Abstract:

This study explored the effects of urbanization on leaf functional traits of mutual species along the urban-suburban-rural gradient in the Peral River Delta. Mutual species of different plant functional groups, including trees, shrubs and herbs, in the evergreen broadleaf forests, were sampled along this gradient. Eleven leaf traits were measured, including leaf thickness (LT), specific leaf area (SLA), leaf tissue density (LTD), leaf dry matter content (LDMC), leaf nitrogen content (LNC), leaf phosphorus content (LPC), leaf N:P, maximum photosynthetic rate (A_max), chlorophyll content (CHL), photosynthetic phosphorus use efficiency (PPUE) and photosynthetic nitrogen use efficiency (PNUE). The results showed that (1) the LT, LTD and LMDC of trees in urban were significantly higher than those in suburban and rural areas (P<0.05). Whereas, the CHL, SLA, A_max and PNUE of trees in the urban were significantly lower than those in suburban and rural areas (P<0.05). The LT and LMDC of shrubs and herbs in the urban were significantly lower than those in suburban and rural areas, while CHL, SLA, A_max and PNUE of shrubs and herbs in urban were significantly higher than those in suburban and rural areas (P<0.05). Urbanization had a significant impact on the LTD of shrubs but had no significant impact on the LTD of herbs. Leaves in different functional groups showed different responses to urbanization. (2) LNC of three plant functional groups in urban was significantly higher than that in suburban and rural areas (P<0.05). LPC of woody plants (trees and shrubs) were not significantly affected by urbanization. However, the LPC of herbs in urban areas was significantly higher than that in suburban and rural areas. The leaf N:P ratios in all plants were higher than 19 and showed an increasing trend along the rural-to-urban gradient, suggesting that urbanization intensified plant P limitation. (3) Leaf traits were significantly correlated with each other. For instance, the CHL was significantly (P<0.05) and negatively correlated with LT and positively correlated with A_max and PNUE. By contrast, the LT, LTD and LDMC were significantly (P<0.05) and positively correlated with each other. Additionally, SLA was positively correlated with A_max, PNUE and LNC (P<0.05). However, correlations among leaf functional traits of different functional groups had different responses to urbanization. (4) Urbanization drove shrubs and herbs to adopt “fast investment-return” growth strategies with larger SLA, thinner leaves and higher photosynthetic rates, while the arbor tended to adopt the conservative growth strategies with thicker leaves and lower photosynthetic rates due to environmental stresses. These results suggested that with the change in the urban environment, plants changed their survival strategies and adjusted resource allocation to maintain the stability of communities. The findings revealed the changes in plant leaf traits and adaptation mechanisms due to urbanization, which will be helpful to guide the management practices of the urban forest.

Key words: Pearl River Delta, evergreen broad-leaved forest, different functional groups, leaf functional traits, urbanization

摘要：

为探讨植物叶片功能性状对城市化的响应与适应机制,以珠江三角洲城乡梯度上天然阔叶林共有优势种为研究对象,分别测定了乔木、灌木和草本3个功能群植物的11项叶片功能性状,包括叶厚度（LT）、比叶面积（SLA）、叶组织密度（LTD）、干物质含量（LDMC）、叶氮含量（LNC）、叶磷含量（LPC）、叶氮磷比（N:P）、最大光合速率（A_max）、叶绿素含量（CHL）、光合磷利用效率（PPUE）和光合氮利用效率（PNUE）。结果表明,（1）在城市化影响下,城区乔木植物的LT、LTD和LDMC都显著高于近郊和远郊（P<0.05）,CHL、SLA、A_max、PPUE和PNUE在城市区显著低于近郊与远郊区（P<0.05）。城区灌木和草本的LT和LDMC显著低于近郊和远郊区域,CHL、SLA、A_max、PPUE和PNUE在城区显著更高（P<0.05）;城市化对灌木的LTD影响显著（P<0.05）,对草本植物LTD无明显影响。（2）城市区域的3个功能群植物LNC均显著高于城市化程度低的区域（P<0.05）;城市化对木本植物（乔木和灌木）的LPC无显著影响,城区草本植物的LPC显著高于城市化低的区域（P<0.05）;所有植物叶片N:P大于19并随城市化程度升高而增大,城市化加剧了磷对植物的限制。（3）叶性状之间普遍存在显著相关性,CHL和LT呈显著负相关,与A_max和PNUE呈显著的正相关关系（P<0.05）;LT、LTD和LDMC相互间呈现显著的正相关关系（P <0.05）;SLA与A_max、PNUE和LNC呈显著的正相关（P<0.05）;但不同功能群植物叶功能性状间的相关性对城市化响应有差异。（4）城市化促使灌木和草本植物趋向于比叶面积更大、更薄和光合速率更大的“快速投资-收益型”生长策略,乔木植物对城市化伴随的环境胁迫响应则偏向叶片增厚、光合速率更小的“缓慢投资-收益型”生长策略。当城市化环境改变时,植物会改变生存策略和重新分配资源,保证群落的相对位置和群落整体结构的稳定性。上述研究结果对城市森林经营与管理具有一定的实践指导意义。

关键词: 珠江三角洲, 常绿阔叶林, 不同功能群植物, 叶片功能性状, 城市化