Daytime depression in temperature‐normalised stem CO₂ efflux in young poplar trees is dominated by low turgor pressure rather than by internal transport of respired CO₂

Abstract

Daytime decreases in temperature-normalised stem CO efflux (E ) are commonly ascribed to internal transport of respired CO (F ) 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 F , 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 CO fluxes was applied to estimate F . Under well-watered control conditions, both turgor pressure and sap flow, as a proxy of F , were reliable predictors of E . After tree manipulation, only turgor pressure was a robust predictor of E . Moreover, F accounted for < 15% of E . Our results suggest that daytime reductions in turgor pressure and associated constrained growth are the main cause of E 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.

Elevated CO2 does not affect stem CO2 efflux nor stem respiration in a dry Eucalyptus woodland, but it shifts the vertical gradient in xylem [CO2]

“…Temperature sensitivity of E A in Eucalyptus trees on a daily basis was relatively low, with Q 10 decreasing from 1.52 to 1.25 along the summer season, which are values slightly lower than those commonly reported (from 1.5 to 2, compiled by Wang, Wang, Zu, Li, & Koike, ). In this line, as soil dried out and constrained stem growth and overall R S , E A progressively decoupled from air temperatures, highlighting the crucial role of additional drivers affecting R S such as water availability and cell turgor (Salomón et al, ). These observations emphasize the need to move beyond temperature‐based predictions of plant respiration commonly applied in global dynamic vegetation models (Fatichi, Leuzinger, & Korner, ; Smith, ).…”

“…It is worth noting that during the 2‐month experimental period, accumulated precipitation was below 70 mm, when precipitation records during summer—from December to February—average ~258 mm (Drake et al, ), and soil VWC progressively decreased during the course of the experiment. Drought stress leads to stem growth cessation and cell turgor loss limiting associated respiratory expenditures (Rodríguez‐Calcerrada et al, ; Salomón et al, ; Saveyn et al, ; Steppe et al, ). In this line, stem growth of monitored trees was already constrained at the onset of the experiment (Figure ).…”

“…This time lag is consistent with previous observations across Northern Hemisphere forests (Cuny et al, ), where time lags between stem girth‐increment and biomass production ranged from 27 to 30 days in boreal, temperate, and subalpine areas to 49 days in drought‐prone Mediterranean regions that would be comparable with our dry Eucalyptus woodland. Delayed cell wall thickening—not registered by our band dendrometers—may enhance growth respiratory expenditures resulting in the comparatively high R S observed during DEC. Lower R S during JAN and FEB could be attributed to the complete cessation of new cell formation and enlargement, and the progressive reduction in soil water content and cell turgor, further limiting maintenance respiratory processes (Salomón et al, ; Saveyn et al, ). Temperature sensitivity of E A in Eucalyptus trees on a daily basis was relatively low, with Q 10 decreasing from 1.52 to 1.25 along the summer season, which are values slightly lower than those commonly reported (from 1.5 to 2, compiled by Wang, Wang, Zu, Li, & Koike, ).…”

Woody tissue photosynthesis reduces stem CO2 efflux by half and remains unaffected by drought stress in young Populus tremula trees

“…Stem CO 2 efflux rates observed here ranged between 0.5 and 2 μmol m −2 s −1 and were two to three times lower compared with other values reported for young poplars. Salomón et al () measured E A of 3–4 μmol m −2 s −1 in 3‐year‐old P. canadensis trees, whereas Steppe, Saveyn, McGuire, Lemeur, and Teskey () reported values of 2–5 μmol m −2 s −1 in 3‐year‐old P. deltoides trees. Differences in E A were likely related to the wood phenological stage at which E A measurements were performed in our study (late summer–early autumn), when stem growth was largely limited and associated growth respiration was therefore low (Steppe et al, ).…”

More than just CO2‐recycling: corticular photosynthesis as a mechanism to reduce the risk of an energy crisis induced by low oxygen

“…Over 80% of diurnal changes in sapwood cO 2 could be explained by respiration rates. The changes in the explanatory power of this relationship with diel time period (Table ) might be the result of diurnal changes in the water status and cell turgor pressure of the stem cells (Etzold et al ., ; Mencuccini et al ., ; Salomón et al ., ). A reduced cell water status and turgor pressure during daytime has been suggested as a constraint on stem respiratory metabolism (Saveyn et al ., ; Steppe et al ., , ; Mencuccini et al ., ; Salomón et al ., ).…”