Investigating leaf gas exchanges of common trees in two urban parks with different periods of establishment in Bangkok, Thailand

Abstract

For sustainable management of urban forests, information regarding species-specific responses of urban trees is essential to determine suitable trees for planting and to maintain the existing trees amid the worsening climate. One approach is to study plant growth and water use and their changes with weather variations in various tree species and sites. Here, we measured leaf photosynthesis (A), transpiration (E), and stomatal conductance (g_s) of Samanea saman, Tabebuia rosea, and Millingtonia hortensis, which were found in a new and an old park in Bangkok. Additionally, we investigated how these parameters and water-use efficiency (WUE), the ratio between A and E, varied with atmospheric conditions, represented by leaf-to-air vapor pressure deficit (VPD_LA). Results indicated decreasing gas exchange rates with increasing VPD_LA in all species, signifying stomatal closure under drying air. In all cases, changes of A and E with VPD_LA were similar to those in g_s, except for E of T. rosea in the new park that showed no response. Of all species, only M. hortensis displayed similar response patterns with VPD_LA across sites, implying its capacity to adapt to various settings. Interestingly, regardless of the periods since establishments of the parks, WUE of these species responded similarly to VPD_LA. This finding may imply that, regardless of their sizes, these trees may retain the capacity of carbon dioxide absorption provided the same unit of water use, despite the changing climate. Nevertheless, further investigations involving more species and sites with various periods since establishment should be performed to confirm these results.