Seasonal and annual differences in net primary production, growth, and nutrient use between evergreen and deciduous trees of the Appalachians following nitrogen and phosphorus amendments

Evergreen and deciduous plants differ in growth habit and resource allocation. Each may have traits best suited for a particular set of growing conditions. Within the eastern deciduous forest region, evergreens may have greater importance on nutrient poor soils. The objectives of this study were to consider seasonal and annual differences of various aspects of above ground growth, productivity and nutrient use between evergreen and deciduous trees following N and P amendments. To conduct this study 12 (13M x 13M) plots were located at each of three elevations (710, 728, 757 M) within the Coweeta Hydrologic Laboratory (Otto, NC). At each elevation, three replicates of four treatments (control, N added, P added, N + P added) were used to compare differences in response by 11 species. Twig and leaf samples were collected at monthly intervals over a three year period. Seasonal differences between treated and untreated plots were considered for 2 evergreen (Rhododendron maximum L. and Kalmia latifolia L.) and 2 deciduous (Acer rubrum L. and Cornus florida L.) trees. Deciduous trees were more productive than evergreens suggesting less "conservative" growth. Evergreens were more N and P efficient. Following fertilization, all species increased net primary production. Evergreens increased leaf size, deciduous trees increased the number of leaves per twig and twig production. Kalmia increased twig production following N additions.; Following fertilization, deciduous trees declined in nutrient use efficiency. Nitrogen amendments resulted in more rapid leaf development. Phosphorus amendments prolonged evergreen leaf retention and P concentrations increased in older evergreen leaves. Trees were most productive when N and P were added together. Percent resorption following fertilizer additions did not change but increased foliar concentrations in deciduous and evergreen plants (P only) resulted in greater resorption. In summary, evergreens exhibit more "conservative" nutrient use and growth than deciduous tress, but are no less responsive to N and P amendments. Older evergreen leaves stored increased amounts of P which was then resorbed in greater quantities.