Growth and water relations of walnut trees (Juglans regia L.) on a mesic site in central Italy: effects of understorey herbs and polyethylene mulching

The establishment of trees and associated herbaceous understorey vegetation during the afforestation of former arable lands can decrease soil erosion, increase soil fertility and diversify plantation income. This study reports on the five-year results from experimental plots of common walnut (Juglans regia L.) established in association with two different herbaceous understoreys in 1994 in central Italy. Treatments included: (i) walnut established with plastic film mulching in association with subclover (Trifolium subterraneum L.); (ii) walnut with subclover; (iii) walnut with a spontaneous herbaceous cover (grassing treatment); (iv) clean-cultivated walnut (control). Stem growth rates and the periodical changes in predawn and midday leaf water potentials of walnut, as well as the annual sward dry matter production, were measured. Over the five-years, the understorey vegetation was competitive towards trees, negatively affecting their leaf water status relative to the control, especially during mid-summer observations, with the onset of summer drought. Tree growth in the grassing treatment was slightly but significantly (p < 0.05) reduced in height in comparison to unmulched trees with subclover. Subclover competitiveness towards walnut was completely masked by the plastic mulching, so that mulched walnut with subclover had the highest cumulative stem diameter and height (+20% than control treatment). This was associated with water potentials that were never higher than the control. The subclover-mulched treatment, due to its three main advantages (highest cumulative stem growth, an annual dry matter fodder production of 6.3 t/ha, and soil erosion protection), appears to be a promising cultural model for walnut cultivation in areas without marked drought.