Isohydric and Anisohydric Water Use Considerations in Species Selection for the Great Green Wall of Africa

Abstract

The hydric model of plant water use behavior is potentially useful in selecting appropriate species in arid to semi-arid condition shelterbelts plantings like the Great Green Wall. Anisohydric species have high “use it or lose it” use at the cost of negative internal water potential, out-competing other plants but possibly inefficiently using water. Isohydric species “save it for a rainy day” strategy conserves water in non-competitive situations but may limit carbon gain and growth. Empirical testing of this model was conducted in two studies involving subtropical herbaceous and tropical woody plants. In Queenland, Australian, container grown Orthosiphon aristatus, Dianella revolata ‘Breeze’, and Ptilotus nobilis plants were stressed through cyclical dry downs or kept well watered while stomatal conductance, predawn water potential, and osmotic adjustment were measured. O. aristatus, a rainforest species, fit the anisohydric model of high water use and more negative water potential during soil drying until stomatal closure and leaf wilting. Deep rooted D. revolata native to Eucalyptus scrub forests fit the isohydric model of reduced transpiration under mild water stress. By contrast, shallow-rooted P. nobilis native to Australia’s very dry interior became water stress by anisohydric behavior to achieve osmotic adjustment that allowed subsequent isohydric behavior. In Bangkok, Thailand, stomatal conductance and predawn water potential was measured in three container-grown, well watered and water stressed monsoonal dry forest species, Pterocarpus indicus, Lagerstroemia loudonii, and Swietiena macrophylla. The deciduous Lagerstroemia loudoni, had very high water use consistent with other deciduous species, but isohydrically closed stomata to moderate internal water potential. The obligate evergreen Swietenia macrophylla fit the isohydric model even more closely, moderating transpiration and water potential under a wide range of dry soil and air. P. indicus, nominally evergreen but deciduous when water stressed, fit the anisohydric model more closely, allowing internal water potential to become more negative during soil drying. Anisohydric species may be more suited for planting in deeper soils with high water holding capacity, while isohydric species may be more suitable for shallower and sandier soils in noncompetitive plantings.