Magnolia and Viburnum Plant Factors at Different Growing Seasons and Allowed Depletion Levels in a Monsoonal Climate

Hongyan Sun,Richard C Beeson,Michael D Dukes,Roger Kjelgren

doi:10.3390/w13131744

Abstract

We investigated seasonal water use, growth and acceptable root-zone water depletion levels to develop tools for the more precise irrigation of two Southeast U.S. landscape species in a monsoonal climate—Magnolia grandiflora and Viburnum odoratissimum. The study was conducted under a rainout shelter consisting of two concurrent studies. One, weighing lysimeter readings of quantified water use (ETA) at different levels of irrigation frequency that dried the root zone to different allowable depletion levels (ADL). Two, planting the same species and sizes inground and irrigating them to the same ADLs to assess the effect of root-zone water depletion on growth. The projected crown area (PCA) and crown volume were concurrently measured every three weeks in both studies as well as reference evapotranspiration (ETo). Plant factor values were calculated from the ratio of ETA (normalized to depth units by PCA) to ETo. The two species had different tolerances for irrigation frequency depending on the season: peak magnolia canopy growth was mid-spring to mid-summer, while peak viburnum canopy growth was summer. Canopy growth for both species was most sensitive to greater ADL-water stress during the peak growth stages of both species. For urban landscape irrigation, these data suggest that 60–75% of available water in magnolia and viburnum root zones can be depleted before irrigation and that they can be irrigated at a plant factor (PF) value of 0.6 of ETo. For landscape situations with high expectations, such as during establishment and especially during peak growth, a wetter water budget that minimizes water stress would be more appropriate: 30–45% ADL and PF values of 0.7–0.8. The results of this study are aimed at water managers and landscape architects and designers in a humid climate who need to account for water demand in planning scenarios.

Highlights

Often the most important component of urban landscapes, woody plants and trees, typically require irrigation to sustain their health and value
Adapted from agriculture, the accepted approach for estimating plant water demand is to use an empirically derived correction factor, Plant Factor (PF) (i.e., actual water use divided by reference evapotranspiration (ETo) which is equivalent to Kc in agriculture), to adjust downward the local ETo for a given plant type [1]
Air was the driest in spring, as the vapor pressure deficit (VPD) peaked at 2.5 kPa in March–May at the end of the dry season, staying below 2 kPa during the wet season, minimally contributing to evapotranspiration

Summary

Introduction

Often the most important component of urban landscapes, woody plants and trees, typically require irrigation to sustain their health and value. Climate change and drought are forcing landscape managers to be more efficient in conserving water while maintaining woody plant health This efficiency is achieved with a water budget: irrigating woody plants with just enough at the right time before they become water stressed [1]. Extant studies have focused largely on trees in temperate-arid to semi-arid climates [2] rather than on shrubs [3,4,5,6,7,8] These water-use studies are rarely paired with an investigation into the amount of root-zone water that can be safely depleted before irrigation is required, an important piece of irrigation information in any climate that determines how much a plant can safely mine root-zone water without stress

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