Differences in Early Growth and Photosynthetic Characteristics of Acacia mangium × A. auriculiformis Clones in Coastal Sandy Land

This study was conducted at the Xiacai State-owned Shelter Forest Farm in Zhangpu, Fujian Province, China. Based on cluster analysis of growth data from 18 Acacia mangium × A. auriculiformis clones during 2023–2025, Clone 18 (fast-growing type), Clone 7 (intermediate type), and Clone 3 (slow-growing type) were selected as representative materials. Photosynthetic pigments, diurnal variations in photosynthetic gas exchange parameters, chlorophyll fluorescence parameters, and energy allocation characteristics were systematically compared among the three clones. Correlation analysis and principal component analysis (PCA) were further conducted in combination with synchronous growth traits to clarify the coupling relationship between photosynthetic characteristics and growth performance among different growth-type clones under coastal sandy conditions. The results showed that Clone 18 exhibited significantly greater tree height, diameter at breast height (DBH), and crown width than Clone 7 and Clone 3 (P<0.05), indicating a clear growth advantage. Although Clone 18 had the lowest chlorophyll content, it showed the highest net photosynthetic rate (P_n), water use efficiency (WUE), density of active reaction centers per cross section (RC/CS_m), and electron transport quantum yield per cross section (ET_o/CS_m), as well as the lowest energy dissipation per reaction center (DI_o/RC). Correlation analysis revealed obvious differences in the relationships between photosynthetic parameters and growth traits among the clones. In Clone 18, P_n was positively correlated with growth traits, while chlorophyll content was negatively correlated with P_n but positively correlated with growth traits. In Clone 7, the correlation strength weakened, and some indices showed no significant correlations. Clone 3 was characterized by relatively high chlorophyll content, whereas the correlations between photosynthetic parameters and growth traits were the weakest. PCA results showed that Clone 18 was mainly distributed along the positive axis of PC₁ and was closely associated with growth and photosynthetic efficiency traits, while Clone 3 was mainly associated with chlorophyll-related traits, and Clone 7 was located in an intermediate transitional position. In conclusion, the fast-growing Clone 18 achieved strong photosynthetic-growth coupling through relatively low chlorophyll investment and high light energy conversion efficiency, thereby exhibiting superior adaptability under coastal stress conditions.