Understanding drought dynamics of carbon and water fluxes from leaf to ecosystem scales in an experimental tropical forest

Ecosystem response to drought present a complex interplay between regulation at the leaf, plant, and ecosystem scale as well as soil-plant-atmosphere interactions and feedbacks. While single leaf or tree fluxes can be continuously measured, other processes such as changes in below ground carbon allocation or shifts in root-water uptake depth under drought are difficult to assess.

To trace ecosystem scale interactions, we implemented a whole-ecosystem labelling approach in the Biosphere 2 Tropical Rainforest. In the Biosphere 2 Water, Atmosphere, and Life Dynamics (B2-WALD) experiment, we applied an ecosystem scale drought and tracing carbon allocation and dynamics of volatile organic compounds (BVOC), CO₂ and H₂O fluxes and their isotopes from leaf, root, trunks, soil and atmospheric scales.

Drought sequentially propagated through the vertical forest strata, with a rapid increase in vapor pressure deficit in the top canopy layer and early dry-down of the upper soil layer but delayed depletion of deep soil moisture. Gross primary production (GPP), ecosystem respiration (Reco), and evapotranspiration (ET) declined rapidly during early drought and severe drought. Interactions between plants and soil led to distinct patterns in the relative abundance of atmospheric BVOC concentrations as the drought progressed, serving as a diagnostic indicator of ecosystem drought stress.

Ecosystem ¹³CO₂-pulse-labeling showed that drought enhanced the mean residence times of freshly assimilated carbon, indicating down-regulation of carbon cycling velocity and delayed transport form leaves to trunk and roots. Despite reduced ecosystem carbon uptake and total VOC emissions, plants continued to allocate a similar proportion of fresh carbon to de novo VOC synthesis, as incorporation of 13C into both isoprene and monoterpenes remained high.

A ²H-labelled deep-water label during severe drought provided a unique opportunity to evaluate transit times and legacy effects during the recovery phase. Combined with an in situ approaches allowed to monitor the isotopic composition in soils, tree xylem and transpiration at high temporal resolution. Drought-sensitive canopy trees strongly reduced water fluxes during early drought, while drought-tolerant trees increased their relative contribution to total water flux. Interestingly, all deep-rooted canopy trees taped into deep-water reserves, but spared deep water reserves until severe drought and exhibited long transit times of 2-6 weeks until d2H-labelled water was transpired. This was partially due to stem water refill exceeding the onset of transpiration after drought release.

These data highlight the importance of quantifying drought impacts on forest functioning beyond the intensity of (meteorological) drought, but also taking dynamics response of hydraulic regulation of different vegetation and soil compounds into account. Such data set can be used for carbon and water partitioning from the metabolic to ecosystem scale and help disentangling belowground processes to better parameterize models.

Werner et al. 2021, Science 374, 1514 (2021), DOI: 10.1126/science.abj6789