Response of desert-adapted landscape trees to deficit irrigation

Nine species of landscape trees commonly planted in the semi-arid southwestern United States were irrigated with three different regimes to determine their growth response and aesthetic functionality. Irrigation treatments started in May 2010 and were based on applying 80% (wet), 60% (medium), and 40% (dry) of reference evapotranspiration (ET_o) from May to October and half of that (40, 30, and 20% of ETo) from November through April. The three irrigation treatments resulted in different irrigation frequencies. Plants in the wet treatment received about twice the number of irrigations compared to plants in the dry treatment. During one 12-month period of the study, the shortest irrigation interval in summer was 6, 8, and 13 days, and the preceding winter the longest interval between irrigations was 109, 156, and 189 days for the wet, medium, and dry treatment, respectively. Trunk area was largest for Parkinsonia hybrid followed by Pistacia 'Red Push', Prosopis velutina, Chilopsis linearis, and Pinus eldarica. Smallest trees based on growth index and trunk area were Ebenopsis ebano, Quercus virginiana and Fraxinus velutina 'Rio Grande'. By fall of 2013 no significant differences in height and trunk growth growth of the same species receiving the different irrigation treatments were recorded. Symptoms of deficit irrigation started to develop on F. velutina 'Rio Grande', Cupressus arizonica, and P. eldarica as marginal leaf burn, thinning canopy, and branch dieback. After 41 months of treatments, trees of similar size were grown with half the amount of water that was applied to a tree of the same species in the wet irrigation treatment without detrimental consequences for six of the nine species.