Foliage, fine-root, woody-tissue and stand respiration in Pinus radiata in relation to nitrogen status

Abstract

We measured respiration of 20-year-old Pinus radiata D. Don trees growing in control (C), irrigated (I), and irrigated + fertilized (IL) stands in the Biology of Forest Growth experimental plantation near Canberra, Australia. Respiration was measured on fully expanded foliage, live branches, boles, and fine and coarse roots to determine the relationship between CO₂ efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues. Efflux of CO₂ from foliage (dark respiration at night) and fine roots was linearly related to biomass and N content, but N was a better predictor of CO₂ efflux than biomass. Respiration (assumed to be maintenance) per unit N at 15 °C and a CO₂ concentration of 400 μmol mol⁻¹ was 1.71 μmol s⁻¹ mol⁻¹ N for foliage and 11.2 μmol s⁻¹ mol⁻¹ N for fine roots. Efflux of CO₂ from stems, coarse roots and branches was linearly related to sapwood volume (stems) or total volume (branches + coarse roots) and growth, with rates for maintenance respiration at 15 °C ranging from 18 to 104 μmol m⁻³ s⁻¹. Among woody components, branches in the upper canopy and small diameter coarse roots had the highest respiration rates. Stem maintenance respiration per unit sapwood volume did not differ among treatments. Annual C flux was estimated by summing (1) dry matter production and respiration of aboveground components, (2) annual soil CO₂ efflux minus aboveground litterfall, and (3) the annual increment in coarse root biomass. Annual C flux was 24.4, 25.3 and 34.4 Mg ha⁻¹ year⁻¹ for the C, I and IL treatments, respectively. Total belowground C allocation, estimated as the sum of (2) and (3) above, was equal to the sum of root respiration and estimated root production in the IL treatment, whereas in the nutrient-limited C and I treatments, total belowground C allocation was greater than the sum of root respiration and estimated root production, suggesting higher fine root turnover or increased allocation to mycorrhizae and root exudation. Carbon use efficiency, the ratio of net primary production to assimilation, was similar among treatments for aboveground tissues (0.43–0.50). Therefore, the proportion of assimilation used for construction and maintenance respiration on an annual basis was also similar among treatments.