Severe Dieback Research Articles

Biological and molecular characterization of seven Diaporthe species associated with kiwifruit shoot blight and leaf spot in China

Diaporthe species are significant pathogens, saprobes, and endophytes, with comprehensive host association and geographic distribution. These fungi cause severe dieback, cankers, leaf spots, blights, and stem-end rot of fruits on different plant hosts. This study, explored the occurrence, diversity and pathogenicity of Diaporthe spp. associated with Actinidia chinensis and A. deliciosa in the main kiwifruit production areas of China. Diaporthe isolates (284) derived from 106 diseased leaf and branch samples were examined. Multi-locus phylogenetic analyses and morphology of 43 representative isolates revealed that seven Diaporthe species were obtained, including D. alangii, D. compactum, D. eres, D. hongkongensis, D. sojae, D. tectonae, and D. unshiuensis. Pathogenicity tests were performed on kiwifruit fruits, leaves and branches. Koch’s postulates confirmed all species were pathogenic. D. alangii and D. tectonae were the most aggressive species, followed by D. eres, D. sojae, D. hongkongensis, D. unshiuensis, and D. compactum. Host range evaluation showed that the seven Diaporthe species could also infect apricot, apple, peach, pear, and plum. This is the first report of D. alangii, D. compactum, D. sojae, D. tectonae, and D. unshiuensis infecting kiwifruit in China, increasing understanding of the Diaporthe complex causing diseases of kiwifruit plants, to assist effective disease management.

Species and season affect response of container-grown shade trees to pre-plant root modifications

Container-grown shade trees make up an increasing proportion of nursery stock, yet arborists and urban foresters are often concerned about root defects associated with trees grown in smooth-sided containers, such as circling roots, persisting in the landscape post-transplant. The objective of this study was to determine the effect of root modifications (shaving or bare-rooting) at planting on establishment, survival, and growth of container-grown Acer rubrum L. ‘October Glory’, Liriodendron tulipifera L. ‘Fastigiatum’, and Platanus x acerifolia (Aiton) Willd. ‘Bloodgood’. Trees were planted on two dates (May and July) in 2018 and evaluated for two growing seasons. Modifying roots before planting resulted in increased occurrence of leaf scorch for L. tulipifera and A. rubrum trees. Nearly all trees bare-rooted before planting in July had severe die-back. Survival was excellent (>75 %) for all A. rubrum and P. x acerifolia trees planted in May regardless of treatment. Survival for L. tulipifera trees that were bare-rooted in May was 50 %; all L. tulipifera trees that were bare-rooted in July died. Bare-rooting increased predawn leaf water potential (Ψw) immediately after planting. However, Ψw did not differ among root treatments for the rest of 2018 and throughout 2019. This suggests that trees with modified root systems achieved a functional equilibrium by adjusting leaf area to reduce whole-tree water loss. Root biomass outside the original root-ball did not differ among root modification treatments two years post-transplant. However, bare-rooting reduced the proportion of circling roots compared to control trees for all species. Shaving root systems reduced circling roots compared to the control for L. tulipifera and P. x acerifolia trees. For practitioners interested in trialing these techniques, we advise performing root modifications in the dormant season and avoiding species known to be difficult to transplant bareroot.

The ecological impact of pest-induced tree dieback on insect biodiversity in Yunnan pine plantations, China

China has recently announced a reform of forestry policy, with a major goal being to transform from plantation to heterogeneous forests, which have higher resistance to pests and disease and house more biodiversity. One driver of reform is increased intensity and frequency of pest-induced tree-dieback events. To inform management, we ask what effects these events have on insect biodiversity in Pinus yunnanensis monocultures in Yunnan province, the province with one of the highest proportions of forest cover in China. We sampled aerial arthropods (mostly insect) biodiversity along gradients of Pinus yunnanensis dieback severity using Malaise traps and used metabarcoding to characterise the insect community. We used MS-GDM (‘multi-site generalized dissimilarity modelling of zeta diversity’), zeta-decline analysis, and iNEXT (‘Interpolation and extrapolation for species diversity’) to assess community change as functions of forest-structure covariates. Metabarcoding of Malaise-trapped insects reveals that bark-beetle induced forest dieback does not result in detectable differences in species diversity but does result in compositional change, with the biggest turnover occurring between 0% and infested-0%-open-canopy forests and 20%-infested-20%-open-canopy forests. Zeta-decline analysis found that the insect community in low-infestation forests is characterized by a stochastic assembly, while in high-infestation forests, the community structure is consistent with niche assembly. Our results thus suggest that bark-beetle dieback mimics natural forest-gap dynamics, consistent with the interpretation of bark beetles as a keystone species in European conifer forests, where it has been proposed that forest heterogeneity can be created efficiently by allowing natural disturbances, including bark-beetle outbreaks, to proceed naturally, without being mitigated by deadwood removal and dense replanting. In Yunnan’s situation, and given predicted increases in bark-beetle dieback severity and frequency, this strategy should probably be supplemented with anthropogenic treatments, such as deadwood enhancement and planting of multiple tree species, to accelerate the succession of plantations into heterogeneous forests.

Eastern Black Walnut (Juglans nigra L.) Originating From Native Range Varies in Their Response to Inoculation With Geosmithia morbida

Thousand cankers disease (TCD) is caused by the walnut twig beetle (Pityophthorus juglandis) vectoring the fungal canker pathogen Geosmithia morbida, which can result in severe dieback and eventual death to species of walnut (Juglans spp.) and wingnut (Pterocarya spp.). This disease is most devastating to the highly valued species J. nigra (black walnut). This species is primarily grown and harvested for timber production in the Central Hardwood Region of the United States, which comprises part of its native range. Management options for TCD are limited; therefore, finding resistant genotypes is needed. Initial studies on black walnut susceptibility to G. morbida documented some genetic variation and suggested potential resistance. Furthermore, G. morbida is thought to be native to the United States, which may have allowed for co-evolution. To capture the representative genetic diversity and screen for resistance to G. morbida, J. nigra families were collected from across the native range. These wild trees, in conjunction with seedlings developed in a black walnut timber improvement program, were planted in a common garden in Fort Collins, Colorado and repeatedly inoculated with G. morbida over the course of four years and three growing seasons. Improved seedlings exhibited larger cankered areas than wild J. nigra of the same provenance. Cankers induced by G. morbida in wild germplasm were smaller on J. nigra collected from the western and central portions of the native range compared to those collected from the eastern portion. Although trees from the western and central part of the range still incurred cankers, our findings indicate that variation in genetic resistance to G. morbida is present in black walnut. This study was performed with G. morbida independent of the walnut twig beetle, but our results suggest the limited G. morbida resistance observed in J. nigra will prevent the full compromise of black walnut to TCD. Results from this study should be taken into consideration in future black walnut breeding programs.

Susceptibility of Endemic Myoporum (Naio) Species and Populations to Klambothirps myopori in Hawai‘i

An invasive thrips species (Klambothrips myopori, Thysanoptera), originally from Australia, attacks and inflicts severe damage to endemic Myoporum species in Hawai‘i. There is concern that the thrips will cause local extinctions of Myoporum in Hawai‘i. This study examined susceptibility of different Myoporum populations from various Hawaiian Islands to K. myopori infestation and dieback of aerial plant parts. Experimental exposures of plants from different populations were conducted in common garden studies. All M. sandwicense populations included in the study were highly susceptible to infestation and severe dieback of leaves and shoots occurred. Plants from a single M. stellatum population were less susceptible to attack and subsequent dieback of stems. Management options for populations under pressure from K. myopori are discussed.

Eco-physiological evaluation of multipurpose tree species to ameliorate saline soils

Salinity is a widespread soil and underground water contaminant threatening food security and economic stability. Phytoremediation is an efficient and environmental-friendly solution to mitigate salinity impacts. The present study was conducted to evaluate the phytoremediation potential of five multipurpose trees: Vachellia nilotica, Concorpus erectus, Syzygium cumini, Tamarix aphylla and Eucalyptus cammaldulensis under four salinity treatments: Control, 10, 20 and 30 dS m−1. Salinity negatively impacted all the tested species. However, E. cammaldulensis and T. aphylla exhibited the lowest reduction (28%) and (35%) in plant height respectively along with a minimal reduction in leaf gas exchange while V. nilotica, S. cumini and C. erectus showed severe dieback. Similarly, the antioxidant enzymes increased significantly in E. cammaldulensis and T. aphylla as Superoxide Dismutase (87% and 79%), Catalase (66% and 67%) and Peroxidase (89% and 81%), respectively. Furthermore, both of these species maintained optimum Na/K ratio reducing the highest levels of soil ECe and SAR, suggesting the best phytoremediation potential. The present study identifies that E. cammaldulensis and T. aphylla showed effective tolerance mechanisms and the highest salt sequestration; therefore, may be used for phyto-amelioration of salinity impacted lands. Novelty statement Although previous studies evaluated the tolerance potential of many tree species, comparative and physiochemical evaluation of multipurpose tree species has been remained unexplored. In this scenario, eco-physiological characterization of multipurpose tree species may inform tree species for phytoremediation of saline soils according to the level of salinity. Optimizing tree species selection also improves the success of wood for energy and revenue generation while restoring degraded soils.

Monitoring ash dieback (Hymenoscyphus fraxineus) in British forests using hyperspectral remote sensing

AbstractLarge‐scale dieback of ash trees (Fraxinus spp.) caused by the fungus Hymenoscyphus fraxineus is posing an immense threat to forest health in Europe, requiring effective monitoring at large scales. In this study, a pipeline was created to find ash trees and classify dieback severity using high‐resolution hyperspectral imagery of individual tree crowns (ITCs). Hyperspectral data were collected in four forest sites near Cambridge, UK, where 422 ITCs were manually delineated and labelled using field‐measurements of species and dieback severity (for ash trees). Four algorithms, namely linear discriminant analysis (LDA), principal components analysis coupled with LDA (PCA‐LDA), partial least squares discriminant analysis (PLS‐DA) and random forest (RF), were used to build classification models for species and dieback severity classification. The effect of dark‐pixel filtering on classification accuracy was evaluated. The best performing models were then coupled with automatic ITC segmentation to map species and ash dieback distribution over 16.8 hectares of woodland. We calculated and partitioned the coefficient of variation (CV) of the reflected ash spectra to find variable wavebands associated with dieback. PLS‐DA and LDA were most accurate for classifying ITC species identifies (overall accuracy >90%), whereas RF was most accurate for classifying ash dieback severity (overall accuracy 77%). Dark pixel filtering further increased the accuracy of species classification (+6%), but not disease classification. The reflectances of narrow blue (415 nm), red‐edge (680 nm) and NIR (760 nm) bands had high CV across disease classes and should be included if multispectral imagery were to be used to monitor ash dieback. The study demonstrates the possibility of using remote sensing to forward epidemiological research by monitoring forest pathogens in landscape scales, which would allow temperate forest managers to control pathogen outbreaks, assess associated impacts and restore affected forests much more effectively.

A century old regeneration problem of Shorea robusta Gaertn. F. in south Asia: SWOT analysis (Review)

Shorea robusta (Sal) Gaertn. F. a commercially and ecologically important forestry species of south Asia, is facing serious regeneration problem since last century. The continuously diminishing natural regeneration is associated with numerous abiotic and biotic factors, like edaphic, micro-climatic, physiological, genetic, anthropogenic, pathogens, insect-pests, etc. Following a good seed year and timely commencement of monsoon, Sal seeds germinate readily, and thousands of seedlings cover the forest floor. Subsequently, these get afflicted with die-back syndrome impeding shoot growth. Regular fire incidences during hot and dry season further aggravates the problem. Among biotic factors, Hoplocerambyx spinicornis , Cylindrocladium floridanum and Inonotus shoreae causes severe heartwood decay, blight and dieback leading to mortality. Moreover, over-exploitation, illegal felling, grazing, etc., have severely depleted the Sal forest. This review systematically explores the factors contributing to regeneration problem in S. robusta and opines appropriate silvicultural operations and management strategies for the conservation of Sal forests through SWOT (Strengths, Weaknesses, Opportunities and Threats) analysis. We emphasized on the ecological aspects, soil characteristics, micro-climatic factors and importance of associated species to develop sustainable management regimes. Highlighting all facets of Sal regeneration problem and SWOT assessment, we suggest comprehensive weaknesses and threat perceptions to formulate strategies to seal the gaps.

Long-Term Responses of Mediterranean Mountain Forests to Climate Change, Fire and Human Activities in the Northern Apennines (Italy)

Fagus sylvatica (beech) dominates the montane forests of the Apennines and builds old-growth high-conservation value stands. However, recent severe drought-induced diebacks raise concern on the future persistence of these forests and of Southern European mesophilous woodlands overall, growing at their dry edge. To explore the history of Apennine beech-dominated forests, we draw on the multiproxy paleoecological record from Lago Verdarolo, which includes a robust vegetation-independent temperature reconstruction. Numerical techniques are used to investigate the drivers of long-term Mediterranean mountain forest dynamics. Specifically, we focus on disentangling the ecological factors that caused the shift from high-diversity mixed forests to beech-dominated stands and on assessing the occurrence of legacy effects on present-day forests. Abrupt climate change largely drove vegetation dynamics during the Late Glacial and Early Holocene. Species-rich mixed Abies alba (silver fir) forests dominated about 10,500—5500 years ago, under rather dry and warmer-than-today conditions (+ 1—2 °C) and limited fire occurrence. Cooler and moister summers and increasing fire activity caused declines in several fire-sensitive temperate deciduous trees (for example, Ulmus, Tilia, Fraxinus) and favored the establishment of fir-beech forests around 5500 years ago. Further enhancement of fire activity and farming around 2000 years ago led to local Abies alba extinction and forest impoverishment. We conclude that the currently widespread monospecific Apennine beech forests are the result of multi-millennial land-use intensification superimposed on Late Holocene cooling and moistening. Given their higher drought-tolerance compared to beech stands, reviving ancient species-rich mixed fir forests represents a feasible and ‘tested’ possibility to adapt forests to climate change.

Fire and rust – the impact of Austropuccinia psidii (myrtle rust) on regeneration of Myrtaceae in coastal heath following wildfire

In April 2010, the exotic rust fungus Austropuccinia psidii was detected for the first time in Australia, with the host range rapidly increasing to >350 species from 58 genera within the family Myrtaceae. Commonly known as myrtle rust, the disease is well established in native ecosystems, particularly in New South Wales (NSW) and Queensland, impacting on a range of species and associated environments. While fire is considered an important selection agent in the development of Australia’s native flora, re-sprouting and germination of seedlings en masse is ideal for the development and spread of A. psidii. Our study took advantage of a unique opportunity to assess the impacts of myrtle rust on species of Myrtaceae regenerating after a wildfire in a northern NSW coastal heath environment. Austropuccinia psidii infection was found on all species of regenerating Myrtaceae with impacts ranging from minor leaf spotting to severe repeated dieback and death of affected trees. This is the first investigation into the impact of myrtle rust in regenerating Myrtaceae following wildfire and highlights a need for more detailed studies.

Rapid and surprising dieback of Utah juniper in the southwestern USA due to acute drought stress

For nearly two decades, the American Southwest has been in the grips of a long-term ‘megadrought’, punctuated by a number of short and severe ‘global change type droughts’ (i.e., negative precipitation anomalies co-occurring with high temperature). These events have caused widespread mortality of the drought-sensitive piñon pine (Pinus edulis) while co-dominant Juniperus spp. have historically been much more drought tolerant and thus have rarely died. However, a severe drought occurred in 2018 that rapidly (<1 year) caused canopy dieback of Juniperus osteosperma across a vast area of southeastern Utah. In order to uncover the etiology behind this surprising dieback event, we set up a series of survey plots that captured gradients in microclimate, topography, and dieback severity. We also quantified xylem hydraulic damage and assessed the presence of various biotic agents in declining junipers to identify the primary causes of this dieback. We observed that juniper canopy dieback was most severe (>60% canopy dieback) at hot, dry, low elevation sites, and was associated with drought-induced hydraulic damage. There was no evidence that biotic agents could be the primary drivers of this dieback, implicating the acute effects of drought as the main causal agent. The speed and scale of this drought-induced juniper dieback seems to be historically unprecedented in the region and foreshadows an uncertain future for piñon-juniper woodlands as the region continues to get warmer and drier.

Mechanisms of severe dieback and mortality in a classically drought-tolerant shrubland species (Arctostaphylos glauca).

Mortality events involving drought and pathogens in natural plant systems are on the rise due to global climate change. In Santa Barbara, California, United States, big berry manzanita (Arctostaphylos glauca) has experienced canopy dieback related to a multi-year drought and infection from fungal pathogens in the Botryosphaeriaceae family. A greenhouse experiment was conducted using Neofusicoccum australe to test the specific influences of drought and fungal infection on A. glauca. A full factorial design was used to compare four treatment groups (drought + inoculation; drought - inoculation; watering + inoculation; and control: watering - inoculation). Data were collected for 10 weeks on stress symptoms, changes in leaf fluorescence and photosynthesis, and mortality. Results indicated significant effects of watering and inoculation treatments on net photosynthesis, dark-adapted fluorescence, and disease symptom severity (P < 0.05), and a strong correlation was found between physiological decline and visible stress (P < 0.0001). Mortality differed between treatments, with all groups except for the control experiencing mortality (43% mortality in drought - inoculation, 83% in watering - inoculation, and 100% in drought + inoculation). A Kaplan-Meier survival analysis showed drought + inoculation to have the least estimated survivorship compared to all other treatment groups. In addition to a possible synergistic interaction between drought and fungal infection in disease onset and mortality rates in A. glauca, these results indicate that young, non-drought-stressed plants are susceptible to mortality from N. australe infection, with important implications for the future of wildland shrub communities.

Outbreak of Phytophthora cinnamomi causing severe decline of avocado trees in southern Turkey

AbstractSince the summer 2017, severe decline symptoms have been observed on 10‐ to 25‐year‐old avocado trees in almost all commercial orchards planted in the Mediterranean coastal region of Turkey. Young, newly planted trees in infected orchards were also affected by the disease. Affected trees showed wilting, leaf discoloration, defoliation and severe dieback. Some trees were completely desiccated. Although fine roots of symptomatic trees usually were decayed, reddish brown cankers also occurred on taproots and lateral roots of heavily infected trees. The pathogens were isolated from necrotic root and soil samples of symptomatic trees, using selective medium and soil baiting, and were identified based on morphological features and DNA sequences of the internal transcribed spacer (ITS) region. One isolate each of Phytophthora cryptogea and P. palmivora was identified, while all other isolates were P. cinnamomi. In addition, a subcortical fan‐shaped mycelium, characteristic of Armillaria spp., was observed in the stem base of a symptomatic tree and identified as Armillaria gallica by DNA sequences of the internal transcribed spacer (ITS) and the translational elongation factor 1‐α (EF 1‐α) gene regions. Pathogenicity of Phytophthora isolates was tested by stem inoculation on one‐year‐old avocado seedlings. Two months after inoculation, canker lesions developed on stems of seedlings inoculated by any of the three Phytophthora spp. In contrast, collenchyma callus formed over the wound points on control plants over the same time period. This is the first report of P. cinnamomi, P. cryptogea, P. palmivora and A. gallica causing root rot of avocado trees in Turkey. In addition, P. cryptogea and A. gallica are reported for the first time associated with disease on this host. Due to the severe symptoms and widespread occurrence, P. cinnamomi should be considered a potential threat to avocado cultivation and natural ecosystems of this region of Turkey.

Pestalotiopsis desseminata PADA JAMBU MENTE BIOLOGI DAN INTERAKSINYA DENGAN Helopeltis antonii

&lt;p&gt;&lt;strong&gt;Pestalotiopsis desseminata on cashew: its biology and interaction with Helopeltis antonii&lt;/strong&gt;&lt;/p&gt;&lt;p&gt;Pestalotiopsis desseminata is one of pathogens causing leaf spots. die-back of shoots and inflorescence of cashew plant. It is assumed there is an interaction between the attack of Helopeltis antonii and the fungus in Ihe ield so that the damage becomes more devastating. The objectives of (his research were to investigate several aspects of fungal biology (pathogenicity, mycelial growth, acervuli production, sensitivity to fungicides) and its interaction with //. antonii. The esearch was carried out from April 1999 to March 2000 at the laboratory and green house of Pest and Disease Department Research Institute for Spice and Medicinal Crops, Bogor. /' desseminata was diectly isolated from cashew leaves. Pathogenicity test of P. desseminata isolate was conducted on cashew seedlings. The fungal growth and fungicide effects were carried out in vitro on agar medium. The interaction between P. desseminata and H. antonii was examined by inoculating the fungus and insect, either individually or in combination, on cashew seedlings. The results showed that P. desseminata isolate fomis black spherical acervuli containing oblongs conidia with 5 cells, and the outermost cell has 3 flagellate. Acervuli could only be produced on the cultue illuminated continuously by 600 lux translucent lamp. Pathogenicity test of several isolates of /' desseminata on cashew seedlings caused spherical leaf spot symptom, individually or coalesced as bigger leaf spots. In vitro fungal mycelia growth could be suppressed by several fungicides, including bcnomyl 50% (I ppm) and Uiiopanate-mcthyl 70% (10 ppm) Combining inoculation between P. desseminata and //. antonii resulted in moe severe (devastating) die-back compared with individually inoculated.&lt;/p&gt;

Phytophthora Species Causing Root and Collar Rot of Pomegranate in Turkey

Pomegranate (Punica granatum) is an important fruit-tree at southwestern Turkey, as this region (Antalya and Mugla provinces) produces and exports the great majority of the pomegranate fruits of the country. Pomegranate orchards showing severe dieback and tree mortality in the region were surveyed to determine the role of Phytophthora spp. in symptom development. Root (from 51 trees) and soil (from 136 trees) samples were collected from symptomatic trees in 113 commercial pomegranate orchards, covering 170 ha. A total of 98 isolates were obtained from the roots of 19 trees and soil samples of 79 trees, using selective medium and soil baiting. Species isolated were identified by morphological characteristics and ITS sequencing. The most frequently isolated species were P. nicotianae (37 isolates) and P. palmivora (26 isolates), followed by P. parvispora (14 isolates), P. inundata (six isolates), P. cryptogea (four isolates), P. niederhauserii (four isolates), P. taxon walnut (two isolates) and P. rosacearum (one isolate), and four unidentified species. Twelve orchards were infected with more than one species. Disease incidence and prevalence were 4.59% and 69.91%, respectively. Pathogenicity tests by soil infestation, revealed that the isolates caused root and collar rot, reducing root weights of pomegranate seedlings. Phytophthora palmivora and P. nicotianae were the most virulent species. This is the first report of any species other than P. palmivora causing root and collar rot of pomegranate trees worldwide.

Hail-Induced Infections of the Chestnut Blight Pathogen Cryphonectria parasitica Depend on Wound Size and May Lead to Severe Diebacks.

This study combined phytosanitary surveys, laboratory analyses, and mathematical modeling to show how hail-induced wounds can foster the infections of the blight pathogen Cryphonectria parasitica, locally associated with extensive dieback of chestnut (Castanea sativa). Orchards and coppices located within and outside the assessed dieback area in a single location in the North West of Italy were inspected to appraise the abundance of hail-induced wounds and C. parasitica infections. The incidence of C. parasitica was significantly higher within the dieback area compared with outside (92% versus 60%; P < 0.05). Hail-induced wounds were observed on small branches and shoots of all trees sampled within the dieback area, whereas they were less abundant outside (20% of trees), suggesting either that the dieback was directly associated with the injuries caused by the hailstorms or that those injuries may have facilitated infections of C. parasitica. Isolations conducted on 359 branches and shoots showed that hail-induced wounds served as infection courts for C. parasitica and that infections depended on the size rather than on the number of hail wounds. We fitted a logistic model showing that hail-induced wounds whose perimeter was larger than 66 mm were at particular risk of C. parasitica infection. A newly designed geometrical-based model is proposed to relate hailstones size, hail wound perimeter, and the risk of infection. We established that hail-induced wounds are entry points for virulent and hypovirulent strains of C. parasitica, since 6.5% of isolates were infected by Cryphonectria hypovirus-1.

A new invasive species of Kerria Targioni-Tozzetti (Hemiptera: Coccomorpha: Kerriidae) on rain tree, Albizia saman (Fabaceae) from India

ABSTRACT A new lac insect pest, Kerria destructor Talukder and Das, sp. nov. (Kerriidae) infesting the rain tree (Albizia saman), is described and illustrated based on the adult female, and a key to the known species of Kerria is provided. Recently, this species has been reported causing severe dieback on the twigs and branches leading to the death of a large number of rain trees in West Bengal, India. No previous records of such a destructive lac insect infestation on rain tree from India are known. Its invasive characters are discussed. http://www.zoobank.org/urn:lsid:zoobank.org:pub:67D8D166-5D7B-47A5-B867-3C9A5B049E35

Weather underground: Subsurface hydrologic processes mediate tree vulnerability to extreme climatic drought.

Drought extent and severity have increased and are predicted to continue to increase in many parts of the world. Understanding tree vulnerability to drought at both individual and species levels is key to ongoing forest management and preparation for future transitions in community composition. The influence of subsurface hydrologic processes is particularly important in water-limited ecosystems, and is an under-studied aspect of tree drought vulnerability. With California's 2013-2016 extraordinary drought as a natural experiment, we studied four co-occurring woodland tree species, blue oak (Quercus douglasii), valley oak (Quercus lobata), gray pine (Pinus sabiniana), and California juniper (Juniperus californica), examining drought vulnerability as a function of climate, lithology and hydrology using regional aerial dieback surveys and site-scale field surveys. We found that in addition to climatic drought severity (i.e., rainfall), subsurface processes explained variation in drought vulnerability within and across species at both scales. Regionally for blue oak, severity of dieback was related to the bedrock lithology, with higher mortality on igneous and metamorphic substrates, and to regional reductions in groundwater. At the site scale, access to deep subsurface water, evidenced by stem water stable isotope composition, was related to canopy condition across all species. Along hillslope gradients, channel locations supported similar environments in terms of water stress across a wide climatic gradient, indicating that subsurface hydrology mediates species' experience of drought, and that areas associated with persistent access to subsurface hydrologic resources may provide important refugia at species' xeric range edges. Despite this persistent overall influence of the subsurface environment, individual species showed markedly different response patterns. We argue that hydrologic niche segregation can be a useful lens through which to interpret these differences in vulnerability to climatic drought and climate change.

A Survey in Natural Forest Ecosystems of Vietnam Reveals High Diversity of both New and Described Phytophthora Taxa including P. ramorum

In 2016 and 2017, surveys of Phytophthora diversity were performed in 25 natural and semi-natural forest stands and 16 rivers in temperate and subtropical montane and tropical lowland regions of Vietnam. Using baiting assays from soil samples and rivers and direct isolations from naturally fallen leaves, 13 described species, five informally designated taxa and 21 previously unknown taxa of Phytophthora were isolated from 58 of the 91 soil samples (63.7%) taken from the rhizosphere of 52 of the 64 woody plant species sampled (81.3%) in 20 forest stands (83.7%), and from all rivers: P. capensis, P. citricola VII, VIII, IX, X and XI, P. sp. botryosa-like 2, P. sp. meadii-like 1 and 2, P. sp. tropicalis-like 2 and P. sp. multivesiculata-like 1 from Phytophthora major phylogenetic Clade 2; P. castaneae and P. heveae from Clade 5; P. chlamydospora, P. gregata, P. sp. bitahaiensis-like and P. sp. sylvatica-like 1, 2 and 3 from Clade 6; P. cinnamomi (Pc), P. parvispora, P. attenuata, P. sp. attenuata-like 1, 2 and 3 and P. ×heterohybrida from Clade 7; P. drechsleri, P. pseudocryptogea, P. ramorum (Pr) and P. sp. kelmania from Clade 8, P. macrochlamydospora, P. sp. ×insolita-like, P. sp. ×kunnunara-like, P. sp. ×virginiana-like s.l. and three new taxa, P. sp. quininea-like, P. sp. ×Grenada 3-like and P. sp. ×Peru 4-like, from Clade 9; and P. sp. gallica-like 1 and 2 from Clade 10. The A1 and A2 mating types of both Pc and Pr co-occurred. The A2 mating type of Pc was associated with severe dieback of montane forests in northern Vietnam. Most other Phytophthora species, including Pr, were not associated with obvious disease symptoms. It is concluded that (1) Vietnam is within the center of origin of most Phytophthora taxa found including Pc and Pr, and (2) Phytophthora clades 2, 5, 6, 7, 8, 9, and 10 are native to Indochina.

Stable isotopes indicate ecosystem restructuring following climate‐driven mangrove dieback

AbstractExtreme climatic events can trigger sudden but often long‐lasting impacts in ecosystems by causing near to complete mortality of foundation (habitat‐forming) species. The magnitude and frequency of such events are expected to rise due to anthropogenic climate change, but the impacts that such events have on many foundation species and the ecosystems that they support remains poorly understood. In many cases, manipulative experimentation is extremely challenging and rarely feasible at a large scale. In late 2015 to early 2016, an extensive area of mangrove forest along ∼ 1000 km of coastline in the Gulf of Carpentaria, Australia, experienced severe dieback, an event associated with climatic extremes. To assess the effect this dieback event had on the mangrove ecosystem, we assessed benthic faunal assemblages and food web structure using stable carbon and nitrogen isotopes in a comparative experiment of impacted forest and adjacent unimpacted forest. Eighteen months after the dieback, the forest that suffered dieback contained significantly fewer crabs that rely on mangrove litter food source but more crabs that rely on microphytobenthos food source than the unimpacted forest. However, the infaunal biomass was largely unaffected by the mortality effect. This is most likely because microphytobenthos was largely unaffected and consequently, this buffered the food web responses. However, overall, the habitat value for mangrove ecosystem services most likely decreased due to lower physical habitat complexity following tree mortality. Longer‐term monitoring could lead to better understanding of biological effects of this extreme event and underlying biological mechanisms that drive changes and recovery.