INTRASEASONAL VARIATION IN THE DELTA-N-15 SIGNATURE OF TAIGA TREES AND SHRUBS

We examined the stable isotope ratios of nitrogen from six dominant ta iga species over three distinct phenological periods during the growin g season. Temporal changes in the isotopic signature varied among spec ies, but were not consistent within a given growth form. Despite large variation between nitrogen concentrations in new, mature, and senesce nt foliage, the seasonal fluctuations in delta(15)N were small with th e exception of aspen, a tree species growing an the most fertile sites . In the absence of strong within-season variation in isotope signatur e, we conclude that this parameter reasonably well integrates the plan t-nitrogen relations over the growing season for most species, with th e caveat that this parameter may show significant temporal variation i n species from high-nitrogen environments. We found a significant, pos itive relationship between nitrogen concentration and delta(15)N value s in mature and newly flushed foliage, suggesting that plant enrichmen t in delta(15)N is associated with increased soil nitrogen turnover.