Tree Water Research Articles

A multifunctional matching algorithm for sample design in agricultural plots

Collection of accurate and representative data from agricultural fields is required for efficient crop management. Since growers have limited available resources, there is a need for advanced methods to select representative points within a field in order to best satisfy sampling or sensing objectives. The main purpose of this work was to develop a data-driven method for selecting locations across an agricultural field given observations of some covariates at every point in the field. These chosen locations should be representative of the distribution of the covariates in the entire population and represent the spatial variability in the field. They can then be used to sample an unknown target feature whose sampling is expensive and cannot be realistically done at the population scale.An algorithm for determining these optimal sampling locations, namely the multifunctional matching (MFM) criterion, was based on matching of moments (functionals) between sample and population. The selected functionals in this study were standard deviation, mean, and Kendall’s tau. An additional algorithm defined the minimal number of observations that could represent the population according to a desired level of accuracy. The MFM was applied to datasets from two agricultural plots: a vineyard and a peach orchard. The data from the plots included measured values of slope, topographic wetness index, normalized difference vegetation index, and apparent soil electrical conductivity. The MFM algorithm selected the number of sampling points according to a representation accuracy of 90% and determined the optimal location of these points. The algorithm was validated against values of vine or tree water status measured as crop water stress index (CWSI). Algorithm performance was then compared to two other sampling methods: the conditioned Latin hypercube sampling (cLHS) model and a uniform random sample with spatial constraints. Comparison among sampling methods was based on measures of similarity between the target variable population distribution and the distribution of the selected sample.MFM represented CWSI distribution better than the cLHS and the uniform random sampling, and the selected locations showed smaller deviations from the mean and standard deviation of the entire population. The MFM functioned better in the vineyard, where spatial variability was larger than in the orchard. In both plots, the spatial pattern of the selected samples captured the spatial variability of CWSI. MFM can be adjusted and applied using other moments/functionals and may be adopted by other disciplines, particularly in cases where small sample sizes are desired.

Prediction of Olive Tree Water Requirements under Limited Soil Water Availability, Based on Sap Flow Estimations

The forecast of irrigation requirements in commercial olive orchards in the era of climate change is at the forefront of scientific research. Simplified models that are based on monitoring soil and plant water status, along with microclimatic variables are well established. In the present study, an attempt was made to correlate the olive tree sap fluxes and the theoretical grass water losses, as expressed by reference evapotranspiration (ETo) in mild to moderate water-stress conditions. The water flow in the soil–plant–air continuum was monitored using soil water and thermal dissipation probes (TDP), which have a comparatively low cost and satisfactory reliability, while microclimatic variability was monitored by a meteorological station placed within the experimental orchard. The assessment of water stress was conducted via a stress coefficient (Ks), which was determined according to soil water availability, and validated with measurements of pre-dawn water potential and stomatal conductance. The results suggest the existence of an exponential correlation (R2 = 0.869) between daily plant transpirational losses and reference evapotranspiration, while the methodology’s applicability is verified by the validation process (R2 = 0.804 and RMSE = 0.579 L per day).

Isotopic fractionation from deep roots to tall shoots: A forensic analysis of xylem water isotope composition in mature tropical savanna trees

Studies of plant water sources generally assume that xylem water integrates the isotopic composition (δ2H and δ18O) of water sources and does not fractionate during uptake or transport along the transpiration pathway. However, woody xerophytes, halophytes, and trees in mesic environments can show isotopic fractionation from source waters. Isotopic fractionation and variation in isotope composition can affect the interpretation of tree water sources, but most studies to date have been greenhouse experiments. Here we present a field-based forensic analysis of xylem water isotope composition for 12 Eucalyptus tetrodonta and Corymbia nesophila trees. We used a 25-tonne excavator to access materials from the trees' maximum rooting depth of 3 m to their highest canopies at 38 m. Substantial within-tree variation occurred in δ2H (−91.1‰ to −35.7‰ E. tetrodonta; −88.8‰ to −24.5‰ C. nesophila) and δ18O (−12.3‰ to −5.0‰ E. tetrodonta; −10.9‰ to −0.3‰ C. nesophila), with different root-to-branch isotope patterns in each species. Soil water δ2H and δ18O dual isotope slopes (7.26 E. tetrodonta, 6.66 C. nesophila) were closest to the Local Meteoric Water Line (8.4). The dual isotope slopes of the trees decreased progressively from roots (6.45 E. tetrodonta, 6.07 C. nesophila), to stems (4.61 E. tetrodonta, 5.97 C. nesophila) and branches (4.68 E. tetrodonta, 5.67 C. nesophila), indicative of fractionation along the xylem stream. Roots of both species were more enriched in 2H and 18O than soil water at all sampled depths. Bayesian mixing model analysis showed that estimated proportions of water sourced from different depths reflected the contrasting root systems of these species. Our study adds evidence of isotopic fractionation from water uptake and along the transpiration stream in mature trees in monsoonal environments, affecting the interpretation of water sources. We discuss the findings with view of interpreting aboveground xylem water isotopic composition, incorporating knowledge of root systems.

Water-use patterns of Chinese wolfberry (Lycium barbarum L.) on the Tibetan Plateau

Chinese wolfberry (Lycium barbarum L.) can efficiently ameliorate land deterioration and increase farmers’ incomes on the Tibetan Plateau. Therefore, it has been widely grown in this region in the past decades. The aims of this study were to clarify the patterns of water sources and water use efficiency under 3 management practices to determine the optimal cultivation strategies. A 2-year field experiment was undertaken in a Chinese wolfberry plantation with 3 management practices, including the conventional flat planting plus surface drip irrigation (CK), flat planting with full-film mulching plus surface drip irrigation (MF) and ridge-furrow full-film mulching plus surface drip irrigation (MR). The soil moisture in shallow (0–20 cm), middle (20–60 cm) and deep (60–100 cm) soil layers were regarded as the trees’ potential water sources. The IsoSource model and two Bayesian mixing models of MixSIR and MixSIAR were employed to calculate the contribution of different water sources to xylem water. The MixSIR model exhibited relatively better performance in quantifying water source contribution for different layers compared with the IsoSource and MixSIAR models. Management practices significantly altered water use patterns of the wolfberry during the growing periods. Under CK the wolfberry preferentially extracted moisture from the middle and deep layers even during rainfall and irrigation. Under MF and MR they switched more flexibly their water source between the three layers; and they used more water from shallow and middle layers when soil moisture availability increased there, which was especially true under MR. Compared with CK, the average yield of MR and WUE were found to increase by 21.5% and 17%, respectively, over the 2-years period. This indicated that film mulching and ridge-furrow altered the water use strategy of Chinese wolfberry and WUE, which can inform the designing of the best management regimes. The response to tree water use in terms of soil nutrients and subsurface irrigation should be investigated to optimize field management practices, including irrigation schedules and modes.

Processing and Extraction of Seasonal Tree Physiological Parameters from Stem Radius Time Series

Radial stem size changes, measured with automated dendrometers at intra-daily resolution, offer great potential to link environmental conditions with tree physiology at the seasonal scale. Such measurements need to be time-aligned, cleaned of outliers and shifts, gap-filled and analysed for reversible (water-related) and irreversible (growth-related) fractions to obtain physiologically meaningful data. Therefore, comprehensive tools are needed for reproducible data processing and analytics of dendrometer data. Here we present a transparent method, compiled in the R package treenetproc, to turn raw dendrometer data into clean, physiologically interpretable information, i.e., stem growth, tree water deficit, growth phenological phases, mean daily shrinkage and their respective timings. The removal of errors is facilitated by additional functions and supported with graphical visualizations. To ensure reproducible data handling, the processing parameters and induced changes to the raw data are documented in the output and, thus, are a step towards a standardized processing of automatically measured stem radius time series. We discuss examples, such as the seasonality of growth or the dependence of growth on atmospheric and soil drought. The presented growth and water-related physiological variables at high temporal resolution offer novel physiological insights into the seasonally varying responses of trees to changing environmental conditions.

Increased hydraulic constraints in Eucalyptus plantations fertilized with potassium.

Fertilization is commonly used to increase growth in forest plantations, but it may also affect tree water relations and responses to drought. Here, we measured changes in biomass, transpiration, sapwood-to-leaf area ratio (As :Al ) and sap flow driving force (ΔΨ) during the 6-year rotation of tropical plantations of Eucalyptus grandis under controlled conditions for throughfall and potassium (K) fertilization. K fertilization increased final tree height by 8m. Throughfall exclusion scarcely affected tree functioning because of deep soil water uptake. Tree growth increased in K-supplied plots and remained stable in K-depleted plots as tree height increased, while growth per unit leaf area increased in all plots. Stand transpiration and hydraulic conductance standardized per leaf area increased with height in K-depleted plots, but remained stable or decreased in K-supplied plots. Greater Al in K-supplied plots increased the hydraulic constraints on water use. This involved a direct mechanism through halved As :Al in K-supplied plots relative to K-depleted plots, and an indirect mechanism through deteriorated water status in K-supplied plots, which prevented the increase in ΔΨ with tree height. K fertilization in tropical plantations reduces the hydraulic compensation to growth, which could increase the risk of drought-induced dieback under climate change.

Rapid stomatal response in lemon saves trees and their fruit yields under summer desiccation, but fails under recurring droughts

Lemon (Citrus limon) is a fruit tree with major agricultural importance around the Mediterranean basin and is considered to be highly drought resistant. In this study, we tested the effect of two months summer-desiccation on physiological and yield parameters of mature lemon trees growing under Mediterranean climate during three consecutive years. We also examined the efficiency of current irrigation regime, which is based on reference evapo-transpiration. We measured leaf gas exchange and water potential (Ѱl), monitored sap flow and soil moisture and followed flowering, fruit set and fruit size. Lemon trees showed an isohydric stomatal regulation, as stomata maintained leaf water potential >-2 MPa. Summer desiccation caused a gradual decrease in diurnal tree water use, starting immediately after cessation of irrigation, with leaf gas exchange practically halted at the end of the drought period. Tree function recovered following re-irrigation, and fruit yields were not reduced, but even mildly increased during the first year. In contrast, summer desiccation during two consecutive years caused long-term effects of tree activity decrease, significantly lower yield, main branch collapse and even tree mortality. Irrigation amounts matched closely tree water-use amounts; soil moisture was maintained around 26% (v/v); and irrigation responded dynamically to meteorological changes, indicating that current irrigation regime represents highly efficient water management. The lemon desiccation protocol relied on the physiological capacity of this species to avoid short-term drought effects through stomatal closure. Still, this protocol must be managed carefully, to reduce risk to trees and save yields.

Second-growth redwood forest responses to restoration treatments

Following 20th century logging, much of the natural coast redwood (Sequoia sempervirens [D.Don.] Endl.) range consists of dense second-growth stands with slow tree growth and low biodiversity. There is a landscape-scale effort in much of coastal northern California to increase tree growth rates and ecosystem biodiversity via thinning treatments, thereby hopefully accelerating the development of old-growth forest characteristics. Redwood National Park has been experimenting with thinning in these forest types since the 1970s. This study investigated short- (1 year post-thinning) and long-term (40 years post-thinning) responses of redwood physiology (water potential and stomatal conductance), redwood growth (via tree-rings), and forest biodiversity (understory plants, birds, and mammals) to restoration thinning treatments. We found that thinning second-growth redwood forests with 40% basal area reduction treatments 1) did not meaningfully influence tree water potential, 2) increased tree stomatal conductance in the short-term, 3) increased tree growth in the long-term, 4) increased understory plant diversity, and 5) did not affect bird or mammal diversity. Collectively, these findings demonstrate that thinning second-growth redwood forests has the potential to accelerate the development of old-growth characteristics. This verification of the efficacy of restoration treatments is important information for land managers, as plans are currently underway to apply these treatments at the landscape-scale. This study can provide useful baseline data to aid future assessments of long-term forest responses to contemporary restoration efforts.

Climate and functional traits jointly mediate tree water-use strategies.

Tree water use is central to plant function and ecosystem fluxes. However, it is still unknown how organ-level water-relations traits are coordinated to determine whole-tree water-use strategies in response to drought, and whether this coordination depends on climate. Here we used a global sap flow database (SAPFLUXNET) to study the response of water use, in terms of whole-tree canopy conductance (G), to vapour pressure deficit (VPD) and to soil water content (SWC) for 142 tree species. We investigated the individual and coordinated effect of six water-relations traits (vulnerability to embolism, Huber value, hydraulic conductivity, turgor-loss point, rooting depth and leaf size) on water-use parameters, also accounting for the effect of tree height and climate (mean annual precipitation, MAP). Reference G and its sensitivity to VPD were tightly coordinated with water-relations traits rather than with MAP. Species with efficient xylem transport had higher canopy conductance but also higher sensitivity to VPD. Moreover, we found that angiosperms had higher reference G and higher sensitivity to VPD than did gymnosperms. Our results highlight the need to consider trait integration and reveal the complications and challenges of defining a single, whole-plant resource use spectrum ranging from 'acquisitive' to 'conservative'.

Сезонный рост сосны обыкновенной на заболоченных почвах Севера

The seasonal rhythm of height growth of young shoots of Scots pine growing in a 33-year-old shrubby-sphagnum pine forest of the northern taiga natural-climatic zone has been studied in a complex with the main physiological processes. The possibility of regulating their intensity by additional introduction of nitrogen fertilizer has been determined. It has been found that on hydromorphic excessively wet soils of the North, seasonal height growth of pine occurs with a maximum rate in late June and early July, when air temperature rises and the root system is released from excessive moisture. Unlike lichen pine forests, in which pine roots are not affected by flooding, on hydromorphic peat soils pine grows much slower and during the growing season has less intensive physiological processes. The low rate of shoot growth and physiological processes in pine in sphagnum forest site conditions is explained by dysfunction of its root system as a result of soil anoxia caused by flooding, as well as by insufficient supply of growing shoots with acronutrients from the crown as a result of violation of their normal outflow and movement in the tree. Nitrogen fertilizers positively affect growth and physiological processes in pine trees in sphagnum forest types. However, their effect in these forest site conditions is much weaker than in the dry lichen forests of the Far North. On water-logged peat soils under the influence of nitrogen fertilizer seasonal height growth of pine trees increases by 20 %, the duration of seasonal shoot growth increases by 8–10 days, the intensity of photosynthesis increases significantly, and, as a result of reduced water onsumption for transpiration, the tree water regime normalizes and the transpiration productivity increases. These positive nitrogen-induced changes ultimately improve the viability and productivity of sphagnum pine forests. For citation: Zarubina L.V., Khamitov R.S. Seasonal Growth of Scots Pine under the Conditions of Water-Logged Soils of the North. Lesnoy Zhurnal [Russian Forestry Journal], 2021, no. 3, pp. 86–100. DOI: 10.37482/0536-1036-2021-3-86-100

Soil oxygen and water dynamics underlying hypoxic conditions in the root-zone of avocado irrigated with treated wastewater in clay soil

Alternative water sources for agriculture are in high demand in a world with diminishing fresh water (FW) availability. Treated wastewater (TWW) offers a reliable alternative, but increasing evidence is pointing to damage to TWW irrigated orchards planted in clay soils related to soil hypoxia. The mechanisms responsible for this hypoxia have not been extensively studied to date. The aim of this study was to elucidate meaningful insights into the mechanisms responsible for the hypoxia in TWW irrigated orchards planted in clay soils using a novel approach whereby parameters describing the soil oxygen and water temporal dynamics are analyzed. To that end, soil oxygen and soil water tension (SWT) measurements from a two year field experiment comparing TWW to FW irrigation in an avocado orchard planted in a clay soil (60 % clay) were used. The deterioration in oxygen levels occurred as the irrigation season progressed, and the oxygen availability decreased with depth (10–35 cm depth). During August-September, when the lowest oxygen concentrations were measured, the water content at which oxygen supply matched oxygen consumption at 35-cm depth did not differ between treatments (∼50 mbar), but the TWW irrigated soil experienced ∼47 % more time at wetter conditions. Lower oxygen decline rates were observed in the TWW irrigated plots which countered the previous concept that TWW leads to increased soil oxygen consumption. The findings point towards the rate of soil drying as the prime cause of differences – TWW irrigated plots dried in a rate which is nearly 4-times smaller than that in FW irrigated plots during the dark and light hours, reflecting slower drainage and water uptake respectively. It is suggested that soil hypoxia induced by the low soil drainage in TWW irrigated clay soils impairs tree water uptake, which further hinders the soil oxygen levels. Based on these results management tools are suggested to allow sustainable irrigation with TWW in the future. Furthermore, the work demonstrates how analysis of parameters describing the oxygen hourly changes can be utilized to gain mechanistic insights unto processes affecting the oxygen regime in the soil.

Paludification reduces black spruce growth rate but does not alter tree water use efficiency in Canadian boreal forested peatlands

BackgroundBlack spruce (Picea mariana (Mill.) BSP)-forested peatlands are widespread ecosystems in boreal North America in which peat accumulation, known as the paludification process, has been shown to induce forest growth decline. The continuously evolving environmental conditions (e.g., water table rise, increasing peat thickness) in paludified forests may require tree growth mechanism adjustments over time. In this study, we investigate tree ecophysiological mechanisms along a paludification gradient in a boreal forested peatland of eastern Canada by combining peat-based and tree-ring analyses. Carbon and oxygen stable isotopes in tree rings are used to document changes in carbon assimilation rates, stomatal conductance, and water use efficiency. In addition, paleohydrological analyses are performed to evaluate the dynamical ecophysiological adjustments of black spruce trees to site-specific water table variations.ResultsIncreasing peat accumulation considerably impacts forest growth, but no significant differences in tree water use efficiency (iWUE) are found between the study sites. Tree-ring isotopic analysis indicates no iWUE decrease over the last 100 years, but rather an important increase at each site up to the 1980s, before iWUE stabilized. Surprisingly, inferred basal area increments do not reflect such trends. Therefore, iWUE variations do not reflect tree ecophysiological adjustments required by changes in growing conditions. Local water table variations induce no changes in ecophysiological mechanisms, but a synchronous shift in iWUE is observed at all sites in the mid-1980s.ConclusionsOur study shows that paludification induces black spruce growth decline without altering tree water use efficiency in boreal forested peatlands. These findings highlight that failing to account for paludification-related carbon use and allocation could result in the overestimation of aboveground biomass production in paludified sites. Further research on carbon allocation strategies is of utmost importance to understand the carbon sink capacity of these widespread ecosystems in the context of climate change, and to make appropriate forest management decisions in the boreal biome.

The Effect of Foliar and Ground-Applied Essential Nutrients on Huanglongbing-Affected Mature Citrus Trees.

The fate of foliar and ground-applied essential nutrients is the least studied topic under citrus greening or Huanglongbing (HLB)-affected citrus, which is inherently suffering from severe root decline because of HLB-associated problems. The objective of this study was to evaluate if ground-applied coupled with foliar spray of essential nutrients can reverse the decline in tree growth and understand the fate of the nutrients in the soil-root-tree interfaces. The treatments were arranged in a split-split plot design in which nitrogen (N) was ground-applied in 20 splits biweekly and Mn, Zn, and B were foliar and /or ground-applied in three splits following the spring, summer, and late summer flush seasons. Soil nutrients in three depths (0–15, 15–30, and 30–45 cm), root, and leaf nutrient concentrations of the essential nutrients, leaf area index (LAI), and tree canopy volume (TCV) data were studied twice (spring and summer) for two years. A significantly higher soil NH4-N and NO3-N concentrations were detected in the topsoil depth than the two lower soil depths (15–30 and 30–45 cm) indicating lesser nutrient leaching as trees received moderate (224 kg ha−1) N rate. Except for soil zinc (Zn) concentration, all the nutrient concentrations were significantly higher in the topsoil (0–15 cm), compared with two lower soil depths indicating that Zn was intricate by changes in soil environmental conditions, root acquisition, and/or leaching to lower soil depth. Leaf N concentration significantly increased over time following seasonal environmental fluctuations, tree growth, and development. Thus, leaf N concentration remained above the optimum nutrient range implying lower N requirement under irrigation scheduling with SmartIrrigation, an App used to determine the daily irrigation duration to meet tree water requirement and split fertigation techniques. Root Manganese (Mn) and Zn concentrations were significantly higher in the root tissues of the treated than the control trees and translocated to the leaves accordingly. Meanwhile, a significantly higher LAI for trees budded on Swingle (Swc) rootstock however, larger TCV for trees budded on Volkameriana (Volk) rootstocks. The trees had significantly larger TCV when the trees received a moderate N rate during early study years and under foliar 9 kg ha−1 coupled with the ground 9 kg ha−1 Mn and Zn treatments during the late study years. Therefore, split ground application of 224 kg ha−1 of N, foliar applied 9 kg ha−1 coupled with ground-applied 9 kg ha−1 Mn and Zn were the suggested rates to sustain the essential leaf nutrient concentration within the optimum ranges and improve the deterioration of vegetative growth associated with HLB-induced problems of citrus trees.

Importance of hydraulic strategy trade-offs in structuring response of canopy trees to extreme drought in central Amazon.

Plant ecophysiological trade-offs between different strategies for tolerating stresses are widely theorized to shape forest functional diversity and vulnerability to climate change. However, trade-offs between hydraulic and stomatal regulation during natural droughts remain under-studied, especially in tropical forests. We investigated eleven mature forest canopy trees in central Amazonia during the strong 2015 El Niño. We found greater xylem embolism resistance ([Formula: see text] = - 3.3 ± 0.8MPa) and hydraulic safety margin (HSM = 2.12 ± 0.57MPa) than previously observed in more precipitation-seasonal rainforests of eastern Amazonia and central America. We also discovered that taller trees exhibited lower embolism resistance and greater stomatal sensitivity, a height-structured trade-off between hydraulic resistance and active stomatal regulation. Such active regulation of tree water status, triggered by the onset of stem embolism, acted as a feedback to avoid further increases in embolism, and also explained declines in photosynthesis and transpiration. These results suggest that canopy trees exhibit a conservative hydraulic strategy to endure drought, with trade-offs between investment in xylem to reduce vulnerability to hydraulic failure, and active stomatal regulation to protect against low water potentials. These findings improve our understanding of strategies in tropical forest canopies and contribute to more accurate prediction of drought responses.

Rootstock–Scion Hydraulic Balance Influenced Scion Vigor and Yield Efficiency of Malus domestica cv. Honeycrisp on Eight Rootstocks

Rootstocks with internal hydraulic limitations can effectively restrict scion growth, influence crop load, and improve yield efficiency in apple production. The characteristics of xylem vessels in rootstock and scion play essential roles in determining the hydraulic properties of the grafted tree; however, much remains unknown for commonly available rootstocks. In this study, we extracted secondary xylem using an increment borer from living Honeycrisp scion (Malus domestica cv. ‘Honeycrisp’), and two Malling rootstocks, one Budagovsky rootstock, and five Geneva rootstocks. The size and density of xylem vessels in rootstocks and scions were analyzed in relation to trunk cross-section area (TCSA), tree–water relations, and fruit dry matter of 2019, as well as with cumulative yield efficiency during 2014–2019. Honeycrisp scion exceeded most of the rootstocks in cross-section size and density of vessel elements. Scion vigor and cumulative yield were positively correlated with TCSA and total vessel cross-section area (VCSA) of the rootstock, with G.202 being the highest, and B.9 being the lowest with small xylem vessels in high density. In the rootstocks with the highest cumulative yield efficiency, the rootstock/scion ratio in VCSA was equal to or slightly higher than 1. Lower scion vessel density in G.214 was associated with lower fruit dry matter weight, more restricted water relations, and worsened leaf chlorosis. G.935 with larger rootstock vessels led to both high yield and high yield efficiency. This suggested that higher scion vessel density and larger rootstock vessel size can be advantageous characteristics for early-stage evaluation.

Eco-physiological response, water productivity and fruit quality of sweet cherry trees under high tunnels

It is known that high tunnels modify sweet cherry physiological and agronomical response; however, horticultural practices such as irrigation have usually not been adapted to this cultivation system because there is no information about the effects of sweet cherry growing under this controlled environment on fruit tree water relations, water productivity and the possibility of saving water. The present work aims to assess the positive and negative impacts of irrigation regime and protective cultivation on tree water status, agronomical and physiological responses of the sweet cherry tree combination ‘Santina’/‘Colt’ in Mediterranean dry weather conditions of Central Valley of Chile. Two treatments were imposed, plastic covered trees under multi-bay high tunnels and uncovered trees under open field conditions. Within each environment, two irrigation treatments were assayed, a conventional treatment, which followed growers’ normal irrigation practices in the region, and a reduced irrigation treatment, which was irrigated the 75 % of the conventional treatment. High tunnel increased maximum air temperature, relative humidity and altered light radiation in relation to the environmental conditions in the open. Overall, trees under high tunnel showed higher values of soil matric potential, midday stem water potential (-0.8 MPa) and stomatal conductance (250 mmol m−2 s−1). In the open, trees under reduced irrigation showed soil matric potential values below -150 kPa. Thus, soil water deficit affected tree water status, decreased vegetative growth and fruit size. Covered trees blossomed 7 d earlier than those in the open and were harvested 10 d earlier. Total tree yield did not show significant differences among treatments (11.9 kg tree−1); however, when the tree was divided into bottom and top canopy layers, it was found that the top layer of covered trees resulted in significantly lower yield. The cherries from the covered trees were significantly larger (10.4 g) and less firm (75 Durofel units) than those from uncovered trees (8.4 g and 82 Durofel units). The reduced irrigation strategy did not negatively affect tree yield or fruit quality of covered trees and increased water productivity compared with those irrigated with the conventional irrigation. The results obtained highlighted the possibility of decreasing the irrigated water applied to sweet cherry trees under high tunnels by 25 % compared to conventional irrigation.

Vertical root distribution in Himalayan trees: about half of roots occur below 30 cm, the generally sampled depth

Almost nothing is known about the vertical root distribution for Himalayan forest trees. By providing an analysis of vertical distribution of root density of ten important central Himalayan tree species (two evergreen conifers, five evergreen broadleaf species and three deciduous broadleaf species), this paper attempts to address this gap. We used trench profile method to measure tree root density, both of fine (≤ 3 mm diameter) and coarse (> 3 mm diameter) roots. Principal Component Analysis was performed to identify the major sources of variation among species in root distribution. In ten study tree species (eg., sal, oaks, pine and Cupressus torulosa) the rooting depth varied from 90 to 150 cm. On average across the study species, about 49% total roots (all diameter classes) occurred below 30 cm depth and 20.6% below 60 cm depth. These percentages are almost identical to that of tropical rain forest of Amazon. The tree species varied in root density (in entire soil column) from 26.04 ± 2.66 root/100 cm2 in Q. floribunda to 52.7 ± 4.96 root/100 cm2 in Machilus duthei. The ordination graph indicated that tree species of a growth form and even a genus did not form groups and differed markedly in root characters. Tree water status and roots were related only partially and in a complex way. The study has shown that (1) root density measured by digging trenches gives a reasonable estimate of proportional root distribution of trees, and (2) studies which consider only top 30 cm depth can grossly misrepresent root distribution and its consequence.

Antioxidant Adjustments of Olive Trees (Olea Europaea) under Field Stress Conditions.

Extreme climate events are increasingly frequent, and the 2017 summer was particularly critical in the Mediterranean region. Olive is one of the most important species of this region, and these climatic events represent a threat to this culture. However, it remains unclear how olive trees adjust the antioxidant enzymatic system and modulate the metabolite profile under field stress conditions. Leaves from two distinct adjacent areas of an olive orchard, one dry and the other hydrated, were harvested. Tree water status, oxidative stress, antioxidant enzymes, and phenolic and lipophilic metabolite profiles were analyzed. The environmental conditions of the 2017 summer caused a water deficit in olive trees of the dry area, and this low leaf water availability was correlated with the reduction of long-chain alkanes and fatty acids. Hydrogen peroxide (H2O2) and superoxide radical (O2•–) levels increased in the trees collected from the dry area, but lipid peroxidation did not augment. The antioxidant response was predominantly marked by guaiacol peroxidase (GPOX) activity that regulates the H2O2 harmful effect and by the action of flavonoids (luteolin-7-O-glucuronide) that may act as reactive oxygen species scavengers. Secoiridoids adjustments may also contribute to stress regulation. This work highlights for the first time the protective role of some metabolite in olive trees under field drought conditions.

Temporal variation and controlling factors of tree water consumption in the thornbush savanna

For a typical encroacher tree of the thornbush savanna, we studied the responses of water consumption to changes in soil water availability within 80 cm depth, vapor pressure deficit (VPD), and global radiation. Therefore, we monitored the sap velocities of Senegalia mellifera trees over 2 years in a maximum of 20 stems (6–15 plant individuals) per measurement period on a site in central Namibia. For this water-restricted ecosystem we aimed to understand the role of an encroacher tree on soil water dynamics and potential groundwater recharge.At the day-to-day scale, soil water was the primary driver of cumulative daily sap velocities (Qn-day). In the dry season, Qn-day decreased with increasing soil drought. Rainy seasons triggered multiple sapflow phases, initiated by soil-wetting rain events and associated with increase of soil water. Maximum attainable Qn-day increased with VPD and radiation. We hypothesize a 3-stage response of tree water use to soil water tension (SWT): (1) SWT < pF 2.6–2.7: soil water is optimal for transpiration; (2) pF 2.6–2.7 < SWT < pF 3.2–3.7: transpiration decreases with soil drought; (3) SWT > pF 3.2–3.7: trees minimize transpiration. Our findings contribute to the understanding of tree–grass coexistence in savannas. The responses of tree water use to soil water within 80 cm depth indicate that S. mellifera exploits the same depths as grasses, in contrast to the classic hypothesis of vertical niche differentiation.

A fragile soil moisture environment exacerbates the climate change-related impacts on the water use by Mongolian Scots pine (Pinus sylvestris var. mongolica) in northern China: Long-term observations

Characterizing the soil moisture environment in the plantation ecosystems may provide substantial information for judging the stability of forests in relation to climate change. In this study, we explored the level and the variations of water availability, as well as their influence on tree water use in the long-term. This was achieved by monitoring in particular the volumetric soil water content (θ), the groundwater level, the atmospheric vapor pressure deficit (VPD), and the sap flux-based forest transpiration (Tr) in a sand-fixation plantation of Mongolian Scots pine in northern China. The results showed a significant inter- and intra-annual variability of annual precipitation (Pr), for an average (±standard deviation) of ~ 476 ± 139 mm over the past 16-year study period. The inter-annual variations of the groundwater level showed a general downward trend, decreasing from −4.82 to −6.13 m. The relative extractable soil water (REW) between 0−1 m and between 1−2 m depth along soil profile was only 0.29 ± 0.14 and 0.16 ± 0.08, respectively, indicating a decreasing of soil moisture capacity and of the moisture depletion patterns with depth. The statistics relative to the past seven years (between 2013−2019) indicate an average Tr of 193 ± 59 mm for Mongolian Scots pines. Notably, their transpiration capability (Tr/VPD) declined with the decrease of REW0−1 m: the boundary curve showed a clear exponential decline. The threshold value of REW0–1 m (deduced from an abrupt decrease of Tr/VPD) was ~ 0.41 and 0.35 at the daily and monthly timescales, respectively. The average Tr/Pr value was 0.42 (maximum = 0.60) and Tr accounted on average for 55% of the total evapotranspiration (maximum = 77%). Soil drought prevailed at the study site; however, we did not notice a heavy use of soil water resources by Mongolian Scots pine. The results, overall, underlined the causes rather than the consequences of the observed soil moisture conditions, which are leading to the degradation of Mongolian Scots pine. Forest management measures causing moderate disturbance are hence recommended for mitigating the water competition between Mongolian Scots pine and sub-canopy vegetation.