Ziyu Yang

Climatic conditions strongly control the geographic distribution of treelines; long-term in situ climatic measurements at the treeline are still rare. While microclimatic differences between trees and shrubs at treeline ecotones are already well established and explained, supporting empirical data remain limited. In this study, we compared in situ microclimate measurements in the canopy, stem, and root zones of tree and shrub plots of Juniperus squamata Buch.-Ham. ex D.Don at the treeline ecotone of Jiaozi Mountain, Yunnan, China, in 2022. Although the shrubs experienced higher wind speeds compared to the trees at the treeline ecotone, the short stature of shrubs created warm stem and soil temperatures that are more suitable for plant growth and survival. Shrub distribution at higher elevations may be due to higher soil temperatures. The inability of shrubs to grow upright may be attributed to the fact that their canopy is limited by lower temperatures above the canopy during the frost season. This can provide new insights into the mechanisms of treeline formation.

Phenological synchronicity of different tree parts and habitats can provide valuable insights into the physiological relationships and regulatory factors of different phenologies. The earlywood (EW) to latewood (LW) transition of the xylem is a critical process closely related to the key functions and physiological processes of trees. This study investigated the phenology phase of the EW–LW transition in branches and stems of Smith fir (Abies forrestii var. smithii Viguié & Gaussen) along an altitude gradient (3600 m, 3800 m, and 4000 m) in Jiaozi Mountain, Yunnan, China, in 2021. The study aimed to test the hypothesis that branches transition earlier than stems, but the elevation does not affect the transition time. We monitored the EW–LW transition dates weekly using microcores and then correlated them to the climatic conditions and developmental processes. Our findings revealed that the EW–LW transition timing varied among the different parts of the tree but was synchronized across the different elevations. Furthermore, the transition always occurred earlier in branches than in the stem, but the difference narrowed with increasing altitude. The EW–LW transition was mainly influenced by photoperiod, which plays a crucial role as a signaling factor. Additionally, the tree crown was more sensitive to environmental changes than the stem. The transition time of stems was less sensitive to environmental factors than that of branches. Therefore, our results suggest that the earlywood to latewood transition is regulated by developmental factors, and the photoperiod may indirectly regulate the developmental process of trees as a signaling factor, thus regulating the earlywood to latewood transition time. Our study provides new insights into the developmental regulation and climate sensitivity of tree ring formation.