Effects of growth space limitation on allometry of Ficus spp. In urban habitats

Abstract

Tree morphological traits are related to the structural stability and their ability to provide ecological benefits such as carbon sequestration and water conservation. The allometric relationship can be used to construct the corresponding prediction equations in forests. However, the urban environment differs from the natural environment for trees. This study aims to enhance the urban tree growth model by classifying urban habitats. We achieved this by collecting morphological traits (DBH, height, crown depth, and width) commonly used in growth model researches of six Ficus spp. in Guangzhou, China, including Ficus virens, Ficus religiosa, Ficus elastica, Ficus benjamina, Ficus altissima, Ficus celebensis. We specifically examine how these trees grow in different urban settings. Urban habitats are classified based on growth space limitations, considering above-ground and underground. Results showed that certain morphologies in urban habitats did not conform to allometry (R2 < 0.4), and the model fitted with height as independent variable is better than that fitted with DBH. The model parameter following allometric relationship with tree height as the independent variable showed no significant difference, but the fit was worse in different urban habitats. On the contrary, models with DBH as the independent variable showed differences, for example, the allometric exponent of maximum crown width relative to DBH were 0.915 and 0.557 in two different habitats. Crown depth and tree height showed significant and stable allometry in the three habitats. The response of tree morphology to different habitats varied, and the allometric relationships among morphological traits showed differences in habitats where the growth space was limited. This underscores the considerable effect of the urban environment on urban tree growth models. Consequently, we recommend developing a series urban tree growth models tailored to different urban habitats to accurately quantify urban ecosystem services for assessing their sustainability in urban development and planning.