Ecohydrological Responses of Different Functional Groups Driving Ecosystem Carbon Cycling under Experimental Climate Change Drought

Carbon and water cycling are tightly linked in forest ecosystems but severe droughts bare the potential to disrupt the sensitive balance and ecohydrological interactions between different species in forest ecosystems. To unravel complex ecosystem carbon-water dynamics we imposed a 9.5-week drought on the Biosphere 2 tropical rainforest, a thirty-year old enclosed forest. We traced ecosystem scale interactions through a whole-ecosystem 13C and 2H-labelling approach in the Biosphere 2 Tropical Rainforest, the B2 Water, Atmosphere, and Life Dynamics (B2WALD) experiment. We analysed total ecosystem exchange, soil and leaf fluxes of H₂O, CO₂ and BVOCs, and their stable isotopes over five months. To trace changes in soil-plant-atmosphere interactions we labelled the ecosystem with a ¹³CO₂-isotope and investigated the importance of deep water sources under drought by 2H-labelling at the end of the drought. The tropical rainforest exhibited highly dynamic, non-linear responses during both dry-down and rewetting phases.

Drought sequentially propagated through the vertical forest strata, with a rapid increase in vapor pressure deficit, the driving force of tree water loss, in the top canopy layer and early dry-down of the upper soil layer but delayed depletion of deep soil moisture. This induced a two-phase response of ecosystem fluxes: gross primary production (GPP), ecosystem respiration (Reco), and evapotranspiration (ET) declined rapidly during early drought and moderately under severe drought.

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. Ecosystem carbon and water fluxes were determined by different ecohydrological responses of the dominant plant functional groups: while drought sensitive canopy trees dominated total ecosystem water fluxes under well-watered conditions, they showed the largest decline in response to top-soil moisture decline. Drought tolerant canopy trees exhibited lower fluxes but also higher resistance to soil water decline. Interestingly, all dominant canopy trees had access to deep water reserves, serving as a crucial water source during drought but not sustaining high transpiration rates.

Recovery of ecosystem carbon and water fluxes was slow after drought release, which reflected the by long water transit times within the soil-plant-atmosphere system. Thus, we found highly diverse responses of carbon and water fluxes, driven by the interplay of hydraulic regulation of different vegetation compounds and ecohydrological feedbacks in the forest. This study highlights the importance of ecohydrological responses for overall ecosystem resilience and carbon sequestration potential, providing valuable insights into the complex interplay between climate change, water availability, and carbon cycling in terrestrial ecosystems. We need to develop a comprehensive understanding of the multifaceted ecohydrological factors shaping ecosystem responses to climate change, with implications for sustainable ecosystem management and carbon mitigation strategies.

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