Evaluating the potential of oxygen isoscapes for tropical timber tracing

<p dir="ltr">Independent verification of timber origin is needed to enforce legislation aimed at combatting illegal tropical<br>timber trade. A potential technique is tracing back the stable isotope signal preserved in wood samples, but the<br>scarcity of reference data currently hampers its operationalization. This can be overcome by creating isoscapes.<br>Here we develop continental isoscapes (at 0.5◦ resolution) for five tropical timbers based on wood δ18O ratios<br>and assess their potential for timber tracing. We compiled a pantropical database of δ18O measurements from<br>712 trees in 20 countries. We tested effects of δ18O in rainfall, potential evapotranspiration (PET), temperature<br>and precipitation on wood δ18O and used these to develop isoscapes based on quantile regression forests. A first<br>indication of the tracing potential of these isoscapes was tested in leave one out cross validation (LOOCV) an-<br>alyses. Across the five isoscapes, ranges in wood δ18O values (10th-90th percentile) averaged 3.9 ‰ and δ18O<br>differences increased with distance. Yet local variability in wood δ18O was substantial compared to large-scale<br>variability. The LOOCV analysis showed that the actual origin was included in the probable origin for<br>59–79 % of the cases. The area of probable origin was large, however, suggesting a low spatial precision of<br>assignment. This study finds limited support for a potential to use wood oxygen isoscapes for tropical timber<br>tracing within continents. Necessary future steps in timber isotope tracing include improving regional repre-<br>sentation, conducting similar analyses for other isotopes, rigorous testing of species differences and conducting<br>blind sample tests.</p>