Light attenuation in a 14-year-old loblolly pine stand as influenced by fertilization and irrigation

We examined empirical and simulated estimates of canopy light attenuation at SETRES (Southeast Tree Research and Education Site), a 2×2 factorial study of water and nutrients. Fertilized plots had significantly lower under-canopy PAR transmittance (T^sub C^) when compared to non-fertilized plots. Light interception efficiency, as measured by the canopy cosine-corrected light extinction coefficient, G, was significantly lower in irrigated plots for all dates examined. Estimates of G ranged from a low of 0.36 in irrigated plots in September to a high of 0.64 in March for control plots. Study-wide analyses indicate that a G of 0.50 and a k (uncorrected light extinction coefficient) of 0.69 may be reasonable parameter estimates of canopy light extinction in intermediate-aged loblolly pine plantations across a range of stand conditions and seasons when site-specific data are unavailable. Simulated T^sub C^ from our version of the BIOMASS model corresponded well to the empirical estimates. Varying the vertical distribution of foliage in simulations (from 10:60:30 to 40:40:10% in the upper, middle, and lower canopy positions, respectively) resulted in only a ±7% change in total PAR intercepted, whereas varying G from 0.3 to 0.7 resulted in a 67% and 31% increase in light intercepted for control and fertilized plots, respectively. Decreased G resulted in an increased proportion of beam radiation intercepted - 63-67% of total PAR intercepted - by the middle canopy where 55-60% of the foliage was found. We hypothesize that proportionally increased productivity observed in irrigated treatments may be attributed to increased beam radiation intercepted deeper into the canopy by a greater foliage area.[PUBLICATION ABSTRACT]