Assimilation and exchange of carbon for arctic ecosystems depend strongly on leaf area index (LAI) and total foliar nitrogen (TFN). For dry-mesic tundras, the seasonality of these characteristics is unexplored. We addressed this knowledge gap by measuring variations of LAI and TFN at five contrasting subarctic heaths during the growing season 2007, from about 2 weeks after bud burst until about 2 weeks before senescence. The communities generally showed an early season LAI and TFN increase, owing to leaf development of deciduous shrubs, and limited variations later on, owing to concurrent leaf growth and mortality of evergreen shrubs. For most of the communities, the TFN:LAI ratio was constant along the entire sampling period, indicating that leaf growth and mortality, triggers for both LAI and TFN, are more important than leaf N retranslocation, which only affects TFN. A constant TFN:LAI ratio facilitates the determination of TFN from LAI, which is easier to measure or simulate. However, a sheltered community showed an increment in the LAI of the deciduous canopy in mid-late season, paralleled by a decreased TFN:LAI ratio.
Key words: leaf traits, leaf area index, foliar nitrogen, seasonal canopy dynamics, northern heath, ecosystem C cycle.