric humidity and CO levels.

The O enrichment (ΔO) of cellulose (ΔO) is recognized as a unique archive of past climate and plant function. However, there is still uncertainty regarding the proportion of oxygen in cellulose (p) that exchanges post-photosynthetically with medium water of cellulose synthesis. Particularly, recent research with C grasses demonstrated that the ΔO of leaf sucrose (ΔO, the parent substrate for cellulose synthesis) can be much higher than predicted from daytime ΔO of leaf water (ΔO), which could alter conclusions on photosynthetic versus post-photosynthetic effects on ΔO via p.

O enrichment of sucrose and photosynthetic and nonphotosynthetic leaf water in a C grass-atmospheric drivers and physiological relations.

The O enrichment (Δ O) of leaf water affects the Δ 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 Δ O of bulk leaf water (Δ O ) and leaf sucrose (Δ O ). We grew Lolium perenne (a C grass) in mesocosm-scale, replicated experiments with daytime relative humidity (50% or 75%) and CO level (200, 400 or 800 μmol mol ) as factors, and determined Δ O , Δ O and morphophysiological leaf parameters, including transpiration (E ), stomatal conductance (g ) and mesophyll conductance to CO (g ).

Contrasting stem water uptake and storage dynamics of water-saver and water-spender species during drought and recovery.

Drought is projected to occur more frequently and intensely in the coming decades, and the extent to which it will affect forest functioning will depend on species-specific responses to water stress. Aiming to understand the hydraulic traits and water dynamics behind water-saver and water-spender strategies in response to drought and recovery, we conducted a pot experiment with two species with contrasting physiological strategies, Scots pine (Pinus sylvestris L.) and Portuguese oak (Quercus faginea L.

Explicitly accounting for needle sugar pool size crucial for predicting intra-seasonal dynamics of needle carbohydrates δ O and δ C.

We explore needle sugar isotopic compositions (δ O and δ C) in boreal Scots pine (Pinus sylvestris) over two growing seasons. A leaf-level dynamic model driven by environmental conditions and based on current understanding of isotope fractionation processes was built to predict δ O and δ C of two hierarchical needle carbohydrate pools, accounting for the needle sugar pool size and the presence of an invariant pinitol pool. Model results agreed well with observed needle water δ O, δ O and δ C of needle water-soluble carbohydrates (sugars + pinitol), and needle sugar δ C (R  = 0.

Do H and O in leaf water reflect environmental drivers differently?

We compiled hydrogen and oxygen stable isotope compositions (δ H and δ O) of leaf water from multiple biomes to examine variations with environmental drivers. Leaf water δ H was more closely correlated with δ H of xylem water or atmospheric vapour, whereas leaf water δ O was more closely correlated with air relative humidity. This resulted from the larger proportional range for δ H of meteoric waters relative to the extent of leaf water evaporative enrichment compared with δ O.

Evidence for distinct isotopic compositions of sap and tissue water in tree stems: consequences for plant water source identification.

The long-standing hypothesis that the isotopic composition of plant stem water reflects that of source water is being challenged by studies reporting bulk water from woody stems with an isotopic composition that cannot be attributed to any potential water source. The mechanism behind such source-stem water isotopic offsets is still poorly understood. Using a novel technique to extract selectively sap water from xylem conduits, we show that, in cut stems and potted plants, the isotopic composition of sap water reflects that of irrigation water, demonstrating unambiguously that no isotopic fractionation occurs during root water uptake or sap water extraction.

Maintaining forest cover to enhance temperature buffering under future climate change.

Forest canopies buffer macroclimatic temperature fluctuations. However, we do not know if and how the capacity of canopies to buffer understorey temperature will change with accelerating climate change. Here we map the difference (offset) between temperatures inside and outside forests in the recent past and project these into the future in boreal, temperate and tropical forests.

Forest microclimates and climate change: Importance, drivers and future research agenda.

Forest microclimates contrast strongly with the climate outside forests. To fully understand and better predict how forests' biodiversity and functions relate to climate and climate change, microclimates need to be integrated into ecological research. Despite the potentially broad impact of microclimates on the response of forest ecosystems to global change, our understanding of how microclimates within and below tree canopies modulate biotic responses to global change at the species, community and ecosystem level is still limited.

Temperature-sensitive biochemical O-fractionation and humidity-dependent attenuation factor are needed to predict δ O of cellulose from leaf water in a grassland ecosystem.

We explore here our mechanistic understanding of the environmental and physiological processes that determine the oxygen isotope composition of leaf cellulose (δ O ) in a drought-prone, temperate grassland ecosystem. A new allocation-and-growth model was designed and added to an O-enabled soil-vegetation-atmosphere transfer model (MuSICA) to predict seasonal (April-October) and multi-annual (2007-2012) variation of δ O and O-enrichment of leaf cellulose (Δ O ) based on the Barbour-Farquhar model. Modelled δ O agreed best with observations when integrated over c.

Asymmetric responses of ecosystem productivity to rainfall anomalies vary inversely with mean annual rainfall over the conterminous United States.

The CONterminous United States (CONUS) presents a large range of climate conditions and biomes where terrestrial primary productivity and its inter-annual variability are controlled regionally by rainfall and/or temperature. Here, the response of ecosystem productivity to those climate variables was investigated across different biomes from 2010 to 2018 using three climate datasets of precipitation, air temperature or drought severity, combined with several proxies of ecosystem productivity: a remote sensing product of aboveground biomass, an net primary productivity (NPP) remote sensing product, an NPP model-based product and four gross primary productivity products. We used an asymmetry index (AI) where positive AI indicates a greater increase of ecosystem productivity in wet years compared to the decline in dry years, and negative AI indicates a greater decline of ecosystem productivity in dry years compared to the increase in wet years.

Partitioning net carbon dioxide fluxes into photosynthesis and respiration using neural networks.

The eddy covariance (EC) technique is used to measure the net ecosystem exchange (NEE) of CO between ecosystems and the atmosphere, offering a unique opportunity to study ecosystem responses to climate change. NEE is the difference between the total CO release due to all respiration processes (RECO), and the gross carbon uptake by photosynthesis (GPP). These two gross CO fluxes are derived from EC measurements by applying partitioning methods that rely on physiologically based functional relationships with a limited number of environmental drivers.

µgreen-db: a reference database for the 23S rRNA gene of eukaryotic plastids and cyanobacteria.

Studying the ecology of photosynthetic microeukaryotes and prokaryotic cyanobacterial communities requires molecular tools to complement morphological observations. These tools rely on specific genetic markers and require the development of specialised databases to achieve taxonomic assignment. We set up a reference database, called µgreen-db, for the 23S rRNA gene.

An explanation for the isotopic offset between soil and stem water in a temperate tree species.

A growing number of field studies report isotopic offsets between stem water and its potential sources that prevent the unambiguous identification of plant water origin using water isotopes. We explored the causes of this isotopic offset by conducting a controlled experiment on the temperate tree species Fagus sylvatica. We measured δ H and δ O of soil and stem water from potted saplings growing on three soil substrates and subjected to two watering regimes.

A novel optimization approach incorporating non-stomatal limitations predicts stomatal behaviour in species from six plant functional types.

The primary function of stomata is to minimize plant water loss while maintaining CO2 assimilation. Stomatal water loss incurs an indirect cost to photosynthesis in the form of non-stomatal limitations (NSL) via reduced carboxylation capacity (CAP) and/or mesophyll conductance (MES). Two optimal formulations for stomatal conductance (gs) arise from the assumption of each type of NSL.

Soil exchange rates of COS and COO differ with the diversity of microbial communities and their carbonic anhydrase enzymes.

Differentiating the contributions of photosynthesis and respiration to the global carbon cycle is critical for improving predictive climate models. Carbonic anhydrase (CA) activity in leaves is responsible for the largest biosphere-atmosphere trace gas fluxes of carbonyl sulfide (COS) and the oxygen-18 isotopologue of carbon dioxide (COO) that both reflect gross photosynthetic rates. However, CA activity also occurs in soils and will be a source of uncertainty in the use of COS and COO as carbon cycle tracers until process-based constraints are improved.

Coupling of ecosystem-scale plant water storage and leaf phenology observed by satellite.

Plant water storage is fundamental to the functioning of terrestrial ecosystems by participating in plant metabolism, nutrient and sugar transport, and maintenance of the integrity of the hydraulic system of the plant. However, a global view of the size and dynamics of the water pools stored in plant tissues is still lacking. Here, we report global patterns of seasonal variations in ecosystem-scale plant water storage and their relationship with leaf phenology, based on space-borne measurements of L-band vegetation optical depth.

Estimating Mesophyll Conductance from Measurements of COO Photosynthetic Discrimination and Carbonic Anhydrase Activity.

Carbonic anhydrase (CA) activity in leaves catalyzes the O exchange between CO and water during photosynthesis. This feature has been used to estimate the mesophyll conductance to CO ( ) from measurements of online COO photosynthetic discrimination (∆O). Based on CA assays on leaf extracts, it has been argued that CO in mesophyll cells should be in isotopic equilibrium with water in most C species as well as many C dicot species.

The interaction of soil phototrophs and fungi with pH and their impact on soil CO, COO and OCS exchange.

The stable oxygen isotope composition of atmospheric CO and the mixing ratio of carbonyl sulphide (OCS) are potential tracers of biospheric CO fluxes at large scales. However, the use of these tracers hinges on our ability to understand and better predict the activity of the enzyme carbonic anhydrase (CA) in different soil microbial groups, including phototrophs. Because different classes of the CA family (α, β and γ) may have different affinities to CO and OCS and their expression should also vary between different microbial groups, differences in the community structure could impact the 'community-integrated' CA activity differently for CO and OCS.

Bryophyte gas-exchange dynamics along varying hydration status reveal a significant carbonyl sulphide (COS) sink in the dark and COS source in the light.

Carbonyl sulphide (COS) is a potential tracer of gross primary productivity (GPP), assuming a unidirectional COS flux into the vegetation that scales with GPP. However, carbonic anhydrase (CA), the enzyme that hydrolyses COS, is expected to be light independent, and thus plants without stomata should continue to take up COS in the dark. We measured net CO (A ) and COS (A ) uptake rates from two astomatous bryophytes at different relative water contents (RWCs), COS concentrations, temperatures and light intensities.

Resilience to seasonal heat wave episodes in a Mediterranean pine forest.

Short-term, intense heat waves (hamsins) are common in the eastern Mediterranean region and provide an opportunity to study the resilience of forests to such events that are predicted to increase in frequency and intensity. The response of a 50-yr-old Aleppo pine (Pinus halepensis) forest to hamsin events lasting 1-7 d was studied using 10 yr of eddy covariance and sap flow measurements. The highest frequency of heat waves was c.

Improvement of water and light availability after thinning at a xeric site: which matters more? A dual isotope approach.

Thinning fosters individual tree growth by increasing the availability of water, light and nutrients. At sites where water rather than light is limiting, thinning also enhances soil evaporation and might not be beneficial. Detailed knowledge of the short- to long-term physiological response underlying the growth responses to thinning is crucial for the management of forests already suffering from recurrent drought-induced dieback.