Effects of different intercropping on walnut growth and stem quality in a plantation close to Pesaro (Marche Region)

The experimental plantation, about 3.0 hectares, was set up in 1999 in a floodplain near Pesaro (Marche Region) on a fertile crop land. Eight theses were compared: 1 thesis: pure walnut, 2: walnut intercropped with autumn olive (50% and 75%), and 5 theses: walnut intercropped with different nurse trees (elm, field maple, white willow, italian alder and plane). Each plot/ thesis was 3,500 m2 wide. Since 2000, the plantation has been monitored: dbh, tree height and stem quality of walnut trees have been measured. The estimate of biomass production has been undertaken for the 3 more competitive species (at the age of 6 for willow and elm and at the age of 7 for plane). Twelve years after planting, biomass production of all the intercropped nurse trees was estimated a second time. This dataset allowed a comparison among walnut trees grown according to different mixtures and in monoculture. In 2012 walnut grown in mixture with 75% of autumn olive tree (N-fixing shrub) showed the best dbh performance reaching on average dbh of 25.8 cm, the same parameter being conversely 14.1cm in monoculture and 12.2 cm in combination with the most competitive spp. (i.e. elm). No strict relationship between walnut growth and stem quality was found. The best stem quality was attained in mixture with plane, while elm confirmed to be an unsuitable nurse tree for walnut. The same nurse trees showed to be interesting for biomass production, too. Especially plane resulted to be an interesting species able to nurse profitably walnut and to have an interesting biomass production around 12.2 t ha-1 in the first rotation and 29.6 t ha-1 in the following one