Robustness of the relationship between needle n-alkane δ²H and leaf water evaporative enrichment, amidst seasonally variable relationships between non-structural carbohydrate δ²H and respiration rate, in a boreal forest

Stable isotopes of n-alkanes are important and rapidly developing tools for understanding paleoecology and past climatic conditions. However, there are knowledge gaps surrounding the physiological and environmental δ²H signals of n-alkanes. In this study, we investigated whether biosynthetic processes interfere with the consistency of the environmental signal in needle n-alkane δ²H. Therefore, we sampled two needle generations (one year-old needles, current-year needles) from five Pinus sylvestris trees at the boreal forest in Hyytiälä, Finland, during the 2019 growing season and analyzed δ²H of leaf non-structural carbohydrates (NSCs), starch, n-alkanes, and combined data with published leaf water isotope and shoot gas exchange measurements. We explored (1) time-integrated relationships between environmental variables and measured organic compound δ²H, and (2) evaluated whether the δ²H data of this study align with model predictions for NSC and leaf n-alkane δ²H values based on a process-based model for NSC δ²H hybridized with different leaf water heavy isotope enrichment and n-alkane models. Our findings suggest that NSC δ²H has temporally variable relationships to environmental variables which are related to needle generation and season, because current-year needle NSC δ²H was more closely correlated to respiration rate than a needle water isotope signal while one year-old needle NSC δ²H was more closely related to a leaf water evaporative enrichment signal. Interestingly, n-alkane δ²H did not exhibit the same seasonally variable relationship to respiration rate and were instead more closely related to a leaf water evaporative enrichment signal. Overall, results suggest that water compartmentalization in leaves can have a prominent enough role during n-alkane synthesis, that its effects can be observed at seasonal scale, which shows promise to the role of leaf n-alkane δ²H as a leaf evaporative enrichment signal without substantial interference by source water δ²H. However, we also highlight the role of signal dampening, by time integration and new needle growth.