EFFECTS OF CARBON-DIOXIDE, FERTILIZATION, AND IRRIGATION ON PHOTOSYNTHETIC CAPACITY OF LOBLOLLY-PINE TREES

Branches of nine-year-old loblolly pine trees grown in a 2 x 2 factori al combination of fertilization and irrigation were exposed for 11 mon ths to ambient, ambient + 175, or ambient + 350 mu mol mol(-1) CO2. Ra tes of light-saturated net photosynthesis (A(max)), maximum stomatal c onductance to water vapor (g(max)), and foliar nitrogen concentration (% dry mass) were assessed monthly from April 1993 until September 199 3 on 1992 foliage (one-year-old) and from July 1993 to March 1994 on 1 993 foliage (current-year). Rates of A(max) of foliage in the ambient + 175 CO2 treatment and ambient + 350 were 32-47 and 83-91% greater, r espectively, than that of foliage in the ambient CO2 treatment. There was a statistically significant interaction between CO2 treatment and fertilization or irrigation treatment on A(max) on only one measuremen t date for each age class of foliage. Light-saturated stomatal conduct ance to water vapor (g(max)) was significantly affected by CO2 treatme nt on only four measurement dates. Light-saturated g(max) in winter wa s only 42% of summer g(max) even though soil water during winter was n ear field capacity and evaporative demand was low. Fertilization incre ased foliar N concentration by 30% over the study period when averaged across CO2 treatments. During the study period, the ambient + 350 CO2 treatment decreased average foliar N concentration of one-year-old fo liage in the control, irrigated, fertilized and irrigated + fertized p lots by 5, 6.4, 9.6 and 11%, respectively, compared with one-year-old foliage in the corresponding ambient CO2 treatments. The percent incre ase in A(max) due to CO2 enrichment was similar in all irrigation and fertilization treatments and the effect persisted throughout the 11-mo nth study period for both one-year-old and current-year foliage.