The molecular composition of leaf lipids changes with seasonal gradients in temperature and light among deciduous and evergreen trees in a sub-humid ecosystem

Forest canopy structure is a fundamental ecosystem attribute affecting regional and global climate through primary production by CO₂ drawdown and evapotranspirative feedback. Environmental changes in temperature and light affect leaf physiology and thus canopy functioning. Leaf physiological changes may be reflected in expressed chemical compounds (e.g., leaf lipid biomarkers), that offer opportunities to characterize and quantify climatic effects on plant canopies in the present and the past. To assess this possibility, we systematically investigated the lipids from leaves of deciduous angiosperm (Quercus buckleyi, April–October 2019) and evergreen gymnosperm (Juniperus ashei, April–October 2019 and January 2020) tree species at monthly sampling intervals over one growing season in a natural sub-humid ecosystem of central Texas. Fatty acid unsaturation in Q. buckleyi and J. ashei leaves was negatively correlated with air temperature. The average chain lengths of leaf wax n-alkanols of Q. buckleyi were strongly correlated with leaf area index (LAI) and absorbed photosynthetically active radiation (APAR) (r² > 0.5). The stigmasterol/β-sitosterol ratio was correlated with light transmittance in the canopy of Q. buckleyi, with values of the sterol ratio three-fold higher in shaded leaves than in sparse canopies. The observed seasonal changes in leaf lipid molecular composition and chain-lengths might be related to their biosynthetic responses to temperature and light stresses. Finally, we developed multi-lipid regression models resolving seasonal differences in temperature, LAI, and APAR. We posit that the specific lipid biosynthetic responses to variations in temperature and light are a basis for reconstructing terrestrial paleoenvironmental changes.