Artificially increasing atmospheric nitrogen deposition does not stimulate growth of Picea mariana (Mill.) BSP in the Canadian boreal forest

Forests provide essential ecosystem services we rely on, including climate regulation and mitigation, biodiversity conservation, clean water and air supply, food and energy production, and improvement of human health and well-being. However, their functioning is significantly altered by different global change drivers related to human activities, such as increasing atmospheric CO2 and pollution, with particular reference to the increase in reactive nitrogen (N) in the atmosphere and their deposition back to the biosphere as N deposition. Global change drivers may particularly affect boreal forests, given that low temperatures, shorter growing seasons, and nutrient (particularly N) limitations strongly limit tree growth in this biome. This study tested the hypothesis that tree growth in boreal forests would increase with the simulated increase in N deposition. To this aim, a dendrochronological study was carried out in a novel manipulation experiment, where an increase in N deposition was simulated for five years (2008-2013) through above-canopy rather than soil fertilization (as in all previous manipulation experiments in this biome). The experiment was carried out in two Picea mariana (Mill.) BSP forests (BER, tree age 130 and SIM, tree age 90), both including treatment plots with aerial N misting (n=3) and control (receiving only water, n=3). From each plot, four wood cores were sampled from the single tree subjected to the N additions for a total of n=3 trees per treatment. We did not observe any significant effect of the added N on tree growth. This result could be related to the short duration of the experiment and the low doses applied (twice as much as the ambient deposition), which did not significantly contribute to altering the growth of a slow-growing tree species like Picea mariana. Moreover, during the last year of the treatment, both stands were interested by the Choristoneura fumiferana attack that likely caused canopy defoliation, thus limiting the potential foliar N uptake, but also photosynthesis and growth. It is likely that the added N reached the soil directly (during the spraying) or indirectly (through litterfall), with a positive effect on the rhizosphere. Conducting long-term manipulation experiments in forest ecosystems is undoubtedly complex, but it is crucial to enhance our understanding of tree growth dynamics and forest carbon sequestration, as well as how they are affected by global change divers and biotic stress.