Effects of drought on nitrogen uptake and carbon dynamics in trees.

Research on drought impact on tree functioning is focussed primarily on water and carbon (C) dynamics. Changes in nutrient uptake might also affect tree performance under drought and there is a need to explore underlying mechanisms. We investigated effects of drought on (a) in situ nitrogen (N) uptake, accounting for both, N availability to fine roots in soil and actual N uptake, (b) physiological N uptake capacity of roots and (c) the availability of new assimilates to fine roots influencing the N uptake capacity using 15N and 13C labelling. We assessed saplings of six different tree species (Acer pseudoplatanus L., Fagus sylvatica L., Quercus petraea (Mattuschka) Liebl., Abies alba Mill., Picea abies (L.) H.Karst. and Pinus sylvestris L.). Drought resulted in significant reduction of in situ soil N uptake in deciduous trees accompanied by reduced C allocation to roots and by a reduction in root biomass available for N uptake. Although physiological root N uptake capacity was not affected by drought in deciduous saplings, reduced maximum ammonium but not nitrate uptake was observed for A. alba and P. abies. Our results indicate that drought has species-specific effects on N uptake. Even water limitations of only 5 weeks as assessed here can decrease whole-plant inorganic N uptake, independent of whether the physiological N uptake capacity is affected or not.

Recovery of trees from drought depends on belowground sink control.

Climate projections predict higher precipitation variability with more frequent dry extremes(1). CO2 assimilation of forests decreases during drought, either by stomatal closure(2) or by direct environmental control of sink tissue activities(3). Ultimately, drought effects on forests depend on the ability of forests to recover, but the mechanisms controlling ecosystem resilience are uncertain(4). Here, we have investigated the effects of drought and drought release on the carbon balances in beech trees by combining CO2 flux measurements, metabolomics and (13)CO2 pulse labelling. During drought, net photosynthesis (AN), soil respiration (RS) and the allocation of recent assimilates below ground were reduced. Carbohydrates accumulated in metabolically resting roots but not in leaves, indicating sink control of the tree carbon balance. After drought release, RS recovered faster than AN and CO2 fluxes exceeded those in continuously watered trees for months. This stimulation was related to greater assimilate allocation to and metabolization in the rhizosphere. These findings show that trees prioritize the investment of assimilates below ground, probably to regain root functions after drought. We propose that root restoration plays a key role in ecosystem resilience to drought, in that the increased sink activity controls the recovery of carbon balances.