Objective The aim of the paper was to study the responses of street trees to impervious land coverage under canopy and to analyze resource utilization and allocation strategies of street trees with different leaf textures by comparing changes and trade-off and synergy in twig and leaf functional traits.
Method Common greening tree species of Sophora japonica, Platanus acerifolia and Ligustrum lucidum in Jinan, Shandong Province of eastern China were selected to represent tree leaf textures of paper, wax and leather leaf, respectively, and 5 gradients of impermeable surface coverage were divided by 20% interval. Samples of twigs and leaves were obtained by typical sampling methods.
Result (1) The sensitivity of branch and leaf traits of tree species with different leaf textures to the response of impervious ground coverage under the crown was different. The sensitivity of twig functional traits to the response of impervious ground coverage under the crown decreased significantly with the increase of thickness of wax layer on the leaf surface, and the leaf functional traits of Platanus acerifolia were more sensitive to the change of impervious ground coverage under the crown. However, the characters (leaf emergence intensity and branch leaf mass ratio) representing the relationship between branches and leaves were sensitive to the impervious ground coverage environment under the canopy. (2) The distribution of photosynthate decreased with the increase of impervious ground coverage, and the growth of branches and leaves, leaf space and leaf area decreased significantly with the increase of impervious ground coverage. With the increase of coverage degree of hardened ground, the three tree species adopted the way of reducing leaf area and maintaining leaf number to adapt to the environment. (3) Allometric growth relationship showed that, under the stress of impervious ground coverage, the resource input of trees to leaves was higher than that of twigs. In the interior of twigs, the resource input for the thickening growth of twigs was more than that for the elongation growth. The utilization mode of internal resources in leaves changed with the change of leaf texture: paper leaf trees tended to allocate more resources to the establishment of leaf resource acquisition organization, while leather leaf trees tended to allocate more resources to the establishment of leaf defense mechanism.
Conclusion Impervious ground coverage environment will significantly affect the growth of branches and leaves of trees. The adaptation strategies adopted by the leaves of tree species with different leaf textures to the hardened surface environment are different. The paper leaf tree species tend to adopt the adaptation strategy of “low consumption-fast income”, while the leather leaf tree species tend to adopt the adaptation strategy of “high consumption- slow income”, and the adaptation strategy of wax leaf tree species is between the two.