A mesocosm study: Carbon release dynamics in young alluvial oak trees affected by flooding stress and elevated temperature

Estuarine alluvial forests, which are characterized by short, intense flooding events, are recognized as global carbon (C) hotspots. However, predicted increases in flooding intensity and prolonged summer droughts due to climate change may alter the timing, quantity and quality of C transfer from alluvial trees to soil, with potential consequences for the C sink strength of alluvial forests. The surplus C hypothesis suggests that trees assimilate a surplus of photosynthates at the onset of resource limitation (Prescott et al. 2020) and that, consequently, frequent shifts from hypoxia to water drainage (or even summer drought) may result in particular high levels of surplus C in alluvial trees, which can be released by alternative root respiration or by root exudation into the rhizosphere. To test this hypothesis for alluvial trees, we conducted an outdoor mesocosm study with young pedunculate oak (Quercus robur L.) trees exposed to different climate change scenarios and examined the consequences for the release of surplus C by alternative root respiration and root exudation. Specifically, we simulated flooding events and increases in average temperature in a full factorial experiment. Over the course of one growing season aboveground performance of trees was monitored, fine roots were sampled to measure alternative root respiration, determined from the isotopic discrimination against ¹⁸O in O₂, and root exudates were repeatedly collected with the culture-based cuvette method, quantified as TOC and later analyzed by LC-MS. We observed that flooding reduced the C sink strengths of aboveground and belowground growth and biomass by up to 40%, independent from temperature. In my presentation I will focus on C release dynamics via root exudation and root respiration and discuss the potential role of flooding and temperature rise on surplus C in alluvial forest trees, and potential consequences for root-microbiome interactions. Our findings will contribute to a broader understanding of the C sink strength of estuarine alluvial forests under climate change.

Prescott CE, Grayston SJ, Helmisaari H-S, Kaštovská E, Körner C, Lambers H, Meier IC, Millard P, Ostonen (2020) Surplus carbon drives allocation and plant–soil interactions. Trends in Ecology & Evolution 35: 1110-1118.