Photosynthesis is limited by three main factors: stomatal conductance (gs), mesophyll conductance (gm) and maximum capacity for Rubisco carboxylation (Vcmax). It is unclear how limiting factors vary under stress, particularly during long-term stress acclimation. In this work, we compared for the first time photosynthesis limitation resulting from long-term acclimation to three major abiotic stresses: drought, salinity and temperature. We used saplings of Ziziphus spina-christi, a thermophilic and drought-tolerant tree, which recently became more abundant in the Mediterranean, presumably due to increased winter temperatures. Stress acclimation was investigated by measuring growth, gas exchange, chlorophyll fluorescence and leaf structure. For each stress, photosynthesis-limiting factors were compared. We developed an integrative stress index that allowed us to precisely define stress level, enabling a comparison between stress types. Photosynthesis under all stresses was limited mostly by gs and gm (80–90%); whereas biochemistry (Vcmax) made a minor contribution (10–20%). The relative contribution of gs and gm on photosynthetic limitation was influenced by stress type. During acclimation to drought or salinity, photosynthesis was limited by a decline in gs, while intolerance to low temperatures was driven by decline in gm. In all the stresses, gm decreased only under progressive reduction in leaf physiological functionality and was associated with low turgor under drought, an increase in leaf Na+ under salinity and low leaf hydraulic conductance (Kleaf) at low temperatures. Mesophyll structure (mesophyll surface area exposed to the intercellular air spaces, leaf thickness, % intercellular air spaces) did not explain gm acclimation to stress. Current work gives methodology for stress studies, and defines the main factors underlying the plant response to climate change. The ability to minimize mesophyll-imposed limitations on photosynthesis was found as a strong indicator of progressive stress tolerance. Moreover, the results demonstrate how warming climate benefits the photosynthetic function in thermophilic species, such as Ziziphus spina-christi. © The Author(s) 2018.

Photoprotection strategies in a Pinus halepensis Mill. forest at the dry timberline that shows sustained photosynthetic activity during 6-7 month summer drought were characterized and quantified under field conditions. Measurements of chlorophyll fluorescence, leaf-level gas exchange and pigment concentrations were made in both control and summer-irrigated plots, providing the opportunity to separate the effects of atmospheric from soil water stress on the photoprotection responses. The proportion of light energy incident on the leaf surface ultimately being used for carbon assimilation was 18% under stress-free conditions (irrigated, winter), declining to 4% under maximal stress (control, summer). Allocation of absorbed light energy to photochemistry decreased from 25 to 15% (control) and from 50% to 30% (irrigated) between winter and summer, highlighting the important role of pigment-mediated energy dissipation processes. Photorespiration or other non-assimilatory electron flow accounted for 15-20% and  10% of incident light energy during periods of high and low carbon fixation, respectively, representing a proportional increase in photochemical energy going to photorespiration in summer but a decrease in the absolute amount of photorespiratory CO loss. Resilience of the leaf photochemical apparatus was expressed in the complete recovery of photosystem II (PSII) efficiency (φPSII) and relaxation of the xanthophyll de-epoxidation state on the diurnal cycle throughout the year, and no seasonal decrease in pre-dawn maximal PSII efficiency (Fv/Fm). The response of CO assimilation and photoprotection strategies to stomatal conductance and leaf water potential appeared independent of whether stress was due to atmospheric or soil water deficits across seasons and treatments. The range of protection characteristics identified provides insights into the relatively high carbon economy under these dry conditions, conditions that are predicted for extended areas in the Mediterranean and other regions due to global climate change. © 2019 The Author(s). Published by Oxford University Press. All rights reserved.

In the future drought events are expected to occur more frequently, with unpredictable rain and heat events. In current research we investigated how different water availability patterns influenced late season plant water status in Vitis vinifera. ‘Cabernet Sauvignon’ grapevines were grown for three consecutive years. We compared the response to five water availability regimes: High, Intermediate, Low (along all season) and High-to-Low (High during the beginning of vegetative seasons switched to low during the rest of season) and Low-to-High (opposite treatment). Midday stem water potential (SWP) was measured weekly to determine the seasonal pattern of drought stress. Xylem anatomy was investigated by trunk vessel diameter measurements, and specific axial xylem conductivity was calculated according to Hagen-Poiseuille's law. Vines exposed to high water availability treatment showed improved seasonal water status along the season, compared to vines in the low treatment. Vines exposed to High-to-Low water regime showed a markedly improved water status at the beginning of the season, but became the most severely stressed toward the end of season. The SWP values were more negative in the High-to-Low regime even when compared to the Low water regime. Water availability at the beginning of the season (during main period of cambial activity) determined the vessel characteristics: high water availability during cambial activity increased vessel diameter and thus specific hydraulic conductivity. Our data strongly indicates that regulated drought stress can be induced by manipulating xylem structural parameters via controlling water availability during the period of stem cambial activity. © 2019 Elsevier B.V.

Most cultivated vineyards worldwide are located in semi-arid and arid regions with a limited water supply. Skilled vineyard water management is considered the main tool for controlling vegetative growth and grape quality and for ensuring vineyard sustainability. Imposing an appropriate drought stress at a suitable phenological stage can improve wine quality with almost no yield reduction. A comprehensive irrigation model enabling precise vineyard irrigation should be based on changes in vine water consumption as a function of climate conditions and canopy area. In 2011, six drainage lysimeters were constructed within a commercial 'Cabernet Sauvignon' vineyard located in the central mountains of Israel. Data were collected during six successive years from 2012 – 2017. The daily vine water consumption, ETc (L day−1), was calculated by subtracting the amount of collected drainage (over a 24 h period) from the amount of applied irrigation during the same time period. Seasonal water consumption (ETc) was 715 mm season−1 on average, while seasonal calculated reference evapotranspiration (ETo) was 1237 mm season−1 on average. Maximal crop coefficient (Kc) was 0.8 – 0.9, meaning that actual water consumption was lower than the calculated reference evapotranspiration. Maximal leaf area index (LAI) was 0.9 to 1.7 m2 m-2. The multi-seasonal linear correlation between LAI and Kc was strongly positive and significant. The robust multiyear relationship between LAI & Kc proves that measuring canopy area of wine grapevines is a reliable approach for estimating their Kc. The LAI to Kc relationship that we have established can be used as a basis for developing a comprehensive irrigation model for wine grapevines that integrates both climatic conditions and canopy area. © 2019 Elsevier B.V.

The study of plant anatomy, which can be traced back to the seventeenth century, advanced hand in hand with light microscopy technology and relies on traditional histologic techniques, which are based on serial two-dimensional (2D) sections. However, these valuable techniques lack spatial arrangement of the tissue and hence provide only partial information. A new technique of whole-mount three-dimensional (3D) imaging termed high-resolution episcopic microscopy (HREM) can overcome this obstacle and generate a 3D model of the specimen at a near-histological resolution. Here, we describe the application of HREM technique in plants by analyzing two plant developmental processes in woody plants: oil secretory cavity development in citrus fruit and adventitious root formation in persimmon rootstock cuttings. HREM 3D models of citrus fruit peel showed that oil cavities were initiated schizogenously during the early stages of fruitlet development. Citrus secretory cavity formation, shape, volume, and distribution were analyzed, and new insights are presented. HREM 3D model comparison of persimmon rootstock clones, which differ in their rooting ability, revealed that difficult-to-root clones failed to develop adventitious roots due to their inability to initiate root primordia. © 2019 The Authors. Plant Direct published by American Society of Plant Biologists, Society for Experimental Biology and John Wiley & Sons Ltd.

Abscission is a developmental process with important implications for agricultural practices. Ethylene has long been considered as a key regulator of the abscission process. The existence of an ethylene-independent abscission pathway, controlled by the complex of INFLORESCENCE DEFICIENT IN ABSCISSION (IDA) peptide and the HAESA (HAE) and HAESA-like2 (HSL2) kinases, has been proposed, based mainly on observations that organ abscission in ethylene-insensitive mutants was delayed but not inhibited. A recent review on plant organ abscission signaling highlighted the IDA-HAE-HSL2 components as the regulators of organ abscission, while the role of auxin and ethylene in this process was hardly addressed. After a careful analysis of the relevant abscission literature, we propose that the IDA-HAE-HSL2 pathway is essential for the final stages of organ abscission, while ethylene plays a major role in its initiation and progression. We discuss the view that the IDA-HAE-HSL2 pathway is ethylene independent, and present recent evidence showing that ethylene activates the IDA-HAE-HSL2 complex. We conclude that the ability of an organ to abscise is tightly linked to cell turgidity in the abscission zone, and suggest that lack of cell turgidity might contribute to the failure of floral organ abscission in the ida mutants. © 2019 The Author(s).

Traditional sheep grazing in natural and planted forests in the Mediterranean basin is based on night penning in seasonal corrals, where excreta accumulate instead of being returned to the grazed range. Lack of planning and unawareness of the long-term effect of abandoned corrals is negatively affecting the landscape and grazing value of the forests. We studied the dynamics of soil nutrients in a chronosequence of abandoned sheep corrals in planted Eucalyptus forests in two semi-arid sites in Israel. Dung decomposition was a slow process lasting 5–10 years. Soluble-N, P and K in the soil beneath the dung layer decreased gradually. Yet, 15–20 years after corral abandonment K and P were still 2 to 3 times higher than in the surrounding range, while soluble-N decreased within 10–15 years. Biomass production in the abandoned corrals was 2–3 times higher than in the grazed range up to 20 years after abandonment. Corrals act as sinks of soil nutrients that are lost to the grazed range. We propose that a balance between nutrient inputs (i.e. atmospheric deposition, N-fixation) and outputs (i.e. grazing and night penning) is reached in the grazed range at a low level of soil nutrients, which may constrain vegetation productivity. © 2019 Elsevier Ltd

Goat herding is an important tool in the ecologically sound management of Mediterranean shrublands and woodlands, although effective levels of woody biomass removal by the goats is neither guaranteed nor easy to predict. Preliminary observations indicated that one reason for this may be poor understanding of plant-herbivore interactions that operate intraspecifically at the local spatial scale. We asked, whether goats show intraspecific preferences among neighboring plants when foraging a small local population of Pistacia lentiscus, a dominant tall shrub. First, we characterized and quantified the profile of stored and emitted volatile organic compounds (VOCs) and the PEG-binding capacity of tannins (a proxy for protein binding capacity) in the foliage of P. lentiscus shrubs, sampled within an area of 0.9 ha. We then tested goat preference between pairs of these shrubs that differed in chemical composition. Almost all sampled P. lentiscus shrubs were allocated to one of two distinct VOC chemotypes: one dominated by germacrene D and limonene (designated chemotype L) and the other by germacrene D and α-pinene (chemotype P). In contrast, continuous moderate variability was found in the binding capacity of tannins in the foliage. Goats showed preference for shrubs of chemotype L over those of chemotype P, and their preference was negatively correlated with the binding capacity of tannins. Possible influences of VOCs on goat preference that may explain the observed patterns are discussed in the light of possible context-dependent interpretation of plant VOC signals by large mammalian herbivores. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

Ambrosia, specifically Ambrosia artemisiifolia, are known throughout the world as invasive, allergenic and noxious weeds. This research leads to the first map of the spread of Ambrosia species in Israel and describes the risk associated with their distribution to the public health. Six Ambrosia species were identified in Israel. There is one invasive species, A. confertiflora DC (Burr ragweed), which is most abundant in central Israel. There are three naturalized species: A. tenuifolia Spreng (Lacy ragweed) which is found in several locations; A. psilostachya DC (Cuman ragweed) and A. grayi (woolly leaf bur ragweed), which are restricted to a single location each. There are two casual species: A. artemisiifolia L. (short ragweed, common ragweed) and A. trifida L. (Giant ragweed). There are pronounced and clear differences between the species in their life cycle, morphology and phenology, which may explain the level of invasion of each one in Israel. The causes of the invasion are mainly anthropogenic. Many populations of Ambrosia are found near fishponds and animal feed centers, indicating that ragweed seeds feasibly arrived to Israel in grain shipments. Human sensitization to local pollen extracts of A. confertiflora and A. tenuifolia was studied by skin test reaction and compared with commercial extracts of A. artemisiifolia and A. trifida. Patient’s response was three times stronger in A. confertiflora with respect to the other three species. The rapid dissemination of A. confertiflora, the manner in which its pollen is dispersed and its allergenic potential indicate risks to public health. © 2018, Springer Nature B.V.

Field dodder is an obligatory stem and leaf plant parasite that causes significant damage in field and vegetable crops in all agricultural regions of the globe. Selective and effective measures to control the parasite are extremely limited. In recent studies, we have shown that granular formulations of dinitroaniline cell division-inhibiting herbicides applied after crop establishment and before dodder germination fit our dodder control strategy and kill the parasite effectively and selectively. The aim of our study conducted from 2014 to 2018 was to evaluate the efficacy and selectivity of granular pendimethalin for dodder control in chickpea under laboratory, greenhouse, and field conditions. Petri dish experiments revealed that the herbicide reduces dodder seed germination while its main effect is a restriction of shoot elongation. Greenhouse experiments demonstrated that the inhibition and distortion of dodder shoot growth impede shoot twining and prevent attachment to the host plant. In dose-response experiments conducted in the greenhouse, we observed that half the recommended rate of granular pendimethalin provides efficient dodder control with no damage to chickpea seedlings. In 3 yr of chickpea field trials, GPM applied across the seeding bed at the recommended rate resulted in high crop yields that were not significantly different from those observed for the untreated no-dodder control, while half of the recommended dose efficiently controlled dodder and other weeds with no damage to the crop, resulting in significantly increased chickpea yields and profitability. These studies indicate that GPM can provide efficient and selective dodder control in chickpea. © Weed Science Society of America, 2019.

We have recently shown that Rickettsia, a secondary facultative bacterial symbiont that infects the whitefly B. tabaci is implicated in the transmission of Tomato yellow leaf curl virus (TYLCV). Infection with Rickettsia improved the acquisition and transmission of the virus by B. tabaci adults. Here we performed a transcriptomic analysis with Rickettsia-infected and uninfected B. tabaci adults before and after TYLCV acquisition. The results show a dramatic and specific activation of the immune system in the presence of Rickettsia before TYLCV acquisition. However, when TYLCV was acquired, it induced massive activation of gene expression in the Rickettsia uninfected population, whereas in the Rickettsia-infected population the virus induced massive down-regulation of gene expression. Fitness and choice experiments revealed that while Rickettsia-infected whiteflies are always more attracted to TYLCV-infected plants, this attraction is not always beneficiary for their offspring. These studies further confirm the role of Rickettsia in many aspects of B. tabaci interactions with TYLCV, and possibly serves as an important factor in the dissemination of the virus. © 2019 The Authors.

Many animal and plant viruses depend on arthropods for their transmission. Virus-vector interactions are highly specific, and only one vector or one of a group of vectors from the same family is able to transmit a given virus. Poleroviruses (Luteoviridae) are phloem-restricted RNA plant viruses that are exclusively transmitted by aphids. Multiple aphid-transmitted polerovirus species commonly infect pepper, causing vein yellowing, leaf rolling, and fruit discoloration. Despite low aphid populations, a recent outbreak with such severe symptoms in many bell pepper farms in Israel led to reinvestigation of the disease and its insect vector. Here we report that this outbreak was caused by a new whitefly (Bemisia tabaci)-transmitted polerovirus, which we named Pepper whitefly-borne vein yellows virus (PeWBVYV). PeWBVYV is highly (95%) homologous to Pepper vein yellows virus (PeVYV) from Israel and Greece on its 5ʹ end half, while it is homologous to African eggplant yellows virus (AeYV) on its 3ʹ half. Koch’s postulates were proven by constructing a PeWBVYV infectious clone causing the pepper disease, which was in turn transmitted to test pepper plants by B. tabaci but not by aphids. PeWBVYV represents the first report of a whitefly-transmitted polerovirus. IMPORTANCE The high specificity of virus-vector interactions limits the possibility of a given virus changing vectors. Our report describes a new virus from a family of viruses strictly transmitted by aphids which is now transmitted by whiteflies (Bemisia tabaci) and not by aphids. This report presents the first description of polerovirus transmission by whiteflies. Whiteflies are highly resistant to insecticides and disperse over long distances, carrying virus inoculum. Thus, the report of such unusual polerovirus transmission by a supervector has extensive implications for the epidemiology of the virus disease, with ramifications concerning the international trade of agricultural commodities. © 2019 American Society for Microbiology. All Rights Reserved.

Females and males of 7 commercial (ComCb) and 3 experimental (ExpCb) crossbreeds were produced by 6 imported parent stocks (PS) and 1 local PS. The ComCb were Dominant Red Barred (DR), Dominant Sussex (DS), Lohmann Brown (LB), Lohmann-Dual, NOVOgen Brown (NB), NOVOgen Color (NC), and local Koekoek (KK). The ExpCb were (dams × sires) DR × KK (R × K), DS × DR (S × R), and KK × DS (K × S). The females were reared to 60 wk of age, and recorded data included BW, BWG, feed intake, egg number, and egg weight, allowing the calculation of egg mass and feed conversion ratio (FCR). The males were reared to 16 wk of age and recoded data included feed intake, BW, FCR, and carcass characteristics. A total of 621 females and 516 males were tested in sex-separated trials, each with 3 replicated floor pens per crossbreed. The overall value of each crossbreed was determined by overall egg production, 60-wk BW and FCR of females, and by 16-wk BW, carcass yield, and FCR of males. The highest laying rate was exhibited by LB (68%) and NB (66%), followed by RxK and K × S (∼62%). The crossbreeds differed in feed intake and in females' FCR, with LB leading (2.74) followed by NB and S × R (3.07) then DR (3.18). In egg production, LB, followed by NB, were the best, as expected from specialized table-egg crossbreeds. The highest 16-wk BW of males and best FCR were exhibited by NC, followed by NB. In summary, LB was the best egg-producing crossbreed, but poor in meat production. Better choice for dual-purpose production would be NB, ranked second in egg production and in males' BW and FCR. NC was the best meat-producing (males and spent hens) crossbreed and the hens were second in egg-mass production. Hence, NC might be the best dual-purpose hybrid where artificial insemination is feasible and the consumers prefer large eggs and birds. Published by Oxford University Press on behalf of Poultry Science Association 2019.

A total of 6 chicken parent stocks (PS) bred by European companies, and 1 local PS, were evaluated under Ethiopian condition for their reproductive performance, to be followed by testing dual-purpose performance of their crossbreed progeny. The imported PS were Lohmann Brown (LB), Lohmann Dual (LD), NOVOgen Brown (NB), NOVOgen Color (NC), Dominant Sussex (DS), and Dominant Red Barred (DR); Koekoek (KK) was obtained locally. They were reared in replicated floor-pens from 16 to 60 wk of age, and evaluated for feed intake, body weight (BW), egg production, fertility, and hatchability. In total, 1,810 females and 261 males were distributed over 4 houses in randomized blocks design. Additionally, 3 experimental crosses, R × K (DR females × KK males), S × R (DS females × DR males), and K × S (KK females × DS males) were evaluated for fertility and hatchability. The PS differed in BW, feed intake, age of sexual maturity, egg production, fertility, and hatchability. Among females, DR and DS had the highest BW, whereas LB, NB, and NC had the lowest BW. Final mean BW of the parental meat-type males of NC and LD were the highest (5,027 and 3,660 g, respectively), whereas the other parental males ranged from 2,585 to 2,955 g. Fertility of NC and LD was low because the heavy parental males had difficulty to mate naturally their small-body female mates. However, with artificial insemination (AI), fertility of NC and LD was between 75 and 80%, similar to the other 5 PS. The fertility and hatchability of eggs laid by DR, DS, and KK hens was improved by 6.3% in the experimental crosses, where these hens were mated with genetically different males. The LD hens exhibited the highest overall laying rate (64.2%) during the study period, and with AI, hatchability of LD eggs (66.6%) was the highest, making it the best chick producer. Thus, despite its high total feed intake (but similar to DR, DS, and KK), LD (followed by DR) was the best PS in this study under floor management in Ethiopia. Published by Oxford University Press on behalf of Poultry Science Association 2019.

The evolutionary route from plant tissue culture (IAPTC) to plant biotechnology (IAPB). Plant biotechnology is an evolutionary scientific process, formulated and maintained by our accumulated cultural-societal knowledge and the invention of new technologies (Altman and Mesoudi submitted). It emerged thousands of years ago when wheat, rice, chickpeas, potatoes, and coffee (and other plants) were first domesticated; when grains were fermented by yeasts to produce bread; and when grape juice, barley, and tubers fermentation resulted in wine, alcohol, and beer. The modern era of plant biotechnology started in the beginning of the twentieth century and is associated with the ability to grow plant cells and tissues in vitro, to regenerate and clone new plants and, later, to modify their genetic characteristics by molecular breeding, including molecular marker-assisted selection (MAS), genetic modification (GM), and, more recently, genome editing. Additional novel procedures will most probably follow in the future. © 2019, The Society for In Vitro Biology.

Since its emergence around 12,000 years ago, agriculture has transformed our species, other species, and the planet on which we all live. Here we argue that the emergence and impact of agriculture can be understood within new theoretical frameworks developing within the evolutionary human sciences. First, the improvement and diversification of agricultural knowledge, practices, and technology is a case of cumulative cultural evolution, with successive modifications accumulated over multiple generations to exceed what any single person could create alone. We discuss how the factors that permit, facilitate, and hinder cumulative cultural evolution might apply to agriculture. Second, agriculture is a prime example of gene-culture co-evolution, where culturally transmitted agricultural practices generate novel selection pressures for genetic evolution. While this point has traditionally been made for the human genome, we expand the concept to include genetic changes in domesticated plants and animals, both via traditional breeding and molecular breeding. Third, agriculture is a powerful niche-constructing activity that has extensively transformed the abiotic, biotic, and social environments. We examine how agricultural knowledge and practice shapes, and are shaped by, social norms and attitudes. We discuss recent biotechnology and associated molecular breeding techniques and present several case studies, including golden rice and stress resistance. Overall, we propose new insights into the co-evolution of human culture and plant genes and the unprecedented contribution of agricultural activities to the construction of unique agriculture-driven anthropogenic biomes. © 2019, Springer Science+Business Media, LLC, part of Springer Nature.

Plants are autotrophic organisms in which there are linear relationships between the rate at which organic biomass is accumulated and many ambient parameters such as water, nutrients, CO2 and solar radiation. These linear relationships are the result of good feedback regulation between a plants sensing of the environment and the optimization of its performance response. In this review, we suggest that continuous monitoring of the plant physiological profile in response to changing ambient conditions could be a useful new phenotyping tool, allowing the characterization and comparison of different levels of functional phenotypes and productivity. This functional physiological phenotyping (FPP) approach can be integrated into breeding programs, which are facing difficulties in selecting plants that perform well under abiotic stress. Moreover, high-throughput FPP will increase the efficiency of the selection of traits that are closely related to environmental interactions (such as plant water status, water-use efficiency, stomatal conductance, etc.) thanks to its high resolution and dynamic measurements. One of the important advantages of FPP is, its simplicity and effectiveness and compatibility with experimental methods that use load-cell lysimeters and ambient sensors. In the future, this platform could help with phenotyping of complex physiological traits, beneficial for yield gain to enhance functional breeding approaches and guide in crop modeling.

Nutrient cycling in most terrestrial ecosystems is controlled by moisture-dependent decomposer activity. In arid ecosystems, plant litter cycling exceeds rates predicted based on precipitation amounts, suggesting that additional factors are involved. Attempts to reveal these factors have focused on abiotic degradation, soil–litter mixing and alternative moisture sources. Our aim was to explore an additional hypothesis that macro-detritivores control litter cycling in deserts. We quantified the role different organisms play in clearing plant detritus from the desert surface, using litter baskets with different mesh sizes that allow selective entry of micro-, meso- or macrofauna. We also measured soil nutrient concentrations in increasing distances from the burrows of a highly abundant macro-detritivore, the desert isopod Hemilepistus reaumuri. Macro-detritivores controlled the clearing of plant litter in our field site. The highest rates of litter removal were measured during the hot and dry summer when isopod activity peaks and microbial activity is minimal. We also found substantial enrichment of inorganic nitrogen and phosphorous near isopod burrows. We conclude that burrowing macro-detritivores are important regulators of litter cycling in this arid ecosystem, providing a plausible general mechanism that explains the unexpectedly high rates of plant litter cycling in deserts.

Carbon dynamics in canopy and roots influence whole tree carbon fluxes, but little is known about canopy regulation of tree-root activity. Here, we assess the patterns and dynamics of canopy-root carbon coupling in tropical trees. We used large aeroponics to directly study the root systems of Ceiba pentandra and Khaya anthotheca saplings at different light intensities. In Ceiba, root respiration (R ) co-varied with photosynthesis (A ) in large saplings (3-to-7-m canopy-root axis) at high-light, but showed no consistent pattern at low-light. At medium-light and in small saplings (c. 1-m axis), R tended to decrease transiently towards midday. Proximal roots had higher R and non-structural carbohydrate concentrations than distal roots, but canopy-root coupling was unaffected by root location. In medium-sized Khaya, no R pattern was observed, and in both species, R was unrelated to temperature. The early-afternoon increase in R suggests canopy-root coupling is based on mass flow of newly fixed carbon in the phloem, while the early-morning rise in R with A indicates an additional coupling signal that travels faster than the phloem sap. In large saplings and potentially also in higher trees, light and possibly additional environmental factors control the diurnal patterns of canopy-root coupling, irrespective of root location. This article is protected by copyright. All rights reserved.

Abstract Drought-related tree mortality had become a widespread phenomenon in forests around the globe. This process leading to these events and its complexity is not fully understood. Trees in the dry timberline are exposed to ongoing drought, and the available water for transpiration in the soil can determine their survival chances. Recent drought years led to 5%–10% mortality in the semi-arid pine forest of Yatir (Israel). The distribution of dead trees was, however, highly heterogeneous with parts of the forest showing >80% dead trees (D plots) and others with mostly live trees (L plots). At the tree level, visible stress was associated with low pre-dawn leaf water potential at the dry season (−2.8 MPa vs. −2.3 MPa in non-stressed trees), shorter needles (5.5 vs. 7.7 mm) and lower chlorophyll content (0.6 vs. 1 mg/g dw). Trends in tree-ring widths reflected differences in stress intensity (30% narrower rings in stressed compared with unstressed trees), which could be identified 15–20 years prior to mortality. At the plot scale, no differences in topography, soil type, tree age or stand density could explain the mortality difference between the D and L plots. It could only be explained by the higher surface rock cover and in stoniness across the soil profile in the L plots. Simple bucket model simulations using the site’s long-term hydrological data supported the idea that these differences could result in higher soil water concentration (m3/m3) in the L plots and extend the time above wilting point by several months across the long dry season. Accounting for subsurface heterogeneity may therefore critical to assessing stand-level response to drought and projecting tree survival, and can be used in management strategies in regions undergoing drying climate trends. A plain language summary is available for this article.