Investigating water relations in a tree (Tabebuia argentea) and a palm (Ptychosperma macarthurii) species in the dry season at a tropical roof garden

Abstract

Ongoing urbanization are increasing in several urban area. The increased urbanization of towns mostly involves concrete buildings, resulting in less green space. Consequently, recent attempts to expand green areas in cities have invaded areas in buildings, such as a roof garden. However, when watering plants on a roof garden, one needs to consider the loading capacity of the roof. This may limit water supply to the plants. To improve the efficiency in irrigation of trees in a roof garden, we evaluated the water status of a tree (Tabebuia argentea, Ta) and a palm (Ptychosperma macarthurii, Pm) species, which are commonly grown in cities, growing in the same roof garden. Midday leaf water potential (ΨMD) and stomatal conductance (gs) were measured to investigate the pattern of tree hydraulic responses of Ta and Pm. The result showed that, Ta had a significance response of gs to ΨMD. Stomata in Ta decreased with increasing leaf water potential, especially under moist condition. This may be explained by the inherent behavior of Tabebuia trees which dislike high soil water. Excessive soil water restricts the root absorption of water leading to decreasing leaf water potential and stomatal conductance. In contrast, Pm had no response of gs to ΨMD. This result implies that Ta should be appropriately watered because excessive watering could reduce ecosystem services by Ta. However, Pm can maintain plant water status and gs regardless of changes in soil moisture. For the response of Ta and Pm to environmental conditions, gs in Ta was more sensitive to change in leaf-to-air vapor pressure deficit (LAVPD) than Pm growing under the same site and environmental conditions. Based on these results, Pm is less sensitive to environmental changes than Ta. Hence, we suggest that Ta should not be irrigated to flood condition occurred while Pm may be watered less frequently to save water. This initial investigation implied that maintaining urban trees using appropriate irrigation that is specific to tree species is the key to maximize benefits from urban trees and optimize urban water use. For this particular roof garden, the result from this study can be applied to adjust irrigation schedule for the tree (Tabebuia argentea) and the palm (Ptychosperma macarthurii).