Time‐integrated δ²<scp>H</scp> in <i>n</i>‐alkanes and carbohydrates from boreal needles reveal intra‐annual physiological and environmental signals

Reliable insights from key δ²H bioindicators, n-alkanes and carbohydrates, are hindered by our limited understanding of isotope fractionation processes related to leaf water and primary assimilates. We addressed this with the first study to investigate time-integrated intra-annual δ²H signals of n-alkanes and carbohydrates in a natural forest. We sampled 1-yr-old needles (1N) and current-year needles (0N) from five Scots Pine trees in a coniferous forest in Hyytiälä, Finland, biweekly during 2019. The δ²H of their n-alkanes (δ²H_alkane), water-soluble carbohydrates (δ²H_WSC) and starch (δ²H_starch) were evaluated for time-integrated physiological (gas-exchange) and environmental (hydrological) signals. Time integration was critical for interpreting δ²H_WSC and δ²H_alkane. Time-integrated net assimilation rate (T:A_n), the strongest signal in δ²H_WSC, correlated negatively with 1N δ²H_WSC (early season) and positively with 0N δ²H_WSC (late season). Unexpectedly, δ²H_alkane exhibited stronger environmental signals in 1N than in 0N, with the most pronounced being physiologically mediated hydrological signals. T:A_n is a major signal in intra-annual δ²H_WSC in a boreal forest, subject to seasonal interactions with needle age and δ²H_starch. There could be enough de novo n-alkane synthesis in situ to enhance the reliability of δ²H_alkane as a hydrological indicator, bringing promise to interpretations of the n-alkane palaeohydrological record.