Linking tree physiology, carbon fluxes, and root growth dynamics under constraining soil water and atmospheric treatments

Combining carbon flux measurements with tree above- and belowground phenology enables a holistic assessment of plant functioning and ecosystem carbon balance under future climatic conditions. The stress caused by rising air temperature and drought affecting the asynchrony in trees above- and belowground phenology might cost more than the carbon gains, weakening future forest carbon sinks. 

Norway spruce seedlings were grown in organic soils (drained Histosol) in climate chamber separately in transparent boxes for 13 weeks in 2023. Ten trees were grown under ambient conditions based on June-July 2021 data, which reflects long term average weather conditions in the Estonian forest. Another ten under low air relative humidity (-10% of ambient) and ten under high temperature (+6°C) treatment, whereas both treatments are equivalent to +30% increase in water vapor deficit. The treatment period lasted for 30 days, whereas half of the trees had 65% of soil moisture from field capacity and other half experienced drought (45%). The recovery period with ambient conditions lasted for 20 days. The ecosystem gas exchange (NEE, Rs) was measured in four key time points during the experiment; photosynthesis, other physiological parameters, shoot length and fine root area was measured weekly. Destructive measurements such as biomass and fine root carbon exudation was measured at the end of the experiment.

Elevated air temperature caused a stronger carbon sink, although there was an increase in soil respiration. However, in the recovery phase the ecosystem-level gas exchange decreased and reached the same level as the ambient condition indicating that some of the physiological changes were strongly tied to the changes in temperature conditions. Soil moisture was a critical constraint to reduced photosynthesis and diminished root relative growth rate highlighting water limitation as a dominant stressor for both carbon assimilation and belowground development. Interestingly, low humidity showed positive effects on fine root growth compared to elevated temperature (but only under 65% of soil moisture), perhaps indicating a compensatory carbon allocation to belowground biomass, which enhances water uptake under drier climate. Meanwhile the aboveground growth increased significantly only under high temperature. Other parameters, including fine root carbon exudation, will be discussed in the context of tree ecosystem carbon flux.