1. Respiration of Amazon tree stems greatly exceeds local CO 2 emission Alon Angert (1), Jan Muhr )2), Robinson Negron Juarez (3), Waldemar Alegria Muñoz (4), Guido Kraemer (4), Jeffrey Q. Chambers (5), Susan E Trumbore (2) References Angert, A., and Y. Sherer (2011), Determining the relationship between tree-stem respiration and CO2 efflux by  O 2 /Ar measurements, Rapid Communications in Mass Spectrometry, 25(12), 1752–1756. Barkan, E., and B. Luz (2003), High-precision measurements of 17O/16O and 18O/16O of O2 and O2/Ar ratio in air, Rapid Communications in Mass Spectrometry, 17(24), 2809-2814. Introduction Respiration in tree stems is an important component of forest carbon balance. The rate of CO 2 efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work has demonstrated that stem CO 2 efflux can either overestimate or underestimate respiration rate, because of emission or removal of CO 2 by transport in xylem water. Here we used the ratio between CO 2 efflux and O 2 influx in stems of tropical forest trees to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. Methods Conclusions Our observations point to “missing” respired CO 2. This can be explained by CO 2 carried upward in the transpiration stream Future work would focus on possible link of this mechanism to drought resilience O 2 uptake is a better measure of stem respiration Results and Discussion (1) The Hebrew University of Jerusalem, (2) MPI for Biogeochemistry, Jena, (3) Tulane University (4) UNAP, Iquitos, Peru (5) LBNL, Berkeley Study site The site was located at the Center for Research and Forest Learning (CIEFOR) of the National University of the Peruvian Amazon (UNAP) which is located 16 km southwest of the city of Iquitos, Peru. CIEFOR is centered over 3°49'53"N, 73°22'28"W, encompasses a forested area of 1300 ha. The mean annual rainfall is ~3000mm, and the maximum, minimum, and average temperatures are 26.3°C, 25.9°C, and 25.2°C, respectively. Respiration rate (  mol O 2 sec -1 m -2 ) Steady state model E C =g C  C E O =g O  O We define ARQ (Apparent Respiratory Quotient): ARQ ≡ - E C /E O ARQ=-0.76(  C /  O ) We found ARQ values of 0.66  0.18 in the stem chamber. These low ARQ values indicate that a large portion of respired CO 2 (~35%) is not emitted locally, and is probably transported upward in the stem. ARQ values of 0.21  0.10 were found by in-stem equilibration probes. These lower values may result from the proximity to the xylem water stream. In contrast, we found ARQ values of 1.00  0.13 for soil respiration. If the transported CO 2 is used in the canopy as a substrate for photosynthesis, it could serve as a mechanism that buffers the response of trees to changing CO 2 levels, and drought. ARQ (unitless) averaged over 9 trees Experimental Specially designed stem chambers (Angert and Sherer, 2011)ensured tight seal and air sampling air without pressure changes (by changing volume). We have installed 9 chambers on trees of 3 species, with varying growth rates and wood densities The CO 2 concentrations were measured in the lab by an IRGA (Licor-840A) connected to a circulating system, as described in Angert and Sherer (2011). The O 2 concentrations were calculated from the O 2 /Ar ratio (expressed as  O 2 /Ar) determined by mass-spectrometric analysis (Barkan and Luz, 2003). Soil air was also sampled next to the trees. Probes were used to sample in-stem air by equilibration.