Tree Water Use Research Articles

Tree density influences ecohydrological drivers of plant–water relations in a larch boreal forest in Siberia

AbstractTranspiration and stomatal conductance in deciduous needleleaf boreal forests of northern Siberia can be highly sensitive to water stress, permafrost thaw, and atmospheric dryness. Additionally, north‐eastern Siberian boreal forests are fire‐driven, and larch (Larix spp.) are the sole tree species. We examined differences in tree water use, stand characteristics, and stomatal responses to environmental drivers between high and low tree density stands that burned 76 years ago in north‐eastern Siberia. Our results provide process‐level insight to climate feedbacks related to boreal forest productivity, water cycles, and permafrost across Arctic regions. The high density stand had shallower permafrost thaw depths and deeper moss layers than the low density stand. Rooting depths and shallow root biomass were similar between stands. Daily transpiration was higher on average in the high‐density stand 0.12 L m−2 day−1 (SE: 0.004) compared with the low density stand 0.10 L m−2 day−1 (SE: 0.001) throughout the abnormally wet summer of 2016. Transpiration rates tended to be similar at both stands during the dry period in 2017 in both stands of 0.10 L m−2 day−1 (SE: 0.002). The timing of precipitation impacted stomatal responses to environmental drivers, and the high density stand was more dependent on antecedent precipitation that occurred over longer periods in the past compared with the low density stand. Post‐fire tree density differences in plant–water relations may lead to different trajectories in plant mortality, water stress, and ecosystem water cycles across Siberian landscapes.

Transpiration dynamics and water sources for selected indigenous trees under varying soil water content

The major route through which water from the earth’s surface re-enters the hydrologic cycle in forested ecosystems is via tree transpiration (T). It is therefore important to have detailed understanding of the quantity and source of water transpired by different tree species. The aims of this study were to i) assess the trends of T for selected tree species (camel thorn, sweet thorn, shepherd’s tree and buffalo thorn) across a range of soil water content conditions and ii) partition the total T of the selected tree species growing in arid environments dominated by open cast mining activities into soil water and groundwater. Tree T was measured using the compensation heat pulse velocity (CHPV) method, while soil water content was monitored using DFM capacitance probes. The soil water content within the upper 50 cm soil profile ranged from 11 mm during the dry season to 20 mm during the wet season. The deeper soil layer (50–120 cm) was generally wetter compared to the top layer with water content was up to >30 mm during the wet season. The measured tree T ranged from 0.2 mm day−1 on buffalo thorn during the dry season to 1.9 mm day−1 on sheperd’s tree in summer. It was also revealed that T of large (diameter at breast height =46 cm) camel thorn trees is not responsive to seasonal variations of soil water availability and remained constant at approximately 1.2 mm day−1. Diurnal patterns of T did not effect changes on the soil water depletions within the top 120 cm soil profile, which indicated that the trees sourced water beyond this zone. Signs of daytime redistribution were observed within the canopy areas of the investigated trees during very limited soil water conditions of the dry season. It was concluded that the water use of trees is inclined to the seasonal variations, which however is not the case in old trees. Close to 100% of the water transpired by trees in the study area is sourced below 1.2 m (vadose zone and water table). We recommended investigation of daytime redistribution among the indigenous tree species of the study area. We also recommended extension of tree water use studies to other species for comprehensive catchment tree water use calculations to inform water budgets.

Water balance and tree water use dynamics in remnant urban reserves

During the expansion of urban areas, small natural reserves have often been left intact within the built environment as central elements of biodiversity conservation, ecological connectivity, landscape sustainability, and quality of life of urban dwellers. Consequently, the surrounding urbanized landscape may impact the environmental conditions of these reserves (e.g., high temperatures, low moisture conditions), resulting in the need for extensive maintenance. This study presents an estimation of the water balance over two years (2017–2018) in three small urban reserves (between 2 and 30 ha) within the Greater Melbourne metropolitan area in Australia, for the purpose of understanding tree water use. Measurements of micrometeorological variables, soil moisture content profiles, water-table levels, sap flow velocities, and stem diameter variations were used to quantify the water sources of tree transpiration in these reserves. Results revealed that, despite the urban surroundings and the climate variations, these reserves have enough water to sustain tree transpiration. In two of the three reserves, groundwater was pivotal in sustaining transpiration rates; specifically, groundwater was estimated to contribute about 30–40% of the total transpiration amount during the driest periods of the year. Groundwater also played an essential role during nights with temperatures above 25 °C, helping trees to maintain night-time water use from 3 to 16% of the daily water use. In the third reserve, the presence of a shallow layer of heavy clay supplied water to the trees, which were able to maintain relatively constant transpiration rates throughout the year. These results demonstrate the importance of understanding the water regime of each urban reserve in order to support government authorities in preserving these ecosystems.

Tree water use strategies and soil type determine growth responses to biochar and compost organic amendments

Background and aimsOrganic matter is often used as an amendment to attempt restoration of degraded soils to improve tree establishment and growth. One key aim is to increase plant available water in the soil profile. The texture of the soil, the type of organic amendment (e.g. compost or biochar), and the native environment of the tree (mesic or xeric) will impact how successful restoration efforts are. We aimed to determine whether compost and biochar amendments, either individually or in combination, would improve plant available water (PAW) in both clay and sand soils. We then aimed to measure whether changes in PAW would translate into increased water use and plant growth of both mesic (Corymbia maculata) or xeric (Eucalyptus torquata) tree species under well-watered (WW) or water deficit (WD) conditions. MethodsClay and sand soils were amended with compost, biochar or a compost and biochar mix, whilst unamended soils acted as controls. Soil moisture characteristics of the soil mixes were determined with samples in the laboratory. Fifteen replicate pots (6 l) of each soil treatment were then planted with either mesic and xeric tree species. All pots and trees were subjected to either a WW or WD irrigation regime for 7 weeks. WD irrigation was a set percentage of the daily WW evapotranspiration (ET). ET was calculated as total pot mass one hour after irrigation (to allow for drainage) minus the total pre-dawn pot mass of the subsequent day. The tree biomass, biomass partitioning, ET, and tree water status were measured throughout the experiment to understand growth and stress responses. Key resultsThe xeric tree species (E. torquata) grown in sand soil had a significant increase in growth with all three OM amendments but did not when grown in clay soil. In contrast, we found no significant growth response for the mesic tree species (C. maculata) when grown in either clay or sand amended with any OM treatment. The ET of the xeric tree species was greater for all the OM amended soils for both WW and WD plants in the sand soil. ConclusionsThis study shows that OM amendments may improve the soil water properties of sand-based soils which in turn can increase the growth of xeric tree species. However, more expensive organic amendments may not be necessary, nor mixing of OM types, as we found no tree growth differences amongst the three different OM treatments.

Water residence times in trees of a neotropical dry forest

Water residence times in trees of a Neotropical dry forest increase with tree diameter and decrease with wood density during the rainy season. In tropical dry forests, tree species with different drought coping strategies co-exist; leaf deciduous and stem-succulent trees are prominent among them. The actual role of stem succulence in tree water use is not fully understood. In a premontane dry forest of southern Ecuador, five tree species were studied in the rainy season including one stem-succulent species (Ceiba trichistandra). All species were studied at the same site and some also across a soil moisture gradient. Deuterium tracing suggests species-specific mean residence times between 11 and 22 days. Mean residence times for Ceiba and two other deciduous tree species were about twice as high as those of two evergreen tree species. Differences in soil moisture did not significantly affect tree water residence times. Our and literature data indicate that across species, residence times increase with tree diameter and decrease with wood density. The stem-succulent Ceiba is characterized by a big stem and a low wood density; the observed residence times in the rainy season were situated on the continuum as derived from a larger data set. The succulent stem may, however, play a special role for leaf flushing at the end of the dry season.

Use of thermal imaging to detect evaporative cooling in coniferous and broadleaved tree species of the Mediterranean maquis

Among forest types, the Mediterranean maquis is specifically exposed to fluctuations in water availability. Therefore, monitoring the water-use patterns of its major tree species is key in quantifying the local and regional water balance. However, the traditional measurement methods of tree water-use at high spatial scales are difficult and labor-intensive, thus indirect methods become useful. Evaporation of water from the stomatal pores on the leaf surface involves evaporative cooling, and hence the differences between the leaf temperature and its surrounding air temperature (ΔTleaf-air) can serve as a reliable estimator for tree water-use.Here, we used direct measurement of transpiration rate (E) with a gas exchange system, simultaneously with ground Thermal Infra-Red (TIR) imaging to study the relationship between ΔTleaf-air and E in dominant tree species of the Mediterranean maquis. Controlled experiments were conducted in parallel with measurements in the forest, on five tree species of contrasting leaf shapes (Conifers: Pinus halepensis; Cupressus sempervirens; Broadleaf: simple: Quercus calliprinos; Ceratonia siliqua; compound: Pistacia lentiscus). Next, we used a quantitative approach, applying a leaf energy balance model to estimate E from the TIR images and compared it to the direct measurement of the gas exchange system.We report evaporative cooling across the five species, replicated in tree saplings and in mature trees in the forest. The conifers were significantly cooler than broadleaves by up to ∼4 °C and produced narrower ΔTleaf-air ranges. Estimations of E from ΔTleaf-air were relatively close to the observed E, with some overestimations. Our observations show that TIR imaging can detect transpiration-related differences in ΔTleaf-air among species and can be used to estimate E in natural environments. Yet, the dependence of ΔTleaf-air on E is species-specific and thus, empirical associations must be developed separately for each of the species.

Determining the independent impact of soil water on forest transpiration: A case study of a black locust plantation in the Loess Plateau, China

Transpiration (Tr) is influenced by environmental factors, vegetation properties, and anthropogenic management. The effect of environmental factors on Tr are taking place from two aspects: evaporative demand (i.e., potential evapotranspiration, PET) and water supply (i.e., soil water). Soil water is one of the most important factors that limit plant transpiration in terrestrial ecosystems, especially in semi-arid and arid regions. Investigating the relationship between Tr and soil water is crucial for an improved understanding of plant survival strategies and predicting hydrological cycles and water resources under climate change. Although the relationships of soil water and Tr have been widely studied, the independent effects of soil water on Tr are difficult to separate because soil water and PET occur concurrently under natural conditions. This study carried out field observations of sapflow density, meteorological factors and soil water in a black locust (Robinia pseudoacacia) plantation in the semi-arid Loess Plateau of China. This information was used to develop a model integrating the daily PET and relative extractable soil water (REW, an index that represents the available soil water): Tr = (0.27 × PET − 0.02 × PET2 − 0.32) × (1 − e−5.70×REW). The model fitted the measured data well (R2 = 0.65 and RMSE = 0.06 mm day−1). We found that the daily Tr increased as the REW increased under varying PET levels. Additionally, the independent effects of soil water on Tr were analysed using the factorial experiment analysis method. The REW was manipulated and PET varied naturally during the measurements to separate the independent effects of the REW on Tr. The results showed that Tr increased with the REW during the study period at a rate of 0.53 mm day−1 per 0.1 REW when the REW < 0.4 and 0.09 mm day−1 per 0.1 REW beyond the threshold (REW = 0.4). Tr increased by 27.3 mm (43.7%) compared to the controlled Tr (observed PET and lowest REW) due to the effects of increasing REW (ranging from 0.14 ∼ 0.81) during the study period (growing seasons of 2015 and 2016). Furthermore, canopy-level stomatal conductance was calculated using a simplified inverted Penman-Monteith equation. The reference canopy-level stomatal conductance (Gsref), which represents the Gs capacity and its sensitivity to soil water, showed an increasing response with increasing REW, which is the possible reason for the Tr dynamics with the REW. Our study provided an improved understanding of the soil water-Tr relationship and predicted effects of climate change on tree water use. Although the method required abundant field observation data, we have provided a feasible method of accurately quantifying the independent contribution of soil water on forest Tr.

Whole tree water use: Effects of tree morphology and environmental factors

Although tree transpiration (T) studies across multiple spatial scales have been conducted, the synthesis of the driving factors of tree water use, especially for a variety of species under different climatic conditions has not yet been made. This paper analyses T data from 94 published studies conducted in various sites between 1970 and 2016, representing 196 data points to seek relations between morphological traits; tree height (H), diameter at breast height (DBH) and environmental factors; mean annual precipitation (MAP), mean annual temperature (MAT) and altitude (Z) on whole tree water use for 130 species of trees. Techniques used in the studies for T measurement were also analysed. Log transformed T (ln T) varied between 0 and 7.1 L day−1. Univariate correlation and regression analysis revealed that ln T was positively and significantly correlated with H (Spearman correlation coefficient (rs) = 0.55) and DBH (rs = 0.62) at P < 0.1. A weak positive correlation was found between ln T and MAP (rs = 0.16) at P < 0.1. The results further showed that during the study period (1970–2016), 82% of the studies used thermodynamic methods to measure T, in particular thermal heat dissipation probes were used by 60% of the studies, while 21% reported use of heat pulse velocity. The results contribute to a better understanding of T in forest ecosystems, and the factors of control to inform global scale modelling and ecosystem management. Thermodynamic methods, especially thermal heat dissipation probes and heat pulse velocity are the most prevalent techniques used for whole tree T measurement.

Water use of Al Samr (Acacia tortilis) forests irrigated with saline groundwater and treated sewage effluent in the hyper-arid deserts of Abu Dhabi

The arid forests planted in Abu Dhabi provide a variety of valuable ecosystem services. The forests need to be irrigated, and currently groundwater (GW) provides the bulk of this water. However GW recharge is very low, and reserves are rapidly dwindling and becoming more saline. In 2016, Law 5 was passed in Abu Dhabi with the objective to set GW extraction limits and define irrigation-usage allowances. Here we sought to define the usage allowance for Al Samr trees, and this work complements our previous research on Al Ghaf and Al Sidr forests. We have measured tree water-use, ETc, using sapflow monitoring of GW-irrigated trees, and trees irrigated with treated sewage effluent (TSE). Maximum rates of tree water-use, ETc, were found to around 10 L d−1, and there are two distinct deciduous periods where ETc briefly dropped below 2 L d−1. The total annual water use of the TSE-irrigated trees was 2.2 kL y−1, which is about 25% higher than the 1.8 kL y−1 for the GW-irrigated trees.For Law 5, we recommend that the irrigation allocation for Al Samr trees be simply based on a constant ETc of 10 L d−1. So for GW irrigation, allowing for a 25% factor-of-safety, and a 25% salt-leaching fraction, the recommended allocation would be 15 L d−1. This represents a saving of 75% from the current practice of irrigation 60 L d−1. For TSE, without the need for salt leaching, the irrigation allocation would only need to be 12.5 L d−1.

2018–2019 Florida Citrus Production Guide: Irrigation Management of Citrus Trees

This 6-page fact sheet is part of the 2018–2019 Florida Citrus Production Guide. This chapter on irrigation management of citrus is largely taken from guidelines provided in SL253 Chapter 9 on trees prior to citrus greening: SL253/SS478: Nutrition of Florida Citrus Trees, 3rd Edition. A section has been added to cover recent findings on water use of trees affected by citrus greening and the impact this would have on the irrigation management considerations. Written by D. M. Kadyampakeni, K. T. Morgan, M. Zekri, R. S. Ferrarezi, A. W. Schumann, and T. A. Obreza, and published by the Agronomy Department, January 2019. CPG12/CG093: 2022–2023 Florida Citrus Production Guide: Irrigation Management of Citrus Trees (ufl.edu)

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.

Coordination of stomatal control and stem water storage on plant water use in desert riparian trees

Populus euphratica Oliv. is a relatively ‘conservative’ water user even when growing in favourable water conditions, but the mechanistic understanding of this has received little attention. We undertook an experiment to determine trees water use by measuring stem and root sap flow (Fs), variation in stem diameter (Ds), leaf stomatal gas-exchange (e.g. conductance, gs and transpiration, Tr) and water potential $$\left( {{\psi _{\text{L}}}} \right)$$ during the growing season for P. euphratica. There was a hysteretic ‘apparent feedforward’ of stomatal response to increasing VPD. Mean of gs was not significantly different among months. $${\psi _{\text{L}}}$$ was negatively related to gs and Tr, but in contrast, stem Fs was positively associated to Tr but not to gs. There was no lag in the daily onset and cessation of Fs between the bottom and top of the trunk, possibly due to the short distance between measurement points (about 2 m), however, the lag time in Fs between the bottom of the trunk and roots, approximately 30 min, suggested that stored water was withdrawn first from the trunk and subsequently the roots. Daily contraction of Ds (− Ds) increased with increasing Fs during both day and night, and expansion of Ds (+ Ds) showed a logarithmic rise to a maximum with increasing Fs during the day. Day Fs and − Ds were also logarithmic with respect to VPD, with a correlation coefficient equal to 0.51 and 0.50, respectively (P < 0.001). This suggests that water use of P. euphratica was determined by the stomatal control and stem water storage together, which has great significance for the species buffers xylem water deficit, maintaining high leaf production, and water use efficiency.

Water-Related Variables for Predicting Yield of Apple under Deficit Irrigation

Predicting apple yield in relation to tree water use is important for irrigation planning and evaluation. The aim of the present study was to identify measurable variables related to tree water use that could predict final fruit yield of apple trees under different strategies of deficit irrigation. Adult ‘Gala’ and ‘Fuji’ apple trees were exposed to conventional irrigation (CI), delivering 100% of crop evapotranspiration; partial root zone drying (PRD), delivering 50% of CI water only on one alternated side of the root-zone; and continuous deficit irrigation (CDI), delivering 50% of CI water on both sides of the root-zone. Integrals of soil (SWDint) and leaf (LWSDint) water deficit along with growth and stomatal conductance (Gsint) were calculated across each season and used to estimate total conductance (GStree) and transpiration (Trtree) per tree, transpiration efficiency on a fruit (GRfruit/Tr) or tree (GRtrunk/Tr) growth basis, and transpiration productivity (Yield/Trtree). ‘Fuji’ trees had higher Yield/Trtree, but had lower GRtrunk/Tr and similar GRfruit/Tr compared to ‘Gala’ trees. In ‘Fuji’, CDI reduced yield, trunk growth, leaf hydration, and gas exchange, while in ‘Gala’, it did not reduce yield and gas exchange. In ‘Fuji’, a linear combination of GRtrunk/Tr, GRfruit/Tr, and Gstree contributed to predicting yield, with GRfruit/Tr explaining nearly 78% of the model variability. In ‘Gala’, a linear combination of LWSDint and Gstree contributed to predicting yield, with Gstree explaining over 79% of the model variability. These results indicate that measuring tree water status or water use may help predict final apple yields only in those cultivars like ‘Gala’ that cannot limit dehydration by closing stomates because of carbon starvation. In more vigorous cultivars like ‘Fuji’, transpiration efficiency based on fruit growth can be a powerful predictor of final yields.

The impact of replacing groundwater by treated sewage effluent on the irrigation requirements of Al Ghaf (Prosopis cineraria) and Al Sidr (Ziziphus spina-christi) forests in the hyper-arid deserts of Abu Dhabi

The arid forests of Abu Dhabi are valuable but they require irrigating. Currently groundwater (GW) is the source of this water, but these subterranean reserves are being over-exploited. Law No 5 of 2016 on the regulation of GW has been passed by the Government of Abu Dhabi to reduce GW abstraction. Abu Dhabi has a supply of tertiary-treated sewage effluent (TSE) that could be used as an alternative for irrigation. We set up experiments near Madinat Zayed in the Al Dhafra region of Abu Dhabi on two arid-forest species: Al Ghaf (Prosopis cineraria) and Al Sidr (Ziziphus spina-christi). The trees were planted at 8 m x 8 m spacing. The TSE is significantly ‘sweeter’ than GW, as its electrical conductivity is < 1 dS m−1, whereas GW is 8–10 dS m−1. The GW in this region is very high in nitrates (38.4 ± 9.2 mg-NO3 L−1), and this was not significantly different from the TSE (53.7 ± 11.8 mg-NO3 L−1). We monitored the actual tree water-use (ETc, L h−1) via heat-pulse devices in both the GW and TSE-irrigated trees. We quantified the differences in the ETc patterns for both the GW and TSE-irrigated trees of both species over 3 years. Both species showed positive growth-responses to TSE, relative to the GW, and we consider this to be due to the lower electrical conductivity of the TSE water. Because of this growth response the ETc of the TSE by the Ghaf trees was, on annual average 17% higher than GW, and for the Sidr it was 39%. Our results were corroborated by leaf conductance and leaf-area inferences. But for TSE there is no need for a salt-leaching fraction. Furthermore, to achieve the same tree-health outcome as with the GW, even less TSE need be applied. Irrigation requirements for TSE were at least 25% less than for GW.

ENSO effects on the transpiration of eastern Amazon trees.

Tree transpiration is important in the recycling of precipitation in the Amazon and might be negatively affected by El Niño-Southern Oscillation (ENSO)-induced droughts. To investigate the relative importance of soil moisture deficits versus increasing atmospheric demand (VPD) and determine if these drivers exert different controls over tree transpiration during the wet season versus the dry season (DS), we conducted sap flow measurements in a primary lowland tropical forest in eastern Amazon during the most extreme ENSO-induced drought (2015/2016) recorded in the Amazon. We also assessed whether trees occupying different canopy strata contribute equally to the overall stand transpiration (Tstand). Canopy trees were the primary source of Tstand However, subcanopy trees are still important as they transpired an amount similar to other biomes around the globe. Tree water use was higher during the DS, indicating that during extreme drought trees did not reduce transpiration in response to low soil moisture. Photosynthetically active radiation and VPD exerted an overriding effect on water use patterns relative to soil moisture during extreme drought, indicating that light and atmospheric constraints play a critical role in controlling ecosystem fluxes of water. Our study highlights the importance of canopy and subcanopy trees to the regional water balance and highlights the resilience to droughts that these trees show during an extreme ENSO event.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'.

Differential use of winter precipitation by upper and lower elevation Douglas fir in the Northern Rockies.

In temperate regions such as the American west, forest trees often exhibit growth sensitivity to climatic conditions of a particular season. For example, annual tree ring growth increments may correlate well with winter precipitation, but not with summer rainfall, suggesting that trees rely more on winter snow than summer rain. Because both the timing and character of seasonal western climate patterns are expected to change considerably over coming decades, variation in the importance of different seasonal moisture sources for trees can be expected to influence how different forest trees respond to climate change as a whole, with shifts in seasonality potentially benefitting some trees while challenging others. In this study, we inferred patterns of tree water use in Douglas fir trees from the Northern Rockies for 2years using stable water isotopes, while simultaneously quantifying and tracking precipitation inputs to soil moisture across a vertical soil profile. We then coupled water source use with daily measurements of radial growth to demonstrate that soil moisture from winter precipitation accounted for 87.5% and 84% of tree growth at low and high elevations, respectively. We found that prevailing soil moisture conditions drive variation in the depth at which trees access soil water, which in turn determines which seasonal precipitation inputs are available to support tree growth and function. In general, trees at lower elevations relied more on winter precipitation sourced from deep soils while trees at higher elevations made better use of summer rains sourced from near-surface soil layers. As both the timing of seasons and phase of precipitation (rain vs. snow) are likely to change considerably across much of the west, such patterns in tree water use are likely to play a role in determining the evolution of forest composition and structure in a warming climate.

Irrigation management with saline groundwater of a date palm cultivar in the hyper-arid United Arab Emirates

The United Arab Emirates has a hyper-arid climate. Irrigation is essential for dates (Phoenix dactylifera L.), an important crop economically and culturally. Groundwater is relied on, yet it is a non-renewable resource at the rate it is being used. Furthermore, as the water-table drops, it is becoming more saline. Law no. 5 has been passed in Abu Dhabi to regulate the use of groundwater and set allocation limits for agriculture. For assessing the allocation of irrigation water to date farms under Law 5, we carried out measurements of tree water-use by the compensation heat-pulse method, complemented by measurements of the changing soil-water dynamics using time domain reflectometry and bulk soil electrical conductivity. Over four years we measured the hourly pattern of Lulu date-palm water use, ETc, at two levels of irrigation-water salinity: Treatment S1 at 5 dS m−1, and S3 at 15 dS m−1. The mid-summer ETc for the S1 Lulu trees is up to 190 L d−1, on average, whereas for the S3 trees ETc is lower at 130 L d−1 (68% of S1) because of the salt. Measurements of canopy radiation interception using a ‘light stick’ showed the S1 trees intercepted 26% of the incident radiation, whereas the S3 trees only intercepted 20% (ratio S3/S1 = 76%). The date yield of the S1 trees was 68 kg tree−1, but was 46 kg tree−1 for the S3 trees (ratio 68%). Current practice is to irrigate trees with 275 L d−1, irrespective of salinity. Our recommendation for Law 5 is to tailor irrigation to the seasonal demand in the reference evapotranspiration of ETo, and allow for a 25% factor-of-safety and a 25% salt leaching fraction. For S1 date palms this would mean an annual average of 210 L d−1, and for S3 just 137 L d−1. This represents savings of 25–50% from current practice.

Better tree performance and water use efficiency through resilient root systems: rapid screening for drought and salinity stress tolerance using relative growth rate

Abiotic stresses, such as soil water deficit and high soil solution salts, can lead to negative impacts on crop growth, productivity and quality, impacting industry profitability. The first contact of the tree with these stresses is the root system, providing the possibility of improving tree resilience by rootstock choice. Identifying almond rootstock genotypes with high stress tolerance is, consequently, of high importance in enabling the Australian almond industry to cope with these environmental challenges. This study developed and tested a potential tool to screen genotypes for tolerance to abiotic stresses, using the response of relative growth rate (RGR) to water deficit or salinity stress. Seedling plants of six almond rootstock cultivars were randomly allocated into three treatments (control, drought or salinity stress) for two months in a glasshouse. Plants in the well-watered treatment were irrigated to the point of run-off daily. The drought treatment was imposed by adding 33% of evapotranspiration of well-watered plants daily. Plants in the salinity treatment were supplied with mixed chloride solution (3.3 dS m-1) to the point of run-off daily. Plants were harvested at three time points to calculate RGR. The percentage of RGR reduction in stressed plants, relative to the non-stressed plants, was calculated and used as an index of stress tolerance. Considerable variation for stress tolerance was found among the six almond rootstock genotypes, with some genotypes being able to maintain growth rates - whereas others stopped growing - under drought or salinity stress. The impacts of the drought treatment used were generally larger than those of salinity stress. The screening approach introduced in this study provides a relatively simple and robust method to compare rootstock genotypes, contributing to horticultural management decisions, whilst also being applicable to the generation of fundamental knowledge in this area.

Mediterranean trees coping with severe drought: Avoidance might not be safe

Plants coexisting in Mediterranean regions differ widely in their sensitivity to drought and in their ability to maintain carbon assimilation and hydraulic function ranging from avoidance to tolerance of stressful periods. We studied the response of three coexisting Mediterranean tree species (Quercus ilex L. spp. Ballota (Desf.) Samp., Quercus faginea Lam. and Pinus nigra ssp. Salzmannii J.F. Arnold) under natural conditions for three years to test their functional strategies in dealing with extremely dry years. Despite P. nigra following a drought-avoiding strategy, it was unable to effectively avoid the hampering effects of a severe drought in the driest year, and experienced significant drops in the efficiency of the hydraulic system, in gas exchange at the leaf level and in tree water use estimated from daily sap flow. In contrast, the two Quercus species showed a more drought-tolerant strategy by maintaining a more profligate water use at the leaf and tree level, with Q. ilex having a larger tolerance threshold to severe drought. The main decrease in carbon assimilation was due to stomatal closure, with a minor effect of photochemistry damage or down regulation in the three species. In the case of the two oaks, seasonal osmoregulation was inferred from changes in the osmotic potential at full turgor (πo), bringing about the lowest leaf osmotic potentials at turgor lost (πtlp) at late summer in both species, and maintenance of leaf turgor under the lowest midday leaf water potential (Ψmid). Seasonal changes in osmotic potentials were observed in the three years, more during the driest year. Plasticity in modulating the tolerance to leaf dehydration as the season advances, especially in response to the intensity of inter-annual summer droughts was higher for the evergreen Q. ilex than in the semi-deciduous Q. faginea. In spite of the intensity of the drought and the associated stress experienced by the three forest tree species, especially by P. nigra, mortality or severe defoliation were not observed after the extreme dry year. This reveals a large operational range of coping with water stress by the three species regardless of the hydric strategy, which is crucial for survival under the increasing drought expected in the most likely climate change scenarios for the region.

Hydraulic Constraints to Whole-Tree Water Use and Respiration in Young Cryptomeria Trees under Competition

Although extensive studies have focused on carbon and water balance from aboveground measurements, the link between the belowground and aboveground processes deserves greater attention. In this context, the aim of this work was to assess the bi-directional feedback between whole-plant respiration and transpiration. The study was performed on 25 saplings of Sugi (Japanese cedar, Cryptomeria japonica D. Don), including dominant and suppressed individuals (total fresh weight ranging between 0.2 and 8.0 kg). During one week, the integrated water use (WU) was determined using the Deuterium dilution method. After this, the trees were uprooted and the root, stem, and leaf respiration were measured using incubation chambers and CO2 infrared sensors. The stem and root respiration followed a power response to mass (power exponent b &lt; 1), implying a decline in mass-specific respiration with size. Conversely, the leaf respiration followed a near-linear increase with size (power exponent b ≈ 1), but was negatively affected by the stem density, indicating the hydraulic limitations of the leaf metabolism. The water use followed a power response with the tree size (b &lt; 1), showing a decline in the transpiration per leaf mass with the tree size, but was also negatively correlated with the stem density. Our results indicate that dominant trees are more efficient in the use of water, and highlight the role of hydraulic limitations to leaf metabolism in suppressed trees.