Whole ecosystem 13CO2 and 2H2O and metabolite labeling to trace drought response from leaf/roots to ecosystem scale and investigate the role of deep-water reserves during drought recovery

Severe droughts endanger forest worldwide, but how plant responses drive ecosystem water, carbon and volatile organic compound (VOC) fluxes remains poorly understood. To disentangle complex ecosystem dynamics, we imposed a 9.5-week drought on the Biosphere 2 tropical rainforest (B2WALD experiment [1]). We continuously measured isofluxes of ecosystem exchange, soil and leaf H₂O, CO₂ and BVOCs, over 5 months and applied an ecosystem ¹³CO₂-pulse during pre-drought and drought. Leaves and roots of different species were labelled with position-specific pyruvate. Finally, a ²H-labelled deep-water labelling provided a unique opportunity to evaluate legacy effects during recovery.

Ecosystem ¹³CO₂-pulse-labeling showed enhanced mean residence times of freshly assimilated carbon during drought and down-regulation of carbon transport to trunk and roots. Plants continued to allocate a similar proportion of fresh carbon to de novo VOC synthesis, demonstrating the fundamental role of VOC in protecting plants from heat stress and photooxidative damage. Soil VOC uptake increased immediately upon rain rewetting. Interestingly, all canopy trees taped into deep water, but spared deep-water reserves until severe drought and exhibited long transit times before transpiring d²H-labelled water. These data highlight the importance of taking dynamics ecophydrological regulations and soil-plant-atmosphere interactions into account.

 [1] Werner et al. 2021, Science 374, DOI: 10.1126/science.abj6789