VARIATION IN FOLIAR NITROGEN CONCENTRATION BY FOREST TYPE AND CLIMATIC GRADIENTS IN NORTH-AMERICA

Published data were used to explore variation in foliar nitrogen (N) c oncentration in relation to forest type, folair life-span, latitude, m ean January temperature, mean July temperature, mean annual temperatur e. annual precipitation, annual actual evapotranspiration, mean monthl y solar radiation in foliar lifetime, and annual soil water deficit. T he hypothesis was that foliar N concentration integrating carbon and N links should be closely correlated with climatic constraints at a bro ad regional scale. The data set included 49 deciduous broadleaf forest s and 58 evergreen coniferous forests distributed across North America . Foliar N concentrations were correlated primarily with mean July tem perature for deciduous forests (negative), but also with mean January temperature, mean annual temperature, latitude, and solar radiation fo r coniferous forests (negative quadratic). These correlations also exi sted for individual forest groups defined by N requirement or genera o r dominant species. Deciduous forests nearly always had higher foliar N concentrations than coniferous forests for given climates, but diffe rences diminished in warm climates. Mean July temperature and latitude each explained 69% of variance in foliar N concentrations of deciduou s and coniferous forests, respectively, and together explained 90% of variance in the total data set. Independent data confirmed these corre lations. It is proposed that foliar N concentrations of deciduous fore sts increase with decreasing mean July temperature because (i) N may b e passively concentrated due to low temperatures reducing net photosyn thesis and foliar life-spans, but not N availability: and (ii) N may b e actively concentrated as a plant adaptation to counteract suboptimum temperature effects on net photosynthesis. In coniferous forests, N m ay concentrate in foliage with decreasing but sufficiently high light availability and temperature, when foliar life-spans are fairly consta nt, and where soil is rarely frozen. As light availability and tempera ture fall below a limit, conifer foliar N concentrations may decrease due to combined effects of prolonged foliar life-spans and restricted N uptake.