Parasitoid and inquiline attack in the galls of four alien, cynipid gall wasps: host switches and the effect on parasitoid sex ratios

1. Four alien cynipid gall wasps of the genus Andricus are established and still spreading in the British Isles. The order, according to the northerliness of their distribution boundary, is: A. corruptrix→A. quercuscalicis→A. lignicola→A. kollari. All four aliens have a sexual generation in spring on Quercus cerris (introduced to Britain) and an agamic generation in autumn on native oak species. 2. For 2 years 1994 and 1995, galls of both generations of the four alien species were sampled at eight sites from the south of England to the north of Scotland to determine the parasitoid and inquiline species that attack the new galls. The spring generations of the invading species shared a parasitoid complex of four pteromalid species. Five species of inquilines and 11 species of parasitoids emerged from the autumn galls. 3. Two colonisation events were recorded for A. lignicola and A. corruptrix. On both occasions, the spring generations were found first at the new sites, indicating that the agamic generation provides the colonisers for these invading species. After colonisation, the galls of both species were attacked by parasitoids in their first season. 4. In spring, the invading species were among the most abundant cynipids at all eight sites. By sampling the whole local community of cynipid galls, it was found that the parasitoid species attacking the spring galls of the invaders seemed to have shifted their attack to the new hosts. 5. The secondary sex ratios of the parasitoid species emerging from the sexual galls of A. quercuscalicis (the smallest of the four) showed a strong and significant male bias at all sites and in both years. Parasitoid emergence from the galls of the sexual generations of the other three species (all about equal in size) was between 60 and 70% male, and variable among sites and between years.     

Invaders on the move: parasitism in the sexual galls of four alien gall wasps in Britain (Hymenoptera: Cynipidae)

Four alien, host-alternating cynipid gall wasps are established in the British Isles: Andricus kollari, A. lignicola, A. quercuscalicis and A. corruptrix. Their current distributions divide Britain into four zones; all four invaders are present in the south and south-east of Britain, whereas only A. kollari is present in the far north of Scotland. The rank order, according to how far north the distributions of the four invading cynipid species reach, is: A. corruptrix→ A. quercuscalicis→ A. lignicola→ A. kollari. The life cycles of all four cynipid species involve a sexual generation in spring on Quercus cerris, and an agamic generation in autumn on Q. robur. Here we studied the parasitoid attack by four pteromalid species on the sexual generations of the invaders. We collected and reared the galls of all four species at eight sites from the south to the north of the country (two sites per zone). The geographical locations of the sites reflect the invasion history and thus the residence time of the alien species in the four zones. At each site we measured the densities of all host galls on Q. cerris and of both obligatory host-tree species. We also took a series of measures, such as host-tree density and mean host-tree size, to further characterize the tree stands. These measures are referred to as local parameters. Host densities varied between sites and between years. In A. kollari, galling rates were highest in the middle of the country (zone 2) in 1994, whereas in 1995 they increased from the south to the north. In A. lignicola, galling rates in both years were lowest at the sites in zone 3 (closest to its distribution boundary). In A. quercuscalicis, galling rates were found to be lowest at the site most to the north-west in both years, again the one furthest away from the area where this species was first recorded. Mortality caused by parasitoid attack differed from less than 10% to as high as 70% and varied between host species, sites and years. In four out of six cases the historical/regional variables (north/south and east/west) correlated significantly with parasitoid attack rates that were characteristically lowest at sites close to the distribution boundaries. Of the local factors, we found parasitoid attack rates correlated negatively in one case with host density, whereas they correlated positively in four cases with the density of alternative hosts of the parasitoids. In one of the models the local density of Q. cerris trees correlated negatively with parasitoid attack on A. quercuscalicis. For all three host species the terms retained in the minimal adequate models obtained for 1994 and 1995 differed, which might indicate that these communities of native parasitoids and invading host have not yet settled in any definite structure.     

Differential success in northwards range expansion between ecotypes of the marble gallwasp Andricus kollari: a tale of two lifecycles

The Marble gallwasp Andricus kollari has a native range divided into two geographically separated lifecycles. In Eastern Europe and Turkey, the lifecycle involves a sexual generation on Turkey oak, Quercus cerris, while in Iberia and North Africa the sexual generation host is cork oak, Q. suber. Over the last 500 years, A. kollari has expanded its range into northern Europe, following human planting of Q. cerris from Italy and the Balkans. We ask: (i) what is the genetic relationship between eastern and western distributions of Andricus kollari? Can we determine which lifecycle is ancestral, and how long ago they diverged? (ii) To what extent have eastern and western native ranges contributed to northwards range expansion? (iii) Is there any evidence for hybridization between the two life cycle types? We present analyses of allozyme data for 13 polymorphic loci and of sequence variation for a 433 bp fragment of the mitochondrial cytochrome b gene. These show: (i) that four haplotype lineages (one in Spain, two in Hungary/Italy and one in Turkey) diverged more or less simultaneously between 1 and 2 million years ago, suggesting the existence of at least four refuges through recent ice age cycles. Our data cannot resolve which lifecycle type is ancestral. (ii) Populations north of putative refuges are divided into two sets. Populations in south‐west France are allied to Spain, while all remaining populations in northern Europe have been colonized from Italy and the Balkans. (iii) The transition from one race to another in south‐west France is marked by abrupt transitions in the frequency of refuge‐specific private alleles and corresponds closely to the northern limit of the distribution of cork oak. Although hybrids were detected in north‐west France, none were detected where the two lifecycles meet in south‐western France. The biology of oak gallwasps predicts that any hybrid zone will be narrow, and limited to regions where Q. cerris and Q. suber meet. Our data suggest that eastern and western A. kollari are effectively separate species.     

Partitioning of herbivore hosts across time and food plants promotes diversification in the Megastigmus dorsalis oak gall parasitoid complex

Communities of insect herbivores and their natural enemies are rich and ecologically crucial components of terrestrial biodiversity. Understanding the processes that promote their origin and maintenance is thus of considerable interest. One major proposed mechanism is ecological speciation through host‐associated differentiation (HAD), the divergence of a polyphagous species first into ecological host races and eventually into more specialized daughter species. The rich chalcid parasitoid communities attacking cynipid oak gall wasp hosts are structured by multiple host traits, including food plant taxon, host gall phenology, and gall structure. Here, we ask whether the same traits structure genetic diversity within supposedly generalist parasitoid morphospecies. We use mitochondrial DNA sequences and microsatellite genotypes to quantify HAD for Megastigmus (Bootanomyia) dorsalis, a complex of two apparently generalist cryptic parasitoid species attacking oak galls. Ancient Balkan refugial populations showed phenological separation between the cryptic species, one primarily attacking spring galls, and the other mainly attacking autumn galls. The spring species also contained host races specializing on galls developing on different host‐plant lineages (sections Cerris vs. Quercus) within the oak genus Quercus. These results indicate more significant host‐associated structuring within oak gall parasitoid communities than previously thought and support ecological theory predicting the evolution of specialist lineages within generalist parasitoids. In contrast, UK populations of the autumn cryptic species associated with both native and recently invading oak gall wasps showed no evidence of population differentiation, implying rapid recruitment of native parasitoid populations onto invading hosts, and hence potential for natural biological control. This is of significance given recent rapid range expansion of the economically damaging chestnut gall wasp, Dryocosmus kuriphilus, in Europe.     

Seasonal variations of gas exchange, photosynthetic pigments, and antioxidants in Turkey oak (Quercus cerris L.) and Hungarian oak (Quercus frainetto Ten.) of different age

Seasonal changes in physiological and biochemical parameters were studied in 35-, 55- and 140-year-old trees of Turkey oak (Quercus cerris L.) and Hungarian oak (Q. frainetto Ten.), growing in natural stands in Eastern Balkan Mountains (Bulgaria). During the seasonal drought period (August), assimilation activity, transpiration rate, stomatal conductance and water potential had a seasonal minimum in all the studied tree ages and species. The foliar concentrations of glutathione, ascorbate, α-tocopherol, as well as photosynthetic pigments in oak leaves were significantly affected by season. With the increasing age of the studied trees, we observed a decrease of the physiological activity and an increase of the antioxidants’ accumulation. Both the species were drought tolerant and anisohydric, where Q. frainetto exhibited higher rates of gas exchange than Q. cerris. Moreover, they differed in the extent of increase in the foliar antioxidants and carotenoids.     

Native Birds and Alien Insects: Spatial Density Dependence in Songbird Predation of Invading Oak Gallwasps

Revealing the interactions between alien species and native communities is central to understanding the ecological consequences of range expansion. Much has been learned through study of the communities developing around invading herbivorous insects. Much less, however, is known about the significance of such aliens for native vertebrate predators for which invaders may represent a novel food source. We quantified spatial patterns in native bird predation of invading gall-inducing Andricus wasps associated with introduced Turkey oak (Quercus cerris) at eight sites across the UK. These gallwasps are available at high density before the emergence of caterpillars that are the principle spring food of native insectivorous birds. Native birds showed positive spatial density dependence in gall attack rates at two sites in southern England, foraging most extensively on trees with highest gall densities. In a subsequent study at one of these sites, positive spatial density dependence persisted through four of five sequential week-long periods of data collection. Both patterns imply that invading galls are a significant resource for at least some native bird populations. Density dependence was strongest in southern UK bird populations that have had longest exposure to the invading gallwasps. We hypothesise that this pattern results from the time taken for native bird populations to learn how to exploit this novel resource.     

Cytosolic ascorbate peroxidase in some species of the Quercus genus

ABSTRACT

Six species of the Quercus genus (Quercus ilex L., Q. coccifera L., Q. suber L., Q. trojana Webb, Q. macrolepis Kotschy, Q. cerris L.) have been screened for cytosolic ascorbate peroxidase (APX) by means of native polyacrylamide gel electrophoresis (PAGE). A single isozyme was found in five species (Q. trojana, Q. suber, Q. cerris, Q. macrolepis and Q. coccifera), while Q. ilex shows two different APX proteins. The data showed marked similarities among Q. trojana, Q. suber, Q. cerris and Q. macrolepis with respect to the electrophoretic mobility. The validity of APX electrophoretic patterns in systematic studies is discussed.     

Geographic and between-generation variation in the parasitoid communities associated with an invading gallwasp,Andricus quercuscalicis (Hymenoptera: Cynipidae)

The knopper gallwasp Andricus quercuscalicis Burgsdorf 1783 (Hymenoptera: Cynipidae) has invaded western and northern Europe from southern and eastern Europe over the last 400 years. A. quercuscalicis has two alternating generations, which differ in phenology, structure, and host oak species. This study describes geographic variation in the community in the tiny catkin galls of the sexual generation on Turkey oak, Quercus cerris, and compares the patterns obtained with those in the community attacking the alternate agamic generation. As predicted from considerations of parasitoid recruitment to the communities of invading phytophagous insects (Cornell and Hawkins 1993), in its native range the sexual generation shows (1) higher parasitoid community species richness, (2) higher total mortality due to parasitoid attack and (3) a higher ratio of specialist to generalist parasitoid species than is evident in the invaded range. Counter to predictions, there is no indication that parasitoid community richness in the invaded range has increased with time since the arrival of the new host. Higher host mortality in the native range is due principally to a single specialist, Aulogymnus obscuripes Mayr 1877 (Hymenoptera: Eulophidae), and is not distributed evenly among parasitoid species which attack the gall-former only in this area. This contrasts with the community in Britain, where three principal generalist parasitoids cause approximately equal mortalities. The agamic gall contains a taxonomically and structurally diverse guild of parasitoid and inquiline species, associated with the changing resource provided by a large, long-lived, complex gall. In contrast, the sexual community includes a taxonomically and structurally narrow guild, associated with a resource which is structurally simple, small in size and short-lived. No parasitoid species attacks the gall-former in both generations. Surprisingly, in spite of these differences in the nature of the gall resource in the two generations, over their entire range (native and invaded) the parasitoid guilds of the two are equally species rich.     

Variations in physiological and biochemical traits of oak seedlings grown under drought and ozone stress

Despite the huge biodiversity characterizing the Mediterranean environment, environmental constraints, such as high sunlight and high temperatures alongside with dry periods, make plant survival hard. In addition, high irradiance leads to increasing ozone (O₃) concentrations in ambient air. In this era of global warming, it is necessary to understand the mechanisms that allow native species to tolerate these environmental constraints and how such mechanisms interact. Three Mediterranean oak species (Quercus ilex, Quercus pubescens and Quercus cerris) with different features (drought tolerant, evergreen or deciduous species) were selected to assess their biometrical, physiological and biochemical responses under drought and/or O₃ stress (80–100 nl l^?1 of O₃ for 5 h day^?1 for 77 consecutive days). Leaf visible injury appeared only under drought stress (alone or combined with O₃) in all three species. Drought × O₃ induced strong reductions in leaf dry weight in Q. pubescens and Q. cerris (?70 and ?75%, respectively). Alterations in physiological (i.e. decrease in maximum carboxylation rate) and biochemical parameters (i.e. increase in proline content and build‐up of malondialdehyde by‐products) occurred in all the three species, although drought represented the major determinant. Quercus ilex and Q. pubescens, which co‐occur in dry environments, were more tolerant to drought and drought × O₃. Quercus ilex was the species in which oxidative stress occurred only when drought was applied with O₃. High plasticity at a biochemical level (i.e. proline content) and evergreen habitus are likely on the basis of the higher tolerance of Q. ilex.     

KISSMig – a simple model for R to account for limited migration in analyses of species distributions

Detecting the legacy of time‐lagged migration in species ranges is an urgent matter for understanding range dynamics. KISSMig is a simple migration model which generates maps of accessibility from areas of origin and allows the generation and testing of hypotheses about the influence of specific spread patterns on species distributions. KISSMig has important applications: 1) uncovering the influence of limited migration relative to other drivers, 2) detecting areas of origin and their importance as sources of migration, and 3) accounting for limited migration in modeling species distributions. Here we introduce KISSMig and use the oak species Quercus cerris to illustrate these applications.     

Fungal Endophytic Communities in Healthy and Declining Quercus robur L. and Q. cerris L. Trees in Northern Italy

Healthy and declining English oak (Quercus robur) and Turkey oak (Q. cerris) in north‐western Italy, in a plain oak forest showing decline for oak puzzle disease, were tested to assess possible variations in the composition of their fungal endophytic communities and their relation to the health status of trees. Samples collected in spring (buds) and in autumn (leaves, annual shoots and twigs) were surface‐sterilized, cut into fragments and placed on potato dextrose agar for a month; 26 fungal species were isolated, the most frequent being Tubakia dryina, Dendrodochium sp., Eutypella sp. and a sterile mycelium. Correspondence analysis showed significant qualitative differences between assemblages inhabiting twigs and herbaceous tissues that were due to the low frequency of Tubakia dryina in twigs and its higher frequency in buds, leaves and shoots. Tubakia dryina was isolated more frequently from leaves of declining oaks and from buds of healthy oaks; Monochaetia monochaeta showed a preference for healthy trees, especially leaves and buds. According to the Shannon–Wiener index, endophytic fungal communities of leaves, twigs and buds of declining English oak were poorer than those of declining Turkey oak, but there were no significant differences between healthy hosts.     

Spread and potential host range of the invasive oak lace bug [Corythucha arcuata (Say, 1832) – Heteroptera: Tingidae] in Eurasia

1. The North American oak lace bug feeds on leaves of ‘white oaks” in its native range. In Europe, it was first discovered in northern Italy in 2000. In recent years, it has subsequently spread rapidly and population outbreaks have been observed in several European countries. In the present study, we summarize the steps of its expansion.
2. To predict its potential host range, we checked 48 oak species in 20 sentinel gardens in seven countries between 2013 and 2018.
3. In total, 27 oak species were recorded as suitable hosts; 13 of them are globally new ones, 23 out of the 29 in section Quercus (～ white oaks, an intrageneric taxonomic unit within genus Quercus), including Asian oaks, native to Japan, Korea and China, and four out of five in section Cerris (another intrageneric unit of the same genus), were accepted as hosts. None of the species in section Lobatae (red oaks) or in the Ilex group was accepted.
4. Host records were also collected in forest stands of 10 countries. We found 11 oak species that were infested. Outbreak populations were most commonly found on Quercus robur, Quercus frainetto, Quercus petraea and Quercus cerris, comprising widespread and outstandingly important oaks species in Europe.
5. Based on our findings, we conclude that suitable hosts for oak lace bug are present in most of Europe and Asia. This means that a lack of hosts will likely not restrict further range expansion.

     

A nuclear DNA barcode for eastern North American oaks and application to a study of hybridization in an Arboretum setting

DNA barcoding has proved difficult in a number of woody plant genera, including the ecologically important oak genus Quercus. In this study, we utilized restrictionsite‐associated DNA sequencing (RAD‐seq) to develop an economical single nucleotide polymorphism (SNP) DNA barcoding system that suffices to distinguish eight common, sympatric eastern North American white oak species. Two de novo clustering pipelines, PyRAD and Stacks, were used in combination with postclustering bioinformatic tools to generate a list of 291 potential SNPs, 80 of which were included in a barcoding toolkit that is easily implemented using MassARRAY mass spectrometry technology. As a proof‐of‐concept, we used the genotyping toolkit to infer potential hybridization between North American white oaks transplanted outside of their native range (Q. michauxii, Q. montana, Q muehlenbergii/Q. prinoides, and Q. stellata) into a horticultural collection surrounded by natural forests of locally native trees (Q. alba and Q. macrocarpa) in the living collection at The Morton Arboretum (Lisle, IL, USA). Phylogenetic and clustering analyses suggested low rates of hybridization between cultivated and native species, with the exception of one Q. michauxii mother tree, the acorns of which exhibited high admixture from either Q. alba or Q. stellata and Q. macrocarpa, and a hybrid between Q. stellata that appears to have backcrossed almost exclusively to Q. alba. Together, RAD‐seq and MassARRAY technologies allow for efficient development and implementation of a multispecies barcode for one of the more challenging forest tree genera.     

Foliar nutrients explain goldspotted oak borer,Agrilus auroguttatus,adult feeding preference among four California oak species

Adults of the invasive goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), consumed foliar weight in no‐choice feeding tests of, in descending order, California black oak Quercus kelloggii Newb., Engelmann oak, Quercus engelmannii Greene, coast live oak, Quercus agrifolia Née, and canyon live oak, Quercus chrysolepis Liebm. (Fagaceae). Furthermore, significantly more foliar area was consumed of Q. kelloggii than of Q. chrysolepis. In dual‐choice feeding tests with isolated leaf disks, A. auroguttatus consumed significantly more foliar weight and area of Q. kelloggii relative to the other three oak species, and more foliar weight of Q. agrifolia than of Q. chrysolepis. In dual‐choice feeding tests with leaves on small branches, A. auroguttatus consumed more foliar weight of Q. kelloggii than of Q. engelmannii and Q. agrifolia. Thus, multiple experiments suggested that adults of A. auroguttatus preferred the foliage of Q. kelloggii over that of the other three oak species, and among the other three species they did not appear to have a strong feeding preference. Factor analysis reduced the quantities of 13 foliar nutrients into two new variables (factor 1 and factor 2). Factor 1 was weighted heavily on the quantities of nitrogen, sulfur, phosphorus, potassium, zinc, and copper, whereas factor 2 was weighted heavily on the quantities of zinc, iron, and aluminum. Factor 1 varied by oak species, with Q. kelloggii having a higher factor 1 nutrient content than the other three species. Factor 2 response was higher in Q. kelloggii, Q. agrifolia, and Q. engelmannii than in Q. chrysolepis. The collective effects of four macronutrients (nitrogen, sulfur, phosphorus, and potassium) and two micronutrients (zinc and copper) suggest that these might be the nutrients directing preferential feeding of A. auroguttatus adults on the foliage of Q. kelloggii. Leaf toughness might also play an important role in feeding preference. Female A. auroguttatus did not show an ovipositional preference among the four oak species.     

Cascading reproductive isolation: Plant phenology drives temporal isolation among populations of a host‐specific herbivore

All organisms exist within a complex network of interacting species, thus evolutionary change may have reciprocal effects on multiple taxa. Here, we demonstrate “cascading reproductive isolation,” whereby ecological differences that reduce gene flow between populations at one trophic level affect reproductive isolation (RI) among interacting species at the next trophic level. Using a combination of field, laboratory and common‐garden studies and long‐term herbaria records, we estimate and evaluate the relative contribution of temporal RI to overall prezygotic RI between populations of Belonocnema treatae, a specialist gall‐forming wasp adapted to sister species of live oak (Quercus virginiana and Q. geminata). We link strong temporal RI between host‐associated insect populations to differences between host plant budbreak phenology. Budbreak initiates flowering and the production of new leaves, which are an ephemeral resource critical to insect reproduction. As flowering time is implicated in RI between plant species, budbreak acts as a “multitrophic multi‐effect trait,” whereby differences in budbreak phenology contribute to RI in plants and insects. These sister oak species share a diverse community of host‐specific gall‐formers and insect natural enemies similarly dependent on ephemeral plant tissues. Thus, our results set the stage for testing for parallelism in a role of plant phenology in driving temporal cascading RI across multiple species and trophic levels.     

Oak gall wasp infections of Quercus robur leaves lead to profound modifications in foliage photosynthetic and volatile emission characteristics

Oak trees (Quercus) are hosts of diverse gall‐inducing parasites, but the effects of gall formation on the physiology and biochemistry on host oak leaves is poorly understood. The influence of infection by four species from two widespread gall wasp genera, Neuroterus (N. anthracinus and N. albipes) and Cynips (C. divisa and C. quercusfolii), on foliage morphology, chemistry, photosynthetic characteristics, constitutive isoprene, and induced volatile emissions in Q. robur was investigated. Leaf dry mass per unit area (M_A), net assimilation rate per area (A_A), stomatal conductance (g_s), and constitutive isoprene emissions decreased with the severity of infection by all gall wasp species. The reduction in A_A was mainly determined by reduced M_A and to a lower extent by lower content of leaf nitrogen and phosphorus in gall‐infected leaves. The emissions of lipoxygenase pathway volatiles increased strongly with increasing infection severity for all 4 species with the strongest emissions in major vein associated species, N. anthracinus. Monoterpene and sesquiterpene emissions were strongly elicited in N. albipes and Cynips species, but not in N. anthracinus. These results provide valuable information for diagnosing oak infections using ambient air volatile fingerprints and for predicting the impacts of infections on photosynthetic productivity and whole tree performance.     

Impacts of elevated atmospheric CO2 on litter quality, litter decomposability and nitrogen turnover rate of two oak species in a Mediterranean forest ecosystem

Elevated CO₂ may affect litter quality of plants, and subsequently C and N cycling in terrestrial ecosystems, but changes in litter quality associated with elevated CO₂ are poorly known. Abscised leaf litter of two oak species (Quercus cerris L. and Q. pubescens Willd.) exposed to long-term elevated CO₂ around a natural CO₂ spring in Tuscany (Italy) was used to study the impact of increasing concentration of atmospheric CO₂ on litter quality and C and N turnover rates in a Mediterranean-type ecosystem. Litter samples were collected in an area with elevated CO₂ (>500 ppm) and in an area with ambient CO₂ concentration (360 ppm). Leaf samples were analysed for concentrations of total C, N, lignin, cellulose, acid detergent residue (ADR) and polyphenol. The decomposition rate of litter was studied using a litter bag experiment (12 months) and laboratory incubations (3 months). In the laboratory incubations, N mineralization in litter samples was measured as well (125 days). Litter quality was expressed in terms of chemical composition and element ratios. None of the litter quality parameters was affected by elevated CO₂ for the two Quercus species. Remaining mass in Q. cerris and Q. pubescens litter from elevated CO₂ was similar to that from ambient conditions. C mineralization in Q. pubescens litter from elevated CO₂ was lower than that from ambient CO₂, but the difference was insignificant. This effect was not observed for Q. cerris. N mineralization was higher from litter grown at elevated CO₂, but this difference disappeared at the end of the incubation. Litter of Q. pubescens had a higher quality than Q. cerris, and indeed mineralized more rapidly in the laboratory, but not under field conditions.     

Long-term radial growth and climate-growth relationships of Quercus petraea (Matt.) Liebl. and Quercus cerris L. in a xeric low elevation site from Hungary

Sessile oak (Quercus petraea [Matt.] Liebl.) and Turkey oak (Quercus cerris L.) dominated mixed forests are common in low montane and hilly regions in Hungary. Here, we aimed to describe the long-term pattern and climatic responses of the radial growth of Q. petraea and Q. cerris in a xeric low-elevation forest, using retrospective tree-ring analysis for the period 1910–2019. We performed separate analyses with time series of full tree-ring (TRW), earlywood (EW) and latewood (LW) widths. Our results showed that the radial growth of the two species was largely synchronous over time, but became transiently divergent for a 20-year period after a drought in 1968, due to the greater-than-expected growth of Q. cerris and the supressed growth of Q. petraea. Precipitation was the major growth-limiting factor for both species, with a strong positive influence on LW particularly during the current early growing season (March-June), on EW in the previous late summer (August-September) and in the current early spring (March), and on all tree-ring traits in the previous December. The radial growth of both species was negatively related to temperature in the spring (May) and late summer (August) of both current and previous years. The climate-growth relationships showed general instability over time: the most striking temporal change was a gradual shift of positive correlations with precipitation and SPEI during the growing season from spring (March-May) to summer (June-August) since the 1980s over the analysed period. The two species had similarly low growth resistance to droughts in four studied pointer years (1968, 1993, 2002 and 2012), but Q. cerris exhibited a greater capacity to recover over the four post-drought years, and thus higher growth resilience, particularly after the drought of 1968. Our results contribute to the better understanding of the role of climate variability and droughts in the growth of the two co-existing species in transitional locations between closed forests and forest-steppes.     

Molecular evidence for hybrid origin of Quercus crenata Lam. (Fagaceae) from Q. cerris L. and Q. suber L.

Abstract

Random Amplified Polymorphic DNA (RAPD) and Inter-Simple Sequence Repeats (ISSR) markers were employed to examine samples from Quercus cerris, Q. suber and Q. crenata in order to test the hypothesis of the hybrid origin of Q. crenata from Q. cerris and Q. suber in a part of its distribution area where the two putative parents do not overlap at present. Leaves from 21 Q. crenata and 37 Q. cerris individual trees were collected at natural sites in northern Italy, where Q. suber is currently lacking; 21 Q. suber and six Q. crenata plants from central Italy were also analysed. Results from Unweighted Pair Group Method with Arithmetic mean (UPGMA) analysis and principal component analysis (PCA) implied that exchange of neutral markers has been considerable between the three species, while differences in morphological characters have remained comparatively stable. The Mantel test indicated low correlation between RAPD- and ISSR-based similarity matrices, showing that the two screening techniques reveal unrelated estimates of genetic relationships. Hybrid indices computed for both markers displayed an intermediate position of Q. crenata individuals between the two putative parents shifted toward Q. cerris. Results from the present study corroborate the hypothesis of a hybrid origin for Q. crenata occurring in northern Italy, and suggest asymmetrical backcrossing with Q. cerris acting as the recurrent parent.     

Suitability of native and ornamental oak species in California for Agrilus auroguttatus

Goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is a new invasive species in southern California, USA. The extent of the host range of this insect is not known, but this knowledge will have a major impact on assessment of the risks that this pest poses to oaks [Quercus spp. (Fagaceae)]. We conducted laboratory tests to determine the potential suitability of native and ornamental oak species for larvae and adults of A. auroguttatus. We infested 179 cut logs (from 163 different trees) with eggs or larvae, measured neonate survival and, after 5 months, counted feeding galleries, and noted the proportion of galleries with late instars. Initial larval survival was generally high when larvae penetrated the phloem (range: 62–98% among oak species), and low by the time larvae began to feed at the phloem/xylem interface (range: 0–25% among oak species). The majority of larvae that established a visible feeding gallery survived to the fourth instar (total of 73% for all oak species). Larval galleries were established with greater frequency in red oaks (Section Lobatae) compared with other oaks (19 vs. 7 or 4%). All red oaks tested (Q. agrifolia Née, Q. kelloggii Newberry, and Q. wislizeni A. DC.) were likely suitable hosts for larvae. Larvae were apparently able to feed on some of the other oaks (Q. chrysolepis Leibmann, Q. suber L., Q. lobata Née, and Q. douglasii Hook & Arn), although it remains unclear whether these species would be preferred hosts under natural conditions. Adult longevity and fecundity varied little by species of oak foliage fed to adults. The host range of A. auroguttatus is likely limited by suitability of oak species for the larval rather than the adult life stage. Results support published field observations that red oaks are more suitable hosts than white oaks.