Can reduced root hydraulic functioning and redistribution due to climate variability impact carbon and water cycling in trees: Establishing a direct link between plant root functioning and carbon fluxes

Background/Question/Methods

Deep root water uptake and hydraulic redistribution (HR) has been shown to play a major role in forest ecosystems during drought, but little is known about the impact of climate change on root-zone processes influencing HR and its consequences on water and carbon fluxes. Using data from two old growth sites of the western USA, three mature sites of the eastern USA, and simulations a process-based model, the objectives of this project were to show that HR can 1) mitigate the effects of soil drying on root functioning, and 2) have important implications for carbon uptake and net ecosystem exchange (NEE).

Results/Conclusions

At the dry old-growth ponderosa pine characterized by deep sandy soil, HR limited the decline in root hydraulic conductivity and increased dry season tree transpiration (T) by up to 30%, and such an increase impacted NEE through major changes in gross primary productivity (GPP). Deep-rooted trees did not necessarily translate into a large volume of HR unless soil texture allowed large water potential gradients to occur, as it was the case at the old-growth Douglas-fir/mixed conifer stand. At the Duke mixed hardwood forest characterized by a shallow clay-loam soil, modeled HR was low but not negligible, representing up to 10% of T, and maintaining root conductance high. At this site, in the absence of HR, it was predicted that annual GPP would have been diminished by 7-19%. At the coastal loblolly pine plantation, characterized by deep organic soil, HR limited the decline in shallow root conductivity by more than 50% and increased dry season T by up to 40%, and such an increase reduced NEE by 400 gC m^-2 yr^-1, showing that HR is an important mechanism for maintaining this ecosystem as a carbon sink. Under future climate conditions, HR was predicted to be reduced by 35%; reducing the resilience of trees to precipitation deficits. Under future conditions, T was predicted to stay the same at the Duke site, but to be marginally reduced at the loblolly pine plantation and slightly increased at the old-growth ponderosa stand. As a consequence, in all sites, water use efficiency was predicted to improve dramatically with future conditions.  We concluded that the predicted reductions in HR under future climate conditions are expected to play an important regulatory role in the land–atmosphere interaction by affecting the whole ecosystem water balance and thus the partitioning of net radiation between sensible and latent heat fluxes.