Growth and Physiology of Deciduous Shade Trees Under Varying Irrigation Regimes

Reduced water availability and increased cost of water have caused nursery producers to rethink water management practices. High-value container grown crops require frequent irrigation to optimize plant growth, and container production of high-value crops, like landscape trees, is increasing relative to field production. Irrigating cyclically, or applying daily water allotments in multiple irrigation events, instead of the traditional once daily application method may lead to increased tree growth and decreased water use. The objective of this study was to quantify the effects of varying irrigation regimes on the growth and physiology of common landscape shade tree species in Pot-in-Pot (PiP) production. Seven species were grown in #25 containers and randomly assigned to irrigation regimes with varying combinations of volumes (low or high) and cycle frequencies (once or 4 times daily). Daily water allowances were based on 75% or 150%, ET₀, and water was applied using pressure compensating spray stakes. Mid-day gas exchange, stem caliper, and tree height were measured monthly in 2008 and 2009. Diurnal gas exchange and water potential of two species were measured on two dates during each growing season, and substrate media temperature and moisture were monitored on one block throughout the study. Trees were more responsive to irrigation rate than cycle frequencies. Across all species, trees receiving the high water volume had greater height and stem radial growth, net assimilation (A), and stomatal conductance (g_s) than trees irrigated with the low volume. A strong species response was observed in most growth and physiological variables. Within species, the difference in stem radial growth between trees irrigated at the high volume and trees irrigated at the low volume was greater in year two than year one. Cyclic irrigation increased height growth in 2009 only in’ Ulmus ‘Morton’ compared to trees irrigated only once daily. Response of tree WUE, calculated as A/g_s, was dependent on date of sampling. Trees irrigated with four cycles had higher WUE than trees irrigated with one cycle on only one sampling date in the 2009 season (14 May). Irrigating cyclically decreased the amplitude of daily fluctuations in substrate moisture. Differences in growth and physiological responses to treatments could be due to inherent differences in species morphology, tree size, and growth rate.