An Assessment of Contemporary and Historic Nitrogen Availability in Contrasting Coastal Douglas-Fir Forests Through δ15N of Tree Rings

Wood nitrogen isotope composition (delta N-15) provides a potential retrospective evaluation of ecosystem N status but refinement of this index is needed. We calibrated current wood delta N-15 of Douglas-fir (Pseudotsuga menziesii), an ectomycorrhizal tree species, against a productivity gradient of contrasting coastal forests of southern Vancouver Island (Canada). We then examined historical delta N-15 via increment cores, and tested whether wood delta N-15 corresponded with climatic fluctuations. Extractable soil N ranged from 11 to 43 kg N ha(-1) along the productivity gradient, and was characterized by a progressive replacement of N forms (amino acids, NH4 (+) and NO3 (-)). Current wood delta N-15 was significantly less depleted (-5.0 to -2.6 aEuro degrees) with increasing productivity, although linear correlations were stronger with Delta delta N-15 (the difference between wood and soil delta N-15) to standardize the extent of isotopic fractionation by ectomycorrhizal fungi. An overall decline in wood delta N-15 of 0.9 aEuro degrees over the years 1900-2009 was detected, but trends diverged widely among plots, including positive, negative and no trend with time. We did not detect significant correlations in detrended wood delta N-15 with mean annual temperature or precipitation. The contemporary patterns in stand productivity, soil N supply and wood delta N-15 were moderately strong, but interpreting historical patterns in delta N-15 was challenging because of potential variations in N uptake related to stand dynamics. The lack of wood delta N-15 correlations with climate may be partly due to methodological limitations, but might also reflect the relative stability in N supply due to the overriding constraints of soil organic matter quantity and quality.