25 year results of a dryland tree trial, and an invasive risk assessment of introduced species

Tree species elimination trials were planted in 1988 and 1989 in Almeria, south-eastern Spain, and the driest part of mainland Europe. The aim was to test the adaptability of exotic species to local conditions and compare their performance with native species. Of the 50 species initially selected, 12 exotics failed to germinate or exhibited poor nursery establishment. Of the 38 species outplanted (574 trees in total, between 14 and 29 of each species), 11 exotics died within the first year, and a further 18 recorded 100% mortality after 24-25 years, when only 9 species remained with a total of only 40 surviving trees. The best performing species were the native/naturalised Pinus pinea and Pinus halepensis with 79% and 65% survival, respectively, followed by the exotic Acacia salicina with 38% survival, and most trees were 2.0-4.5 m tall. Apart from these dryland trials, trees were also planted on better terrace sites. Of these, Acacia salicina and Casuarina spp. were the largest (to 15 m tall), with an 11 m Prosopis chilensis and a Robinia pseudoacacia of 10 m. On the edge of the nationally recognised ‘Karst en Yesos de Sorbas’ Natural Area, special measures should be taken regarding potentially invasive alien species, and all non-native trees should be removed from the dryland site if further self-seeding is observed. This paper reports the first records of naturally regenerating Prosopis chilensis and P. velutina in Europe. On the terrace sites where natural regeneration were pronounced, it is strongly recommended that all exotic and potentially invasive species of Acacia, Casuarina, Leucaena, Gleditsia, Parkinsonia, Prosopis and Robinia are cut down immediately and stumps removed. On the irrigated ‘arboretum’ site, however, some stumps could be retain be retain for educational purposes, though regrowth must be cut as soon as they flower and before any further fruiting.