Respiration of Amazon tree stems greatly exceeds local CO 2 emission

1. Respiration of Amazon tree stems greatly exceeds local CO2 emission Alon Angert (1), Jan Muhr)2), Robinson Negron Juarez (3), WaldemarAlegriaMuñoz (4), Guido Kraemer (4), Jeffrey Q. Chambers (5), Susan E Trumbore (2) • (1) The Hebrew University of Jerusalem, (2) MPI for Biogeochemistry, Jena, (3) Tulane University (4) UNAP, Iquitos, Peru (5) LBNL, Berkeley Steady state model Results and Discussion Introduction Respiration in tree stems is an important component of forest carbon balance. The rate of CO2 efflux from the stem has often been assumed to be a measure of stem respiration. However, recent work has demonstrated that stem CO2 efflux can either overestimate or underestimate respiration rate, because of emission or removal of CO2 by transport in xylem water. Here we used the ratio between CO2 efflux and O2 influx in stems of tropical forest trees to better understand respiration in an ecosystem that plays a key role in the global carbon cycle. EC=gCDC EO=gODO We define ARQ (Apparent Respiratory Quotient): ARQ ≡ - EC/EO ARQ=-0.76(DC/DO) Methods 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. ARQ (unitless) averaged over 9 trees We found ARQ values of 0.660.18 in the stem chamber. These low ARQ values indicate that a large portion of respired CO2 (~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 CO2 is used in the canopy as a substrate for photosynthesis, it could serve as a mechanism that buffers the response of trees to changing CO2levels, and drought. 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 CO2 concentrations were measured in the lab by an IRGA (Licor-840A) connected to a circulating system, as described in Angert and Sherer(2011). The O2 concentrations were calculated from the O2/Ar ratio (expressed as O2/Ar) determined by mass-spectrometric analysis (Barkanand Luz, 2003). Soil air was also sampled next to the trees. Probes were used to sample in-stem air by equilibration. Respiration rate (mmol O2 sec-1 m-2) • Conclusions • Our observations point to “missing” respired CO2. • This can be explained by CO2 carried upward in the transpiration stream • Future work would focus on possible link of this mechanism to drought resilience • O2uptake is a better measure of stem respiration References Angert, A., and Y. Sherer(2011), Determining the relationship between tree-stem respiration and CO2 efflux by dO2/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.