Comparing growth rate in a mixed plantation (walnut, poplar and nurse trees) with different planting designs: results from an experimental plantation in northern Italy

Results of a mixed plantation with poplar, walnut and nurse trees established in winter 2003 in Northern Italy, are reported. Main tree species (poplar and walnut) were planted according to a rectangular design (10 x 11m), with different spacings and alternate lines. The experimental trial was carried out to verify the following working hypotheses: ( i ) possibility to combine main trees with different growth levels (common walnut, hybrid walnut, and different poplar clones) and test two different poplar and walnut spacings (5.0 and 7.4 m) in the same plantation; ( ii ) opportunity to reduce cultivation’s workload, in comparison with poplar monoculture, using mixtures with different poplar clones and N-fixing nurse trees; ( iii ) verifying the growth pattern of two new poplar clones in comparison with the traditional clones cultivated for different purposes in Italy.The use of different valuable crop trees’ mixtures intercropped with nurse trees and shrubs (including N-fixing trees) allows to decrease the cultivation’s workload. In fact, a heavy reduction of cultural practices - fertilizers, weed control, irrigation and pesticides applications (-61%) are the main concurrent, supplementary benefits. The best growth performances (DBH and tree height), associated with the higher competition towards walnuts, were recorded with the new clones Lena and Neva in comparison with the I214 and Villafranca. The closer spacing (5 m between poplar and walnut trees) was found to be unsuited to get merchantable poplars sized 30 cm without developing a heavy competition towards walnut trees. The wider spacing (7.4 m) resulted vice versa suitable to get poplar trees sized as requested by veneer factories and to maintain an acceptable competitive level with walnut. Within this plantation design, a shorter rotation (8 yrs) is needed for Lena and Neva clones in comparison with I214 and Villafranca (10 yrs). Walnut intercropped with poplar showed cone-shaped crowns, light branching and a good stem quality in comparison with walnut grown in pure plantations. This model of mixed plantation can become an interesting optional choice to walnut's and poplar's monoculture with notable advantages both for farm economics, landscape quality and environment preservation.