Does atmospheric nitrogen deposition cause critical nutrient imbalances in temperate tree regeneration?

Key messageNitrogen deposition and precipitation analyzed along independent gradients did not cause major foliar nutrient imbalances that affected tree health in temperate tree regeneration of Fagus sylvatica, Pseudotsuga menziesii and Abies alba.AbstractWhile drought tolerance receives great attention in the scope of climate change research, the response of trees to high N deposition, which is also part of global change, has remained less studied. We investigated 45 temperate forest stands across Germany along a gradient of atmospheric N deposition of 22 kg N ha-1 a-1 (range 10-32 kg N ha-1 a-1) and analyzed foliar nutrient concentrations and ratios in saplings of European beech, Douglas fir, and silver fir. We also measured relative shoot length increment and (as a proxy for intrinsic water use efficiency, WUEi, and stomatal conductance, gs) foliar delta 13C signatures to detect potential effects of nutrient imbalances on productivity and plant water relations. Although all species responded on deposition with increased foliar N concentrations, reduced foliar nutrient concentrations were only observed for P, but not K, Ca, Mg, and Fe. The effect on foliar P concentrations was only observed at sites with low mean annual precipitation (range 565-1690 mm). Reduced growth was not observed. Low foliar P concentrations were associated with high gs in Douglas fir, which poses a potential risk under dry conditions and is consistent with the literature, but lacks a mechanistic explanation. The lack of nutrient imbalance at high N supply can at least partly be explained by the relatively low absolute amounts of nutrients needed by the small-sized tree regeneration.