Height growth and flushing in common walnut (Juglans regia L.): 5-year results from provenance trials in Great Britain

A collection of common walnut (Juglans regia L.) genotypes from the species' natural and introduced ranges was assembled in 1997 to test their suitability for timber production in Britain. Field trials were established across three sites in southern England during winter 1998, containing 18 provenances. Tree height was measured annually, allowing height increment to be calculated and, during spring 2003, flushing (leaf-emergence) and frost injury were assessed. Survival was high with 98.7 per cent of trees alive after five growing seasons. At the end of the fifth growing season (2003) overall mean tree height was 138 cm but significantly different between sites (P ≤ 0.001) and provenances (P ≤ 0.001); provenance × site interaction also occurred (P ≤ 0.001). Overall growth was best at the Somerset site and worst at the Gloucestershire site. Height increment averaged 25 cm per year but varied significantly (P ≤ 0.001) between provenances, sites and for provenance × site interaction. The Turkish provenance T1 grew 285 per cent more in height in Somerset than in Gloucestershire. Progression of flushing was significantly different (P ≤ 0.001) for provenances at one site and a positive correlation (P ≤ 0.001; r2 = 78 per cent) was identified between early flushing and frost injury. In the earliest flushing provenance (T2, Turkey), 55 per cent of trees were injured by late frosts during April and May 2003. Multi-trait ranking, based on vigour and late-flushing, indicated that provenances from more northerly latitudes, and from within the introduced range of the species, performed best after 5 years.