We measured diel patterns of stem surface CO₂ efflux (E_s, µmol m⁻² s⁻¹), sap velocity (v_s, mm s⁻¹) and xylem CO₂ concetration ([CO₂]) (X_s, %) in 8‐year‐old loblolly pine trees during the spring to determine how v_s and X_s influence E_s. All trees showed a strong diel hysteresis between E_s and stem temperature, where at a given temperature, E_s was lower during the day than at night. Diel variations in temperature‐independent E_s were correlated with v_s (R² = 0.54), such that at maximum v_s, E_s was reduced between 18 and 40%. However, this correlation may not represent a cause‐and‐effect relationship. In a subset of trees, v_s was artificially reduced by progressively removing the tree canopy. Reducing v_s to near zero had no effect on E_s and did not change the diel hysteretic response to temperature. Diel X_s tended to decrease with v_s and increase with E_s, however, in defoliated trees, large increases in X_s, when v_s ≈ 0, had no effect on E_s. We conclude that at this time of the year, E_s is driven primarily by respiration of cambium and phloem tissues and that sap flow and xylem transport of CO₂ had no direct influence on E_s.

Stem respiration and growth in 10-year-old loblolly pine (Pinus taeda L.) plantations were measured monthly during the third year of fertilization and irrigation treatments to determine whether soil resource availability differentially altered growth and respiration in stem tissue. Fertilized trees had significantly greater stem biomass, stem nitrogen concentration ([N]) and growth rate than unfertilized trees. Stem respiration (R_t) was significantly greater in fertilized trees when expressed on a per unit surface area (R_t,a, μmol CO₂ m⁻² s⁻¹), sapwood volume (R_t,v, μmol CO₂ m⁻³ s⁻¹), or mass (R_t,w, nmol CO₂ g⁻¹ s⁻¹) basis; however, there was no difference between treatments when expressed as a function of stem N content (R_t,n, μmol CO₂ (mol N)⁻¹ s⁻¹). Irrigation had no significant effect on R_t 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 seasonal variation in temperature-normalized R_t,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 correlation between winter R_t,v, a measure of basal R_m, and sapwood N content; however, R_t,v per unit N was greater in January before diameter growth started than in the following December after growth ceased, indicating that R_t,v declined as stem diameter increased. Consequently, estimates of annual maintenance respiration (R_M) based on January data were 44% higher than estimates based on December data. Growth respiration was correlated with stem growth rate (r² = 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-use efficiency (CUE)—the ratio of stem growth to stem growth plus respiration—averaged 0.63 and was unaffected by fertilization.

We examined effects of a first nitrogen (N) fertilizer application on upper-canopy needle morphology and gas exchange in ∼20-m-tall loblolly pine (Pinus taeda L.) exposed to elevated carbon dioxide concentration ([CO₂]) for 9 years. Duke Forest free-air CO₂ enrichment (FACE) plots were split and half of each ring fertilized with 112 kg ha⁻¹ elemental N applied in two applications in March and April 2005. Measurements of needle length (L), mass per unit area (LMA), N concentration (N_l) on a mass and an area basis, light-saturated net photosynthesis per unit leaf area (A_a) and per unit mass (A_m), and leaf conductance (g_L) began after the second fertilizer application in existing 1-year-old foliage (F_O) and later in developing current-year first-flush (F_C1) and current-year second-flush (F_C2) foliage. Elevated [CO₂] increased A_a by 43 and 52% in F_O and F_C1 foliage, respectively, but generally had no significant effect on any other parameter. Fertilization had little or no significant effect on L, LMA, A or g_L in F_O foliage; although N_l was significantly higher in fertilized trees by midsummer. In contrast, fertilization resulted in large increases in L, N_l, and A in F_C1 and F_C2 foliage, increasing A_a by about 20%. These results suggest that, although both needle age classes accumulate N following fertilization, they use it differently—current-year foliage incorporates N into photosynthetic machinery, whereas 1-year-old foliage serves as an N store. There were no significant interaction effects of elevated [CO₂] and fertilization on A. Elevated [CO₂] increased the intercept of the A:N_l relationship but did not significantly affect the slope of the relationship in either foliage age class.

We used whole-tree, open-top chambers to expose 13-year-old loblolly pine (Pinus taeda L.) trees, growing in soil with high or low nutrient availability, to either ambient or elevated (ambient + 200 μmol mol⁻¹) carbon dioxide concentration ([CO₂]) for 28 months. Branch growth and morphology, foliar chemistry and gas exchange characteristics were measured periodically in the upper, middle and lower crown during the 2 years of exposure. Fertilization and elevated [CO₂] increased branch leaf area by 38 and 13%, respectively, and the combined effects were additive. Fertilization and elevated [CO₂] differentially altered needle lengths, number of fascicles and flush length such that flush density (leaf area/flush length) increased with improved nutrition but decreased in response to elevated [CO₂]. These results suggest that changes in nitrogen availability and atmospheric [CO₂] may alter canopy structure, resulting in greater foliage retention and deeper crowns in loblolly pine forests. Fertilization increased foliar nitrogen concentration (N_M), but had no consistent effect on foliar leaf mass (W_A) or light-saturated net photosynthesis (A_sat). However, the correlation between A_sat and leaf nitrogen per unit area (N_A = W_AN_M) ranged from strong to weak depending on the time of year, possibly reflecting seasonal shifts in the form and pools of leaf nitrogen. Elevated [CO₂] had no effect on W_A, N_M or N_A, but increased A_sat on average by 82%. Elevated [CO₂] also increased photosynthetic quantum efficiency and lowered the light compensation point, but had no effect on the photosynthetic response to intercellular [CO₂], hence there was no acclimation to elevated [CO₂]. Daily photosynthetic photon flux density at the upper, middle and lower canopy position was 60, 54 and 33%, respectively, of full sun incident to the top of the canopy. Despite the relatively high light penetration, W_A, N_A, A_sat and R_d decreased with crown depth. Although growth enhancement in response to elevated [CO₂] was dependent on fertilization, [CO₂] by fertilization interactions and treatment by canopy position interactions generally had little effect on the physiological parameters measured.

• We examined the relationships between stem CO₂ efflux (E_s), diameter growth, and nonstructural carbohydrate concentration in loblolly pine trees. Carbohydrate supply was altered via stem girdling during rapid stem growth in the spring and after growth had ceased in the autumn. We hypothesized that substrate type and availability control the seasonal variation and temperature sensitivity of E_s. • The E_s increased and decreased above and below the girdle, respectively, within 24 h of treatment. Seasonal variation in E_s response to girdling corresponded to changes in stem soluble sugar and starch concentration. Relative to nongirdled trees, E_s increased 94% above the girdle and decreased 50% below in the autumn compared with a 60% and 20% response at similar positions in the spring. • The sensitivity of E_s to temperature decreased below the girdle in the autumn and spring and increased above the girdle but only in the autumn. Temperature-corrected E_s was linearly related to soluble sugar (R² = 0.57) and starch (R² = 0.62) concentration. • We conclude that carbohydrate supply, primarily recently fixed photosynthate, strongly influences E_s in Pinus taeda stems. Carbohydrate availability effects on E_s obviate the utility of applying short-term temperature response functions across seasons.