Patterns Of Sap Flow Research Articles

A Copula Approach for Predicting Tree Sap Flow Based on Vapor Pressure Deficit

While using sap-flow sensor measurements is a well-established technique for quantifying leaf water transpiration in tree species, installing and maintaining a large number of sensors and data loggers in large-scale plantations to obtain accurate measurements is both costly and time-consuming. We developed a copula-based approach to predict sap flows based on readily available vapor pressure deficits (VPDs) and found that the Normal copula was the best among five commonly used copulas. The Normal-copula approach was validated using our field-measured eastern cottonwood (Populus deltoides (Bartr. ex Marsh.)) sap flow data, yielding solid statistical measures, including Mann–Kendall’s τ = 0.59, R2 = 0.81, and p-value < 0.01. The approach was applied to predict sap flows of eastern cottonwood during the growing period from 1 March to 31 October 2015 as well as the 5-year growing period from 2019 to 2023. It successfully replicated the characteristic diurnal sap flow pattern, with rates increasing during the day and decreasing at night, as well as the typical seasonal pattern, with rates rising from winter to summer and decreasing from summer to next winter. Our study suggests that the copula-based approach is a reliable tool for estimating sap flows based on VPD data.

Large variation in the radial patterns of sap flow among urban trees

Quantifying sap flow patterns along the sapwood radius is necessary for accurate up-scaling and estimation of tree and stand transpiration based on sap flow measurements. However, little information on radial patterns of sap flow exists in urban environments where precise estimates of tree water uptake and transpiration are important to quantify ecosystem services and plan tree management. Because of the heterogeneous conditions in urban environment, upscaling approaches specific to urban environments and species may be required to estimate whole-tree transpiration.We measured sap flow at four sapwood depths for Acer platanoides, Acer saccharinum and Tilia cordata, in two contrasting urban environments (site types): parks and streets, in Montreal, Canada. We then estimated the potential importance of tree-to-tree and temporal variation of radial patterns by comparing generalized additive models (GAM) with or without tree ID or a temporal effect, and using the models to upscale sap flow measurements conducted at the sapwood surface to tree-level daily water use estimates.The radial pattern of sap flow diverged between park and street trees, with a slightly more monotone pattern in park than street trees in the two Acer species. However, tree-to-tree variation was large even within a given species and site type, and partly associated with tree size. The radial pattern of sap flow varied in time in five of the twelve trees that were measured twice over the growing season. Sap flow in deeper layers relative to the sapwood surface correlated with vapor pressure deficit, solar radiation and the sap flow density at the sapwood surface. When comparing the two sources of uncertainty (tree-to-tree and temporal variation) in upscaling daily water use, a larger uncertainty was associated with tree-to-tree variation than with temporal variation. Thus, understanding the inter-tree variability in radial patterns of sap flow patterns should be prioritized in future studies.

Sap flow of two typical woody halophyte species responding to the meteorological and irrigation water conditions in Taklimakan Desert

Understanding plant water consumption is crucial for artificial afforestation under drought environments and water stress in desert regions. However, the water consumption characteristics of desert species responding to the irrigation regimes are often neglected. By conducting a field test in the Taklimakan Desert Highway shelterbelt, this study examines the sap flow traits of two typical woody halophyte species (Calligonum mongolicum and Haloxylon ammodendron) and how they react to weather conditions and watering practices. Under the same irrigation treatment, the stem flux of C. mongolicum on sunny days was 1.5–5.3 times that on dusty days, while the stem flux of H. ammodendron on sunny days was 3.5–5.5 times that on dusty days. Both species demonstrated some sap flow during the night, representing 14.3%–24.9% and 7.3%–10.4% of the total sap flow for C. mongolicum and H. ammodendron, respectively. H. ammodendron maintained a higher stem flow during daytime and was more drought resistant than C. mongolicum. The daily sap flow patterns of these two species varied, showing both ‘single’ and ‘double peak’ curves depending on the watering conditions. A delay was also observed between the sap flow of these two species and the environmental factors. The factors influencing plant sap flow were found to be in the order of solar radiation, temperature, relative humidity, and saturated water vapor pressure difference. A BP-neural network proved highly effective for accurately simulating the sap flow of these two species. This research provides insights into how two common desert tree species adapt their water use in response to drought conditions, which is vital for artificial forest creation in desert areas.

Environmental Drivers of Gross Primary Productivity and Light Use Efficiency of a Temperate Spruce Forest

AbstractVarious environmental variables drive gross primary productivity (GPP) and light use efficiency (LUE) of forest ecosystems. However, due to their intertwined nature and the complexity of measuring absorbed photosynthetically active radiation (APAR) of forest canopies, the assessment of LUE and the importance of its environmental drivers are difficult. Here, we present a unique combination of measurements during the 2021 growing season including eddy covariance derived GPP, sap flow, Sentinel‐2 derived canopy chlorophyll content and in situ measured APAR. The importance of environmental variables for GPP models is quantified with state‐of‐the‐art machine learning techniques. A special focus is put on photosynthesis‐limiting conditions, which are identified by a comparison of GPP and sap flow hysteretic responses to Vapor pressure deficit (VPD) and APAR. Results demonstrate that (a) LUE of the canopy's green part was on average 4.0% ± 2.3%, (b) canopy chlorophyll content as a seasonal variable for photosynthetic capacity was important for GPP predictions, and (c) on days with high VPD, tree‐scale sap flow and ecosystem‐scale GPP both shift to a clockwise hysteretic response to APAR. We demonstrate that the onset of such a clockwise hysteretic pattern of sap flow to APAR is a good indicator of stomatal closure related to water‐limiting conditions at the ecosystem‐scale.

Radial and seasonal variation of sap flow and its response to meteorological factors in sandy Pinus sylvestris var. mongolica plantations in the Three North Shelterbelt of China

Mongolian Scots pine (Pinus sylvestris var. mongolica) is one of the main afforestation tree species in the Three North shelterbelt project in China, and an accurate estimation of its transpiration is of importance for plantation scientific density management especially under future climate change scenarios. While radial variation in tree species sap flow is an important factor for estimating transpiration, it is usually neglected. Thus, this study selected Mongolian Scots pine of different ages (10a, 20a, 30a, 40a) to study its radial variation pattern of sap flow using the multipoint (probes of different specifications) thermal dissipation method; additionally, its response characteristics to meteorological factors in northeast China were also discussed. The results show that the radial distribution of the sap flux density varied with age and growing season. It gradually decreased from the outside to the inside of the sapwood of the 10a stand. In contrast, the sap flux densities of the 20a, 30a and 40a stands had all typical single-peak patterns, with the highest value at 15 mm from the outside of the sapwood, and then sharply decreased with increasing distance. The sap flux density in both the 10a and 20a stands slightly changed, while it a significantly change in both the 30a and 40a stands between the middle and end of the growing season occurred. Further estimation of transpiration by single point and multipoint analyses was undertaken. The single point analysis resulted in large errors of -18.81%-11.02%, -52.44%-133.47%, -10.26%-130.38% and -48.78%-162.11% in the 10a, 20a, 30a and 40a stands, respectively. However, Js15mm was significantly correlated with the mean sap flux density of a sapwood area (Jsmean) and exhibited a constant conversion coefficient. The sap flux densities of separate sapwood depths were significantly affected by meteorological factors and varied according to different meteorological factors.

Temporal Variation and Hysteresis of Soil Respiration and Sap Flow of Pinus densiflora in a Cool Temperate Forest, Japan

Soil respiration (Rs) consists of autotrophic (Ra) and heterotrophic (Rh) respiration, and the metabolic responses of Ra and Rh are supposedly affected by environmental factors. Our hypothesis was that the contribution of Ra and Rh would be affected by seasons. To characterize seasonal patterns of Rs, sap flow (as an indicator of photosynthetic activity), and environmental factors, we continuously measured temporal variation in Rs using an automated opening and closing chamber system in a cool temperate forest. Rs had counterclockwise hysteresis (Rs: spring < autumn) with soil temperature at a depth of 5 cm. Daily maximum Rs had a significant positive relationship with daily maximum sap flow over three seasons, and daily maximum sap flow was lower in autumn than in spring (Ra: spring > autumn). The amount of leaf litterfall increased significantly from August to October; the high Rs in autumn would be due to an increase in Rh (Rh: spring < autumn). These results suggest that Ra contributes more in spring than in autumn, and the contribution of Rh is high from summer through autumn.

Bark and sapwood water storage and the atypical pattern of recharge and discharge of water reservoirs indicate low vulnerability to drought in Araucaria araucana.

Stored water in inner tissues influences the plant water economy, which might be particularly relevant for trees facing increasing dry conditions due to climate change. We studied the water storage in the inner bark and the sapwood of Araucaria araucana (Molina) K. Koch. This species has an extremely thick inner bark and thus it can be used as a model system to assess the impact of internal water storage on plant water balance. Specifically, we analyzed the water circulation pathways in and out of the elastic water storages by using simultaneously frequency domain moisture sensors and dendrometers inserted in the inner bark and in the sapwood, and sap flow determinations during the dry season. The daily patterns of water content and expansion and contraction of the stem tissues were similar to the sap flow pattern. The whole-stem water content and diameter increased in the morning and decreased in the afternoon, contrary to the typical pattern observed in most tree species. An osmotic gradient favoring the water influx from sapwood to inner bark was observed in the morning. There were no lags in the onset of sap flow between different stem heights at the time that recharge of reservoirs occurred. Sap flow at 6m height was higher than basal sap flow in the afternoon, when the sapwood water content started to decline followed by the water content of the inner bark. Inner bark and sapwood contributed 5-11% to total daily transpiration, allowing the maintenance of high water potentials in the dry season. Our results suggest that the stored water in the stems, the atypical dynamic of recharge and discharge of water from reservoirs and the high tissue capacitance may make an important contribution to the survival of A. araucana during drought periods by maintaining the water balance.

Towards Continuous Stem Water Content and Sap Flux Density Monitoring: IoT-Based Solution for Detecting Changes in Stem Water Dynamics

Taking advantage of novel IoT technologies, a new multifunctional device, the “TreeTalker”, was developed to monitor real-time ecophysical and biological parameters of individual trees, as well as climatic variables related to their surrounding environment, principally, air temperature and air relative humidity. Here, IoT applied to plant ecophysiology and hydrology aims to unravel the vulnerability of trees to climatic stress via a single tree assessment at costs that enable massive deployment. We present the performance of the TreeTalker to elucidate the functional relation between the stem water content in trees and respective internal/external (stem hydraulic activity/abiotic) drivers. Continuous stem water content records are provided by an in-house-designed capacitance sensor, hosted in the reference probe of the TreeTalker sap flow measuring system, based on the transient thermal dissipation (TTD) method. In order to demonstrate the capability of the TreeTalker, a three-phase experimental process was performed including (1) sensor sensitivity analysis, (2) sensor calibration, and (3) long-term field data monitoring. A negative linear correlation was demonstrated under temperature sensitivity analysis, and for calibration, multiple linear regression was applied on harvested field samples, explaining the relationship between the sample volumetric water content and the sensor output signal. Furthermore, in a field scenario, TreeTalkers were mounted on adult Fagus sylvatica L. and Quercus petraea L. trees, from June 2020 to October 2021, in a beech-dominated forest near Marburg, Germany, where they continuously monitored sap flux density and stem volumetric water content (stem VWC). The results show that the range of stem VWC registered is highly influenced by the seasonal variability of climatic conditions. Depending on tree characteristics, edaphic and microclimatic conditions, variations in stem VWC and reactions to atmospheric events occurred. Low sapwood water storage occurs in response to drought, which illustrates the high dependency of trees on stem VWC under water stress. Consistent daily variations in stem VWC were also clearly detectable. Stem VWC constitutes a significant portion of daily transpiration (using TreeTalkers, up to 4% for the beech forest in our experimental site). The diurnal–nocturnal pattern of stem VWC and sap flow revealed an inverse relationship. Such a finding, still under investigation, may be explained by the importance of water recharge during the night, likely due to sapwood volume changes and lateral water distribution rather than by a vertical flow rate. Overall, TreeTalker demonstrated the potential of autonomous devices for monitoring sap density and relative stem VWC in the field of plant ecophysiology and hydrology.

Dry Season Transpiration and Soil Water Dynamics in the Central Amazon.

With current observations and future projections of more intense and frequent droughts in the tropics, understanding the impact that extensive dry periods may have on tree and ecosystem-level transpiration and concurrent carbon uptake has become increasingly important. Here, we investigate paired soil and tree water extraction dynamics in an old-growth upland forest in central Amazonia during the 2018 dry season. Tree water use was assessed via radial patterns of sap flow in eight dominant canopy trees, each a different species with a range in diameter, height, and wood density. Paired multi-sensor soil moisture probes used to quantify volumetric water content dynamics and soil water extraction within the upper 100 cm were installed adjacent to six of those trees. To link depth-specific water extraction patterns to root distribution, fine root biomass was assessed through the soil profile to 235 cm. To scale tree water use to the plot level (stand transpiration), basal area was measured for all trees within a 5 m radius around each soil moisture probe. The sensitivity of tree transpiration to reduced precipitation varied by tree, with some increasing and some decreasing in water use during the dry period. Tree-level water use scaled with sapwood area, from 11 to 190 L per day. Stand level water use, based on multiple plots encompassing sap flow and adjacent trees, varied from ∼1.7 to 3.3 mm per day, increasing linearly with plot basal area. Soil water extraction was dependent on root biomass, which was dense at the surface (i.e., 45% in the upper 5 cm) and declined dramatically with depth. As the dry season progressed and the upper soil dried, soil water extraction shifted to deeper levels and model projections suggest that much of the water used during the month-long dry-down could be extracted from the upper 2–3 m. Results indicate variation in rates of soil water extraction across the research area and, temporally, through the soil profile. These results provide key information on whole-tree contributions to transpiration by canopy trees as water availability changes. In addition, information on simultaneous stand level dynamics of soil water extraction that can inform mechanistic models that project tropical forest response to drought.

Some information obtained from diurnal pattern of sap flow

Some information obtained from diurnal pattern of sap flow

Prediction model for sap flow in cacao trees under different radiation intensities in the western Colombian Amazon

In this study, we measured diurnal patterns of sap flow (Vs) in cacao trees growing in three types of agroforestry systems (AFs) that differ in the incident solar radiation they receive. We modeled the relationship of Vs with several microclimatic characteristics of the AFs using mixed linear models. We characterized microclimatic variables that may have an effect on diurnal patterns of sap flow: air relative humidity, air temperature, photosynthetically active radiation and vapor pressure deficit. Overall, our model predicted the differences between cacao Vs in the three different AFs, with cacao plants with dense Musaceae plantation and high mean diurnal incident radiation (HPAR) displaying the highest differences compared to the other agroforestry arrangements. The model was also able to predict situations such as nocturnal transpiration in HPAR and inverse nocturnal sap flows indicative of hydraulic redistribution in the other AFs receiving less incident radiation. Overall, the model we present here can be a useful and cost-effective tool for predicting transpiration and water use in cacao trees, as well as for managing cacao agroforestry systems in the Amazon rainforest.

Response of linden tree to nocturnal simulation of daylight conditions

Although the importance of the internal clock for plant development and fitness was extensively studied in the last few decades, the majority of work was conducted in laboratory conditions using some small plants as model species. The objective of this study was to determine the response of sap flow to the circadian regulation physiological state of the entire tree. Monitoring a variety of relevant parameters and sap flow in different tree organs, we illuminated the crown of a linden tree, Tilia cordata, with artificial daylight mimicking solar conditions during three consecutive nights. The patterns of stimulated nocturnal sap flow were compared to sap flow patterns observed during daytime. The response of sap flow to light stimuli was weak compared to daytime and diminished further as night progresses. Light induced sap flow during the night was very low and the responses to changes in light intensity were highly non-linear and sluggish. While sap flow correlated directly with changes in VPD during the day, it did not respond to occasional fluctuations in VPD at night. The results convincingly demonstrated the effectiveness of the internal regulation at the level of the entire large tree through gating of external stimuli.

Assimilate, process and analyse thermal dissipation sap flow data using the TREX r package

Abstract A key ecophysiological measurement is the flow of water (or sap) along the tree's water‐transport system, which is an essential process for maintaining the hydraulic connection within the soil–plant–atmosphere continuum. The thermal dissipation method (TDM) is widespread in the scientific community for measuring sap flow and has provided novel insights into water use and its environmental sensitivity, from the tree‐ to the forest‐stand level. Yet, methodological approaches to determine sap flux density (SFD) from raw TDM measurements remain case‐specific, introducing uncertainties and hampering data syntheses and meta‐analyses. Here, we introduce the r package TREX (TRee sap flow EXtractor), incorporating a wide range of sap flow data‐processing procedures to quantify SFD from raw TDM measurements. TREX provides functions for (a) importing and assimilating raw measurements, (b) data quality control and filtering and (c) calculating standardized SFD outputs and their associated uncertainties according to different data‐processing methods. A case study using a Norway spruce tree illustrates TREX's functionalities, featuring interactive data curation and generating outputs in a reproducible and transparent way. The calculations of SFD in TREX can, for instance, use the original TDM calibration coefficients, user‐supplied calibration parameters or calibration data from a recently compiled database of 22 studies and 37 species. Moreover, the package includes an automatic procedure for quantifying the sensitivity and uncertainty of the obtained results to user‐defined assumptions and parameter values, by means of a state‐of‐the‐art global sensitivity analysis. Time series of plant ecophysiological measurements are becoming increasingly available and enhance our understanding of climate change impacts on tree functioning. TREX allows for establishing a baseline for data processing of TDM measurements and supports comparability between case studies, facilitating robust, transparent and reproducible large‐scale syntheses of sap flow patterns. Moreover, TREX facilitates the simultaneous application of multiple common data‐processing approaches to convert raw data to physiological relevant quantities. This allows for robust quantification of the impact (i.e. sensitivity and uncertainty) of user‐specific choices and methodological assumptions, which is necessary for process understanding and policy making.

Optimisation of CAG technique: An algorithm to estimate low and reverse sap flow in roots

The present study aims at developing a specific algorithm to automatize the calibrated average-gradient (CAG) heat pulse method and estimating low and reverse sap flow in roots, termed as CAGALG. The innovative algorithm is based on an automatic data processing to make easier the complexity in interpreting data from sap flow sensors, without losing the reliability on low and even reverse sap flows quantification. The CAGALG algorithm resorts to two mathematical tools to indicate reverse sap flow occurrence and clustering the roots per reverse sap flow patterns. Results showed a good correlation between the average temperature gradient (ΔTavg) and the corrected heat pulse velocity (VC) that was confirmed by the simulation study. The simulation provided a model aiming to estimate the thermal diffusivity, which has been shown to be below the default values usually attributed. Quantitative and temporal reverse sap flow occurrences were identified along the two years in which presented the results from the CAGALG in accordance with the traditional CAG.

Analytical approach extending the Granier method to radial sap flow patterns

The Granier thermal dissipation (TD) method is probably the most applied method to compute the transpiration flux of trees, due to its simplicity and effective compromise between theory and data availability. Starting from the heat transfer equations at the basis of Granier’s method, the objective of this paper is to derive an analytical solution for the transpiration flux to extend the sap flow equations to the radial domain. We adopted a flexible approach to cope with the differences in radial sapflow density (SFD) profile shapes that are known to occur in relation to wood anatomy (diffuse porous vs. ring- or non-porous xylem). With this purpose, we investigated the robustness of the equations developed on some experimental and reliable radial SFD measurements available in literature to test the influence of considering or not considering the active zone close to the cambium, where most of the species-specific differences are likely to be observed. Moreover, the parameters derived by the extended formulation, are interpreted as descriptive of species-specific radial sap flow patterns. The reliability of the suggested procedure was checked against several experimental SFD profiles from literature: i) monotonically increasing SFD from the centre of the stem towards the cambium, ii) increasing SFD from the centre of the stem and then constant SFD towards the cambium, and iii) increasing SFD from the centre of the stem to a maximum SFD and then decreasing towards the cambium. Results show that according to the suggested procedure, an increasing number of parameters depending on the SFD profile complexity are required to synthetically describe the transpiration flux of different tree species. For the simplest case of monotonically increasing SFD, which could be assumed as standard under conditions of a diffuse porous tree structure, only two parameters with a clear physical meaning are required.

Sap flow of Salix psammophila and its principal influencing factors at different slope positions in the Mu Us desert.

The changes in sap flow of Salix psammophila growing on a gentle slope (lower slope, P1), a middle slope (P2), and an upper slope (P3), and the response of sap flow to meteorological factors at the different slope positions were studied using the continuous and synchronized observations, the instrument were wrapped stem flowmeter EMS 62 sap-flow heat-balance-based system and the LSI-LASTEM automatic weather station. The results revealed that the soil moisture content was the highest and the growth conditions of Salix psammophila were the best at P1, followed by P2. At P3, however, although good apical dominance was observed, the proportion of dead branches was the highest. Furthermore, the daily variation patterns of sap flow on the three slopes presented as multi-peak bell-shaped curves. The daily accumulation changes in sap flow showed a trend of P1 > P3 > P2, and within the same diameter range, the sap flow at P1 was significantly different from that at P2 and P3, whereas the sap flow at P2 and P3 did not vary significantly. All the three slopes showed a significant and positive correlation with photosynthetically active radiation, atmospheric temperature, and vapor pressure difference, and a significant and negative correlation with relative humidity; however, the degrees of correlation varied slightly. The stepwise regression analysis showed that, at different slopes, different variables were selected for different branch diameters, but photosynthetically active radiation and atmospheric temperature played dominant roles on all slopes. This study reveals the sap flow pattern of Salix psammophila on different slopes and its response mechanism to meteorological factors, which was essential for understanding the restoration ability, physiological adaptability, and ecosystem stability of Salix psammophila communities.

Hydraulic Coupling of a Leafless Kauri Tree Remnant to Conspecific Hosts.

SummaryTrees are commonly regarded as distinct entities, but the roots of many species fuse to form natural root grafts allowing the exchange of water, carbon, mineral nutrients, and microorganisms between individuals. Exploiting the phenomenon of leafless (photosynthetically inactive) tree remnants being kept alive by conspecifics, we show tight physiological coupling of a living kauri (Agathis australis) stump to conspecific neighbors. The trunk remnant displayed greatly reduced, inverted daily sap flow patterns compared with intact kauri trees. Its stem water potential showed strong diel variation with minima during daytime and maxima at night, coinciding with peak and minimal sap flow rates in neighbors, respectively. Sudden atmospherically driven changes in water relations in adjacent kauri trees were very rapidly and inversely mirrored in the living stump's water status. Such intimate hydrological coupling suggests a “communal physiology” among (conspecific) trees with far-reaching implications for our understanding of forest functioning, particularly under water shortage.

Changes in xylem conducting capacity and water storage across species: how can variable air content of xylem cells affect sap flow?

Sap flow sensors and other techniques are commonly used across species and plant organs to quantify water use and storage, detect stress, and evaluate the contribution of various tissues to plant/organ water balance. Sap flow methods often rely upon modelling or assumptions about how heat delivered by the sap flow sensors is partitioned into convection and conduction into active sapwood xylem and surrounding tissues. Dynamic changes in tissue water content over space and time can impact the interpretation of plant and organ water use and how various compartments contribute to an integrated response to plant stress. Here, we first summarize results from a variety of studies that used a combination of synchrotron-based X-ray microCT and MRI imaging to demonstrate how water content of various organs and xylem cell types can change temporally and how the spatial distribution of air-filled tissue may impact patterns of sap flow within the xylem network. Results from visualization techniques were compared to that from traditional hydraulic and sap flow methods to illustrate potential discrepancies particularly when comparing data from excised stems versus intact plants. Using a spatially explicit model, we demonstrate how changes in the water content of various cell types can impact resulting interpretation of sensor output. Implications for the interpretation of sap flow and other sensor data based on these results is discussed.

Nocturnal sap flow of Hedysarum scoparium and its response to meteorological factors in semiarid Northwest China

Nocturnal sap flow of shrub may indicate transpiration and water recharge storage at night. Hedysarum scoparium is an important sand fixation shrub in semiarid regions. This paper deals with the water use of Hedysarum scoparium at night in the semiarid Northwest China. Our primary goal was to reveal and understand the nature of nocturnal sap flow and its functional significance. Granier’s stem heat balance method was used to determine the nocturnal sap flow of Hedysarum scoparium (stem of 25, 16, 13 and 9mm). Nocturnal sap flow was substantial and showed seasonal variation similar to the patterns of daytime sap flow. Mean nocturnal sap flow was higher in more precipitation months of year 2011 for more daytime transpiration and high soil water availability which can be the cause of more nocturnal sap flow. And vapour deficit pressure, mean air temperature and wind velocity were significantly correlated with nocturnal sap flow, they could explain a small fraction of the variance in nocturnal sap flow of studied trees. And the meteorological factors have greater impact on nocturnal sap flow rate of Hedysarum scoparium with smaller stem diameter than other diameter classes of Hedysarum scoparium.

Anatomical explanations for acute depressions in radial pattern of axial sap flow in two diffuse-porous mangrove species: implications for water use.

Mangrove species have developed uniquely efficient water-use strategies in order to survive in highly saline and anaerobic environments. Herein, we estimated the stand water use of two diffuse-porous mangrove species of the same age, Sonneratia apetala Buch. Ham and Sonneratia caseolaris (L.) Engl., growing in a similar intertidal environment. Specifically, to investigate the radial patterns of axial sap flow density (Js) and understand the anatomical traits associated with them, we measured axial sap flow density in situ together with micromorphological observations. A significant decrease of Js was observed for both species. This result was accompanied by the corresponding observations of wood structure and blockages in xylem sapwood, which appeared to influence and, hence, explained the acute radial reductions of axial sap flow in the stems of both species. However, higher radial resistance in sapwood of S. caseolaris caused a steeper decline of Js radially when compared with S. apetala, thus explaining the latter's more efficient use of water. Without first considering acute reductions in Js into the sapwood from the outer bark, a total of ~55% and 51% of water use would have been overestimated, corresponding to average discrepancies in stand water use of 5.6 mm day-1 for S. apetala trees and 2.5 mm day-1 for S. caseolaris trees. This suggests that measuring radial pattern of Js is a critical factor in determining whole-tree or stand water use.