Influence of soil warming and N-addition on sap flux density and stem radius variation in boreal stands in Quebec, Canada

Climate change is projected to intensify the global hydrological cycle and increase temperatures. Excess N deposition could limit plant productivity because of a lack of plant-soil system efficiency with reduced water availability. However, the effects of soil warming and N-addition on tree water transport and water use remain poorly understood. This study aimed to quantify the effects of soil warming and N-addition associated with meteorological variables on the sap flux density and stem radius variation in black spruce (Picea mariana [Mill.] B.S.P.). From 2008 to 2014, we conducted a long-term experiment on mature trees growing in two stands (Bernatchez [BER] and Simoncouche [SIM]) at different latitudes to evaluate the effect of soil warming (H) and N-addition. During 2014, we investigated soil and plant water status between May and August, sap flux density from July to August and stem radius variation over the whole year. At the colder, northern site (BER), we detected a significant increase in sap flux density with the H-treatment, whereas no effect was observed at the warmer, southern site (SIM). At BER, diurnal sap flux density in H x N-addition treatment showed a significant effect in the afternoon near the peak of day compared with the other treatments. We found no significant effects of independent single or combined factors on soil and plant water status and stem radius variation at either site. Under climate change, black spruce at more northern sites could have a better water use and storage for wood productivity than those at more southern sites.