Habitat and leaf habit as determinants of growth, nutrient absorption, and nutrient use by Alaskan taiga forest species

Four evergreen and four deciduous trees and shrubs were sampled from habitats with differing soil temperature regimes in interior Alaskan forests to examine the relative importance of habitat and leaf habit in determining seasonal patterns of shoot growth, tissue nutrient concentration, respiration rate, and phosphate absorption rate. Leaf habit was the primary determinant of shoot growth, with deciduous species producing leaf area and leaf biomass earlier in the season than evergreens. Deciduous trees produced more biomass per shoot and per unit ground area than did evergreens. The seasonal pattern of leaf nitrogen and phosphorus concentration was correlated closely with patterns of leaf growth, declining through the growing season in deciduous species first as nutrient concentrations were diluted by increasing leaf biomass and later as nutrients were retranslocated from senescing leaves. In evergreens the seasonal decline in nutrient concentration was entirely due to dilution by increasing leaf biomass, and there was no evidence of autumn retranslocation from 1st-year leaves. In contrast to seasonal pattern, the magnitude of leaf phosphorus and root nitrogen and phosphorus concentrations was correlated more closely with habitat than with leaf habit, generally being lower in cold sites. Leaf respiration was highly correlated with leaf nitrogen concentration, so that the seasonal pattern of leaf respiration was determined primarily by leaf habit, whereas the magnitude of respiration was more closely correlated with habitat. Root respiration showed no consistent correlation with either habitat or leaf habit but was lower than leaf respiration, as would be expected from low root nitrogen concentration. Phosphate absorption rate was determined more strongly by habitat than by leaf habit, being lower in cold sites characterized by slow plant growth and consequently low annual nutrient requirement. Evergreen species were more effective at absorbing phosphate at low solution concentrations than were deciduous species. Phosphate absorption was less temperature sensitive than root respiration, so that roots of all species absorbed more phosphorus per unit of carbon respired at low root temperature.