Canopy Conductance Research Articles

Increased atmospheric moisture demand induced a reduction in the water content of boreal forest during the past three decades

Widespread forest mortality has been observed in recent years which may be related to the forest water content. Changes in atmospheric demand for water (vapour pressure deficit, VPD) and soil moisture due to climate change affect plants in terrestrial ecosystems. Hence, it is crucial to determine the impact of variations in soil-atmosphere water content on forest water content to enhance our predictive capability of future forest dynamics. Here, we explored the dynamics of forest water content in the boreal forest (BFC) based on remote sensing Ku-band vegetation optical depth (VOD) data and found a declining trend from 1988 to 2016. The study revealed a strong negative correlation between VPD and VOD and the negative trend of BFC water content increased with increasing annual VPD. Further analysis found a negative correlation between monthly transpiration and VOD, and a positive correlation between canopy conductance and VOD. The relationship exhibited greater significance in regions characterized by a more rapid increase in VPD suggesting elevated VPD led to a higher transpiration, resulting in a decline in forest water content and subsequent plant physiological regulation. This occurred consistently in intact and managed boreal forest, implying common climatic drivers on a large scale. The decrease of annual mean VOD was slower in the intact boreal forest than in the managed forest, which may be due to the diversity and richness of the intact forest. The findings enhanced the understanding of the dynamics of forest water content under atmospheric drought caused by climate change, which requires the adoption of strategies to prevent forest mortality.

Scaling solar-induced chlorophyll fluorescence by using VPD0.5 improves the simulation of reference crop evapotranspiration in the arid and semiarid regions of northern China

Accurately estimating reference crop evapotranspiration (ET0) is crucial for decision-making regarding irrigation, improving farmland water use efficiency, planning water resources, and simulating the global water cycle. However, because traditional ET0 estimation models often require a large number of meteorological parameters as input variables, their application potential is limited, especially when simulating arid and semiarid regions where meteorological data are lacking. Therefore, it is still very challenging to estimate ET0 at large scales in arid and semiarid areas. In this study, we first proposed a semi-mechanistic model for estimating ET0 by deriving the relationship between canopy conductance to water vapor (gw) and solar-induced chlorophyll fluorescence (SIF) based on Fick's law and the light use efficiency (LUE) model, combining theories on optimal stomatal behaviour. Second, the relationship between SIF × VPD0.5 (VPD0.5: 0.5-power of the vapor pressure deficit) and ET0 in the model was verified at the site scale. On this basis, the semi-mechanistic model was applied to three machine learning models to simulate ET0. Four statistical indicators, including the coefficient of determination (R2), root mean square error (RMSE), normalized root mean square error (NRMSE) and mean absolute error (MAE), were used to verify the model performance and screen out the optimal model. Finally, the optimal model was compared with five traditional empirical models. The results showed that (1) the relationship between ET0 and the product of SIF and the 0.5-power vapor pressure deficit (SIF × VPD0.5) was closer than the relationship between ET0 and SIF at the site scale. (2) Compared to the case in which SIF was used as the only input variable, three machine learning models performed better when SIF was scaled using VPD0.5 as the input variable, and the random forest regression (RFR) model in which SIF × VPD0.5 was the input variable was the optimal model (ET0opt). (3) The total mean value of ET0 at 22 sites simulated by ET0opt is 15.46 mm/4 days. The total mean ET0 value for the 22 sites calculated by the five traditional empirical models vary from 10.70 mm/4 days to 20.96 mm/4 days, while our results were in this range and close to their mean value. Our study provides a valuable reference basis for large-scale ET0 estimations using remote sensing in arid and semiarid areas.

Vapour pressure deficit modulates hydraulic function and structure of tropical rainforests under nonlimiting soil water supply.

Atmospheric conditions are expected to become warmer and drier in the future, but little is known about how evaporative demand influences forest structure and function independently from soil moisture availability, and how fast-response variables (such as canopy water potential and stomatal conductance) may mediate longer-term changes in forest structure and function in response to climate change. We used two tropical rainforest sites with different temperatures and vapour pressure deficits (VPD), but nonlimiting soil water supply, to assess the impact of evaporative demand on ecophysiological function and forest structure. Common species between sites allowed us to test the extent to which species composition, relative abundance and intraspecific variability contributed to site-level differences. The highest VPD site had lower midday canopy water potentials, canopy conductance (gc ), annual transpiration, forest stature, and biomass, while the transpiration rate was less sensitive to changes in VPD; it also had different height-diameter allometry (accounting for 51% of the difference in biomass between sites) and higher plot-level wood density. Our findings suggest that increases in VPD, even in the absence of soil water limitation, influence fast-response variables, such as canopy water potentials and gc , potentially leading to longer-term changes in forest stature resulting in reductions in biomass.

Spatial patterns and recent temporal trends in global transpiration modelled using eco-evolutionary optimality

Transpiration from vegetation accounts for about two thirds of land evapotranspiration (ET), and exerts important effects on of global water, energy, and carbon cycles. Resistance-based ET partitioning models using remote sensing data are one of the main methods to estimate global land transpiration, overcoming the limitation by the sparse distribution and short observation periods of site-level measurements. However, the uncertainties of estimated transpiration for these models mainly come from the resistance parameterization based on specific empirical parameters across different plant functional types (PFT). A model based on eco-evolutionary optimization (P model) has recently been proposed to simulate stomatal conductance without the need of calibrated parameters. Here, we calculated global long-term (1982–2018) monthly transpiration with the Penman-Monteith (PM) equation using canopy conductance estimated by the P model (PM-P) and Ball-Berry-Leuning model (PM-BBL). Using the observations of SAPFLUXNET and FLUXNET sites as reference, the performance of PM-P was comparable with that of PM-BBL and Global Land Evaporation Amsterdam model (GLEAM). Multi-year mean and trends in growing season transpiration estimated by GLEAM and the PM-P model revealed a similar spatial distribution globally. Both GLEAM and the PM-P model showed a widespread increasing trend of growing season transpiration over 72.06%∼80.38% of global land, especially for some main greening hotspots with >3.0 mm/year. The good performance of the P model indicated that it could avoid the uncertainties emerging from the resistance parameterization with too many empirical parameters and had the potential to accurately estimate global transpiration.

Asymmetric response of Amazon forest water and energy fluxes to wet and dry hydrological extremes reveals onset of a local drought-induced tipping point.

Understanding the effects of intensification of Amazon basin hydrological cycling-manifest as increasingly frequent floods and droughts-on water and energy cycles of tropical forests is essential to meeting the challenge of predicting ecosystem responses to climate change, including forest "tipping points". Here, we investigated the impacts of hydrological extremes on forest function using 12+ years of observations (between 2001-2020) of water and energy fluxes from eddy covariance, along with associated ecological dynamics from biometry, at the Tapajós National Forest. Measurements encompass the strong 2015-2016 El Niño drought and La Niña 2008-2009 wet events. We found that the forest responded strongly to El Niño-Southern Oscillation (ENSO): Drought reduced water availability for evapotranspiration (ET) leading to large increases in sensible heat fluxes (H). Partitioning ET by an approach that assumes transpiration (T) is proportional to photosynthesis, we found that water stress-induced reductions in canopy conductance (Gs ) drove T declines partly compensated by higher evaporation (E). By contrast, the abnormally wet La Niña period gave higher T and lower E, with little change in seasonal ET. Both El Niño-Southern Oscillation (ENSO) events resulted in changes in forest structure, manifested as lower wet-season leaf area index. However, only during El Niño 2015-2016, we observed a breakdown in the strong meteorological control of transpiration fluxes (via energy availability and atmospheric demand) because of slowing vegetation functions (via shutdown of Gs and significant leaf shedding). Drought-reduced T and Gs , higher H and E, amplified by feedbacks with higher temperatures and vapor pressure deficits, signaled that forest function had crossed a threshold, from which it recovered slowly, with delay, post-drought. Identifying such tipping point onsets (beyond which future irreversible processes may occur) at local scale is crucial for predicting basin-scale threshold-crossing changes in forest energy and water cycling, leading to slow-down in forest function, potentially resulting in Amazon forests shifting into alternate degraded states.

Interannual variation in evapotranspiration in an urban forest reserve with respect to drought

IntroductionA warming global climate is expected to perturb the hydrological cycle, resulting in deviations in both frequency and duration of drought and thus being hypothesized to lead to interannual variation in evapotranspiration (ET). Interannual variation in ET in urban forest ecosystems in response to drought remains poorly understood.MethodsHere, ET in an urban forest reserve in the megalopolis of Beijing was investigated using eddy-covariance measurements collected over six consecutive years (2012–2017).ResultsThe mean annual cumulative ET was 462 ± 83mm (±first standard deviation), with a coefficient of variation of 18%. Interannual variation in both annual and monthly ET was shown to be largely controlled by canopy conductance (gs), affected by environmental factors. The main factors affecting interannual variation in monthly ET varied seasonally, namely, soil volumetric water content (VWC) and normalized difference vegetation index (NDVI) in spring, precipitation and soil temperature in summer, and VWC and net radiation (Rn) in autumn. Interannual variation in annual ET was driven largely by spring and mid-summer droughts induced by insufficient precipitation during the non-growing and mid-growing seasons, respectively. Spring drought reduced annual ET by restricting leafing out, shortening growing season length (GSL), and reducing the normalized difference vegetation index (NDVI). The summer drought reduced annual ET by reducing stomatal conductance.Discussion and conclusionResults from this study point to the importance of precipitation timing and volume and the soil moisture carry-over effect in controlling interannual variation in ecosystem ET. Irrigation during the early spring and mid-summer is viewed as a practical management measure for sustaining growth and better ecosystem services in urban forests in Northern China.

The Linkage Between Methane Fluxes and Gross Primary Productivity at Diurnal and Seasonal Scales on a Rice Paddy Field in Eastern China

AbstractCarbon dioxide (CO2) and methane (CH4) are the most important two greenhouse gases in the atmosphere which were closely coupled between terrestrial ecosystems and the atmosphere. However, the relationship between CO2 and CH4 fluxes of rice paddy at different temporal scales is still unclear. Based on 6 years of eddy covariance measurements on a flooded rice paddy field in Eastern China, the relationship between gross primary productivity (GPP) and CH4 fluxes and the effects of biophysical and environmental factors on daily CH4/GPP were investigated. CH4 fluxes and GPP were tightly linked at diurnal and seasonal scales. On average, half‐hourly CH4 fluxes lagged GPP by 1.5 hr while daily GPP lagged CH4 fluxes by 19 days over the growing season. Leaf area index, water table depth (WTD), air temperature, vapor pressure deficit, and canopy conductance significantly affect daily CH4/GPP. A semi‐empirical multiplicative model was able to capture 77% and 84% seasonal variability of daily CH4/GPP for the training data set and testing data set, respectively. WTD is a crucial input affecting the performance of the model. Without WTD included, the agreement between estimated and observed daily CH4/GPP was weakened. Under the condition without WTD data available, separate calibration of the semi‐empirical multiplicative model before and after panicle initiation could improve the estimation of daily CH4/GPP to some extent. The findings in this study enhance our understanding regarding the complex interactions between CH4 fluxes and GPP in the rice paddy and are helpful for better estimation of CH4 fluxes from GPP.

A modified Jarvis model to improve the expressing of stomatal response in a beech forest

AbstractThe Jarvis‐type model, which incorporates stress functions, is commonly used to describe the physiological behaviour of stomatal response in various vegetation species. However, the model has been criticized for its empirically formulated multiplicative equation, which may not accurately capture the mutual impact of intercorrelated stress factors, for example, vapour pressure deficit (VPD) and air temperature (Ta). This study proposed a modified Jarvis model that introduces reduction factors in the stress functions of VPD and Ta to provide the description of canopy conductance. We used sap flow data from a beech forest in the mid‐latitude region of Centre Europe to inversely estimate the canopy conductance with optimized stress functions. Our findings reveal that two recommended parameterization strategies for general deciduous broadleaf forest (DBF) significantly overestimated the transpiration rate, with a maximum value of ~2 mm/day on rainless days. This suggested that the beech forest exhibited a distinct stomatal response compared to the general DBF category. By applying boundary line analysis to fit the parameters, both the unmodified and modified Jarvis models provided better simulations of transpiration, with relatively high Nash‐Sutcliffe Efficiency (NSE) values of 0.75 and 0.77, respectively. These results indicated that modelling transpiration can be improved by refining the parameterization of canopy conductance, particularly for vegetation species with unique stomatal behaviours that deviated from the characteristics of their general vegetation type. The modified Jarvis model offers a more accurate description of canopy conductance and enhances the modelling of transpiration in vegetated areas, especially under dry environment conditions with relatively high VPD.

Empirical insights on the use of sun-induced chlorophyll fluorescence to estimate short-term changes in crop transpiration under controlled water limitation

Knowledge of actual crop transpiration (T) is important for advanced crop management but challenging to obtain due to the large spatial and temporal variation of T. Remote sensing offers various possibilities to assess T dynamics, while particularly sun-induced chlorophyll fluorescence (SIF) has been demonstrated as a sensitive empirical proxy for T. Despite this success, the advancement of the mechanistic understanding of how SIF relates to T dynamics is key for the future development and implementation of robust and reliable SIF-based T products. This study aims to contribute insights by experimentally assessing the sensitivity of several SIF-based T estimation strategies for evolving soil water limitation. We investigated extensive in situ and airborne data acquired during a water limitation experiment in a maize canopy in northern Italy. We evaluated five empirical strategies to integrate SIF in a T modelling framework based on the Penman-Monteith (PM) and the Ball-Berry-Leuning (BBL) concepts. Our results indicate that replacing model parameters sensitive to canopy conductance with SIF results in the best agreement between modelled and measured T under evolving water limitation. Our study contributes expanding existing knowledge with empirical insights on the sensitivity of SIF based T approaches under increasing soil water limitation at short time scales.

High vapour pressure deficit enhances turgor limitation of stem growth in an Asian tropical rainforest tree.

Tropical forests are experiencing increases in vapour pressure deficit (D), with possible negative impacts on tree growth. Tree-growth reduction due to rising D is commonly attributed to carbon limitation, thus overlooking the potentially important mechanism of D-induced impairment of wood formation due to an increase in turgor limitation. Here we calibrate a mechanistic tree-growth model to simulate turgor limitation of radial stem growth in mature Toona cilitata trees in an Asian tropical forest. Hourly sap flow and dendrometer measurements were collected to simulate turgor-driven growth during the growing season. Simulated seasonal patterns of radial stem growth matched well with growth observations. Growth mainly occurred at night and its pre-dawn build-up appeared to be limited under higher D. Across seasons, the night-time turgor pressure required for growth was negatively related to previous midday D, possibly due to a relatively high canopy conductance at high D, relative to stem rehydration. These findings provide the first evidence that tropical trees grow at night and that turgor pressure limits tree growth. We suggest including turgor limitation of tree stem growth in models also for tropical forest carbon dynamics, in particular, if these models simulate effects of warming and increased frequency of droughts.

Remote sensing of daily evapotranspiration and gross primary productivity of four forest ecosystems in East Asia using satellite multi-channel passive microwave measurements

Satellite multi-channel passive microwave observations provide valuable information of canopy vertical structure and vegetation water content, while their potential in the simultaneous estimation of forest evapotranspiration (ET) and gross primary productivity (GPP) has not been explored. This study combined satellite multi-channel microwave Emissivity Difference Vegetation Indices (EDVIs) from 6.9 to 36.5 GHz with the Penman-Monteith (PM), Priestley-Taylor (PT) and light use efficiency (LUE) models to estimate daily ET and GPP over four forest ecosystems in East Asia. In-situ measurements of daily ET, GPP and meteorological variables from ChinaFLUX network (2003 to 2010) were used to calculate and validate the method. An exponential relationship was found between EDVIs and canopy conductance across the forests. By inverting EDVIs-based LUE model coupled with PM and PT models when canopy carbon uptake and water content reached a maximum in growing seasons, the maximum LUE (EDVI-LUEmax) was derived and found to agree with other independent studies. Site validations showed that the EDVI-based method performed a good simultaneous estimation of daily ET and GPP at a tropical forest and three sub-tropical and temperate forests (R2 of 0.64 to 0.94 and RMSE of −11.7% to 19.6% for ET, R2 of 0.39 to 0.73 and RMSE of 31.2% to 50.4% for GPP). The modeled relationship between GPP and ET also agreed that from in-situ measurements. The results were also found to correlate with microwave frequencies. In-situ ET and GPP showed a stronger correlation with EDVI at 18.7 and 36.5 GHz than that at 10.7 and 18.7 GHz across the forests. EDVI-LUEmax of a temperate mixed forest increased as EDVI incorporated lower frequency microwave emissions from deeper canopy layers, which reduced the negative bias in GPP estimation. Overall, this study demonstrated the potential of multi-channel microwave measurements in estimating ET and GPP with the potential effects of vegetation vertical structure.

Effects of soil moisture and vapor pressure deficit on canopy transpiration for two coniferous forests in the Loess Plateau of China

Soil moisture (VWC) and atmospheric dryness (vapor pressure deficit, VPD) have an increasing impact on tree transpiration and productivity with climate change. However, the interaction of VWC and VPD on canopy transpiration (Ec) of plantations is still unclear, especially in water-scarce areas. Here, we selected Pinus tabulaeformis Carr. and Platycladus orientalis (L.) Franco plantations in the semiarid loess hills of China to examine the impacts of soil and atmospheric water conditions on Ec during the growing seasons (May–September) of 2015–2020. The results showed that the magnitudes of Ec were similar in the two plantations throughout the study period. The lowest Ec occurred in September because of little precipitation and low atmospheric evaporative demand, while Ec was highest in July due to the increase in soil water availability and atmospheric evaporative demand. There were obvious inflection points in the response relationship between VWC and VPD with Ec for both plantations. Under non-drought conditions, Ec reached a maximum in both plantations and was mainly impacted by solar radiation (Rs) and air temperature. As VPD increased, Rs still was the dominant factor, but canopy conductance decreased significantly, resulting in negative correlations between Ec and VPD. Under soil drought and compound drought conditions, Ec was mainly controlled by Rs and VWC, and the inhibitory effect of VPD on Ec was enhanced. In addition, the ratio of Ec to precipitation was close to 50% for both plantations at the interannual scale. Considering that soil water depletion in the Loess Plateau will accelerate with climate change and increasing drought events, the increasing water use stress will further hinder tree gas exchange, inhibiting tree growth. It is thus necessary to pay special attention to the impact of soil and atmospheric drought on water use for forest management in the Loess Plateau and similar regions of the world.

Validation of a simple canopy conductance model for estimating transpiration of different citrus species under non-limiting soil water conditions

Validation of a simple canopy conductance model for estimating transpiration of different citrus species under non-limiting soil water conditions

The counteracting effects of large-scale vegetation restoration and increased precipitation on drought in the Huang-Huai-Hai-Yangtze River basin

Quantifying the causes of drought trends is critical for disaster prevention and water resource management. In recent years, not only climatic variables, such as temperature and precipitation (PRE), have changed, but also environmental factors, including vegetation restoration, increased CO2 concentration, and land cover conversion. However, the role of diverse factors, particularly large-scale vegetation restoration, in influencing drought trends is still indeterminate. In this study, a Ball-Berry canopy conductance model was incorporated into the Shuttleworth–Wallace (S-W) model to estimate the potential evapotranspiration (PET) in the Huang-Huai-Hai-Yangtze River basin from 2001 to 2019. The standardized precipitation evapotranspiration index (SPEI), which considers both water supply (from PRE) and demand (from PET), was adopted to identify drought conditions. Given the complex non-linear relationships in the model, an interpretable machine learning model was applied to the attribution of PET and SPEI trends. The results were as follows: (1) The PRE and PET showed significant upward trends, whereas the change rate of PET (60 mm/10a, p < 0.01) was higher than that of PET (44 mm/10a, p < 0.1) in the region; (2) The SPEI at an annual scale non-significantly decreased at a rate of −0.12/10a (p > 0.1). Increasing PET dominated SPEI trends with a contribution of −0.28/10a (p < 0.01), whereas increasing PRE contributed 0.16/10a (p > 0.1); (3) The interpretable machine learning model performed better than multiple linear regression in attributing PET trends; (4) Large-scale vegetation restoration (the increased leaf area index, LAI) and wind speed, were the two dominant factors influencing PET trends with contributions of 32.4 mm/10a (32%) and 19.6 mm/10a (20%), respectively; (5) In terms of the trends in drought across the entire region, the increase in LAI almost offset the positive contributions of PRE to SPEI variations, hence, combined with the influence of the other factors, drought was non-significantly accelerated.

Stand transpiration and canopy conductance dynamics of Populus popularis under varying water availability in an arid area

As a tree species of shelterbelts, Populus popularis maintains significant ecological functions in arid and semiarid areas. However, stand transpiration (T) and canopy conductance (gc) dynamics of P. popularis are unclear in arid irrigated areas with shallow groundwater fluctuations. To better understand the responses of T and gc to meteorological factors, soil water, and shallow groundwater in arid areas, we observed the environmental conditions and sap flow of P. popularis, and quantified T and gc in three growing seasons of 2018–2020 in a typical arid area of China. Results showed T and gc ranged from 0.18 to 6.11 mm day−1 and 2.26–12.54 mm s−1 in 2018–2020, respectively. Solar radiation and vapor pressure deficit (VPD) were major drivers of T at daily scales. It was consistently found that T exponentially decreased with increasing groundwater table depth (GTD) and decreasing reference evapotranspiration in three years. gc is primarily influenced by VPD and is positively related to soil water content in 0–30 cm soil layer (SWC0–30 cm). Moreover, low SWC0–30 cm and deepening GTD jointly decreased T and gc by 22.45 % and 30.41 %, respectively. The response of gc to VPD was susceptible to groundwater fluctuations, and the synergistic influences of VPD and GTD on gc could be well described by the logarithmic function, especially in 2019. The sensitivity of gc to VPD and its variations under different environmental conditions suggested that a flexible stomatal regulation of transpiration occurred in the observed P. popularis with the arid climate and shallow groundwater. These findings provided the essential basis for the water use strategy of P. popularis and stand water resources management in arid regions.

Daytime stomatal regulation in mature temperate trees prioritizes stem rehydration at night.

Trees remain sufficiently hydrated during drought by closing stomata and reducing canopy conductance (Gc ) in response to variations in atmospheric water demand and soil water availability. Thresholds that control the reduction of Gc are proposed to optimize hydraulic safety against carbon assimilation efficiency. However, the link between Gc and the ability of stem tissues to rehydrate at night remains unclear. We investigated whether species-specific Gc responses aim to prevent branch embolisms, or enable night-time stem rehydration, which is critical for turgor-dependent growth. For this, we used a unique combination of concurrent dendrometer, sap flow and leaf water potential measurements and collected branch-vulnerability curves of six common European tree species. Species-specific Gc reduction was weakly related to the water potentials at which 50% of branch xylem conductivity is lost (P50 ). Instead, we found a stronger relationship with stem rehydration. Species with a stronger Gc control were less effective at refilling stem-water storage as the soil dries, which appeared related to their xylem architecture. Our findings highlight the importance of stem rehydration for water-use regulation in mature trees, which likely relates to the maintenance of adequate stem turgor. We thus conclude that stem rehydration must complement the widely accepted safety-efficiency stomatal control paradigm.

Stomatal regulation and xylem hydraulics of limber pine and Engelmann spruce in Great Basin sky-island ecosystems

Integration of whole-plant stomatal regulation and xylem hydraulics is of critical importance for predicting species response to drought stress. Yet intraspecific variability of stomatal and hydraulic traits, and how these variabilities interact, remain largely unknown. We hypothesized that drought can drive less stomatal regulation but increase xylem hydraulic safety, resulting in stomatal-hydraulic coordination within a species. We estimated sensitivity of whole-tree canopy conductance to soil drying together with xylem hydraulic traits of two dominant conifers, i.e. limber pine (Pinus flexilis) and Engelmann spruce (Picea engelmannii). Our study was conducted using sub-hourly measurements over five consecutive years (2013–2017) at three instrumented sites with different elevations within the Nevada Eco-hydrological Assessment Network (NevCAN) in Great Basin sky-island ecosystems. Both conifers showed a reduction of stomatal sensitivity to soil drying at lower elevations, indicating an active stomatal acclimation to drought. While limber pine increased xylem embolism resistance in parallel with reduced stomatal sensitivity to soil drying, an opposite hydraulic adjustment was detected in Engelmann spruce. Our results provide evidence that mature trees can respond to climatic changes using coordinated shifts in stomatal regulation and xylem hydraulics, but such changes can differ within and between species in ways that need to be examined using in situ data. Deciphering intraspecific variability in whole-plant stomatal and hydraulic traits ultimately contributes to defining drought tolerance and vulnerability, particularly for tree species that inhabit a wide range of landscapes.

Stomatal ozone uptake of a Quercus serrata stand based on sap flow measurements with calibrated thermal dissipation sensors

The amount of ozone absorbed by the tree leaves is a critical factor determining the ozone effects on forest trees. Stomatal ozone uptake of a forest canopy can be estimated from the ozone concentration and canopy conductance (gc) determined by the sap-flow-based method. This method measures sap flow as a metric of crown transpiration and then derives gc. The thermal dissipation method (TDM) has been used to measure sap flow in most studies adopting this approach. However, recent studies have indicated that TDM may underestimate sap flow, especially in ring-porous tree species. In the present study, the accumulated stomatal ozone uptake (AFST) of a stand of Quercus serrata, a typical ring-porous tree species in Japan, was estimated by measuring sap flow using species-specific calibrated TDM sensors. Laboratory calibration of the TDM sensors revealed that the parameters (α and β) in an equation converting outputs from the sensors (K) to sap flux density (Fd) were substantially larger for Q. serrata than those originally proposed by Granier (1987). The Fd measured in the Q. serrata stand using calibrated TDM sensors were significantly larger than those obtained using non-calibrated sensors. The diurnal average of gc and daytime AFST (10.4 mm s−1 and 10.96 mmol O3 m−2 month−1) of the Q. serrata stand estimated by using calibrated TDM sensors in August 2020 were similar to those of forests dominated by Quercus species estimated by micrometeorological measurements in previous studies. In contrast, the gc and daytime AFST of the Q. serrata stand estimated by non-calibrated TDM sensors were remarkably lower than those estimated by micrometeorological measurements in previous studies, indicating severe underestimation. Therefore, it is strongly recommended that sap flow sensors are species-specifically calibrated when estimating the canopy conductance and ozone uptake of forests dominated by ring-porous trees based on sap flow measurements using TDM.

Balancing water and radiation productivity suggests a clue for improving yields in wheat under combined water deficit and terminal heat stress.

Sustaining crop yield under abiotic stresses with optimized resource use is a prerequisite for sustainable agriculture, especially in arid and semi-arid areas. Water and heat stress are major abiotic stresses impacting crop growth and yield by influencing complex physiological and biochemical processes during the life cycle of crops. In a 2-year (2015-2017) research, spring wheat cv. HD-2967 was grown under deficit irrigation and delayed sowing conditions to impose water and terminal heat stresses, respectively. The data were analyzed for seasonal crop water use, radiation interception, water productivity (WP), and radiation productivity (RP) under combined water deficit and terminal heat stresses. Seasonal crop water use was significantly affected by stresses in the order of water + terminal heat > water > terminal heat. Water stress showed minimal effect on the light extinction coefficient and consequently on seasonal intercepted photosynthetically active radiation (IPAR). However, seasonal IPAR was primarily affected by combined water + terminal heat and terminal heat stress alone. The slope of crop water use and IPAR, i.e., canopy conductance, an indicator of canopy stomatal conductance, was more influenced by water stress than by terminal heat stress. Results showed that linear proportionality between WP and RP is no longer valid under stress conditions, as it follows a curvilinear relation. This is further supported by the fact that independent productivity (either water or radiation) lacked the ability to explain variability in the final economic yield or biomass of wheat. However, the ratio of RP to WP explained the variability in wheat yield/biomass under individual or combined stresses. This suggests a clue for improving higher wheat yield under stress by managing WP and RP. The highest biomass or yield is realized when the ratio of RP to WP approaches unity. Screening of genotypes for traits leading to a higher ratio of RP to WP provides an opportunity for improving wheat productivity under stressed environments.

Decoupling factor, aerodynamic and canopy conductances of a hedgerow olive orchard under Mediterranean climate

The degree of coupling between canopy and atmosphere, through the decoupling factor Ω, well describes the behaviour of a crop concerning its water use and carbon dioxide exchange. Super high-density hedgerow olive orchard system is in great expansion all over the world and, since it has a complex field structure in rows of adjacent trees, investigations are necessary to assess the Ω patterns, as well as aerodynamic (ga) and canopy (gc) conductances in different water conditions. In this study, in a hedgerow olive orchard (cv. “Arbosana”) submitted to full (FI) and regulated deficit irrigation (RDI), cropped under a Mediterranean semi-arid climate (southern Italy), Ω has been determined using gc, as deduced by inverting the Penman-Monteith equation, and ga, by upscaling the wind speed measured in a close station to the canopy; the transpiration has been measured by sap flow thermal dissipation method. The results showed that this olive orchard results very well coupled to the atmosphere, in any soil water conditions; Ω is generally very low, being during daytime equal in mean to 0.021±0.003 ms-1 and 0.018±0.004 ms-1 for RDI and FI, respectively. This condition is linked to ga and gc values; in fact, canopy conductance is much smaller than the aerodynamic one in any water and climatic conditions, except when all canopy surfaces are saturated in water. In this latter case, the gc assumes the highest values due to the contribution of the part of conductance attributable to the structure of the orchard.