Nitrogen isotopes of individual tree-ring series - The validity of middle- to long-term trends

Although requiring laborious analytical treatment, tree-ring series of nitrogen isotopes (delta N-15) have gained popularity amongst researchers for their potential as environmental indicators as anthropogenic emissions increase globally with potential effects on forest N cycles. Previous studies suggested that tree-ring series correlate with climatic and air quality parameters. However, none discussed the level of replication required for expressing the population signals of specific species of trees. In this investigation, we studied 27 white spruce trees from two sites under distinct environmental conditions to evaluate the appropriate protocol for preparing consistent treering delta N-15 series. The produced series indicate that high frequency (short-term,< 7 years) delta N-15 changes at a replication as high as 10 trees cannot serve environmental purposes. Conversely, the low frequency (middle-, 7-15 years, to long-term, > 15 years) delta N-15 trends show coherence between arithmetic means of individual series at replication levels as low as three trees, whereas middle-term pooled trends do not perform as coherently. The low frequency mean trends of individual series obtained for the two sites suggest that local biogeochemical soil conditions modified by anthropogenic emissions modulate the delta N-15 responses in trees. Hence, we propose that long-term tree-ring delta N-15 series constitute reliable environmental indicators.