Growth and Physiology of Container-Grown Conifers Under Cyclic Irrigation Regimes

Water use regulations and the cost of applying water are increasing as the availability of potable water is decreasing. Irrigating ornamental nursery stock cyclically, or applying daily water allotments in multiple irrigation events, instead of the traditional once daily application may lead to increased water use efficiency (WUE). The objective of this study was to quantify the effects of varying irrigation regimes on the growth and physiology of common container-grown conifer species in Pot-in-Pot production. Four species of conifers (Abies fraseri, Picea pungens, Picea glauca var.  densata, Pinus strobus), grown in #3 and #7 containers, were randomly assigned to irrigation regimes with varying combinations of amounts (low, medium, high) and cycle frequencies (once or 4 times daily). Daily amounts for #3 containers were based on 75%, 150%, or 225% ET₀, and amounts for #7 containers were based on 75% or 150% ET₀. Water was applied using pressure-compensating drip emitters or spray stakes. Mid-day gas exchange and monthly tree caliper and height were measured in 2008 and 2009. Diurnal gas exchange and water potential of A. fraseri grown in #7 containers were measured on two dates during each growing season, and substrate media temperature and moisture of #7 containers were monitored throughout the season. Across all species, trees receiving 150% and 225% ET₀ had higher stem radial growth in 2009 than trees irrigated with 75% ET₀, regardless of container size. Response to irrigation varied among species.  Pinus strobus grown in #7 containers had increased stem radial growth in 2009 when irrigation was applied in four cycles compared to a one cycle; however, the number of irrigation cycles had no effect on growth of trees in #3 containers. Responses of assimilation, conductance, and WUE differed depending on sampling date. Mid-day net assimilation of P. glauca and P. pungens and stomatal conductance of P. pungens grown in #7 containers increased with cyclic irrigation on one sampling date (6 August 2009); however, WUE of trees grown in #3 and #7 containers was unaffected by cyclic irrigation. Multiple irrigation applications have potential to alleviate mid-day water stress. Differences in responses between species could be due to tree size, growth rate, and morphological differences.