Widespread Dieback Research Articles

Elevation‐dependent patterns of borer‐mediated snow‐gum dieback are associated with subspecies' trait differences and environmental variation

AbstractSubalpine forests worldwide face the synergistic threats of global warming and increased biotic attack, and the collapse or transition of subalpine forests is predicted in south‐eastern Australia under future climates. The recent widespread dieback of subalpine snow‐gum forests due to increased activity of a native wood‐boring longicorn beetle, Phoracantha mastersii, suggests this process may already be underway. We investigated how variation in tree tissue traits and environmental conditions correlated with elevation‐dependent spatial patterns of forest mortality. We hypothesized that increased vulnerability of subalpine snow gums to wood‐borer‐mediated dieback at intermediate elevations was associated with poorly resolved differences in traits between montane (Eucalyptus pauciflora subsp. pauciflora) and subalpine (E. pauciflora subsp. niphophila) snow‐gum subspecies. We first sought to characterize variation and elevation‐dependent transitions in 20 structural and drought‐related functional traits among 120 healthy trees distributed along a 1000 m elevation transect that spanned the subspecies transition zone. Secondly, we surveyed 774 trees across 53 sites between 1280 and 1980 m a.s.l. to explore associations between borer‐damage severity, elevation, subspecies and a subset of traits that differed between subspecies. We observed evidence for both continuous trait variation in response to changing elevation (10/20 traits) and discrete shifts in mean trait values across the transition between subspecies distributions (5/20 traits). Increased borer‐damage severity across the montane‐to‐subalpine subspecies transition was correlated with lower bark thickness, whereas reduced borer damage at the highest elevations was associated with greater precipitation and lower temperatures. Our results suggest that due to possessing distinct traits associated with increased borer susceptibility, subalpine snow‐gum forests may be subject to an increased risk of severe borer‐mediated forest dieback under warmer and drier future climates. Identifying traits contributing to species' distribution limits and biotic‐agent vulnerability remains critical for predicting, monitoring and possibly mitigating forest and vegetation declines under future climates.

Transcriptome Analysis Reveals the Response of Cryptomeria japonica to Feeding Stress of Dendrolimus houi Lajonquière Larvae

The Japanese fir, Cryptomeria japonica, is ecologically and commercially vital in China. However, infestations by Dendrolimus houi Lajonquière larvae cause widespread dieback, mimicking mechanical damage effects, with unclear differential impacts and mechanisms. To address this, 6th instar larvae of D. houi were introduced to three-year-old C. japonica seedlings to induce pest infestation, while mechanical damage and non-infested pests were controlled. Then, next-generation sequencing techniques were employed, and transcriptome sequencing was conducted to analyze the distinct damage mechanisms responding to C. japonica. This study revealed 10,412 DEGs between pest infestation and controls and 5535 DEGs comparing pest infestation to mechanical damage. Functional enrichment analysis highlighted the involvement of these DEGs in crucial processes such as photosynthesis, amino acid and nucleotide metabolism, flavonoid biosynthesis, and plant hormone signaling pathways. In particular, 786 unique DEGs were discerned in pest-infested samples. Key enriched pathways, such as pyruvate and propionate metabolism, were pinpointed, with flavonoid biosynthesis potentially closely linked to pest-feeding inductions. These findings provided valuable insights into the molecular responses of C. japonica to D. houi infestation, laying the foundation for future research aimed at developing pest-resistant varieties of Japanese fir.

Bird predation on Roseau cane scale as revealed by a web image search and querying a citizen monitoring database

AbstractAn invasion of Roseau cane scale, Nipponaclerda biwakoensis, native to East Asia, is associated with recent widespread dieback of common reed stands in the Mississippi River Delta, Louisiana. A previous study suggested that, in its native range, the combined mortality from Common Reed Bunting Emberiza schoeniclus and parasitoid wasps may control the abundance of Roseau cane scale. However, there is limited knowledge about which other bird species may prey on Roseau cane scale or the feeding patterns of these birds. Here we used web search and citizen science monitoring data to tabulate bird predation events on Roseau cane scales across sites in Japan and determine which bird species may be the most important predators and when the observed feeding typically occurs. We scanned web entries using both Google image searches and the social networking services including Twitter, Instagram, and Facebook. Additionally, we accessed long‐term field observation records obtained by volunteer monitoring conducted by the members of the Kanagawa Branch of the Wild Bird Society of Japan. By using these citizen science approaches, we detected 16 bird species, mainly Common Reed Bunting but also other migratory and resident species, preying on the Roseau cane scale in reed beds throughout central Japan and most detection occurred during the winter. This study demonstrates that biological information obtained from publicly available data from citizen observers can be useful for exploring ecological patterns of various species including potential predators against invasive species.

Phylogenomics resolves the etiology of dieback disease and deciphers Ceratocystis dalbergicans sp.nov., causal agent of Dalbergia sissoo decline.

Dalbergia sissoo is one of the most economically important trees in forestry, agroforestry, and horticulture. This tree species is severely threatened by dieback. Widespread dieback outbreaks and infestations have drastically destroyed billions of D. sissoo trees. Hence, we attempted to resolve the dieback etiology through phylogenomics associated with D. sissoo mortality. The Ceratocystis species was evaluated using morphologically investigated fungal isolates collected from dieback-affected tissue plants. Based on the symptomatology, we have differentiated dieback from Fusarium wilt and concluded that the Ceratocystis fimbriata sensu lato complex is causing shisham dieback in Pakistan. As the Ceratocystis species complex is a cryptic species complex, we used genomics and phylogenetic analysis for deciphering its evolutionary hierarchical order. The pathogen's operational taxonomy was unlocked with the help of phylogenomics, and it was discovered that isolates from D. sissoo represent a species distinct from the other species in the C. fimbriata sensu lato species complex. The name Ceratocystis dalbergicans sp. nov. has been given to the fungus causing dieback disease in D. sissoo.

FUNGI OBTAINED FROM OLIVE TWIG DIEBACK AND ADULTS OF THE ALIEN PEST XYLOSANDRUS COMPACTUS (EICHHOFF) (COLEOPTERA CURCULIONIDAE SCOLYTINAE)

Olive (Olea europaea L.) is a traditional crop of great socioeconomic importance in the Mediterranean Basin. In 2021, a survey was performed in an Italian nursery located in Central Italy where unusual symptoms and widespread dieback had been observed on young potted olive trees. Twigs and branches of all plants affected by dryness showed the presence of the alien bark beetle Xylosandrus compactus. Woody tissues colonized by X. compactus were transferred into the laboratory and several fungi such as Cladosporium perangustum and Neofusicoccum parvum were then identified. In addition to these species, Geosmithia pallida was also reported for the first time in Italy on this plant. Studies on this bark beetle and its associated fungi are currently in progress to further evaluate potential risks for olive cultivation. Key Words: Olea europea, fungi, bark beetles.

Effects and response of the Cerrado ground-layer to frost along the canopy cover gradient.

Frost effects on savanna plant communities have been considered as analogous to those from fire, both changing community structure and filtering species composition. However, while frost impacts have been well-studied for the woody component of savannas, it is still poorly explored for the ground-layer community. Here, we investigated effects of frost in the Cerrado along a gradient of tree cover, focusing on ground-layer plant species, near the southern limit of the Cerrado in Brazil. We aimed to elucidate if the pattern already described for the tree layer also extends to the ground layer in terms of mimicking the effects of fire on vegetation structure and composition. We assessed how damage severity differs across species and across the tree-cover gradient, and we examined the recovery process after frost in terms of richness and community structure along the canopy cover gradient. Frost caused immediate and widespread dieback of the perennial ground-layer, with greatest impact on community structure where tree cover was lowest. However, frost did not reduce the number of species, indicating community resilience to this natural disturbance. Although frost mimicked the effects of fire in some ways, in other ways it differed substantially from fire. Unlike fire, frost increases litter cover and decreases the proportion of bare soil, likely hindering crucial processes for recovery of plant populations, such as seed dispersal, seed germination and plant resprouting. This finding calls attention to the risk of misguided conclusions when the ground layer is neglected in ecological studies of tropical savannas and grasslands.

Exploratory surveys in Taiwan of the roseau cane scale Nipponaclerda biwakoensis Kuwana (Hemiptera: Aclerdidae) and its associated parasitoids

AbstractRoseau cane (Phragmites australis (Cav). Trin. ex Steud.) is the dominant plant species of the Mississippi River Delta in Louisiana, USA, and protects the coastline from erosion and storm‐related impacts, maintaining shipping channels and oil infrastructure. Widespread dieback and thinning of P. australis were noted in the Mississippi River Delta in the fall of 2016. Invasive populations of the roseau cane scale (Nipponaclerda biwakoensis Kuwana), which is native to Asia, were found at outbreak levels in the stands. Subsequent research implicated N. biwakoensis in the diebacks. As a safe, cost‐effective and long‐term approach for managing this invasive scale, classical (importation) biological control methods are being considered. As the first step to developing biological control, research evaluating the natural enemies of the N. biwakoensis in the native range is necessary. This study investigated the population dynamics of N. biwakoensis and its associated parasitoids on P. australis in Taiwan from July to November 2019 at five study sites. Scale densities across sites increased over the growing season, peaking in September at 85.77 ± 6.36 scales/stem, which is half as dense as found in Louisiana. Mean parasitism of adult female scales across sites and season was 14.00%, while parasitism on immature scales was lower (3.00%). Four parasitoid wasp species were reared from the scale—Asytymachus lasallei Noyes and Higashiura, Boucekiella depressa Hoffer, Neastymachus japonicus Tachikawa and Aprostocetus sp. Westwood. Neastymachus japonicus was the dominant species parasitizing adult female scales, while A. lasallei was the dominant wasp species parasitizing immature scales. Astymachus lasallei and B. depressa were frequently reared from the same individual host, suggesting multiparasitism or hyperparasitism. Overall, this study provides important information on the identity and role of parasitoids of N. biwakoensis in its native range, which will aid in developing a classical biological control programme for the invasive N. biwakoensis.

Extreme Precipitation and Flooding Contribute to Sudden Vegetation Dieback in a Coastal Salt Marsh.

Climate extremes are becoming more frequent with global climate change and have the potential to cause major ecological regime shifts. Along the northern Gulf of Mexico, a coastal wetland in Texas suffered sudden vegetation dieback following an extreme precipitation and flooding event associated with Hurricane Harvey in 2017. Historical salt marsh dieback events have been linked to climate extremes, such as extreme drought. However, to our knowledge, this is the first example of extreme precipitation and flooding leading to mass mortality of the salt marsh foundation species, Spartina alterniflora. Here, we investigated the relationships between baseline climate conditions, extreme climate conditions, and large-scale plant mortality to provide an indicator of ecosystem vulnerability to extreme precipitation events. We identified plant zonal boundaries along an elevation gradient with plant species tolerant of hypersaline conditions, including succulents and graminoids, at higher elevations, and flood-tolerant species, including S. alterniflora, at lower elevations. We quantified a flooding threshold for wetland collapse under baseline conditions characterized by incremental increases in flooding (i.e., sea level rise). We proposed that the sudden widespread dieback of S. alterniflora following Hurricane Harvey was the result of extreme precipitation and flooding that exceeded this threshold for S. alterniflora survival. Indeed, S. alterniflora dieback occurred at elevations above the wetland collapse threshold, illustrating a heightened vulnerability to flooding that could not be predicted from baseline climate conditions. Moreover, the spatial pattern of vegetation dieback indicated that underlying stressors may have also increased susceptibility to dieback in some S. alterniflora marshes.Collectively, our results highlight a new mechanism of sudden vegetation dieback in S. alterniflora marshes that is triggered by extreme precipitation and flooding. Furthermore, this work emphasizes the importance of considering interactions between multiple abiotic and biotic stressors that can lead to shifts in tolerance thresholds and incorporating climate extremes into climate vulnerability assessments to accurately characterize future climate threats.

Tree growth decline as a response to projected climate change in the 21st century in Mediterranean mountain forests of Chile

Global Climate Models project that observed climate trends are likely to be preserved and the number of extreme events will be increasing during the rest of the 21st century, which may have a detrimental impact on forest ecosystems. These impacts may include forest decline and widespread dieback of the most vulnerable biomes, such as the Mediterranean Forest of Central Chile (MFCC). Nothofagus macrocarpa and Austrocedrus chilensis are two canopy-dominant, endangered tree species in the mountains of MFCC. Here, we project tree growth of these species based on tree-ring width chronologies, a simplified version of a process-based model, and climate change projections. We used the tree ring information derived from ~400 trees from 12 sites distributed across MFCC in combination with the simplified version of process-based Vaganov-Shashkin tree-growth model (VS-Lite) to forecast changes in tree growth for the next four decades. Tree growth projections were made on the basis of monthly values of temperature and precipitation from the output of 35 climate models based on two ensembles of CO 2 emission scenarios of the IPCC AR5 (RCP 8.5: higher-emission scenario, and RCP 2.6: lower-emission scenario). For the MFCC region these scenarios result in temperature rise ranging between 0.5 °C and 2.0 °C, and a precipitation decrease between 5% and 20% by the year 2065, as related to historical conditions. Our results showed that the VS-Lite model is capable of reproducing tree growth decline during the recent extreme dry period, i.e. 2010–2018, which supports its use for tree growth projections in the MFCC region. According to the modeling results, we find that tree growth in both N. macrocarpa and A. chilensis forests distributed in the MFCC region will be adversely affected by future climate changes, mainly starting from the year 2035, under both scenarios. Our work provides evidence of the degree of vulnerability of Mediterranean mountain forests in central Chile according to current climate change projections. The projected decline in tree growth indicates serious risks in the dynamics and survival of these forests relatively soon, so alerts are given about this situation which may require to counteract the deleterious effects of global change on vegetation in this region. • A VS-Lite model was used to project radial growth from two mountain tree species of Central Chile Mediterranean forests. • The tree-growth model was robustly calibrated and validated for its use in future growth projections under climate change. • A significant tree growth decline in response to climate change is projected in Central Chile by 2065.

Widespread dieback in a foundation species on a sub‐Antarctic World Heritage Island: Fine‐scale patterns and likely drivers

AbstractUnder anthropogenic climate change, emerging diseases and pathogens are increasingly prevalent in high latitude and altitude regions that were previously protected by cold winter temperatures. Ongoing island‐wide dieback of a foundation species, the cushion plant Azorella macquariensis, on World Heritage listed Macquarie Island provides the first sub‐Antarctic example. To better understand the island‐wide progression of cushion dieback and its drivers, we established and quantified plant condition classes and measured microclimate across 62 sites. We then tested whether the drivers of cushion dieback were associated with (i) water stress: represented by vapour pressure deficit, wind exposure and gravel content, (ii) pathogen virulence: using freezing days and extreme humidity as empirically supported surrogates, or (iii) both. There was a strong north‐south progression in cushion condition, with dieback most active in the centre of the island and advanced in the north. Dieback was most extensive at sites with fewer freezing days and high humidity. Natural southern refugia were explained by the significantly colder temperatures, associated with a north‐south temperature gradient. It is expected that under current climate change trajectories, where Macquarie is likely to continue to become warmer and wetter, cushion dieback will remain pervasive, expanding most slowly in the south and potentially outpacing recovery. We emphasise the need for increased awareness to prevent the establishment of pathogens into and across the landscapes of newly susceptible high latitude and altitude regions. Areas of high conservation significance need to be prioritised for management, to prevent further landscape‐scale change under current climate trajectories.

High resilience, but low viability, of pine plantations in the face of a shift towards a drier climate

Plantations of Mediterranean pines will face warmer and drier climate conditions during the 21st century. However, we lack field assessments of how severe droughts could threaten the viability of many of these plantations, often located at xeric sites and lacking adaptive management measures such as thinning. In this work, we related radial-growth variability to recent defoliation, to evaluate the resilience and vulnerability of Maritime pine (Pinus pinaster) plantations from xeric sites in south-eastern Spain to three recent droughts (1995, 2005, and 2012). Tree diameter and growth resistance after the most severe drought, which occurred in 1995, were positively related to growth. In addition, the current defoliation level, a proxy for tree vigor, was related to growth, decreasing as tree-to-tree competition increased. These findings indicate that increasing drought will reduce growth and increase the needle loss and mortality rate across many dense pine plantations in the Mediterranean Basin, suggesting that they are more prone to die. Thus, limited resilience of plantations may explain their higher defoliation and indicate that future severe droughts could lead to widespread dieback and high mortality at marginal sites. Therefore, if effective management is not implemented, warmer and drier conditions will likely threaten the viability of many of these drought-prone plantations and their ability to deliver key ecosystem services.

Multiple Potential Stressors and Dieback of Phragmites australis in the Mississippi River Delta, USA: Implications for Restoration

In 2016, widespread dieback of Phragmites australis was reported in the Mississippi River Delta (MRD), Louisiana. We conducted two common-garden experiments to investigate several potentially important factors associated with this dieback: scale insects, water depth, fertilization and P. australis genetics (three lineages: Gulf, Delta and invasive EU). Predictions tested were scale abundance is lower in high water, at low fertilization, and for EU; plant biomass is negatively impacted by scales, high water and high fertilization; and EU suffers the least damage from the three potential stressors. Scale abundance was 41% lower in high water and decreased 2.7 fold as fertilization increased. Also, EU had 1.5–2.6 times fewer scales than Gulf, but had similar scale abundance to Delta. Impacts of scales, water depth and fertilizer on plant biomass depended strongly on lineage. Scales reduced biomass of Delta, EU and Gulf by 38%, 32% and 10%, respectively. In comparison, biomass was 30% higher for EU, 46% lower for Gulf and unchanged for Delta in high versus low water. Finally, at high fertilization levels, Gulf produced 57% more biomass than EU. Owing to its greater tolerance to scales and high water, EU may be most suitable for use in restoration of the MRD.

Long-term monitoring of the recruitment and dynamics of large wood in Kamienica Stream, Polish Carpathians

Studies presenting long-term observations of the recruitment and mobility of large wood in mountain watercourses are scarce, but they can considerably contribute to the knowledge of river/riparian forest interactions and the assessment of flood hazard resulting from wood mobility during floods. Widespread dieback of riparian forest along the headwater course of Kamienica Stream in the Polish Carpathians, caused by bark beetle infestation of spruce trees, has raised concerns about potential increases of large wood recruitment to the stream and of the flood hazard to downstream valley reaches. In October 2009, 429 trees growing along three sections of the stream were tagged with numbered metal plates and monitored over 10 years to determine the timing and causes of their delivery to the channel and the lengths of their displacement during individual flood events. Moreover, in 2012 the mode of location of wood deposits and a degree of wood decay were determined in the second- to fourth-order stream reaches. The monitoring of tagged trees indicated that trees were recruited to the channel during highintensity meteorological and hydrological events, mostly as a result of bank erosion during floods or windthrow. With 22% of tagged trees recruited to the channel during 10 years, the rate of turnover of the riparian trees was estimated at 45 years. As the riparian area is overgrown with trees with ages up to ~160 years, the rate evidences substantial intensification of large wood recruitment to the channel in the recent period. Results of large wood inventory and the 10-year-long monitoring of tagged trees indicated variable mobility of large wood along the upper course of the stream. Wood mobility was negligible in the second-order stream reach, very small in the third-order reach, and greater, but still limited in the fourth-order reach. Wood is transported longer distances only during major floods. However, the advanced state of decay of most pieces leads to their disintegration during floods, precluding distant transport. Thus, large wood retained in the upper stream course does not constitute an important flood hazard to downstream, inhabited valley reaches.

A new evaluation of the influence of climate change on Zagros oak forest dieback in Iran

Zagros forest, the second most important renewable source of cellulose for Iran, is one of the best known oak forest reserves worldwide. These vital ecosystems hold great environmental and ecological importance in protecting water resources in the climatic dry conditions of the country. In recent years, however, oak dieback has decimated these forests. The purpose of our work is to evaluate and analyze the relationship of climate variables and the dieback of the Zagros oak forest in Lorestan Province, Iran. Dieback is defined by dieback indices, such as DVI, GEMI, IPVI, MSAVI2, NDVI, EVI, OSAVI, Sarvi2, SR, and VHI, which were obtained for 110 locations in the study area in the period between 2001 and 2017. On the other hand, daily climate variables influencing oak tree dieback were extracted in the same time period. As a first step, the most important climate variables affecting oak tree dieback were detected using the factor analysis method. Next, the so-called G* spatial correlation index was used to compute the covariance of dieback indices with climate variables as well as the spatial patterns of the changes of these covariances. The results of factor analysis showed that three factors, namely, humidity, temperature, and precipitation, were the most significant variables affecting dieback indices. Spatial autocorrelation patterns indicated that humidity played a major role regarding the widespread dieback in the study area. The mismatch of areas of low humidity and dieback indices, however, prompts that other factors, such as fungal diseases, affected the dieback alongside climatic variables. A similar investigation relating temperature with dieback indices indicated that changes in the match of spatial autocorrelation patterns of temperature and other dieback indices were less than 30% in terms of matching area except for NDVI and EVI. The effect of temperature on dieback indices was more limited to the southeast part of the study area. However, the covariance change pattern relating precipitation with dieback indices was related to northwest areas. Among all studied dieback indices, the NDVI was most correlated with all three factors of humidity, temperature, and precipitation, and temperature along humidity was found to play the most important role concerning the dieback of the Zagros oak forest.

Biodiversity Conservation Targets: How to Allocate Resources

In this issue of One Earth, Chauvenet et al. identify strategies for the near-term allocation of protected areas on an ecoregion basis. However, the biodiversity challenge extends beyond ecoregions. More resources are needed to maximize the potential co-benefits between conservation, climate-change mitigation, indigenous communities, and sustainable land-management practices.

Triggers of widespread dieback and mortality of poplar (Populus spp.) plantations across northern China

Drought-induced mortality has been reported in many forest biomes around the world. In recent years, large-scale forest mortality has been observed across northern China, with forests exhibiting widespread crown dieback symptoms. Previous reports have attributed dieback and mortality to drought stress. In this study, a field survey was undertaken along two transects in northern China. Our aims were to clarify the underlying mechanisms of the widespread dieback and mortality of poplar (Populus spp.) plantations under various environmental stresses. Under these conditions, we observed narrower tree-ring width, and decreased soil water content, indicating that forest growth increased drought stress, and drought stress played a prominent role in triggering dieback and mortality. Bacterial canker disease and low soil nutrient were also linked to dieback and mortality. We observed xylem damage, showing that latent bacterial canker disease was present in many poplar tree stands across northern China, which may increase their susceptibility to drought stress. The results showed that widespread dieback and mortality of poplar forests were related to the interaction of drought stress, bacterial canker and low soil nutrient. The results contributed to understanding the causes for poplar plantation deaths, and could help in the prevention of large-scale death of poplar plantations.

Large-scale dieback of mangroves in Australia

This study records and documents the most severe and notable instance ever reported of sudden and widespread dieback of mangrove vegetation. Between late 2015 and early 2016, extensive areas of mangrove tidal wetland vegetation died back along 1000km of the shoreline of Australia’s remote Gulf of Carpentaria. The cause is not fully explained, but the timing was coincident with an extreme weather event; notably one of high temperatures and low precipitation lacking storm winds. The dieback was severe and widespread, affecting more than 7400ha or 6% of mangrove vegetation in the affected area from Roper River estuary in the Northern Territory, east to Karumba in Queensland. At the time, there was an unusually lengthy period of severe drought conditions, unprecedented high temperatures and a temporary drop in sea level. Although consequential moisture stress appears to have contributed to the cause, this occurrence was further coincidental with heat-stressed coral bleaching. This article describes the effect and diagnostic features of this severe dieback event in the Gulf, and considers potential causal factors.

Widespread dieback of riparian trees on a dammed ephemeral river and evidence of local mitigation by tributary flows.

Ephemeral rivers act as linear oases in drylands providing key resources to people and wildlife. However, not much is known about these rivers’ sensitivities to human activities. We investigated the landscape-level determinants of riparian tree dieback along the Swakop River, a dammed ephemeral river in Namibia, focusing on the native ana tree (Faidherbia albida) and the invasive mesquite (Prosopis spp.). We surveyed over 1,900 individual trees distributed across 24 sites along a 250 km stretch of the river. General linear mixed models were used to test five hypotheses relating to three anthropogenic threats: river flow disruption from damming, human settlement and invasive species. We found widespread dieback in both tree populations: 51% mortality in ana tree, with surviving trees exhibiting 18% canopy death (median); and 26% mortality in mesquite, with surviving trees exhibiting 10% canopy death. Dieback in the ana tree was most severe where trees grew on drier stretches of the river, where tributary flow was absent and where mesquite grew more abundantly. Dieback in the mesquite, a more drought-tolerant taxon, did not show any such patterns. Our findings suggest that dieback in the ana tree is primarily driven by changes in river flow resulting from upstream dam creation and that tributary flows provide a local buffer against this loss of main channel flow. The hypothesis that the invasive mesquite may contribute to ana tree dieback was also supported. Our findings suggest that large dams along the main channels of ephemeral rivers have the ability to cause widespread mortality in downstream riparian trees. To mitigate such impacts, management might focus on the maintenance of natural tributary flows to buffer local tree populations from the disruption to main channel flow.

Diversity of Phytophthora Species from Declining Mediterranean Maquis Vegetation, including Two New Species, Phytophthora crassamura and P. ornamentata sp. nov.

The Mediterranean basin is recognized as a global biodiversity hotspot accounting for more than 25,000 plant species that represent almost 10% of the world’s vascular flora. In particular, the maquis vegetation on Mediterranean islands and archipelagos constitutes an important resource of the Mediterranean plant diversity due to its high rate of endemism. Since 2009, a severe and widespread dieback and mortality of Quercus ilex trees and several other plant species of the Mediterranean maquis has been observed in the National Park of La Maddalena archipelago (northeast Sardinia, Italy). Infected plants showed severe decline symptoms and a significant reduction of natural regeneration. First studies revealed the involvement of the highly invasive wide-host range pathogen Phytophthora cinnamomi and several fungal pathogens. Subsequent detailed research led to a better understanding of these epidemics showing that multiple Phytophthora spp. were involved, some of them unknown to science. In total, nine Phytophthora species were isolated from rhizosphere soil samples collected from around symptomatic trees and shrubs including Asparagus albus, Cistus sp., Juniperus phoenicea, J. oxycedrus, Pistacia lentiscus and Rhamnus alaternus. Based on morphological characters, growth-temperature relations and sequence analysis of the ITS and cox1 gene regions, the isolates were identified as Phytophthora asparagi, P. bilorbang, P. cinnamomi, P. cryptogea, P. gonapodyides, P. melonis, P. syringae and two new Clade 6 taxa which are here described as P. crassamura sp. nov. and P. ornamentata sp. nov. Pathogenicity tests supported their possible involvement in the severe decline that is currently threatening the Mediterranean maquis vegetation in the La Maddalena archipelago.

Contrasting effects of two exotic invasive hemipterans on whole‐plant resource allocation in a declining conifer

AbstractInvasive herbivores can cause widespread dieback of naïve native hosts in the invaded range. Some consume leaves, some bore through wood, whereas others, such as piercing‐sucking insects, alter plant resource allocation through changes to source‐sink dynamics and depletion of long‐term stores. Invasive sap‐sucking herbivores that target cells critical to resource transport and storage may have particularly large effects. Herbivory by two exotic hemipterans, hemlock woolly adelgid (HWA), Adelges tsugae Annand (Adelgidae), and elongate hemlock scale (EHS), Fiorinia externa Ferris (Diaspididae), have very different effects on eastern hemlock, Tsuga canadensis (L.) Carrière (Pinaceae). Although these insects differ in both timing and feeding site on their hemlock host, the reasons for their differential effects are poorly understood. Here, using potted seedlings in a common garden, we examined the effects of these two herbivores on resource uptake and allocation immediately after an initial attack. We labeled the plants with a single pulse of 13CO2 and a supply of 15NH415NO3 every third day to obtain a whole‐plant perspective on resource uptake and allocation. After 10 weeks of controlled infestation, plants were measured and divided into tissue types (needles, branches, main stem, and roots). In each tissue we quantified biomass, 13C, 15N, total carbon (C), nitrogen (N), protein, and starch pools. Hemlock woolly adelgid feeding decreased new needle biomass by 34%, increased 13C allocation to roots and main stems by 130%, and increased 15N allocation to old foliage by 18%. Hemlock woolly adelgid infestation also resulted in increased starch storage in old branches over new needles, and marginally increased protein content plant‐wide. Elongate hemlock scale infestation resulted in a different growth pattern, with a 27% increase in biomass allocation to the main stem. Elongate hemlock scale also caused a 23% increase in N allocation to roots and main stem. Increases in resource allocation to main stem and belowground may indicate herbivore‐induced changes to storage patterns, or compensatory increased fine root growth to facilitate nutrient and water uptake. These resource allocation effects likely underlie the rapid and dramatic decline of hemlock in response to HWA feeding, and the considerably milder effects of EHS feeding.