Differential nitrogen isotope variation of tree-rings in two coniferous forests under climate change, Japan

Tree-ring δ¹⁵N provide long-term reliable information of soil available nitrogen, especially under increasing atmospheric N deposition upon forested ecosystems. In this study tree-ring δ¹⁵N values of Larix kaempferi and Cryptomeria japonica were measured for five trees each from a heavy snowfall region in north-eastern Japan, respectively. Larch and cedar tree-ring δ¹⁵N showed a range of -1.2 to 3.8 ‰ and -1.6 to 1.6 ‰, respectively. Larch trees showed a stable increase in δ¹⁵N values, while cedar trees showed a steady decreasing trend for the period 1970-2017. However, the divergence in the two tree series started in the early 1990’s. Both tree stands are exposed to the same atmospheric N deposition, therefore in principle both should have been affected equally. Similarly, an increase or a decrease in the δ¹⁵N value of the soil available N cannot be the reason since tree-rings values showed contrasting trends, unless this difference exists in each forest stand, however this seems unlikely. Another possibility could be the canopy uptake of depleted N from the atmosphere in cedar as the N demand increases could be responsible for cedar tree-ring δ¹⁵N temporal decrease but it does not explain the increase observed in larch. We speculate that low N demand and the increase in root biomass as tree grows could have decreased ¹⁵N discrimination by the EM fungi. The most plausible explanation for the contrasting results is that fractionation by the mycorrhiza fungi (Ecto and Arbuscular, respectively) during N root uptake was lower in larch (3.2 ‰) than in cedar (4.7 ‰) trees, which was related to the lower N demand as tree-ring wood %N was in average 0.06 and 0.10, respectively.