Current predictions of forest's functioning under drought and thus forest mortality under future climatic conditions are constrained by a still incomplete picture of the trees' physiological reactions that allows some trees to survive drought periods while others succumb. Concerning the effects of drought on the carbon balance, interactions between abiotic factors and pests and diseases as well as the interaction between carbon and nutrient balances as factors affecting drought-induced impairment of trees’ functioning are not well understood.
To assess the interaction between carbon allocation and nitrogen nutrition, we performed a short-term pulse labelling study using 13C-CO2 and 15N- NO3 on mature (100-year-old) Scots pine under drought conditions. The field site is located in a xeric mature Scots pine forest, called Pfynwald, at the bottom of the Rhone valley, one of the driest inner-Alpine valleys of the European Alps. To study the performance of mature Scots pine (Pinus sylvestris L.) under chronic drought conditions in comparison to drought release, a controlled long-term, irrigation experiment has been set up in 2003 and will be continued until 2022. The amount of irrigation corresponds to additional 700 mm/yr. Trees in the control plots grow under naturally dry conditions (518 mm/yr). We exposed whole canopies of trees to 3-hour pulses of 13CO2 to investigate the short-term carbon dynamics and the allocation of new assimilates within the plant. Moreover, we incubated fine roots with 15N nitrate solution to assess the effect of drought stress on N uptake capacity.
Results/Conclusions
We observed a distinct difference in the carbon and nitrogen dynamics, and allocation pattern between drought stressed and irrigated pine trees.
Irrigated plots assimilated more CO2 in relation to drought stressed and still allocated significant amounts of the new assimilates to the roots. Instead, only small amounts of 13C labelled new assimilate arrived in the roots in the drought treatments. Moreover, the mean residence time of recent assimilates under drought stress was found to be 2.5-fold of that of the irrigated trees. 15N uptake was reduced in the drought treatments. We hypothesis that the reduced assimilate allocation to the roots and thus the reduced availability of energy adversely affect the nutrient uptake and balance of trees under drought. Impairment of carbon uptake and allocation together with reduced nutrient uptake might contribute on the long-term to the impairment of tree functioning and might play a central role in predisposing trees to mortality.