Stem growth and respiration in loblolly pine plantations differing in soilresource availability

Stem respiration and growth in 10-year-old loblolly pine (Pinus taeda L.) p lantations were measured monthly during the third year of fertilization and irrigation treatments to determine whether soil resource availability diff erentially altered growth and respiration in stem tissue. Fertilized trees had significantly greater stem biomass, stem nitrogen concentration ([N]) a nd growth rate than unfertilized trees. Stem respiration (R-t) was signific antly greater in fertilized trees when expressed on a per unit surface area (R-t,R-a mu mol CO2 m(-2) s(-1)), sapwood volume (R-t,R-v, mu mol CO2 m(-3 ) s(-1)), or mass (R-t,R-w, nmol CO2 g(-1) s(-1)) basis; however, there was no difference between treatments when expressed as a function of stem N co ntent (R-t,R-n mu mol CO2 (mol N)(-1) s(-1)). Irrigation had no significant effect on R, or annual stem growth. Daily total respiration (R-d, mol CO2 m(-2) day(-1)) and stem diameter growth both had a seasonal bimodal pattern with peaks in early spring and midsummer. Stem [N] declined significantly during the growing season. Stem growth rate and [N] explained 75% of the se asonal variation in temperature-normalized R-t,R-a. The mature tissue method was used to partition total stem respiration (R-t) into maintenance (R-m) and growth (R-g) components. There was a linear cor relation between winter R-t,R-v, a measure of basal R-m, and sapwood N cont ent; however, R-t,R-v per unit N was greater in January before diameter gro wth started than in the following December after growth ceased, indicating that R-t,R-v declined as stem diameter increased. Consequently, estimates o f annual maintenance respiration (R-m) based on January data were 44% highe r than estimates based on December data. Growth respiration was correlated with stem growth rate (r(2) = 0.55). The growth respiration coefficient (r( g))-the slope of the relationship between R-g and stem growth rate-was 0.24 . Respiration accounted for 37% of annual stem carbon budget. Stem carbon-u se efficiency (CUE)-the ratio of stem growth to stem growth plus respiratio n-averaged 0.63 and was unaffected by fertilization.