Tracing recent carbon from photosynthesis to stem and soil respiration in an experimental tropical rainforest in response to drought

Tropical rainforests play a major role in the terrestrial carbon (C) cycle. However, to date little is known about the mechanisms and processes controlling C fluxes in tropical forests. Within the C cycle of a forest, trees allocate a substantial amount of photoassimilates belowground, and fuel respiration by stems, roots and microorganisms. This link between assimilation and respiration represents a significant pathway by which assimilated C is quickly returned to the atmosphere. However, the nature of this coupling, including the speed of above- to below-ground C allocation and the proportion of rapidly metabolized assimilates is yet unknown for mature tropical rainforest systems. Furthermore, the role of tree species and size and the relative roles of canopy versus understory plants are still unresolved.

Drought spells can exert a major control on the C balance of tropical forest ecosystems by altering C uptake, the partitioning of C and the dynamics of C allocation and belowground utilization. As such responses are difficult to measure in tropical rainforest, the consequences of drought for the dynamics of recent C in stem and soil respiration in this biome remain unclear.

To assess and quantify these processes, we made use of the Tropical Rain Forest at the Biosphere 2 research complex in Arizona, US. This infrastructure provides unique opportunities to study drought effects on the C dynamics in a controlled environment. We simulated a drought spell for eight weeks and continuously measured stem and soil CO₂ fluxes using isotope laser spectroscopy before and during the drought as well as during the subsequent rewetting period. Our study is part of a large-scale experiment that aims to disentangle C- and water-cycle processes underpinning ecosystem responses to drought from a molecular to an ecosystem-scale level, with particular focus on plant-soil and plant-atmosphere interfaces.

We performed two canopy-scale ¹³CO₂ pulse labeling campaigns under ambient environmental conditions and towards the end of the experimental drought. We traced the allocation dynamics of recently assimilated C to soil respiration and to stem respiration of dominant tree species. First results show that the allocation of assimilates from the canopy to soil-respired CO₂ took several days and was affected by tree size and species identity. Under drought, tracer efflux from stems and soils was  slowed down, with strong species-specific differences. Our results will allow novel insights into the combined effects of tree size, species identity and drought on the allocation dynamics and respiratory utilization of photoassimilates in tropical rainforest.