Abstract
The study was carried out in experimental plots located in the Arno valley (Cavriglia, province of Arezzo - Italy). The experimental plots, walnut (Juglans regia L.) plantations both pure and mixed to accessory trees (Alnus cordata Loisel., Corylus avellana L., Eleagnus angustifolia L. and Robinia pseudoacacia L.), were established in 1986 and 1989. Data on grown and architectural characteristics were collected in different periods on trees aged 9, 13 and 16. Walnut trees grown with accessory trees showed the best performances, especially if associated with N-fixing species. These positive effects are probably due to many co-occuring favourable causes, a remarkable aspect of which is the higher availability of N. To better investigate the above effect, N concentration in the soil and in walnut leaves was measured in plantations both pure and mixed with Italian alder (Alnus cordata) or black locust (Robinia pseudoacacia), and N concentration values were compared with growth characteristics. The results showed that Juglans regia trees associated with Italian alder grow faster, with straighter stems and better stem shapes for qualified wood production. Even if walnut trees growing in plantations with black locust show similar N concentration values, growth performances are worse.
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