Stressed Trees Research Articles

European Spruce Bark Beetle, Ips typographus (L.) Males Are Attracted to Bark Cores of Drought-Stressed Norway Spruce Trees with Impaired Defenses in Petri Dish Choice Experiments

The European spruce bark beetle, Ips typographus (L.), is a major pest of Norway spruce. During outbreaks, the beetles can colonize moderately stressed trees via mass attacks mediated by aggregation pheromones, while at endemic population levels, beetles infest trees with impaired defenses. I. typographus introduces ophiostomatoid fungi into the phloem, which can support host colonization. Low-density fungal infections are locally contained by hypersensitive wound reactions; larger necrotic lesions indicate lower tree resistance. Here, we made links between drought stress, susceptibility to fungal infections, and the attractiveness of spruce for host-searching I. typographus males. We sampled bark cores from roofed, non-roofed and untreated control trees of a rainfall exclusion field site. Drought stress was assessed using pre-dawn twig water potentials, and tree defenses were assessed using inoculations with Grosmannia penicillata. Subsequently, we performed Petri dish arena choice tests in the lab, where male beetles could choose between the bark samples of differentially stressed trees. We found that the attractiveness of bark cores increased with drought stress and the extent of hypersensitive wound reactions to fungal infection. Furthermore, beetles stayed longer in those Petri dish sections with the sample of their final choice. The bioassays provide evidence for the primary attraction of male I. typographus to tissues of Norway spruce and preference of beetles for stressed trees.

Numbers matter: how irruptive bark beetles initiate transition to self-sustaining behavior during landscape-altering outbreaks.

Irruptive forest insects such as bark beetles undergo intermittent outbreaks that cause landscape-scale tree mortality. Despite their enormous economic and ecological impacts, we still have only limited understanding of the dynamics by which populations transition from normally stable endemic to irruptive densities. We investigated density-dependent changes in mountain pine beetle reliance on stressed hosts, host selection, spatial configuration of attacks, and the interaction of host selection and spatial configuration by performing a complete census of lodgepole pine across six stands and 6 years. In addition, we compared the dynamics of mountain pine beetle with those of other bark beetles. We found that as population size increased, reliance on stressed trees decreased and new attacks shifted to larger trees with thicker phloem and higher growth rates that can support higher offspring production. Moreover, the spatial configuration of beetle-attacked trees shifted from random to spatially aggregated. Further, we found evidence that beetle utilization of larger trees was related to aggregation behavior as the size of tree attacked was positively correlated at 10-25m, within the effective distance of pheromone-mediated signaling. In contrast, non-irruptive bark beetle species did not exhibit such density-dependent spatial aggregation at the stand scale or switches in host selection behavior. These results identify how density-dependent linkages between spatial configuration and host utilization can converge to drive population transitions from endemic to irruptive phases. Specifically, a combination of stand-level spatial aggregation, behavioral shifts, and higher quality of attainable hosts defines a critical threshold beyond which continual population growth becomes self-driving.

Early detection of spruce vitality loss with hyperspectral data: Results of an experimental study in Bavaria, Germany

Vitality loss of trees caused by extreme weather conditions, drought stress or insect infestations, are expected to increase with ongoing climate change. The detection of vitality loss at an early stage is thus of vital importance for forestry and forest management to minimize ecological and economical damage. Remote sensing instruments are able to detect changes over large areas down to the level of individual trees. The scope of our study is to investigate whether it is possible to detect stress-related spectral changes at an early stage using hyperspectral sensors. For this purpose, two Norway spruce (Picea abies) forest stands, both different in age and maintenance, were monitored in the field over two vegetation periods. In parallel, time series of airborne hyperspectral remote sensing data were acquired. For each stand 70 trees were artificially stressed (ring-barked) and 70 trees were used as control trees. The data collected in south-eastern Germany consists of measurements at multiple times and at different scales: (1) crown conditions were visually assessed in the field (2) needle reflectance spectra were acquired in the laboratory using a FieldSpec spectrometer, and (3) hyperspectral airborne data (HySpex) were flown at 0.5 m spatial resolution. We aimed for a simultaneous data acquisition at the three levels. This unique data set was investigated whether any feature can be discriminated to detect vitality loss in trees at an early stage. Several spectral transformations were applied to the needle and tree crown spectra, such as spectral derivatives, vegetation indices and angle indices. All features were examined for their separability (ring-barked vs. control trees) with the Random Forest (RF) classification algorithm. As result, the younger, well maintained forest stand only showed minor changes over the 2-year period, whereas changes in the older forest stand were observable both in the needle and in the hyperspectral tree crown spectra, respectively. These changes could even be detected before changes were visible by field observations. The tree spectral reactions to ring-barking were first noticeable 11 months after ring-barking and 6 weeks before they were visible by field inspection. The most discriminative features for separating the two groups were the reflectance spectra and the spectral derivatives, over the VIs or angle indices. The tree crown spectra of the two groups could be separated by the RF classifier with a 79% overall accuracy at the beginning of the second vegetation period and 1 month later with 92% overall accuracy with high kappa index. The results clearly demonstrate the great potential of hyperspectral remote sensing in detecting early vitality changes of stressed trees.

Modeling Climate Impacts on Tree Growth to Assess Tree Vulnerability to Drought During Forest Dieback.

Forest dieback because of drought is a global phenomenon threatening particular tree populations. Particularly vulnerable stands are usually located in climatically stressing locations such as xeric sites subjected to seasonal drought. These tree populations show a pronounced loss of vitality, growth decline, and high mortality in response to extreme climate events such as heat waves and droughts. However, dieback events do not uniformly affect stands, with some trees showing higher symptoms of drought vulnerability than other neighboring conspecifics. In this study, we investigated if trees showing different vulnerabilities to dieback showed lower growth rates (Grs) and higher sensitivities to the climate in the past using dendroecology and the Vaganov-Shashkin (VS) process-based growth model. We studied two Pinus pinaster stands with contrasting Grs showing recent dieback in the Iberian System, north-eastern Spain. We compared coexisting declining (D) and non-declining (ND) trees with crown defoliation values above and below the 50% threshold, respectively. The mean growth rate was lower in D than in ND trees in the two stands. The two vigor classes showed a growth divergence prior to the dieback onset and different responsiveness to climate. The ND trees were more responsive to changes in spring water balance and soil moisture than D trees, indicating a loss of growth responsiveness to the climate in stressed trees. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. Such an interaction between water availability and vigor was reflected by the VS-model simulations, which provided evidence for the observation that growth was mainly limited by low soil moisture in both sites. The presented comparisons indicated different stand vulnerabilities to drought contingent on-site conditions. Further research should investigate the role played by environmental conditions and individual features such as access to soil water or hydraulic traits and implement them in process-based growth models to better forecast dieback.

Sesquiterpenes and oxygenated sesquiterpenes dominate the VOC (C<sub>5</sub>–C<sub>20</sub>) emissions of downy birches

Abstract. Biogenic volatile organic compounds (BVOCs) emitted by the forests are known to have strong impacts in the atmosphere. However, lots of missing reactivity is found, especially in the forest air. Therefore better characterization of sources and identification/quantification of unknown reactive compounds is needed. While isoprene and monoterpene (MT) emissions of boreal needle trees have been studied quite intensively, there is much less knowledge on the emissions of boreal deciduous trees and emissions of larger terpenes and oxygenated volatile organic compounds (OVOCs). Here we quantified the downy birch (Betula pubescens) leaf emissions of terpenes, oxygenated terpenes and green leaf volatiles (GLVs) at the SMEAR II boreal forest site using in situ gas chromatographs with mass spectrometers. Sesquiterpenes (SQTs) and oxygenated sesquiterpenes (OSQTs) were the main emitted compounds. Mean emission rates of SQTs and OSQTs were significantly higher in the early growing season (510 and 650 ng gdw-1 h−1, respectively) compared to in the main (40 and 130 ng gdw-1 h−1, respectively) and late (14 and 46 ng gdw-1 h−1, respectively) periods, indicating that early leaf growth is a strong source of these compounds. The emissions had a very clear diurnal variation with afternoon maxima being on average 4 to 8 times higher than seasonal means for SQTs and OSQTs, respectively. β-Caryophyllene and β-farnesene were the main SQTs emitted. The main emitted OSQTs were tentatively identified as 14-hydroxy-β-caryophyllene acetate (M=262 g mol−1) and 6-hydroxy-β-caryophyllene (M=220 g mol−1). Over the whole growing season, the total MT emissions were only 24 % and 17 % of the total SQT and OSQT emissions, respectively. A stressed tree growing in a pot was also studied, and high emissions of α-farnesene and an unidentified SQT were detected together with high emissions of GLVs. Due to the relatively low volatility and the high reactivity of SQTs and OSQTs, downy birch emissions are expected to have strong impacts on atmospheric chemistry, especially on secondary organic aerosol (SOA) production.

Physiological and anatomical responses to drought stress differ between two larch species and their hybrid

Key MessageHybrid saplings were more reactive to soil water deficit than Japanese and European larch. European larch had hydraulically safer wood and anisohydric behavior, Japanese and hybrid larch showed isohydric strategy.Deciduous larch species could be an alternative to evergreen conifers in reforestation, but little is known about drought sensitivity of their saplings. The effect of an experimental drought on hydraulics and quantitative wood anatomy was tested on saplings of European larch (EL, Larix decidua), Japanese larch (JL, Larix kaempferi) and their hybrid (HL). Across species, biomass, transpiration rate and relative water content were higher in controls than in drought stressed trees, but transpiration efficiency was lower. JL had the highest transpiration efficiency under drought, and EL the lowest, coinciding with slower growth of EL. Wood of EL formed before drought was hydraulically safer as shown by higher wall/lumen ratio and lower pit cavity area. EL neither had a significant increase in transpiration efficiency nor a reduction in transpiration rate under drought, suggesting that the stomata remained open under soil water deficit. HL saplings were the most reactive to water shortage, indicated by intra-annual density fluctuations and a decrease in relative water content of the sapwood. Significant reduction in transpiration by HL suggested a higher stomatal sensitivity, while the same leaf surface area was maintained and radial growth was still similar to its best parent, the JL. The latter showed a significantly lower leaf surface area under drought than controls. EL, with its hydraulically safer wood, followed an anisohydric behavior, while JL and HL revealed an isohydric strategy. Altogether, our results suggest species dependent acclimations to drought stress, whereby HL followed the strategy of JL rather than that of EL.

Ambrosia beetle response to ethanol concentration and host tree species

AbstractEthanol emitted by stressed trees is an olfactory cue used by ambrosia beetles (Coleoptera; Curculionidae; Scolytinae) to locate susceptible hosts to colonize. In addition, ethanol enhances the growth of ambrosia beetle fungal symbionts, improving colonization. Whether host selection and colonization are affected also by the amount of ethanol produced by stressed trees and by tree species is unclear. To investigate these mechanisms, we induced attacks by ambrosia beetles in bolts of eight tree species by coring and filling them with either 5% or 90% ethanol solutions in water. For each ethanol concentration, bolts of the eight different tree species were replicated six times in a randomized complete block design. Entry holes were used as a proxy for host selection whereas gallery development stage was used as a proxy for colonization. Ethanol concentration differentially affected host selection of the three ambrosia beetles that were active during this study. Anisandrus dispar Fabricius preferentially attacked bolts with 90% ethanol concentration, Xylosandrus crassiusculus (Motschulsky) preferentially attacked bolts with 5% ethanol concentration, and Xyleborinus saxesenii (Ratzeburg) attacked bolts irrespective of ethanol concentration. Colonization of X. crassiusculus reflected the same pattern observed for entry holes. The effect of host tree species on host selection was most prominent for X. saxesenii, while X. crassiusculus established a higher number of developed galleries in Ostrya carpinifolia Scopoli bolts than on five of the other tested tree species. Our results suggest that ethanol concentration and host tree species may influence ecological niche partitioning among ambrosia beetle species.

Estimation of evapotranspiration and crop coefficient of drip-irrigated orange trees under a semi-arid climate

A sustainable agricultural system requires increasing water use efficiency and enhancing knowledge of crop water use. This prerequisite is more pronounced in the regions with inadequate water resources and limited observational data such as southern Iran. Therefore, this study aimed at finding the water requirement of mature orange trees (Citrus sinensis (L.) Osbeck, cv. Tarocco Ippolito) by identifying standard evapotranspiration rate and crop coefficients (single and dual). Seventy-two orange trees in a drip-irrigated orchard with loam soil were classified into six treatments and irrigated at 100%, 90%, 80%, 70%, 60%, and 50% of reference evapotranspiration rate during 2017 and 2018. Soil moisture variability and crop physiological responses, including stem water potential (Ψstem), net photosynthesis (An), and stomatal conductance (gs) were measured. Our results showed that irrigating at 90% ETo provided the full water requirements of the trees. The mean crop evapotranspiration rate was calculated as 5.2 mm day−1, with the crop coefficient ranging from 0.65 to 0.95. The average irrigation rate performed traditionally in the region was 19% higher than the actual requirement. Analysis of physiological response highlighted the controlling role of stomata in regulating transpiration and maintaining leaf turgor. During the peak water-stress, gs ranged from 0.11 to 0.12 molm−2 s−1 in fully irrigated trees, to 0.04–0.08 molm−2 s−1 in highly stressed trees. Our findings will provide a useful guideline for the local growers and agencies to achieve better irrigation scheduling and higher water productivity for the region.

Early detection of forest stress from European spruce bark beetle attack, and a new vegetation index: Normalized distance red & SWIR (NDRS)

The European spruce bark beetle (Ips typographus [L.]) is one of the most damaging pest insects of European spruce forests. A crucial measure in pest control is the removal of infested trees before the beetles leave the bark, which generally happens before the end of June. However, stressed tree crowns do not show any significant color changes in the visible spectrum at this early-stage of infestation, making early detection difficult. In order to detect the related forest stress at an early stage, we investigated the differences in radar and spectral signals of healthy and stressed trees. How the characteristics of stressed trees changed over time was analyzed for the whole vegetation season, which covered the period before attacks (April), early-stage infestation (‘green-attacks’, May to July), and middle to late-stage infestation (August to October). The results show that spectral differences already existed at the beginning of the vegetation season, before the attacks. The spectral separability between the healthy and infested samples did not change significantly during the ‘green-attack’ stage. The results indicate that the trees were stressed before the attacks and had spectral signatures that differed from healthy ones. These stress-induced spectral changes could be more efficient indicators of early infestations than the ‘green-attack’ symptoms.In this study we used Sentinel-1 and 2 images of a test site in southern Sweden from April to October in 2018 and 2019. The red and SWIR bands from Sentinel-2 showed the highest separability of healthy and stressed samples. The backscatter from Sentinel-1 and additional bands from Sentinel-2 contributed only slightly in the Random Forest classification models. We therefore propose the Normalized Distance Red & SWIR (NDRS) index as a new index based on our observations and the linear relationship between the red and SWIR bands. This index identified stressed forest with accuracies from 0.80 to 0.88 before the attacks, from 0.80 to 0.82 in the early-stage infestation, and from 0.81 to 0.91 in middle- and late-stage infestations. These accuracies are higher than those attained by established vegetation indices aimed at ‘green-attack’ detection, such as the Normalized Difference Water Index, Ratio Drought Index, and Disease Stress Water Index. By using the proposed method, we highlight the potential of using NDRS with Sentinel-2 images to estimate forest vulnerability to European spruce bark beetle attacks early in the vegetation season.

Aggressive tree killer or natural thinning agent? Assessing the impacts of a globally important forest insect

Invasive insects and pathogens are prominent tree mortality agents in forests around the world, and the magnitude of their impacts is increasing. Comparative studies across multiple populations can be helpful for the development of new insights and innovative management strategies. We used the Sirex woodwasp, Sirex noctilio Fabricius, as a model system to compare invasion impacts across a range of ecological contexts around the globe: native woodwasps colonizing native trees, invasive woodwasps in non-native plantation trees, and invasive woodwasps attacking native trees. Across 133 stands of eight pine species on four continents, tree mortality associated with S. noctilio attack was positively correlated with stand basal area and stand density, and was mostly confined to smaller, suppressed trees. Larger average tree size and greater distances between trees were linked to lower levels of tree mortality. To more deeply assess the impacts of tree loss due to this pest, we examined mortality in vigorous trees, defined as those with a stem diameter greater than or equal to 90% of the mean diameter for trees in the stand. Sirex noctilio- related mortality in vigorous trees was rare, with one exception where Pinus contorta stands in Argentina lost as many as 300 vigorous trees ha−1. Pine species varied dramatically in their susceptibility to S. noctilio: for example, these losses in P. contorta were in stark contrast to very low mortality in P. ponderosa, the other pine species grown in Argentina. Surprisingly, location did not alter patterns in the influence of stand basal area on tree susceptibility for individual species. Most notably, Pinus radiata had the same relationship between basal area and tree mortality when grown in Spain (where S. noctilio is native and not considered a meaningful forest pest) and South Africa (where S. noctilio is a problematic invasive). Our findings suggest that the availability of optimal pine hosts is a key driver of S. noctilio-related tree mortality across continents and management regimes. Important variables that influence host availability include species-specific susceptibility and environmental and management-related factors that promote or limit the number of stressed trees present both within stands and across the regional forest or plantation landscape.

Impact of braconid wasps on larval performance of longhorned borer Coptocercus rubripes Boisduval (Coleoptera: Cerambycidae) in Eucalyptus forest of southwestern Australia

AbstractIn southwestern Australia, outbreaks of cerambycids have been associated with infestation of drying and stressed trees. Felled timber provides an abundant habitat for a variety of subcortical beetles where their development can be modified by attack from natural enemies, including parasitoids. There are a variety of parasitoid wasps of cerambycids native to Australia, yet their effectiveness in controlling the larval performance of borers has been little studied. Approximately 700 Eucalyptus trees felled to realign a forest road provided an opportunity to determine how a mass of drying logs influences borer infestation rates and whether parasitoid wasps were effective in controlling borer survival. The native borer Coptocercus rubripes (Coleoptera: Cerambycidae) caused an extensive infestation of fallen trees at the clearfell site. These infestations were subsequently limited by the direct effect of parasitism from the native braconid wasps Callibracon limbatus and Syngaster lepidus emerging from a single hole in a white cocoon. The effects of parasitism showed inverse density dependency where high borer larval densities increased the chances of borer larval survival. The absence of parasitism in caged logs promoted high borer larval densities and greatly improved adult borer survival, and while this increased the demand for sapwood resources, it promoted early adult borer emergence. Control by native braconid wasps of high infestations of Co. rubripes borers attacking drying and stressed trees limits adult borer emergence and has the potential to partially regulate borer populations.

Friendly neighbours: Hydraulic redistribution accounts for one quarter of water used by neighbouring drought stressed tree saplings.

Hydraulic redistribution (HR) can buffer drought events of tree individuals, however, its relevance for neighbouring trees remains unclear. Here, we quantified HR to neighbouring trees in single- and mixed-species combinations. We hypothesized that uptake of HR water positively correlates with root length, number of root tips and root xylem hydraulic conductivity and that neighbours in single-species combinations receive more HR water than in phylogenetic distant mixed-species combinations. In a split-root experiment, a sapling with its roots split between two pots redistributed deuterium labelled water from a moist to a dry pot with an additional tree each. We quantified HR water received by the sapling in the dry pot for six temperate tree species. After 7 days, one quarter of the water in roots (2.1 ± 0.4 ml), stems (0.8 ± 0.2 ml) and transpiration (1.0 ± 0.3 ml) of the drought stressed sapling originated from HR. The amount of HR water transpired by the receiving plant stayed constant throughout the experiment. While the uptake of HR water increased with root length, species identity did not affect HR as saplings of Picea abies ((L.) Karst) and Fagus sylvatica (L.) in single- and mixed-species combinations received the same amount of HR water.

Water Stress Alter Leaf Hydric Status and Flower Bud Development in Apricot cv. “Royal”

Aims: The apricot (Prunus armeniaca L.), is a drought-sensitive deciduous fruit. This concept arises from the fact that soil moisture stress can: Decrease the number and quality of flower buds differentiated; delay the time of flower differentiation and decrease the number of flower buds per shoot. The objectives of this investigation were to determine: The extent to which drought influences water status in the leaves; its effect on flower buds development and on bloom in apricot cv. “Royal”.
 Study Design: Trees were divided into 6 groups of six replicate each under a random block design. Results were analyzed using the statistical program 'RStudio' for Windows version 10 and data obtained subjected to a comparison of means with the Tukey (P≤0.05) test.
 Place and Duration of Study: The experiment was conducted at the Department of Horticulture in Universidad Autónoma Agraria Antonio Narro, Saltillo, Mexico, during 2018-2019.
 Methodology: Seven-year-old apricot trees growing in containers were subjected to a 4 to 5week period of water stress at different times during the growing season. Leaf water potential was periodically measured and flower bud development was followed from early differentiation up to full bloom.
 Results: Leaf water potential in water stressed trees was constantly low. Water stress early in the season induced a delay in bud development during late summer and fall. Water stress late in the season did not appreciably affect the rate of bud development. Full bloom was delayed when water stress was applied in late summer and fall. Water stress at flower bud initiation and differentiation, together with high temperatures, may have induced flowers with double pistils. Water stress from April through October did not induce flower drop.
 Conclusion: Soil water stress severely affect leaf water potential; delays flower bud development and may induce flowers with double pistils without flower drop.

A parameter of thermographic difference used in sunny summer midday and its significance

A large number of thermo-image measurements are useful in determining the water and energy status of ornamental tree or shrubs, especially when using certain improved parameters and methods. With these parameters and proper methods, data comparability in thermo-image analyses of ornamental tree/shrubs have been improved. In a large number of thermographic measurements, we found a significantly high-image temperature or ‘’fever” status in stressed tree or shrubs in sunny summer midday because of their transpiration cooling failure. Leaf withering, scorching, reddening, shedding and green losing can be the typical symptom patterns after persistent “fever” which is a dangerous signal unadapted to local environment and originate from persistent water and energy imbalance. The utilization of parameter of thermo-difference between leaf and finger of observer (TDlf) supports this idea.

How does varying water supply affect oxygen isotope variations in needles and tree rings of Scots pine?

In many regions, drought is suspected to be a cause of Scots pine decline and mortality, but the underlying physiological mechanisms remain unclear. Because of their relationship to ecohydrological processes, δ18O values in tree rings are potentially useful for deciphering long-term physiological responses and tree adaptation to increasing drought. We therefore analyzed both needle- and stem-level isotope fractionations in mature trees exposed to varying water supply. In a first experiment, we investigated seasonal δ18O variations in soil and needle water of Scots pine in a dry inner Alpine valley in Switzerland, comparing drought-stressed trees with trees that were irrigated for more than 10years. In a second experiment, we analyzed twentieth-century δ18O variations in tree rings of the same forest, including a group of trees that had recently died. We observed less 18O enrichment in needle water of drought-stressed compared with irrigated trees. We applied different isotope fractionation models to explain these results, including the Péclet and the two-pool correction, which considers the ratio of unenriched xylem water in the needles to total needle water. Based on anatomical measurements, we found this ratio to be unchanged in drought-stressed needles, although they were shorter. The observed lower 18O enrichment in needles of stressed trees was therefore likely caused by increased effective path length for water movement within the leaf lamina. In the tree-ring study, we observed lower δ18O values in tree rings of dead trees compared with survivors during several decades prior to their death. These lower values in declining trees are consistent with the lower needle water 18O enrichment observed for drought-stressed compared with irrigated trees, suggesting that this needle-level signal is reflected in the tree rings, although changes in rooting depth could also play a role. Our study demonstrates that long-term effects of drought are reflected in the tree-ring δ18O values, which helps to provide a better understanding of past tree physiological changes of Scots pine.

The Effect of Plant Water Status on the Chemical Composition of Pistachio Nuts (Pistacia vera L. Cultivar Bianca)

Pistachio nuts are worldwide appreciated for their chemical and organoleptic profiles. There are several studies on the influence of irrigation on pistachio productivity, whereas there are little available data on the influence on nut quality. In this study we characterized some qualitative traits of pistachio nuts cultivar Bianca in Mediterranean environment and how plant water status affected them. Water status had a positive and significant influence on the chlorophylls content, nuts from less stressed trees showed higher values of chlorophyll a (14.7 mg/100 g) and b (21.1 mg/100 g) compared than more stressed trees (9.3 and 11.5 mg/100 g for a and b, respectively). Solid phase microextraction technique in headspace followed by gas chromatography/mass spectrometry (HS-SPME GC/MS) identified seventeen different compounds with terpenes being the major class of volatiles; the most abundant were α-Pinene (range 26.2–35 μg/g), D-Limonene (2.8–3.3 μg/g), 2-Carene (1.8–3 μg/g) and β-Myrcene (0.6–1.4 μg/g). Overall, we found higher level of terpenes in less stressed trees and for α-Pinene and β-Myrcene differences were significant. The fatty acid composition analysis revealed oleic acid (70.1–71.1%), linoleic acid (13.5–14.4%) and palmitic acid (9.6–9.8%) as the most abundant compounds, but tree water status did not influence their concentration. Overall, the data reported proved that supplemental irrigation contributes to increase pistachio nut quality.

Identification of Anoplophora glabripennis (Moschulsky) by its emitted specific volatile organic compounds

Explorative experiments were done to figure out differences in the emission of volatile organic compounds (VOCs) of not infested trees and trees infested by Anoplophora glabripennis (Asian longhorn beetle, ALB), a quarantine pest. Therefore, VOCs from some native insect species, Anoplophora glabripennis infested Acer, stressed Acer, healthy Acer, Populus and Salix were obtained by enrichment on adsorbents. Qualitative analysis was done by thermal desorption gas chromatography coupled with a mass selective detector (TD-GC/MS). Altogether 169 substances were identified. 11 substances occur from ALB infested or mechanically damaged trees i.e. stressed trees, but not from healthy trees. (+)-Cyclosativene, (+)-α-longipinene, copaene and caryophyllene are detectable only from ALB-infested Acer not from mechanically damaged or healthy Acer. However, these substances are also emitted by healthy Salix. 2,4-Dimethyl-1-heptene is among all tree samples exclusively present in the ambience of ALB-infested trees. It´s rarely detectable from native insect species’ samples.

How do herbivorous insects respond to drought stress in trees?

Increased frequency and severity of drought, as a result of climate change, is expected to drive critical changes in plant-insect interactions that may elevate rates of tree mortality. The mechanisms that link water stress in plants to insect performance are not well understood. Here, we build on previous reviews and develop a framework that incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought. Using this framework, we investigate in greater depth how insect performance responds to increasing drought severity for: (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. We outline how intermittent and moderate drought can result in increases of carbon-based and nitrogen-based chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. We predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap-sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. We argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. Current evidence suggests that insect outbreaks are shorter and more spatially restricted in tropical than in temperate forests. We suggest that future research on the impact of drought on insect communities should include (i) assessing how drought-induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores; (ii) food web implications for other insects and those that feed on them; and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO2 levels. There is a need for larger, temperate and tropical forest-scale drought experiments to look at herbivorous insect responses and their role in tree death.

The cause of bark stripping of young plantation trees

Herbivorous mammals, from small voles to large ungulates, strip and eat the bark of young plantation trees. They do this most frequently at times when sources of protein food that can support their reproduction and lactation are in short supply. Furthermore, they preferentially attack—often repeatedly—trees that have experienced some form of environmental stress, leaving neighbouring trees untouched. Such stressed trees carry higher levels of amino acids in their phloem. These facts, coupled with the similarly timed and selective harvesting of bark phloem by some Australia marsupials and Northern Hemisphere woodpeckers indicate that it is the trees’ protein-enriched phloem that the bark strippers are seeking.

Plant Hydraulic Stress Explained Tree Mortality and Tree Size Explained Beetle Attack in a Mixed Conifer Forest

AbstractDrought predisposes conifer forests to bark beetle attacks and mortality. Although plant hydraulic stress mechanistically links to tree mortality, its capacity to predict trees' susceptibility to beetle attacks has not been evaluated. Further, both tree size and water supply could influence plant hydraulic stress, but their relative importance remained unknown. In this study, we modeled plant hydraulic stress of individual trees in a mixed forest of Lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), and Subalpine fir (Abies lasiocarpa) in southern Wyoming, using an integrated model of plant hydraulics and hydrology, ground surveys of tree size as well as physiological and geophysical measurements. Based on the established link between plant hydraulic stress and tree mortality, we found interspecific differences in the relative importance of water availability and tree size. Pine mortality was best explained by the combination of tree size and water supply, and fir mortality was best explained by variations in water supply. We next compared the prediction of beetle attack by modeled plant hydraulic stress versus tree size and found tree size best explained beetle attack consistently for all three species. Taken together, our results suggested beetle attack was primarily influenced by beetle preference for large trees, potentially as food sources, rather than more hydraulically stressed trees. These findings highlighted the importance of integrated understanding of biotic/abiotic factors and their mechanistic pathways in order to accurately predict the sustainability of forests susceptible to drought and beetle outbreaks.