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In recent studies, the ratio of tree stem CO 2  efflux to O 2  influx has been defined as the apparent respiratory quotient (ARQ). The metabolism of carbohydrates, the putative respiratory substrate in trees, is expected to yield an ARQ of 1.0. However, previous studies have reported ARQ values ranging between 0.23 and 0.90. These interesting results may indicate internal transport of respired CO 2  within stems; yet no simple field applicable methods for ARQ measurement have been available. Here, we report on the assembly of a closed circulating system called ‘ Hampadah ’, which uses CO 2  and O 2  analyzers to measure air samples from stem chambers. We tested the performance of the  Hampadah  with samples from 36 trees ( Tetragastris panamensis  (Engl.) Kuntze). Additionally, we showed the feasibility of measuring ARQ directly from stem chambers, using portable CO 2  and O 2  sensors, in both discrete and continuous modes of operation. The  Hampadah  measurement proved to be consistent with CO 2  gas standards ( R   2  = 0.999) and with O 2  determined by O 2 /Ar measurements with a mass spectrometer ( R   2  = 0.998). The  Hampadah  gave highly reproducible results for ARQ determination of field samples (±0.01 for duplicates). The portable sensors measurement showed good correlation with the  Hampadah  in measuring CO 2 , O 2  and ARQ ( n  = 5,  R   2  = 0.97, 0.98 and 0.91, respectively). We have demonstrated here that the  Hampadah  and the sensors’ methods enable accurate ARQ measurements for both laboratory and field research.

Measuring the ratio of CO 2 efflux to O 2 influx in tree stem respiration