An Urban Tree (Tabebuia argentea) Exhibits Higher Sensitivity to Environmental Conditions than an Urban Palm (Ptychosperma macarthurii) Growing in the Same Roof Garden: An Implication for Sustainable Urban Water Use

Abstract

Roof gardening is popular for increasing green space in cities due to the restricted urban areas. However, when watering plants on a roof garden, one should consider loading capacity of the roof, which may limit water supply to the plants therein. To improve the efficiency in irrigating trees in a roof garden, we evaluated plant water status, represented by midday leaf water potential (Ψ_L), and leaf gas exchange parameters including stomatal conductance (g_s) and net photosynthesis (A_n) of a tree (Tabebuia argentea, Ta) and a palm (Ptychosperma macarthurii, Pm) species, which dominate the garden. The Ψ_L mediated responses of g_s and A_n, regulating plant water use and growth, to soil moisture. Results showed that Ψ_L of Ta significantly varied with changes in soil moisture, being low at low and high soil moisture. Nevertheless, g_s of Ta linearly decreased when Ψ_L increased, suggesting a stronger response of g_s to atmospheric demand. In contrast, no significant responses among the study variables were observed in Pm. For both species, A_n initially increased with g_s and saturated after g_s reached 200 mmol m^-2 s^-1, indicating similar stomatal regulation on atmospheric carbon dioxide absorption. The limited space for root growth and the location with surrounded building walls contributed to unconventional response patterns observed in Ta. Nonetheless, our results suggested that Ta was more sensitive to changing environments, especially the atmospheric demand, than Pm. Additionally, Ta should be irrigated during daytime and when atmospheric vapor pressure deficit is less than 2.6 kPa to allow high photosynthesis and evaporative cooling while Pm may be watered less frequently to reduce the roof’s loading and 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.