Arboriculture & Urban Forestry 46(3): May 2020 179 Table 2. Comparison of the climatic variables between the two study sites from 2002 to 2018. Minimum (min), maximum (max), mean, and standard deviation (SD) refer to the monthly values, which were obtained from the average of daily values. Tmax = average of maximum temperatures; Tm Roccapalumba Tmax Tm Tmin Rainfall (mm) (°C) (°C) (°C) min 0 8.4 4.9 1.9 max 198.2 33.9 26.5 20.4 mean 46.5 21.3 15.7 11.3 = average of mean temperatures; Tmin Palermo SD 44.4 7.3 6.1 5.1 min 0 13.4 9.5 5.6 max 294 33.9 28.8 23.9 = average of minimum temperatures. Kruskal-Wallis mean 62.2 23.2 18.8 14.6 SD 56.8 5.6 5.5 5.3 Z -2.55 -2.94 -5.26 -5.99 P 0.01 0.003 < 0.001 < 0.001 DISCUSSION Environmental Constraints for Naturalization As far as the naturalization process of Juglans regia in Europe is concerned, we report a summary of the current state of knowledge (Table 3). It can be seen that, despite the increasing trend of the walnut natu- ralization reports in the last few years in more than 10 European countries, very rarely do these reports include detailed information because data are primar- ily inferred from presence/absence check-lists. The only exception is represented in the works by Lenda et al. (2012, 2018), which are focused on walnut nat- uralization and its main driving factors, and where all these aspects are interestingly and thoroughly discussed. Juglans regia grows well in mild and warm-mild climates, with mean annual temperatures of 10 °C to 17 °C, and on fresh, fertile, and deep soils, with a pH ranging from 6.5 to 7.5 (Bosco et al. 1997; Fernandez- Lopez et al. 2002). The main climatic constraints for its natural regeneration are the vulnerability of seed- lings to both early and late frost (Loacker et al. 2007) and the relatively high water requirements. Most wal- nut varieties require approximately 800 hours of tem- peratures below 7.2 °C (winter chilling requirement for a regular bud dormancy completion)(Barone and Zappia 1993), and the annual precipitation should be at least of 700 to 800 mm (Lestrade et al. 2013). Indeed, J. regia is very sensitive to water stress, which can negatively affect seed germination and root development (Vahdati et al. 2009), as well as competitive ability against other plants (de Rigo et al. 2016). Concerning the first limiting factor, neither of the two sites experienced frost events in the consid- ered time frame (2002 to 2018), with minimum tem- peratures never falling below 1.9 °C, and in such a case, only for a short period. Moreover, no decreasing or increasing trends in the minimum temperatures have been detected, suggesting that frosts or winter chilling have not represented a limiting factor for walnut regeneration. The water supply, crucial for seedling survival and growth (Janku et al. 2017), should not represent a constraint in the tested condi- tions due to the irrigation practices carried out in “Palermo.” Nevertheless, in abandoned orchards where irrigation has been given up, walnut seedlings must face serious survival risks (personal observa- tion). We observed a fast growth in the early life stages of walnut seedlings in the “Palermo” orchard, as previously detailed. Then, most of the juvenile individuals were destroyed during the mechanical control of weeds, although some of them were able to resprout (Figure 2A), whereas only a few individuals were left to grow deliberately (Figure 2C). However, this fast growth under irrigation may be not enough, as young walnuts are usually outcompeted by faster- growing woody species present in the orchard, such as Celtis australis and Laurus nobilis. Thus, competi- tion also has to be considered as a serious factor impeding walnut establishment and naturalization in abandoned orchards. In “Roccapalumba,” the lower regeneration den- sity we observed seemed to be primarily caused by the low water availability. Such water shortage may be the combined result of a lower precipitation regime, the lack of irrigation, and the presence of hard clay soils, all factors determining less suitable ecological conditions for walnut growth and perfor- mance (Fernandez-Lopez et al. 2002). In this context, the role played by single prickly pear plants (Figure 3) in creating a suitable microhabitat for walnut regeneration appeared to be crucial. In a scenario of ongoing climate change, contrast- ing patterns may arise. In the inner Alpine valleys of ©2020 International Society of Arboriculture
May 2020
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