Ordered rise and disordered fall: dynamic changes of membrane lipids during girdling-induced tree mortality in Populus yunnanensis.

Understanding the mechanisms behind drought-induced tree mortality is crucial for predicting the impact of global climate change on forests. We studied the mechanism at the cellular level in Populus yunnanensis by profiling membrane lipid molecules in leaves, branch phloem, top and bottom trunk phloem under trunk-girdling-induced drought conditions. We found that both lipid composition and content changed, depending on the tree's tissue positions and the progression of the girdling effect. The compositional changes were similar between the leaves and branches and between the top and bottom trunk phloem. The lipid content initially increased and then decreased until complete degradation, with similar fold increases between leaves and branch phloem, and between top and bottom trunk phloem. However, the fold increase in the former two was significantly lower than that in the latter two. The lipid composition remained stable during the increase but changed during the decrease. The decrease in phloem lipids occurred later than in leaves and simultaneously across positions. Our findings provide novel insights into the mechanisms of water deficit and carbohydrate allocation in drought-induced tree mortality, and suggest that the onset of phloem lipid degradation could serve as a threshold for predicting tree mortality.