Distribution of non-structural nitrogen and carbohydrate compounds in mature oak trees in a temperate forest at four key phenological stages

Abstract

The distribution of nitrogen (N) and carbon (C) pools in 100-year-old sessile oaks was investigated in situ at four key periods of tree phenology. Leaves, twigs, trunk and roots were sampled, and total non-structural nitrogen compounds (TNNC) and total non-structural carbohydrates (TNC) were quantified. TNC concentrations decreased more than 50 % between February and May, especially in the above-ground parts. During the same period, TNNC concentrations drastically decreased especially in twigs (more than 55 % below the winter baseline). This indicates high sink strength of new organs at bud break. TNC concentrations increased in summer in all tree compartments highlighting the TNC storage. TNNC deposition for storage constitution began in autumn during leaf senescence. The first organs acting as a N store were coarse roots followed by twigs; whereas all oak tree compartments were potential C store organs except phloem tissue. Arginine and asparagine were the two prevailing amino acids involved in both storage and transport. Besides identifying the compounds involved in the storage and transportation of N, our results highlight that oak seasonal cycles of C and N are not in phase: after intensive use of C and N stores in spring, the replenishment of N stores lags behind that of C stores.