Arboriculture & Urban Forestry 46(3): May 2020 by grafting (patch budding, T-budding, or whip graft- ing) or micropropagated, whereas seeds are mainly used to obtain seedling rootstocks (Grauke and Thompson 2003) or for timber production in forest plantations and agroforestry systems (Dupraz et al. 1999; Eichhorn et al. 2006; Paris et al. 2019). In these cases, large seeds are preferred for planting, because seedling size is generally correlated to kernel weight of the seed (Lagerstedt 1979). The seeds of Juglans species are included among recalcitrant and/or short- lived seeds (Hartmann et al. 2002), having a low genetic potential to tolerate storage. Saline habitats and water stress may severely and negatively affect walnut germination (Vahdati et al. 2009). Further- more, germination may be delayed or prevented due to the mechanical barrier offered by seed-enclosing structures (shells), which may play a significant role on the release from embryo dormancy. As a whole, such biological traits, together with the attractiveness of nuts for a number of predators, reduce the likeli- hood of naturalization. Nevertheless, natural seed dispersal of Juglans spp. largely relies on birds (primarily corvids) and some rodents through a mechanism known as “sinzo- ochory” (Vander Wall 2001; Beck and Vander Wall 2010). In particular, this biotic interaction occurs when the potential animal vector caches the nuts in the soil for later consumption, especially during peri- ods of low resource availability (e.g., winter). Since not all of the buried nuts are then rescued, the condi- tion for germination and subsequent seedling recruit- ment is determined by disperser behavior. Despite its history of cultivation, J. regia seemed to face many constraints to becoming naturalized in areas far from its origin. Nevertheless, in previous decades, naturalization events have been increasingly reported in several temperate regions of Europe, including Germany (Keil and Loos 2005), Belgium (Verloove 2011), Poland (Lenda et al. 2012), Slova- kia (Uhliarová et al. 2012), and Ukraine (Baranovski et al. 2016). The remarkable increase in the frequency of naturalized individuals observed in Poland, also within forest habitats, has been attributed to the increased activity of seed dispersers, accompanied by the progressive abandonment of traditional agricul- tural activities (Lenda et al. 2012, 2018). Despite the naturalization of the walnut already being reported for Mediterranean-type climate regions (Casasayas 1990; Knops et al. 1995), there is 175 limited information concerning the causes and the extent of this process. In the present work, in order to clarify some of these aspects, we studied the regeneration of walnut in two traditional Mediterranean orchards of western Sicily (Italy): a mixed, tree-dominated orchard, and a prickly pear (Opuntia ficus-indica [L.] Mill.) orchard. The aim of this study was to deter- mine whether the different agroecological conditions of the two study sites could affect walnut naturaliza- tion and eventually reveal the major factors involved in this process. MATERIALS AND METHODS The field surveys were carried out in Sicily in a 100 ha mixed suburban orchard (“Fondo Micciulla”— 38°06’20.74” N, 13°19’25.38” E, 90 m above sea level; hereafter “Palermo”) and in a 1 ha prickly pear orchard (37°48’52.39” N, 13°39’26.34” E, 455 m above sea level, hereafter “Roccapalumba”) located inland in a rural area about 70 km south-east of the first site (Figure 1). Following the Köppen climatic classification, both sites fall within the warm-temperate climate, with dry and hot summers (code Csa: Beck et al. 2006). The “Palermo” orchard is a traditional orchard irrigated through water channels, a tradi- tional gravitational system inherited from Arabs with a “forest-like” structure, located at about 90 m above sea level in the Conca d’Oro plain. This area sur- rounding the city of Palermo has been characterized over the centuries by the presence of many natural environments of great ecological and faunal impor- tance, in a context characterized by minimal and sus- tainable urban development and large agriculture areas, dedicated to the development of fruit tree crops (Sparacio et al. 2017). The dominant tree species in the surveyed area are Citrus spp., Eriobotrya japonica (Thunb.) Lindl., Celtis australis L., and Laurus nobilis L. (La Mantia 2006, 2007). Mean annual precipitation in this area is 747 mm; mean annual temperature is 18.8 °C; and the soils are Typic rhodoxeralfs (Fierotti 1988; SIAS [Accessed 2019]). Weed control in the “Palermo” orchard until the 1970s was made through soil tillage. Year after year, this cultural practice has been gradually substituted by the use of herbicides applied repeatedly during the growing season. More recently, during the last decade, these practices were progressively aban- doned in favor of mechanical weed control. Eleven mature walnut trees, with scattered distribution, are interspersed with other trees in the orchard (Figure 1). ©2020 International Society of Arboriculture
May 2020
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