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Bark of mature black walnut (A), and bark of young black walnut (B). Photo A, Sally Weeks, Purdue University, Photo B, Lenny Farlee, HTIRC, Purdue University
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... and their leaf scars have no moustaches (Fig. 3). Mature butternut bark is platy and ash-gray with dark gray fis- sures between the platy ridges. The bark of young trees is smooth and gray or greenish-gray (Fig. 4). Mature black walnut bark is dark brown and heavily ridged or blocky. The bark of young black walnut is brown and ridged or flaky (Fig. 5). Some butternuts have very dark gray bark that closely resembles bark of black walnuts, (Ostry and Woeste, 2004) but it usually retains the light gray, platy surfaces on the top of bark ridges (Fig. 6). The fruit of a butternut is generally more elongated than the round-shaped fruit of a black walnut and is covered with sticky hairs ...
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... Hybridization is a relatively common phenomenon among Juglans species in the same section, and a rather rare one among species from different sections [5]. The question of hybridization is a key topic not only in the conventional walnut breeding [6], but also from the point of view of conservation biology strategies, as it is the case for the North-American butternut, Juglans cinerea L., for instance [7]. Furthermore, there is a well-defined group of hybrid walnuts that hold great potential for future walnut management in a broader context, namely the hybrids between the different North-American black walnut species markers, as this method proved to be a reliable tool for the identification of hybrids and the 'parental' species [14,19]. ...
The question of the hybrid walnut (Juglans × intermedia Carr.) is still under debate in the
Central European region. There is not simply just an underutilization, rather, even the existence of these hybrid forms is not broadly accepted. On the contrary, there is an intensive cross-breeding activity in the western part of Europe resulting in commercially available hybrid clones. In Hungary, several individuals have been reported with intermediate morphology from different old black walnut plantations. Due to the lacking information, a preliminary study was conducted in order to prove the difference of these selected trees and to test the hybrid state. For this purpose, DNA fingerprinting was used by applying 13 simple sequence repeat (SSR) markers for the identification
of 22 hybrid genotypes selected from one study plot. A comparative analysis with a reference sample set of the ‘parental’ species and other known hybrids was performed as well. The genetic analysis resulted distinct, unique genotypes for all of the samples. Based on the genetic pattern, the analyzed hybrid group was clearly distinguishable from the other two walnut groups. The result of this study also highlights the hidden potential in walnut breeding in the Central European region. Future concepts concerning hybrid walnut utilization in plantation forestry, agroforestry or as breeding material are also discussed.
... To identify J. ailantifolia and infer an identification key, several references were used in the absence of a monography of Juglans worldwide: the original description by Carrière (1878), Flora of North America (Whittemore & Stone 1997), Flora of China (Lu et al. 1999), Grimshaw (2003), Flora of Japan (Ohba 2006), a US field guide (Farlee et al. 2010), Flora of Woody Plants (Roloff & Bärtels 2018), and the identification key by De Langhe (2006)(2007)(2008)(2009)(2010)(2011)(2012). For the wood anatomy we used Miller (1976). ...
... While further studies are needed to understand any potential impacts of J. ailantifolia from wild stands, such as in Piario, it is unlikely to expect hybridization with the archaeophyte J. regia. This was not the case in the United States: as J. ailantifolia and its cultivar Cordiformis were introduced already in the mid-19 th century and became widely cultivated for the heartshaped nuts, they have hybridized with the native J. cinerea (Hoban et al. 2009) originating J. ×bixbyi Rehder to such an extent to raise concern for its conservation (Farlee et al. 2010;Pike et al. 2020). No hybrids seem to occur with J. regia (Farlee et al. 2010), although J. hopeiensis Hu seems to have originated as a hybrid between J. regia and J. mandshurica (Mu et al. 2017). ...
... This was not the case in the United States: as J. ailantifolia and its cultivar Cordiformis were introduced already in the mid-19 th century and became widely cultivated for the heartshaped nuts, they have hybridized with the native J. cinerea (Hoban et al. 2009) originating J. ×bixbyi Rehder to such an extent to raise concern for its conservation (Farlee et al. 2010;Pike et al. 2020). No hybrids seem to occur with J. regia (Farlee et al. 2010), although J. hopeiensis Hu seems to have originated as a hybrid between J. regia and J. mandshurica (Mu et al. 2017). ...
We present a new species of Juglans (walnuts, Juglandanceae), J. ailantifolia Carrière, a tree native to Japan and Sakhalin and naturalized in southern Switzerland and northern Italy. First found in Canton Ticino in 2019, its identification has been difficult, due to the (unresolved) taxonomic complexity of this genus. The species was documented and morphologically characterized using field surveys, photographs, and by taking voucher herbarium specimens. A dichotomous key to identify Juglans species in Switzerland and Italy is presented, including similar species. To assess vitality and potential for resprouting, a three-ring girdling experiment was carried out on 30 trees. Juglans ailantifolia is characterized by compound leaves up to 90 cm long with 11-19 densely hairy and only weakly asymmetric leaflets, erect red-pink female inflorescences, and up to 20 fruits. In Switzerland, the species was found in the areas of Bellinzona and Mendrisio, in moist lowland environments. It went unnoticed for decades, as the largest stands derive from abandoned private plantings dating back several decades. In Italy, it occurs in Lombardy, with young individuals also found along rivers, but it might be present also in Piedmont and Veneto. Its biological characteristics (relatively few fruits dispersed by gravity, but nuts can float, no vegetative reproduction) and its relatively limited spread in Canton Ticino and northern Italy (despite the age of the stands) do not indicate an invasive behavior in the Insubric area. Furthermore, its regenerative capacity after girdling is weak: trees die within a year and resprouting is low. Nevertheless, we recommend to control the species especially along waterways, avoid uncontrolled cutting and/or girdling, and to prefer J. regia for timber plantations.
Juglans ailantifolia - Una nuova specie di noce naturalizzata in Svizzera e Italia
Riassunto esteso:
Introduzione: In questo studio presentiamo Juglans ailantifolia Carrière (Juglandaceae), una nuova specie alloctona di noce naturalizzata in Svizzera meridionale e Italia settentrionale, originaria di Giappone e Sakhalin. È stata trovata prima in Cantone Ticino nel 2019 e in seguito in Italia. Poiché la sua identificazione è stata complessa a causa di una tassonomia ancora irrisolta, proponiamo una chiave dicotomica per l'identificazione delle specie di Juglans presenti in Svizzera e Italia, incluse le specie simili J. cinerea (America settentrionale) e J. mandshurica (Asia orientale). Viene valutata inoltre la sua vitalità e la sua capacità di formare ricacci e discusso l'eventuale potenziale invasivo in base alle sue caratteristiche biologiche e alla sua diffusione geografica.
Materiale e metodi: La specie è stata documentata e caratterizzata mediante rilievi in campo durante l'intera stagione vegetativa, fotografie e campioni d'erbario. Per l'identificazione sono state consultate le opere tassonomiche e le flore di riferimento. Per la distribuzione in altri paesi europei sono state consultate banche dati online e sono stati contattati esperti locali. Per valutarne la vitalità e il potenziale di formazione di ricacci, su 30 alberi è stato applicato il metodo di cercinatura con tre anelli, seguito da rilievi di controllo.
Risultati: Juglans ailantifolia presenta foglie composte lunghe fino 90 cm, con 11-19 foglioline densamente pelose e con base debolmente asimmetrica, infiorescenze maschili verdi e pendule, femminili erette e di colore rosso-rosa, fino a 20 frutti indeiscenti, con esocarpo tomentoso-ghiandoloso e noce legnosa relativamente spessa. Fiorisce in aprile e fruttifica a fine estate. Viene discusso il problema tassonomico con J. mandshurica. In Svizzera, J. ailantifolia è stata trovata unicamente nel Bellinzonese e nel Mendrisiotto, in ambienti freschi. I due popolamenti più estesi sembrano derivare da impianti privati risalenti a vari decenni fa. In Italia, è presente in Lombardia, con individui giovani in diffusione lungo i fiumi, e potrebbe essere naturalizzata anche in Piemonte e Veneto. Altrove, in Europa, sembra trovarsi allo stato selvatico unicamente in un paio di località della Germania e del Belgio. La cercinatura ha mostrato una debole capacità rigenerativa: gli alberi muoiono entro un anno e la formazione dei ricacci è bassa. Inoltre, molti degli alberi indeboliti sono stati attaccati dal bostrico esotico Xylosandrus germanus.
Conclusioni: Il processo di naturalizzazione di J. ailantifolia in Svizzera e Italia è in corso già da alcuni decenni, ma questo al-bero è rimasto inosservato fino ad ora. Le sue caratteristiche biologiche (pochi frutti, dispersi soprattutto per gravità a ridosso degli alberi maturi, nessuna riproduzione vegetativa) e la sua diffusione geografica limitata (nonostante l'età degli alberi più grandi) non indicano attualmente un comportamento invasivo nella regione insubrica. Tuttavia, per la capacità delle noci di galleggiare, si raccomanda di sorvegliare la specie soprattutto lungo i corsi d'acqua, di evitare il taglio e la cercinatura incon-trollata e di preferire J. regia per gli impianti da legno.
... Complicating efforts to identify hybrids in the field, F 1 , F 2 and backcross hybrids may resemble either parent or have traits of both parents, which can mask their genetic identity. Purdue University developed a guide to assist with identifying pure versus hybrid butternut and describes characteristics of Japanese walnut and butternut (Farlee et al. 2010b). The guide is useful for identifying many putative hybrids in the wild, but given the overlap in species phenotypes, DNA-based genotyping is required to accurately detect evidence of interspecific ancestry. ...
... Forest managers with access to state and local nurseries that will buy butternuts for nursery stock are encouraged to collect butternut seed in the fall, usually in September or October (Bonner 2008, Farlee et al. 2010b) in locations where competition is intensively controlled. The nuts are usually mature when the fruit starts to fall; good nuts have a pronounced egg shape. ...
Butternut is a relatively uncommon hardwood tree native to eastern North America. The species’ abundance has declined over the past 50 years, primarily because of an invasive pathogen (Ophiognomonia clavigignenti-juglandacearum [Oc-j]) and loss of suitable habitat for regeneration. Although genetic diversity of butternut is highest along the southern range edge, genetic diversity rangewide is fairly high, except in small and isolated populations. Although there is little evidence for even moderate resistance in native butternut, hybrids with Japanese walnut, a closely related species, display enough tolerance to infection to persist on the landscape and bear abundant nut crops year after year. Cryostorage of native embryogenic axes has yielded promising initial results as a strategy for gene conservation, but additional action is needed to conserve the remaining native gene pool. We describe a strategy for canker-resistance breeding in butternut using naturally occurring hybrids, hybrids in research orchards, and sources of native trees from as many regions as possible. Forest managers are encouraged to find surviving trees and collect seed for planting in suitable habitat to develop actionable knowledge that will enable the restoration of butternut with enough resistance to be self-sustaining on the landscape.
... For the Juglans  bixbyi system (buartnut; Juglandaceae; a hybrid of native J. cinerea and Asian J. ailantifolia), seeds of the invasive hybrid trees in the USA are collected for food, moved and planted in new locations by the public. The hybrid taxon is more vigorous and higher yielding than the USA native species (Farlee et al. 2010) though this is not described quantitatively and has better resistance to canker (Orchard et al. 1982). They are also a source of genetic pollution to the native butternut J. cinerea, which is becoming threatened and rare due to attack by the fungus Sirococcus clavigignenti-juglandacearum (Woeste et al. 2009). ...
Hybridization events can generate additional genetic diversity upon which natural selection can act and at times enhance invasiveness of the species. Invasive tree species are a growing ecological concern worldwide, and some of these invasions involve hybridization events pre- or post-introduction. There are 20 hybrid invasive tree taxa in 15 genera (11 plant families) discussed here. When reported, abundance of hybrids comprised 10-100% of an invasion, the remainder being parental taxa. In seven hybrid taxa researchers identified phenotypes that may make hybrids better invaders. Twelve hybrid tree taxa involved introgression and more hybrids involved all non-native taxa than native x non-native taxa. Three hybrid tree taxa were the result of intentional crosses and all hybrid taxa involved intentional introduction of either one or more parental taxon or the hybrid itself. The knowledge gaps present in some hybrid tree taxa can weaken our effectiveness in predicting and controlling invasions, as hybrids can add a level of complexity to an invasion by being morphologically cryptic, causing genetic pollution of a native parental taxon, presenting novel genotypes for which there may not be coevolved biological control agents, or evolving adaptive traits through increased genetic variation.
... The majority of the vegetative characters were taken from the Descriptors for Walnut (Juglans spp.) (IJD: IPGRI, 1994): leaf color, margin, and shape; rachis color and pubescence; average number of leaflets per leaf; and stem pubescence. To ensure distinction between J. cinerea and J. ailantifolia, three traits were chosen from Farlee et al. (2010): lenticel shape, lenticel density, and leaf scar notching. Lenticel density was determined quantitatively in a 1- cm 2 area, between the fourth and fifth node from the shoot apex. ...
Hybridization has been implicated as a driver of speciation, extinction, and invasiveness, but can also provide resistant breeding stock following epidemics. However, evaluating the appropriateness of hybrids for use in restoration programs is difficult. Past the F1 generation, the proportion of a progenitor’s genome can vary widely, as can the combinations of parental genomes. Detailed genetic analysis can reveal this information, but cannot expose phenotypic alterations due to heterosis, transgressive traits, or changes in metabolism or development. In addition, because evolution is often driven by extreme individuals, decisions based on phenotypic averages of hybrid classes may have unintended results. We demonstrate a strategy to evaluate hybrids for use in restoration by visualizing hybrid phenotypes across selected groups of traits relative to both progenitor species. Specifically, we used discriminant analysis to differentiate among butternut (Juglans cinerea L.), black walnut (J. nigra L.), and Japanese walnut (J. ailantifolia Carr. var. cordiformis) using vegetative characters and then with functional adaptive traits associated with seedling performance. When projected onto the progenitor trait space, naturally occurring hybrids (J. × bixbyi Rehd.) between butternut and Japanese walnut showed introgression toward Japanese walnut at vegetative characters but exhibited a hybrid swarm at functional traits. Both results indicate that hybrids have morphological and ecological phenotypes that distinguish them from butternut, demonstrating a lack of ecological equivalency that should not be carried into restoration breeding efforts. Despite these discrepancies, some hybrids were projected into the space occupied by butternut seedlings’ 95% confidence ellipse, signifying that some hybrids were similar at the measured traits. Determining how to consistently identify these individuals is imperative for future breeding and species restoration efforts involving hybrids. Discriminant analysis provides a useful technique to visualize past selection mechanisms and current variation in hybrid populations, especially when key ecological traits that distinguish progenitors are unknown. Furthermore, discriminant analysis affords a tool to assess ecological equivalency of hybrid populations and breeding program efforts to select for certain traits and monitor the amount of variability of those traits, relative to progenitors.
