Arboriculture & Urban Forestry 46(3): May 2020 Tyrol (Austria), milder winters and higher annual temperatures have been identified as key factors in favoring the recent spread of walnut (Loacker et al. 2007). Such findings support the hypothesis of a pro- gressive northward shift in the European distribution of the species (Hemery et al. 2010) as well as that of other woody species (Walther 2003). At the same time, this would suggest a concurrent decrease of the naturalization potential in a scenario of a hotter and drier Mediterranean climate, although the recent data we assessed (the last 16 years) seem not to show any increasing temperature or decreasing precipitation trends. In a potential similar climatic environment like California, walnut has been reported as natural- ized for more than 20 years (Knops et al. 1995), yet only occasional natural regeneration has been reported to date (Whittemore 2012). Our observations suggest that in environmental Mediterranean conditions not affected by frost events, irrigated agroecosystems may offer favorable condi- tions for the naturalization of walnut once the water stress is avoided. Seedling emergence and survival, at least in the early developmental tree stages, may also considerably benefit from plant canopies acting as a shelter against the excessive heat in summer, thus improving microclimatic and edaphic conditions as observed in “Roccapalumba.” Dispersal Mechanisms The transport of viable propagules towards suitable microsites is one necessary step for the naturalization process of a plant. Seed dispersal in walnut species is generally performed by animals through sinzoochory, in which predation is the main goal, but dispersal may arise as a beneficial outcome (Vander Wall 2001). Firstly, animals have to open the hard nut shell to reach its nutritious contents. For instance, the ability of corvids to feed on walnuts using different strate- gies to open their fruits has long been reported (Foer- stel 1993; Cristol 2005). In suburban areas, crows may surprisingly take advantage of cars and other vehicles to crack nuts (Mainardi 2006). Within the “Villa Borghese” urban park in Rome, carrion crows have carried the Juglans nigra L. nuts in their beaks and dropped them on the asphalt, as recently reported (Fraticelli 2000). Additionally, rooks (Corvus frugile- gus L.) and jays (Garrulus glandarius L.) have been reported to play a major role for the invasion of wal- nut from commercial plantations to rural fields and 181 forest habitats in Poland (Lenda et al. 2018). Interest- ingly, the recent arrival of jays in the “Palermo” orchard has triggered the natural regeneration and spread of the holm oak (Quercus ilex L.)(La Mantia and Bueno 2016). Hence, we hypothesize that similar outcomes may arise following the fluctuations of seed disperser populations. Important plant-animal interactions have been already described for other walnut species. For instance, the role of the carrion crows for the disper- sal of Juglans nigra has been recognized, as well as for the dispersal of pecan nuts (Carya illinoensis [Wangenh.] K. Koch) and Juglans regia (Termine et al. 2013). Walnuts are also avidly preyed upon by rats (Rattus norvegicus Berkenhout and black rat Rattus rattus L.)(Bueno and La Mantia, personal observa- tions), which are able to move the nuts above the trees and on the main branches, thus potentially con- tributing to walnut spread in relatively small scales. CONCLUSIONS Juglans regia has been widely cultivated outside its native range for a long time. Despite some reports of its naturalization in Europe dating back to 1950, just recently this process was confirmed by specific inves- tigations in some temperate areas, with no informa- tion for the Mediterranean region. We provide the first field surveys about the walnut naturalization pro- cess within two different Mediterranean agroecosys- tems, characterized by distinct agroecological features. This preliminary survey highlights the need for fur- ther studies to better understand the diffusion of this phenomenon and to better identify the main environ- mental and biotic drivers, including climate and land use changes as well as the role of animal dispersers. LITERATURE CITED Aikio S, Duncan RP, Hulme PE. 2010. Lag-phases in alien plant invasions: separating the facts from the artefacts. Oikos. 119:370-378. Bacchetta G, Mayoral O, Podda L. 2009. Catálogo de la flora exótica de la isla de Cerdeña (Italia). Flora Montiberica. 41:35-61. Badalamenti E, Gristina L, La Mantia T, Novara A, Pasta S, Lauteri M, Fernandes P, Correia O, Máguas C. 2014. Rela- tionship between recruitment and mother plant vitality in the alien species Acacia cyclops A. Cunn. ex G. Don. Forest Ecology and Management. 331:237-244. Badalamenti E, La Mantia T, Quatrini P. 2015. Arbuscular mycorrhizal fungi positively affect growth of Ailanthus altis- sima (Mill.) Swingle seedlings and show a strong association ©2020 International Society of Arboriculture
May 2020
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