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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.

Soil CO2 efflux (SR) is the second largest carbon flux on earth. We investigated the driving factors of the seasonal change and short-distance spatial variation in SR in a short-rotation plantation of willow (Salix viminalis Orm). Total annual SR ranged from 723 to 1149 g Cm(-2) year(-1). Both an exponential and a logistic model were fitted to the data, with soil temperature at a depth of 5 cm as the independent variable. The R2 values for individual sampling points ranged from 0.83 to 0.95 and from 0.85 to 0.93 for the exponential and logistic models, respectively, indicating that soil temperature largely determined the seasonal variation in SR. Modeled soil SR at 10 degrees C ranged from 1.22 to 1.95 micromol m(-2) s(-1), whereas modeled annual Q(10) values were between 3.31 and 6.13. These high Q(10) values were attributed to the absence of drought during the study in 2005. When the coefficients of the general SR models were replaced by linear dependencies on soil and vegetation-related characteristics, the resulting spatially explicit exponential and logistic SR models explained 85 and 86%, respectively, of the variability within the dataset. The analysis indicated that soil carbon concentration, leaf area index, soil pH and root biomass caused differences in SR at the short distances considered in this study. However, incorporating information on variables considered to account for spatial variability in the model did not result in a higher R2 compared with a simple temperature function. When the general SR models were applied to independent datasets from the same plantation, the logistic model provided a better fit than the exponential model when drought occurred. Drought greatly reduced the annual Q(10) values of SR.

A young potted oak (Quercus robur L.) tree was subjected to drought by interrupting the water supply for 9 days. The tree was placed in a growth chamber in which daily patterns of temperature and radiation were constant. The effects of drought on the water and carbon status of the stem were examined by measuring stem sap flow rate, stem water potential, stem diameter variations, stem CO(2) efflux rate (F(CO2)) and xylem CO(2) concentration ([CO(2)*]). Before and after the drought treatment, diurnal fluctuations in F(CO2) and [CO(2)*] corresponded well with variations in stem temperature (T(st)). Daytime depressions in F(CO2) did not occur. During the drought treatment, F(CO2) still responded to stepwise changes in temperature, but diurnal fluctuations in F(CO2) were no longer correlated with diurnal fluctuations in T(st). From the moment daily growth rate of the stem became zero, diurnal fluctuations in F(CO2) became closely correlated with diameter variations, exhibiting clear daytime depressions. The depressions in F(CO2) were likely the result of a reduction in metabolic activity caused by the lowered daytime stem water status. Xylem [CO(2)*] showed clear daytime depressions in response to drought. When the tree was re-watered, F(CO2) and [CO(2)*] exhibited sharp increases, coinciding with an increase in stem diameter. After resumption of the water supply, daytime depressions in F(CO2) and [CO(2)*] disappeared and diurnal fluctuations in F(CO2) and [CO(2)*] corresponded again with variations in T(st).

Background and Aims: Daytime CO2 efflux rates (FCO2) from tree stems are often reported to be lower than expected from the exponential relationship between temperature and respiration. Explanations of daytime depression in FCO2 have focused on the possible role of internal CO2 transport in the xylem. However, another possible cause that has been overlooked is the daily dynamics of the water status in the living stem tissues and its influence on stem growth rate and thus respiration. The objective of this study was to assess the daily dynamics of stem water status and growth rate and to determine the extent to which they may be linked to daily variations in stem FCO2., Methods: FCO2 of young beech and oak stems were measured under controlled conditions. Relative stem turgor pressure (Psi(p)), obtained from simulations with the 'RCGro' model, was used as an indicator of the water status in the living stem tissues. Daily dynamics of stem growth were derived from Psi(p): growth was assumed to occur when Psi(p) exceeded a relative threshold value., Key Results: There was a strong correspondence between fluctuations in FCO2 and simulated Psi(p). The non-growth conditions during daytime coincided with depressions in FCO2. Moreover, FCO2 responded to changes in Psi(p) in the absence of growth, indicating also that maintenance processes were influenced by the water status in the living stem tissues., Conclusions: Daytime depressions in stem FCO2 correlate with the daily dynamics of turgor, as a measure of the water status in the living stem tissues: it is suggested that water status of tree stems is a potentially important determinant of stem FCO2, as it influences the rate of growth and maintenance processes in the living tissues of the stem.

Oxidative respiration is strongly temperature driven. However, in woody stems, efflux of CO2 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 CO2 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 CO2 concentrations ([CO2*]) 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 CO2 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 [CO2*] was found. [CO2*] varied diurnally and increased substantially at night and during periods of rain. Changes in [CO2*] were related to changes in f s but not T st. We conclude that changes in both respiration and internal CO2 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. [ABSTRACT FROM AUTHOR]