Contents Summary 1 I. Introduction 2 II. Quantity of CO₂ in tree stems 2 III. Sources of CO₂ found in stems 4 IV. Fate of the CO₂ released by respiring cells in stems: loss, transport and fixation 6 V. What do we know about woody tissue respiration? 10 VI. Why do trees have the capacity for internal capture of CO₂? 12 VII. Conclusions 13 Acknowledgements 13 References 13

We monitored sap flux density (v) diurnally in nine mature southeastern pine (Pinus spp.) trees with a thermal dissipation probe that spanned the sapwood radius. We found the expected pattern of high v near the cambium and decreasing v with depth toward the center of the tree; however, the pattern was not constant within a day or between trees. Radial profiles of trees were steeper earlier in the day and became less steep later in the day. As a result, time-dependent changes in the shape of the radial profile of v were sometimes correlated with daily changes in evaporative demand. As the radial profile became less steep, the inner xylem contributed relatively more to total tree sap flow than it did earlier in the day. We present a 3-parameter Gaussian function that can be used to describe the radial distribution of v in trees. Parameters in the function represent depth in the xylem from the cambium, maximum v, depth in the xylem where maximum v occurs, and the rate of radial change in v with radial depth (β). Values of β varied significantly between trees and with time, and were sometimes correlated with air vapor pressure deficit (D). We hypothesize that this occurred during periods of high transpiration when the water potential gradient became great enough to move water in the inner sapwood despite its probable high hydraulic resistance. We examined discrepancies among estimates of daily water use based on single-point, two-point and multi-point (i.e., every 20 mm in the sapwood) measurements. When radial distribution of v was not considered, a single-point measurement resulted in errors as large as 154% in the estimate of daily water use relative to the estimate obtained from a multi-point measurement. Measuring v at two close sample points (10 and 30 mm) did not improve the estimate; however, estimates derived from v measured at two distant sample points (10 and 70 mm) significantly improved the estimate of daily water use, although errors were as great as 32% in individual trees. The variability in v with depth in the xylem, over time, and between trees indicates that measurements of the radial distribution of v are necessary to accurately estimate water flow in trees with large sapwood areas.

Summary: Here, we investigated the effect of different heat‐wave intensities applied at two atmospheric CO₂ concentrations ([CO₂]) on seedlings of two tree species, loblolly pine (Pinus taeda) and northern red oak (Quercus rubra). Seedlings were assigned to treatment combinations of two levels of [CO₂] (380 or 700 μmol mol⁻¹) and four levels of air temperature (ambient, ambient +3°C, or 7‐d heat waves consisting of a biweekly +6°C heat wave, or a monthly +12°C heat wave). Treatments were maintained throughout the growing season, thus receiving equal heat sums. We measured gas exchange and fluorescence parameters before, during and after a mid‐summer heat wave. The +12°C heat wave, significantly reduced net photosynthesis (A_net) in both species and [CO₂] treatments but this effect was diminished in elevated [CO₂]. The decrease in A_net was accompanied by a decrease in F_v′/F_m′ in P. taeda and Φ_PSII in Q. rubra. Our findings suggest that, if soil moisture is adequate, trees will experience negative effects in photosynthetic performance only with the occurrence of extreme heat waves. As elevated [CO₂] diminished these negative effects, the future climate may not be as detrimental to plant communities as previously assumed.

Complementary laboratory and field experiments showed that theinternal transport of carbon dioxide (CO₂) in the xylemof trees is an important pathway for carbon movement. Carbon dioxidereleased by respiration dissolves in sap and moves upward in thetranspirational stream. The concentration of CO₂ in xylemsap can be up to three orders of magnitude greater than that foundin the atmosphere. In the present experiments, diffusion outwardof a portion of xylem‐transported CO₂ caused a substantialoverestimation of the apparent rate of stem and branch respiration.Rates of CO₂ efflux were linearly related to sap CO₂ concentration.Direct manipulations of xylem sap CO₂ concentration producedrapid and reversible changes in CO₂ efflux from stemsand branches, in some cases quadrupling the rate of efflux. Theseresults demonstrated that apparent rates of stem and branch respirationof trees are in large part a by‐product of the rate of CO₂ diffusionfrom xylem sap.