Developmental patterns and nutrition impact radiation use efficiency components in southern pine stands

A number of contemporary forest productivity models use some variation of a growth efficiency (epsilon) approach. Typically, these models predict production (aboveground net primary production, ANPP in units of dry mass biomass per unit of area per unit of time) as the product of two terms: radiation use efficiency (epsilon, in units of dry mass biomass per megajoule of photosynthetically active radiation [PAR] intercepted or absorbed by the plant canopy) and the sum of PAR intercepted or absorbed by the canopy (Phi(par), in units of megajoules per unit of radiation area per unit of time). Predicting productivity in a biologically realistic manner requires an understanding of how model components are affected by natural and anthropogenic environmental factors, as well as other influences such as aging or stand development. We measured or calculated all components of the epsilon model (aboveground woody biomass increment, I-WB; foliage biomass increment, I-FB; aboveground net primary production, ANPP; leaf area index, LAI; Phi(par); and aboveground radiation use efficiency, epsilon(A)) from ages 4 to 18 yr in loblolly and slash pine stands in north-central Florida grown under replicated fertilizer and understory vegetation control treatments that induced a large gradient in soil nutrient availability. Treatments impacted all measured components, which in turn led to strong responses in aboveground radiation use efficiency. Age 6-9 yr epsilon(A), averaged across species, ranged from 0.78 g/MJ to 0.83 g/MJ in treatments receiving fertilizer or vegetation control vs. 0.53 g/MJ in untreated plots. Stand developmental processes modified these responses, however, with epsilon(A), declining by over 40% in treated plots from age 6-9 yr to age 15-16 yr. Variation in epsilon(A) in both species was linked to development of stand basal area (BA), with a positive, linear relationship between e, and BA for values of BA < 18 M-2/ha and a declining, linear relationship for values of BA > 18 M-2/ha. Loblolly pine epsilon(A) was positively correlated with foliar nitrogen concentration, [N]. These data highlight the dynamic nature of epsilon(A) and suggest that both stand developmental and nutritional processes drive changes in ANPP and epsilon(A) in southern pines.