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 CO(2) efflux, tissue temperature, and biomass or nitrogen (N) content of individual tissues. Efflux of CO(2) from foliage (dark respiration at night) and fine roots was linearly related to biomass and N content, but N was a better predictor of CO(2) efflux than biomass. Respiration (assumed to be maintenance) per unit N at 15 degrees C and a CO(2) concentration of 400 micro mol mol(-1) was 1.71 micro mol s(-1) mol(-1) N for foliage and 11.2 micro mol s(-1) mol(-1) N for fine roots. Efflux of CO(2) 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 degrees C ranging from 18 to 104 micro mol 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 CO(2) 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.

The growth rates of woody plants depend on both the rate of photosynthetic carbon gain and the availability of essential nutrients. Instantaneous carbon gain is known to increase in response to increasing atmospheric CO2 concentration, but it is uncertain whether this will translate into increased growth in the longer term under nutrient‐limited conditions. An analytical model to address this question was developed by Comins & McMurtrie (1993, Ecological Applications 3, 666–681). Their model was further tested and analysed. Manipulation of various assumptions in the model revealed its key assumptions and allowed a more confident prediction of expected growth responses to CO2 enrichment under nutrient‐limited conditions. The analysis indicated that conclusions about the CO2 sensitivity of production were strongly influenced by assumptions about the relationship between foliar and heartwood nitrogen concentrations. With heartwood nitrogen concentration proportional to foliar nitrogen concentration, the model predicted a strong response of plant productivity to increasing CO2 concentration, whereas with heartwood nitrogen concentration set constant, the model predicted only a very slight growth response to changing CO2 concentration. On the other hand, predictions were only slightly affected by: (1) assumptions about the extent of nitrogen retranslocation out of senescing roots and foliage or wood during heartwood formation; (2) the effects of nitrogen status on specific leaf area or (3) leaf longevity; (4) carbon allocation between different plant parts; or (5) changes in the N:C ratio of organic matter sequestered in the passive pool of soil organic matter. Modification of the effect of foliar nitrogen concentration on the light utilization coefficient had only a small effect on the CO2 sensitivity for pines. However, this conclusion was strongly dependent on the chosen relationship between single‐leaf photosynthesis and leaf nitrogen concentration. Overall, the analysis suggested that trees growing under nitrogen‐limited conditions can respond to increasing atmospheric CO2 concentration with considerable increases in growth.

We compared independent estimates of total root carbon allocation (TRCA) and fine root net primary production $(NPP_{fr})$ for temperate forests to total root carbon allocation (TRCA—RN) estimates derived from an empirical model developed by J. W. Raich and K. J. Nadelhoffer for world forests. Measured total root carbon allocation for temperate forests ranged from 233 to 1013 g $C\cdot m{—2}\cdot yr{—1}$ and was not correlated to TRCA calculated from the Raich and Nadelhoffer model for world forests. Fine root NPP was not correlated to TRCA—RN; however, a strong correlation was found between $NPP_{fr}$ and TRCA—RN for five control Pinus resinosa natural forests and plantations. The poor correlation between measured TRCA or $NPP_{fr}$ and the estimates may be due to interspecies differences in belowground carbon allocation, variation in foliage: root carbon allocation ratios due to water and nutrient availability, and violations in the carbon budget approach. Using the Raich and Nadelhoffer model to estimate total root carbon allocation and to construct stand carbon budgets for a stand is not an acceptable substitute for measuring soil carbon fluxes.