Sap Velocity Research Articles

Field sugarcane transpiration based on sap flow measurements and root water uptake simulations: Case study on Tanegashima Island, Japan

In agriculture, soil evaporation is considered a loss of water, and crop transpiration is a crucial factor affecting crop growth and yield. Although there have been numerous studies on evapotranspiration (ET) on farmland, only a few have been conducted on field crop transpiration because of the difficulties in the direct measurement of transpiration and in separate evaluation of evaporation and transpiration under field conditions. In this study, we used the heat-pulse method to measure the sap velocity in sugarcane stems to evaluate field transpiration during summer at the study site on Tanegashima Island (at about 31° N) in Japan. The reference crop ET was determined by the Food and Agriculture Organization (FAO) Penman–Monteith method from on-site meteorological data and was compared with the measured transpiration. The daily transpiration in August ranged from 3.3 mm d−1 to 6.6 mm d−1; the corresponding sap flow varied from 0.27 L d−1 to 0.55 L d−1 per stalk. The daily analysis of ET showed that a standard value of 1.25 for the crop coefficient presented by the FAO is acceptable as the crop coefficient for the mid-season growth stage of sugarcane in the study site. Numerical simulations of Richards equation with the sink term were conducted to examine the response of root water uptake to atmospheric transpiration demand throughout 20 consecutive clear and dry days in August. The decrease in the soil moisture in the root zone depends on the transpiration demand during daytime, and soil moisture is redistributed and recovered during the nighttime. The hourly numerical results in August support a standard value of 1.20 for the basal crop coefficient in the mid-season growth stage, as proposed by FAO. The simulations showed that roots extract water preferentially from the deep and wet soil layers to meet the transpiration demand; transpiration predicted from the sink term agreed well with the actual transpiration measured by the heat-pulse method. Therefore, the numerical analysis of soil moisture dynamics under the atmospheric transpiration demand based on the reference crop ET can be an effective tool to evaluate field sugarcane water consumption.

Estimates of tree root water uptake from soil moisture profile dynamics

Abstract. Root water uptake (RWU), as an important process in the terrestrial water cycle, can help us to better understand the interactions in the soil–plant–atmosphere continuum. We conducted a field study monitoring soil moisture profiles in the rhizosphere of beech trees at two sites with different soil conditions. We present an algorithm to infer RWU from step-shaped, diurnal changes in soil moisture. While this approach is a feasible, easily implemented method for moderately moist and homogeneously textured soil conditions, limitations were identified during drier states and for more heterogeneous soil settings. A comparison with the time series of xylem sap velocity underlines that RWU and sap flow (SF) are complementary measures in the transpiration process. The high correlation between the SF time series of the two sites, but lower correlation between the RWU time series, suggests that soil characteristics affect RWU of the trees but not SF.

Uncertainty in sap flow of Brazilian mahogany determined by the heat ratio method

The tropical arboreal species Brazilian mahogany (Swietenia macrophylla) is very important economically and ecologically, for which understanding ecophysiological variables such as sap flow will improve understanding of the species and its cultivation. This paper aims to measure uncertainties (U) involved in the application of the heat ratio method for determining sap flow in Brazilian mahogany using sets of heating probes and thermometers installed on plants of 18 months of age, cultivated in Yellow Latosol, under a weighing lysimeter and located in a protected environment. The uncertainty in sap flow was calculated as the combination of uncertainty in the thermal diffusivity (Uk), conductive section (USc) and corrected sap velocity (UVc). Uk had greater weight in determining the flow of sap in Brazilian mahogany, when compared to USc and UVc. The thermal diffusivity during the cycle, or period evaluated, must be adjusted to improve the accuracy of the heat ratio method because the sap flow overestimated transpiration by 15.0%. When soil water was optimal In addition, the vapor pressure deficit linearly and indirectly influenced the SF with a difference of 14.6%.

Effect of transpiration on the physiological vitality of Zelkova serrata

This study aimed to investigate the effect of suppressing transpiration on the physiological characteristics of trees and identify suitable indicators to assess tree vitality. Three seedlings of Zelkova serrata were used: a control, a unit treated on the upper sides of the leaves (TU), and a unit treated on double sides of the leaves (TD). Changes in photosynthetic rate, transpiration rate, sap velocity, and bark temperature following the suppression of transpiration were measured. The transpiration rate and other physiological characteristics of TU were not significantly different from those of the control except for photosynthetic rate. However, TD showed a significantly lower photosynthetic rate, transpiration rate, and sap velocity compared to the control. The bark temperature of TD was not significantly higher than that of the control and TU. Our findings indicate that photosynthetic rate and sap velocity were positively correlated with transpiration rate, whereas bark temperature was negatively correlated. Furthermore, photosynthetic rate showed the most significant linear relationship with transpiration rate, suggesting the photosynthetic rate is a suitable physiological indicator to evaluate the vitality of Z. serrata.

Temporal and spatial variation in water content within living tree stems determined by electrical resistivity tomography

Stem water content (θ) is an important state variable in the soil-plant-atmosphere continuum (SPAC), and varies temporally and spatially in response to environmental factors and plant growth stages. However, it is difficult to measure θ distribution in living trees in a manner that is not destructive. In this study, temporal and spatial variations in θ within living tree stems were examined based on minimally destructive electrical resistivity tomography. Measurements of tree bulk electrical resistivity (ρ), stem temperature, sap flow and stem water potential were taken on an Australian native tree species, Allocasuarina verticillata. The results show that θ estimated from adjusted resistivity (ρ*) based on a reference ρ-θ relationship approach agrees well with θ estimated from predawn stem water potential. Sapwood θ gradually increased during the wet season and substantially decreased during the dry season as predawn stem water potential and sap velocity increased and decreased, respectively, and ρ*-estimated θ reveals water replenishment and loss within the sapwood during the wet and dry seasons, respectively. During the dry period, mean sapwood θ for the study trees decreased to 0.23 cm3/cm3 (January), and daily maximum sap velocity was only about 10 % of that in wet season, suggesting that the trees were suffering heavy water stress. The spatially exhaustive method to estimate stem water content within living trees proposed here provides an additional approach to investigating tree response to water stress.

Maize transpiration and water productivity of two irrigated fields with varying groundwater depths in an arid area

A better understanding of crop transpiration and its influencing factors in arid areas where irrigation is required contributes to improving agricultural water productivity and water resources management. However, previous studies from different regions have not come to a consensus on which factors and how they control crop transpiration, and little is known for areas with a shallow groundwater table. In this study, maize (Zea mays L.) transpiration was investigated under two groundwater table depths (WTDs) during two growing seasons. Sap velocity (SV), micrometeorological conditions and water regimes were monitored to evaluate transpiration rates under different WTDs and their relationships to environmental and hydrologic conditions. Results show that the combination of photosynthetic active radiation (PAR) and vapor pressure deficit (VPD) mainly controlled the diurnal dynamics of SV. PAR better explained the temporal variability of transpiration than solar radiation at hourly scale. At daily scale, the two-segment transpiration response to VPD was observed, suggesting that the studied species likely has a capacity of drought-tolerance. On average, SV was found to be larger during midday for the shallower WTD in 2016 than the deeper one. For different seasons, the shallower WTD in 2017 also led to larger transpiration than the deeper one in 2016. Given similar yields for all cases, the maize field with the deeper WTD in 2016 was estimated to have higher water productivity. An optimum water use efficiency WTD was determined by assessing the normalized transpiration difference with the varying WTD and it was about 2.1–2.2 m for maize in this area. These results shed light on the most effective utilization of the valuable water confronting the changing climate in arid areas with shallow groundwater.

Ecohydrology of urban trees under passive and active irrigation in a semiarid city.

The infiltration of stormwater runoff for use by urban trees is a major co-benefit of green infrastructure for desert cities with limited water resources. However, the effects of this passive irrigation versus regular, controlled moisture inputs, or active irrigation, is largely unquantified. We monitored the ecohydrology of urban mesquite trees (Prosopis spp.) under these contrasting irrigation regimes in semiarid Tucson, AZ. Measurements included soil moisture, sap velocity, canopy greenness, and leaf-area index. We expected both irrigation types to provide additional deep (>20 cm) soil moisture compared to natural conditions, and that trees would depend on this deep moisture for transpiration and phenological activity. Results show that active irrigation supported higher soil moisture throughout the study than passive irrigation. Passive irrigation only provided additional deep moisture when green infrastructure features received impervious runoff from a city street. Sap velocity and greenness were similar under both irrigation types, outside of isolated periods of time. These differences occurred during the extremely wet summer 2017 when passively irrigated trees exhibited a greenness peak, and the dry conditions of spring when actively irrigated trees had higher sap flow and relative greenness. Finally, it was not determined that deep soil moisture had a stronger relationship with mesquite productivity than shallow moisture, but both relationships were stronger in the spring, before summer rains. This study aims to contribute empirical observations of green infrastructure performance for urban watershed management.

The impact of climate change on water fluxes in a Macaronesian cloud forest

AbstractFog phenomena and their associated meteorological variables were continuously monitored during 4 years in an evergreen laurisilva cloud forest of the Anaga Massif Biosphere Reserve (Tenerife, Canary Islands), in order to establish its current dynamics. Fog was more frequent during night through early morning and in the afternoon, and particularly from May until September, coincidental with a frequent immersion of the 1025 m a.s.l. experimental site in the cloud layer of wind‐driven stratocumulus. The concomitant meteorological conditions during different fog regimes, characterized according to visibility (Ω) ranges, were compared with those when fog was absent. The presence of fog was associated with a significant reduction in global solar radiation, Rg, increased wind speed, and lower and more stable ambient temperatures. The foggy versus fog‐free hourly medians of Rg were found to be linearly related, whereas the proportion of median Rg reduction due to fog varied logarithmically with Ω. However, foggy versus fog‐free extreme values of the hourly Rg distributions departed from such a linear trend. By contrast, hourly temperatures during foggy versus fog‐free periods behaved linearly for most of the Ω range, except for very dense fog, Ω ≤ 100 m. Transpiration of the canopy, intermittently wetted due to interception of both rain and fog water droplets, was determined by quantifying the water balance at leaf scale with a mathematical model for the two representative hypostomatous species present at the site: the arboreal shrub Erica platycodon, with needle‐like leaves, and the laurophyll tree Myrica faya. Both tree transpiration and evaporation of the intercepted fog water were predictively higher during summer. By contrast, transpiration was reduced during February, in agreement with a 1 year period of sap velocity measurements, and was not appreciably affected by soil moisture content. The consequences of an anticipated downward shift of the stratocumulus cloud layer and of various projected Representative Concentration Pathways (RCPs) scenarios in the Macaronesian area were simulated, yielding in all cases a significant rise in transpiration for both species. Particularly, the simulated RCPs scenarios implied 29%–73% increments in transpiration from the actual values. Because fog is concomitant with lower temperatures and vapour pressure deficit, the modification of its current distribution as a consequence of climate change may have a direct effect on such associated meteorological variables, and therefore a meaningful impact in the water relations of the laurel cloud forests.

Spatio-temporal transpiration patterns reflect vegetation structure in complex upland terrain

Topography exerts control on eco-hydrologic processes via alteration of energy inputs due to slope angle and orientation. Further, water availability varies with drainage position in response to topographic water redistribution and the catena effect on soil depth and thus soil water storage capacity. Our understanding of the spatio-temporal dynamics and drivers of transpiration patterns in complex terrain is still limited by lacking knowledge of how systematic interactions of energy and moisture patterns shape ecosystem state and water fluxes and adaptation of the vegetation to these patterns. To untangle the effects of slope orientation and hillslope position on forest structure and transpiration patterns, we measured forest structure, sap flux, soil moisture, throughfall and incoming shortwave radiation along two downslope transects in a forested head water catchment in south-east Australia. Our plot locations controlled for three systematically varying drainage position levels (topographic wetness index: 5.0, 6.5 and 8.0) and two levels of energy input (aridity index: 1.2 and 1.8). Vegetation patterns were generally stronger related to drainage position than slope orientation, whereas sap velocity variations were less pronounced. However, in combination with stand sapwood area, consistent spatio-temporal transpiration patterns emerged in relation to landscape position, where slope orientation was the primary and drainage position the secondary controlling factor. On short temporal scales, radiation and vapor pressure deficit were most important in regulating transpiration rates, whereas soil water limitation only occurred on shallow soils during summer. The importance of stand structural parameters increased on longer time scales, indicating optimization of vegetation in response to the long-term hydro-climatic conditions at a given landscape position. Thus, vegetation patterns can be conceptualized as a ‘time-integrated’ predictor variable that captures large fractions of other factors contributing to transpiration patterns.

Bamboo Water Transport Assessed with Deuterium Tracing

Bamboo water transport comprises the pathway rhizomes-culms-leaves as well as transfer among culms via connected rhizomes. We assessed bamboo water transport in three big clumpy bamboo species by deuterium tracing. The tracer was injected into the base of established culms, and water samples were collected from leaves of the labeled culms and from neighboring culms. From the base of labeled culms to their leaves, the average tracer arrival time across species was 1.2 days, maximum tracer concentration was reached after 1.8 days, and the tracer residence time was 5.6 days. Sap velocities were high (13.9 m d−1). Daily culm water use rates estimated by the tracer method versus rates measured by a calibrated sap flux method were highly correlated (R2 = 0.94), but the tracer estimates were about 70% higher. Elevated deuterium concentrations in studied neighbor culms point to deuterium transfer among culms, which may explain the difference in culm water use estimates. We found no differences in deuterium concentrations between neighbor-established and neighbor freshly sprouted culms of a given species. In two species, elevated concentrations in both neighbor-established and neighbor freshly sprouted culms were observed over an extended period. An applied mixing model suggests that five neighbor culms received labeled water. In contrast, for the third species, elevated concentrations in neighbor culms were only observed at the earliest sampling date after labeling. This could indicate that there was only short-term transfer and that the tracer was distributed more widely across the rhizome network. In conclusion, our deuterium tracing experiments point to water transfer among culms, but with species-specific differences.

Species-Specific Shifts in Diurnal Sap Velocity Dynamics and Hysteretic Behavior of Ecophysiological Variables During the 2015-2016 El Niño Event in the Amazon Forest.

Current climate change scenarios indicate warmer temperatures and the potential for more extreme droughts in the tropics, such that a mechanistic understanding of the water cycle from individual trees to landscapes is needed to adequately predict future changes in forest structure and function. In this study, we contrasted physiological responses of tropical trees during a normal dry season with the extreme dry season due to the 2015–2016 El Niño-Southern Oscillation (ENSO) event. We quantified high resolution temporal dynamics of sap velocity (Vs), stomatal conductance (gs) and leaf water potential (ΨL) of multiple canopy trees, and their correlations with leaf temperature (Tleaf) and environmental conditions [direct solar radiation, air temperature (Tair) and vapor pressure deficit (VPD)]. The experiment leveraged canopy access towers to measure adjacent trees at the ZF2 and Tapajós tropical forest research (near the cities of Manaus and Santarém). The temporal difference between the peak of gs (late morning) and the peak of VPD (early afternoon) is one of the major regulators of sap velocity hysteresis patterns. Sap velocity displayed species-specific diurnal hysteresis patterns reflected by changes in Tleaf. In the morning, Tleaf and sap velocity displayed a sigmoidal relationship. In the afternoon, stomatal conductance declined as Tleaf approached a daily peak, allowing ΨL to begin recovery, while sap velocity declined with an exponential relationship with Tleaf. In Manaus, hysteresis indices of the variables Tleaf-Tair and ΨL-Tleaf were calculated for different species and a significant difference (p < 0.01, α = 0.05) was observed when the 2015 dry season (ENSO period) was compared with the 2017 dry season (“control scenario”). In some days during the 2015 ENSO event, Tleaf approached 40°C for all studied species and the differences between Tleaf and Tair reached as high at 8°C (average difference: 1.65 ± 1.07°C). Generally, Tleaf was higher than Tair during the middle morning to early afternoon, and lower than Tair during the early morning, late afternoon and night. Our results support the hypothesis that partial stomatal closure allows for a recovery in ΨL during the afternoon period giving an observed counterclockwise hysteresis pattern between ΨL and Tleaf.

Finding behavioral parameterization for a 1-D water balance model by multi-criteria evaluation

Abstract Evapotranspiration is often estimated by numerical simulation. However, to produce accurate simulations, these models usually require on-site measurements for parameterization or calibration. We have to make sure that the model realistically reproduces both, the temporal patterns of soil moisture and evapotranspiration. In this study, we combine three sources of information: (i) measurements of sap velocities; (ii) soil moisture; and (iii) expert knowledge on local runoff generation and water balance to define constraints for a “behavioral” forest stand water balance model. Aiming for a behavioral model, we adjusted soil moisture at saturation, bulk resistance parameters and the parameters of the water retention curve (WRC). We found that the shape of the WRC influences substantially the behavior of the simulation model. Here, only one model realization could be referred to as “behavioral”. All other realizations failed for a least one of our evaluation criteria: Not only transpiration and soil moisture are simulated consistently with our observations, but also total water balance and runoff generation processes. The introduction of a multi-criteria evaluation scheme for the detection of unrealistic outputs made it possible to identify a well performing parameter set. Our findings indicate that measurement of different fluxes and state variables instead of just one and expert knowledge concerning runoff generation facilitate the parameterization of a hydrological model.

Does the Normalized Difference Vegetation Index explain spatial and temporal variability in sap velocity in temperate forest ecosystems?

Abstract. Understanding the link between vegetation characteristics and tree transpiration is a critical need to facilitate satellite-based transpiration estimation. Many studies use the Normalized Difference Vegetation Index (NDVI), a proxy for tree biophysical characteristics, to estimate evapotranspiration. In this study, we investigated the link between sap velocity and 30 m resolution Landsat-derived NDVI for 20 days during 2 contrasting precipitation years in a temperate deciduous forest catchment. Sap velocity was measured in the Attert catchment in Luxembourg in 25 plots of 20×20 m covering three geologies with sensors installed in two to four trees per plot. The results show that, spatially, sap velocity and NDVI were significantly positively correlated in April, i.e. NDVI successfully captured the pattern of sap velocity during the phase of green-up. After green-up, a significant negative correlation was found during half of the studied days. During a dry period, sap velocity was uncorrelated with NDVI but influenced by geology and aspect. In summary, in our study area, the correlation between sap velocity and NDVI was not constant, but varied with phenology and water availability. The same behaviour was found for the Enhanced Vegetation Index (EVI). This suggests that methods using NDVI or EVI to predict small-scale variability in (evapo)transpiration should be carefully applied, and that NDVI and EVI cannot be used to scale sap velocity to stand-level transpiration in temperate forest ecosystems.

Diurnal dynamics of phloem loading: theoretical consequences for transport efficiency and flow characteristics.

Phloem transport is the process by which plants internally distribute assimilates. The loading of assimilates near the photosynthetic source is responsible for generating enough osmotic pressure to drive sap flow towards the sink tissues where assimilates are consumed. Phloem loading is variable and subject to a diurnal cycle. It is dominated by photosynthesis during the day and by degradation of leaf starch to sugars at night. Most studies ignore the effect of the loading cycle on transport and assume that sugar flow operates at equilibrium. In this study, phloem transport was simulated for three successive days using a finite element model of time-dependent Münch-Horwitz equations. The spatial and temporal distributions of phloem pressure, sucrose concentration, sap velocity and sucrose flux were predicted for five different time variations in sucrose loading. Results showed that periodic loading induces an alternance of two distinct transport phases: one with high pressure, concentration and sucrose flux magnitudes and another with low magnitudes. In contrast, phloem water velocity remained remarkably stable. The alternating phases persisted over time and, under source-driven variation, transport did not reach steady-state conditions for the tested configuration. However, the impact of loading dynamics on transport was mitigated by pathway effects. Oscillations were not only delayed as one travelled away from the source, their amplitude was also reduced over distance. That behaviour stabilized the supply of sucrose to the sink, which continued at moderate levels during the dark cycles. This finding suggests that transport would assist night conversion of starch to sugars in the leaf to prevent carbon starvation at distant sinks in the early morning. The propagation velocity of pressure/concentration waves in phloem was predicted to vary by a factor up to 2.5 depending on the time series chosen to describe the dynamics of loading. Finally, the model predicted that up to 87% of the amount of sucrose loaded over 48 h would be unloaded under time-dependent loading, whereas only 76% would under constant-rate loading. This additional efficiency was periodic. It did not increase significantly the overall efficiency of the system but could be responsible for inducing rhythms in sink activity.

Scaling of phloem hydraulic resistance in stems and leaves of the understory angiosperm shrub Illicium parviflorum.

Recent studies in canopy-dominant trees revealed axial scaling of phloem structure. However, whether this pattern is found in woody plants of the understory, the environment of most angiosperms from the ANA grade (Amborellales-Nymphaeales-Austrobaileyales), is unknown. We used seedlings and adult plants of the understory tropical shrub Illicium parviflorum, a member of the lineage Austrobaileyales, to explore the anatomy and physiology of the phloem in their aerial parts, including changes through ontogeny. Adult plants maintain a similar proportion of phloem tissue across stem diameters, but larger conduit dimensions and number cause the hydraulic resistance of the phloem to decrease toward the base of the plant. Small sieve plate pores resulted in an overall higher sieve tube hydraulic resistance than has been reported in other woody angiosperms. Sieve elements increase in size from minor to major leaf veins, but were shorter and narrower in petioles. The low carbon assimilation rates of seedlings and mature plants contrasted with a 3-fold higher phloem sap velocity in seedlings, suggesting that phloem transport velocity is modulated through ontogeny. The overall architecture of the phloem tissue in this understory angiosperm shrub scales in a manner consistent with taller trees that make up the forest canopy. Thus, the evolution of larger sieve plate pores in canopy-dominant trees may have played a key role in allowing woody angiosperms to extend beyond their understory origins.

Contrasting water use patterns of two important agroforestry tree species in the Mt Elgon region of Uganda

ABSTRACTLack of information on water use of key agroforestry species is an obstacle to understanding their influence on crop productivity. Cordia africana and Albizia coriaria are the dominant tree species of smallholder farming systems in the Mt Elgon region of Uganda and have multiple uses in agroforestry systems. This study deployed six sap flow meters on stems of three selected trees each of C. africana and A. coriaria on-farm. The objective of the study was to assess the daily water use patterns of these agroforestry tree species at different times of the year. We measured the daily sap flow of these two species using the heat ratio method over a period of 18 months. There was a significant main effect of the interaction between tree species and season on daily water use. The two species show contrasting patterns of seasonal water use across leaf shedding stages characterised by episodes of reverse flow in A. coriaria at specific periods of the year. We propose that reverse flows in A. coriaria were triggered by leaf shading while the zero flows in C. africana, which occurred during rainfall events, could have resulted from a lag phase, an indication that the two species may have different water-use strategies. Although C. africana uses 12–15 l day−1 and A. coriaria uses 20–32 l day−1 based on the study trees, C. africana generally uses 12% more water than A. coriaria on a standardised daily basis. Albizia coriaria exhibited radial variation of sap velocities between the inner and outer thermocouples at different periods of measurement, a phenomenon worth investigating further. The leaf shedding patterns of the two trees provide an opportunity for maximising the temporal complementarities of agroforestry systems where these trees exist. This knowledge of C. africana and A. coriaria tree water use provides critical insight for developing successful long-term tree monitoring and management programs in agroforestry systems.

Determining mid-day stem water potential from sap flow measurements

Trees tend to stabilize mid-day sap velocity through apparently coordinated active canopy conductance (Gc) responses to atmospheric evaporative demand. We recently reported a model describing this stabilization based on the linear relationship between canopy resistance (Rc) and vapor pressure deficit (VPD). The model was able to accurately reproduce the daily and seasonal courses of sap flow in nectarine trees for different leaf water potentials. Here, we studied inversion of the model to determine mid-day stem water potential, Ψmd, from sap flow measurements. The study was in a nectarine orchard in the hot Hula valley of northern Israel. Measurements included sap flux, weather variables, mid-day stem water potential and other parameters. Twelve trees were measured, where four were well-watered and irrigation was withheld from eight trees for eight weeks in the hot and dry summer. Ψmd declined from -1.05 to -2 MPa, and pre-dawn Ψ from -0.42 to -0.89 MPa. Similar to previous results, we found that the slope of the linear relationship between canopy resistance and VPD increased as Ψmd decreased in a monotonic manner, which was described by a linear relationship. Slopes of the relationship of canopy resistance to VPD varied linearly from 1.2 to 2.2 s mm-1 kPa-1 over the range of Ψmd values. Re-calculation of Ψmd using the relationship was very successful, giving an r2 of 0.83 between calculated and measured values.

Dry and hot: the hydraulic consequences of a climate change-type drought for Amazonian trees.

How plants respond physiologically to leaf warming and low water availability may determine how they will perform under future climate change. In 2015-2016, an unprecedented drought occurred across Amazonia with record-breaking high temperatures and low soil moisture, offering a unique opportunity to evaluate the performances of Amazonian trees to a severe climatic event. We quantified the responses of leaf water potential, sap velocity, whole-tree hydraulic conductance (Kwt), turgor loss and xylem embolism, during and after the 2015-2016 El Niño for five canopy-tree species. Leaf/xylem safety margins (SMs), sap velocity and Kwt showed a sharp drop during warm periods. SMs were negatively correlated with vapour pressure deficit, but had no significant relationship with soil water storage. Based on our calculations of canopy stomatal and xylem resistances, the decrease in sap velocity and Kwt was due to a combination of xylem cavitation and stomatal closure. Our results suggest that warm droughts greatly amplify the degree of trees' physiological stress and can lead to mortality. Given the extreme nature of the 2015-2016 El Niño and that temperatures are predicted to increase, this work can serve as a case study of the possible impact climate warming can have on tropical trees.This article is part of a discussion meeting issue 'The impact of the 2015/2016 El Niño on the terrestrial tropical carbon cycle: patterns, mechanisms and implications'.

Diurnal and seasonal coupling of conifer sap flow and vapour pressure deficit across topoclimatic gradients in a subalpine catchment

AbstractThe hydroclimatic controls on transpiration often follow hillslope‐ or catchment‐scale topography in water‐limited ecosystems; however, rates of transpiration may deviate from these patterns due to microtopographic variation in environmental conditions or physiology. Here, we assessed the microtopographic effects on water use of five conifer species within four subcatchments in western Montana along a water availability gradient that was driven by aspect and elevation. To infer physiological processes at both diurnal and seasonal time scales, we analysed the relationship between sap velocity, Vs, and vapour pressure deficit, D, using instantaneous (half‐hourly) and aggregated (daily mean) values of Vs and D. Both within and across species, daily mean Vs was more tightly coupled to D at higher elevation sites (1,720 m), whereas trees 350 m lower in elevation became decoupled from D as snowmelt‐derived soil moisture declined. At the diurnal scale, we found that the degree of decoupling of Vs from D during soil moisture deficits decreased when a time lag between Vs and D was considered. Additionally, contrary to the common inference of plant hydraulic capacitance based on Vs lagging behind D at the diurnal scale, half‐hourly Vs tended to lead D in three of the five conifer species we studied. Predawn and midday branch water potential measurements provided additional evidence that topography influenced plant water status. These results suggest that improved understanding of soil–vegetation–atmosphere coupling in mountainous terrain requires further inquiry into the spatial variability of plant hydraulic regulation.

Plant water use responses along secondary forest succession during the 2015–2016 El Niño drought in Panama

Tropical forests are increasingly being subjected to hotter, drier conditions as a result of global climate change. The effects of drought on forests along successional gradients remain poorly understood. We took advantage of the 2015-2016 El Niño event to test for differences in drought response along a successional gradient by measuring the sap flow in 76 trees, representing 42 different species, in 8-, 25- and 80-yr-old secondary forests in the 15-km2 'Agua Salud Project' study area, located in central Panama. Average sap velocities and sapwood-specific hydraulic conductivities were highest in the youngest forest. During the dry season drought, sap velocities increased significantly in the 80-yr-old forest as a result of higher evaporative demand, but not in younger forests. The main drivers of transpiration shifted from radiation to vapor pressure deficit with progressing forest succession. Soil volumetric water content was a limiting factor only in the youngest forest during the dry season, probably as a result of less root exploration in the soil. Trees in early-successional forests displayed stronger signs of regulatory responses to the 2015-2016 El Niño drought, and the limiting physiological processes for transpiration shifted from operating at the plant-soil interface to the plant-atmosphere interface with progressing forest succession.