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) concentration in relation to forest type, folair life-span, latitude, mean January temperature, mean July temperature, mean annual temperature. annual precipitation, annual actual evapotranspiration, mean monthly solar radiation in foliar lifetime, and annual soil water deficit. The hypothesis was that foliar N concentration integrating carbon and N links should be closely correlated with climatic constraints at a broad regional scale. The data set included 49 deciduous broadleaf forests and 58 evergreen coniferous forests distributed across North America. Foliar N concentrations were correlated primarily with mean July temperature for deciduous forests (negative), but also with mean January temperature, mean annual temperature, latitude, and solar radiation for coniferous forests (negative quadratic). These correlations also existed for individual forest groups defined by N requirement or genera or dominant species. Deciduous forests nearly always had higher foliar N concentrations than coniferous forests for given climates, but differences diminished in warm climates. Mean July temperature and latitude each explained 69% of variance in foliar N concentrations of deciduous and coniferous forests, respectively, and together explained 90% of variance in the total data set. Independent data confirmed these correlations. It is proposed that foliar N concentrations of deciduous forests increase with decreasing mean July temperature because (i) N may be passively concentrated due to low temperatures reducing net photosynthesis and foliar life-spans, but not N availability: and (ii) N may be actively concentrated as a plant adaptation to counteract suboptimum temperature effects on net photosynthesis. In coniferous forests, N may concentrate in foliage with decreasing but sufficiently high light availability and temperature, when foliar life-spans are fairly constant, and where soil is rarely frozen. As light availability and temperature fall below a limit, conifer foliar N concentrations may decrease due to combined effects of prolonged foliar life-spans and restricted N uptake.