light attenuation in a W - 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 2x2 factorial study of water and nutrients. Fertilized plots had signiticantly lower under-canopy PAR transmittance (T,) when compared to non-fertilized plots. Light interception efftciency. 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 pammeter estimates of canopy light extinction in intermediate-aged lohlolly pine plantations across a range of stand condittons and seasons when site-specific data are unavailable. Simulated T,. f om f our version of the BIOM.ASS model tit:<responded well to the empirical estimates. Varying the vertical distribution of foliage in simulations t from 10:6():30 to W-W: IO% in the upper. middle. and loHer cantlpy positions. respectively) resulted in only a +7% change in total PAR intercepted. whereas varying G from 0.3 to 0.7 resulted in 9 67%and 3 I % increase in light intcrccptcd for control :und fertilized plots. respeciivcly. Dccr&cd G r&uItcd in 311 increased proportion of hcrrn~ ritJiution intcrccptcd -63-675; of‘ total P.4R intcrccptcd’ by the rniddlc canopy’ where 5.5-605~ of the foliage vv;~\ I’oun~J. b’c hypothesize that proportionally increased prodiicli\,ity ohscr\.cd in iriifatrd treatments may he atrributcd 10 inc‘reascd hc~n rtldiation intercepted derpcr into the canop\ hy ;I grc’;ltcr Ii~liibgC ;ire;L.