Differentiation of Acclimation Responses and Seedling Source Effects in Deciduous and Evergreen Conifer Seedlings Under Cross-Altitude Transplantation

Understanding the acclimation responses of conifer species with different functional types in response to altitudinal gradient changes is of significance for deepening our knowledge of how plants cope with global climate change. This study focused on seedlings of larch (Larix principis-rupprechtii) and spruce (Picea meyeri) within the Pangquangou Nature Reserve in North-central China. By setting two seedling source altitudes (1600 m, 2400 m) and four transplant altitudes (1600 m, 1900 m, 2100 m, 2400 m), we compared the response characteristics of 20 functional traits, including light response, instantaneous photosynthesis, leaf morphology and nutrient storage. The results showed that: (1) Tree functional type was the core factor determining the altitudinal response pattern of seedlings. Larch exhibited higher sensitivity to altitude change, and its photosynthetic (Amax, AQE, LSP, E) and morphological strategy traits (PA, SL, SW, SLA) changed significantly with altitude. (2) The regulation of phenotypic plasticity by seedling source altitude showed a significant functional-type dependence. The larch was significantly regulated by seedling source altitude, with low-altitude seedling sources displaying higher plasticity in traits such as H:D ratio, Tr and NSC. The spruce showed smaller differences between seedling sources, yet exhibited stronger environmental dynamic plasticity than the larch in carbon storage and utilization strategies (Sugars, NSC). (3) The two tree species coped with altitudinal gradient changes through differentiated trait variation pathways. Larch was primarily driven by carbon metabolism and protein synthesis (C, Protein), which was manifested in adjustments of photosynthetic capacity (Pn) and morphology (leaves and branches), thereby achieving rapid adaptation. In spruce, functional traits such as Pn, SLA and NSC were highly coupled; regardless of seedling source, photosynthetic and leaf morphological traits served as the core drivers, and coordinated changes in multiple traits were relied upon to maintain physiological homeostasis.