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Rates of CO2 efflux of stems and branches are highly variable among and within trees and across stands. Scaling factors have only partially succeeded in accounting for the observed variations. In this study, the resistance to radial CO2 diffusion was quantified for tree stems of an eastern cottonwood (Populus deltoides Bartr. ex Marsh.) clone by direct manipulation of the CO2 concentration ([CO2]) of xylem sap under controlled conditions. Tree-specific linear relationships between rates of stem CO2 efflux (JO) and xylem [CO2] were found. The resistance to radial CO2 diffusion differed 6-fold among the trees and influenced the balance between the amount of CO2 retained in the xylem v. that which diffused to the atmosphere. Therefore, we hypothesised that variability in the resistance to radial CO2 diffusion might be an overlooked cause for the inconsistencies and large variations in woody tissue CO2 efflux. It was found that transition from light to dark conditions caused a rapid increase in JO and xylem [CO2], both in manipulated trees and in an intact tree with no sap manipulation. This resulted in an increased resistance to radial CO2 diffusion during the dark, at least for trees with smaller daytime resistances. Stem diameter changes measured in the intact tree supported the idea that higher actual respiration rates occurred at night owing to higher metabolism in relation to an improved water status and higher turgor pressure.

Oxidative respiration is strongly temperature driven. However, in woody stems, efflux of CO(2) to the atmosphere (E (A)), commonly used to estimate the rate of respiration (R (S)), and stem temperature (T (st)) have often been poorly correlated, which we hypothesized was due to transport of respired CO(2) in xylem sap, especially under high rates of sap flow (f (s)). To test this, we measured E (A), T (st), f (s) and xylem sap CO(2) concentrations ([CO(2)*]) in 3-year-old Populus deltoides trees under different weather conditions (sunny and rainy days) in autumn. We also calculated R (S) by mass balance as the sum of both outward and internal CO(2) fluxes and hypothesized that R (S) would correlate better with T (st) than E (A). We found that E (A) sometimes correlated well with T (st), but not on sunny mornings and afternoons or on rainy days. When the temperature effect on E (A) was accounted for, a clear positive relationship between E (A) and xylem [CO(2)*] was found. [CO(2)*] varied diurnally and increased substantially at night and during periods of rain. Changes in [CO(2)*] were related to changes in f (s) but not T (st). We conclude that changes in both respiration and internal CO(2) transport altered E (A). The dominant component flux of R (S) was E (A). However, on a 24-h basis, the internal transport flux represented 9-18% and 3-7% of R (S) on sunny and rainy days, respectively, indicating that the contribution of stem respiration to forest C balance may be larger than previously estimated based on E (A) measurements. Unexpectedly, the relationship between R (S) and T (st) was sometimes weak in two of the three trees. We conclude that in addition to temperature, other factors such as water deficits or substrate availability exert control on the rate of stem respiration so that simple temperature functions are not sufficient to predict stem respiration.

Stem photosynthesis can contribute significantly to woody plant carbon balance, particularly in times when leaves are absent or in 'open' crowns with sufficient light penetration. We explored the significance of woody tissue (stem) photosynthesis for the carbon income in three California native plant species via measurements of chlorophyll concentrations, radial stem growth, bud biomass and stable carbon isotope composition of sugars in different plant organs. Young plants of Prunus ilicifolia, Umbellularia californica and Arctostaphylos manzanita were measured and subjected to manipulations at two levels: trunk light exclusion (100 and 50%) and complete defoliation. We found that long-term light exclusion resulted in a reduction in chlorophyll concentration and radial growth, demonstrating that trunk assimilates contributed to trunk carbon income. In addition, bud biomass was lower in covered plants compared to uncovered plants. Excluding 100% of the ambient light from trunks on defoliated plants led to an enrichment in 13 C of trunk phloem sugars. We attributed this effect to a reduction in photosynthetic carbon isotope discrimination against 13 C that in turn resulted in an enrichment in 13 C of bud sugars. Taken together our results reveal that stem photosynthesis contributes to the total carbon income of all species including the buds in defoliated plants.