We developed a pioneering research facility (PTR-TOF-IRIS, Fig.1) coupling emergent technology from geosciences, such VOC mass spectrometry and 13CO2 isotope spectroscopy to resolve high temporal dynamics in VOCs and CO2 emissions. Combined with radically new approaches of 13C-position-specific feeding experiments of central metabolites, it enabled an unprecedented understanding of the metabolic linkage of plant CO2 and VOC emission. We gained novel insights into biochemical synthesis of VOC in response to environmental stress, such as the enhanced use of cytosolic pyruvate for plastidial isoprene biosynthesis under heat stress, and metabolic cross-talk between cytosolic and chloroplastic pathways for VOC biosynthesis. It enabled the quantification of the often-underestimated contribution of day-time CO2 emission during secondary metabolism and regulation of de novo and storage pool VOC emissions under stress.
This innovative setup enabled a series of climate change experiments, simulating e.g. heat waves and droughts on Mediterranean, temperate, boreal and tropical plant species. We further explored the impact of natural droughts during the severe 2018 heat wave, which significantly affected mid European forests. Recurrent hot drought shifted a pine forest to its tipping point, resulting in enhanced VOC emissions and significant tree mortality.
VOCO did set a new dimension with a world-wide first whole ecosystem labelling in a large-scale interdisciplinary drought experiment in the unique Biosphere 2 tropical rainforest (B2WALD campaign, Arizona, Fig. 2,3) with over 70 researchers covering molecular to ecosystem and atmospheric sciences. It opened new frontiers for assessing biogenic emissions of greenhouse gases and their isotopic compositions from different ecosystem components in an unparalleled 5-month drought experiment including a deep-water labelling during the recovery phase. B2WALD received substantial press, radio and TV coverage (> 40 reports; including Science Magazine News).
The overarching publication (Werner et al., 2021 Science, highlighted by an editorial) identifies significant flux dynamics as drought cascaded through different forest strata and the importance of diverse plant functional groups with different hydraulic strategies as key processes to increase ecosystem drought resistance. Drought-sensitive canopy trees strongly reduced water loss, but spared deep-water reserves until late drought thereby preventing rapid desiccation of the forest. Carbon allocation was down-regulated by increasing mean residence times, though fresh carbon investment in VOCs remained high, indicating their important functions. Atmospheric VOC dynamics traced increasing drought impact and dynamic soil-plant-atmosphere interactions. This has important implications for coupled climate-vegetation models, which mostly neglect diverse plant hydraulic responses and VOC dynamics, despite their impotent climate feedbacks. Notably, we achieved the first proof on the origin of independent regulation of different VOC enantiomers, i.e. the chiral versions of a VOC, emitted by plants (Byron et al. under revision in Nature). These deliver important information for global change related aspects, as these greenhouse gases can affect atmospheric chemistry and enhance global warming.
Thus, VOCO very successfully completed its objectives and went significantly beyond, delivering important new insights on key processes for ecosystem functioning from metabolic to ecosystem scale, which enhance our understanding of climate change impact and calls for advanced action to protect our pristine forest ecosystems revealing their important role for climate regulatory functions.
