Early growth and form of common walnut (Juglans regia L.) in mixture with tree and shrub nurse species in southern England

Common walnut (Juglans regia L.) is usually grown in pure stands or as individual trees, rather than within mixed woodlands. Previous studies indicate that walnut can benefit from being planted in mixed stands, particularly with nitrogen-fixing (N-fixing) species. The effects of establishing walnut with various combinations of tree and shrub nurse species were investigated, including those capable of fixing nitrogen. After 6 years, walnut survival exceeded 99 per cent. Tree nurses did not significantly effect walnut growth; however, walnut height growth with autumn olive (Elaeagnus umbellata Thunb.) was significantly greater (P < 0.05), with an average increment of 17 cm year(-1) across all sites at year six and 32 cm year(-1) at one site at year seven. The architecture (height, width and estimated volume) of Corylus avellana L. and E. umbellata had a significant positive (P < 0.05) effect on walnut height. Foliar nitrogen levels for walnuts with at least one N-fixing nurse were within the reported optimal range, whereas those walnuts in control plots were deficient in nitrogen. Walnut trees grown with E. umbellata also had fewer multiple stems and finer branches. The results indicate that there are significant early benefits from establishing walnut with E. umbellata.