Daytime depression in temperature-normalised stem CO(2) efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO(2).

Daytime decreases in temperature-normalised stem CO2 efflux (EA_D ) are commonly ascribed to internal transport of respired CO2 (FT ) or to an attenuated respiratory activity due to lowered turgor pressure. The two are difficult to separate as they are simultaneously driven by sap flow dynamics. To achieve combined gradients in turgor pressure and FT , sap flow rates in poplar trees were manipulated through severe defoliation, severe drought, moderate defoliation and moderate drought. Turgor pressure was mechanistically modelled using measurements of sap flow, stem diameter variation, and soil and stem water potential. A mass balance approach considering internal and external CO2 fluxes was applied to estimate FT . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of FT , were reliable predictors of EA_D . After tree manipulation, only turgor pressure was a robust predictor of EA_D . Moreover, FT accounted for < 15% of EA_D . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of EA_D in young poplar trees. Turgor pressure is determined by both carbohydrate supply and water availability, and should be considered to improve our widely used but inaccurate temperature-based predictions of woody tissue respiration in global models.

Salomón RL ，De Schepper V ，Valbuena-Carabaña M ，Gil L ，Steppe K ... -  《-》

Temporal and spatial patterns of internal and external stem CO2 fluxes in a sub-Mediterranean oak.

To accurately estimate stem respiration (RS), measurements of both carbon dioxide (CO2) efflux to the atmosphere (EA) and internal CO2 flux through xylem (FT) are needed because xylem sap transports respired CO2 upward. However, reports of seasonal dynamics of FT and EA are scarce and no studies exist in Mediterranean species under drought stress conditions. Internal and external CO2 fluxes at three stem heights, together with radial stem growth, temperature, sap flow and shoot water potential, were measured in Quercus pyrenaica Willd. in four measurement campaigns during one growing season. Substantial daytime depressions in temperature-normalized EA were observed throughout the experiment, including prior to budburst, indicating that diel hysteresis between stem temperature and EA cannot be uniquely ascribed to diversion of CO2 in the transpiration stream. Low internal [CO2] (<0.5%) resulted in low contributions of FT to RS throughout the growing season, and RS was mainly explained by EA (>90%). Internal [CO2] was found to vary vertically along the stems. Seasonality in resistance to radial CO2 diffusion was related to shoot water potential. The low internal [CO2] and FT observed in our study may result from the downregulation of xylem respiration in response to a legacy of coppicing as well as high radial diffusion of CO2 through cambium, phloem and bark tissues, which was related to low water content of stems. Long-term studies analyzing temporal and spatial variation in internal and external CO2 fluxes and their interactions are needed to mechanistically understand and model respiration of woody tissues.

Salomón RL ，Valbuena-Carabaña M ，Gil L ，McGuire MA ，Teskey RO ，Aubrey DP ，González-Doncel I ，Rodríguez-Calcerrada J ... -  《-》

Woody tissue photosynthesis reduces stem CO(2) efflux by half and remains unaffected by drought stress in young Populus tremula trees.

A substantial portion of locally respired CO2 in stems can be assimilated by chloroplast-containing tissues. Woody tissue photosynthesis (Pwt ) therefore plays a major role in the stem carbon balance. To study the impact of Pwt on stem carbon cycling along a gradient of water availability, stem CO2 efflux (EA ), xylem CO2 concentration ([CO2 ]), and xylem water potential (Ψxylem ) were measured in 4-year-old Populus tremula L. trees exposed to drought stress and different regimes of light exclusion of woody tissues. Under well-watered conditions, local Pwt decreased EA up to 30%. Axial CO2 diffusion (Dax ) induced by distant Pwt caused an additional decrease in EA of up to 25% and limited xylem [CO2 ] build-up. Under drought stress, absolute decreases in EA driven by Pwt remained stable, denoting that Pwt was not affected by drought. At the end of the dry period, when transpiration was low, local Pwt and Dax offset 20% and 10% of stem respiration on a daily basis, respectively. These results highlight (a) the importance of Pwt for an adequate interpretation of EA measurements and (b) homeostatic Pwt along a drought stress gradient, which might play a crucial role to fuel stem metabolism when leaf carbon uptake and phloem transport are limited.

De Roo L ，Salomón RL ，Steppe K 《-》

Stem respiration and carbon dioxide efflux of young Populus deltoides trees in relation to temperature and xylem carbon dioxide concentration.

Saveyn A ，Steppe K ，McGuire MA ，Lemeur R ，Teskey RO ... -  《OECOLOGIA》

Elevated CO(2) does not affect stem CO(2) efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO(2) ].

To quantify stem respiration (RS ) under elevated CO2 (eCO2 ), stem CO2 efflux (EA ) and CO2 flux through the xylem (FT ) should be accounted for, because part of respired CO2 is transported upwards with the sap solution. However, previous studies have used EA as a proxy of RS , which could lead to equivocal conclusions. Here, to test the effect of eCO2 on RS , both EA and FT were measured in a free-air CO2 enrichment experiment located in a mature Eucalyptus native forest. Drought stress substantially reduced EA and RS , which were unaffected by eCO2 , likely as a consequence of its neutral effect on stem growth in this phosphorus-limited site. However, xylem CO2 concentration measured near the stem base was higher under eCO2 , and decreased along the stem resulting in a negative contribution of FT to RS , whereas the contribution of FT to RS under ambient CO2 was positive. Negative FT indicates net efflux of CO2 respired below the monitored stem segment, likely coming from the roots. Our results highlight the role of nutrient availability on the dependency of RS on eCO2 and suggest stimulated root respiration under eCO2 that may shift vertical gradients in xylem [CO2 ] confounding the interpretation of EA measurements.

Salomón RL ，Steppe K ，Crous KY ，Noh NJ ，Ellsworth DS ... -  《-》