Photosynthetic Performance of Newly Planted Trees in Urban Roadside Habitats in Hong Kong

Climate change is one of the great challenges to all living creatures on the planet. There has been a growing interest in the potential of photosynthetic CO2 uptake and carbon sequestration by plants to mitigate anthropogenic CO2 emissions. Many previous studies on photosynthetic CO2 uptake have focused on forest trees, our understanding of the photosynthetic performance of urban trees remains limited. Urban population accounts for more than half of the world's population in 2018, and urban living is believed to become a dominant lifestyle by 2050 (UN, 2019). As urbanization is expected to increase, urban trees play a significant role in carbon removal and storage (McGovern & Pasher, 2016; Nowak & Crane, 2002).

Plant functional traits (morpho-physio-phenological characteristics) determine a species' ability to survive, grow, and reproduce, closely link to ecosystem functions like carbon cycling (Meng et al., 2007). They can be divided into response traits and effect traits. Environmental factors can be considered as filters constraining which individuals bearing specific attributes of response traits can be sorted out and persist in a community (Keddy, 1992). Ecosystem properties depend on effect traits weighted according to the relative abundance of species. The complex network of interactions among traits affects CO2 assimilation (Marino et al., 2010). The leaf economic spectrum (LES) describes the correlations among different leaf traits such as photosynthetic capacity (Amax) and nitrogen (N) content, which reflects a gradient of conservative and acquisitive strategies adopted by plants (Wright et al., 2004).

Leaf N and chlorophyll (Chl) content have been suggested as proxies for photosynthetic parameters, yet few studies have been carried out to examine the suitability of using the two traits to estimate Amax in urban environments (Weissert et al., 2016). Measurement of leaf spectral reflectance has also been used to predict photosynthetic rates based on the derived leaf Chl (Sims & Gamon, 2002; Vilfan et al., 2019). However, the spectral signatures of trees in Hong Kong stored in worldwide spectral libraries are limited because of the tall and dense city structure and hilly topography. Abbas et al. (2021) acquired in-field hyperspectral images of 19 urban tree species in Hong Kong, however, only seasonality and phenological traits were analyzed with spectral properties of plants.

This research aims to (i) quantify the photosynthetic performance of newly planted trees in urban roadside habitats in Hong Kong, (ii) examine the LES as well as the relationships between soil properties and leaf photosynthetic traits of the trees, (iii) study the relationships among central leaf economic traits and access the suitability of using leaf N and Chl levels, and leaf spectral reflectance as proxies to estimate Amax.