How nitrogen deposition hampers common juniper regeneration in heathlands

Aims: Common Juniper (Juniper communis) populations in western Europe are rapidly declining due to a lack of regeneration, which has been linked to nitrogen (N) deposition. However, the mechanisms by which N deposition affects juniper are not clear. Nitrogen deposition can lead to N eutrophication and soil acidification, which cause nutrient leaching and metal mobilisation with potential negative impacts on juniper regeneration. We investigated associations among soil element concentration (soil [E]), needle element concentration (needle [E]) and plant performance in terms of height growth and viable seed production. Methods: We sampled adult females and juveniles across 6 natural heathland areas in the Netherlands and Belgium, and applied linear mixed models for eleven elements to test associations among plant performance, soil [E], and needle [E]. Results: Soil [E] was reflected in needle [E], which subsequently was associated with juniper growth. However, direct associations between soil [E] and growth were absent. Seed viability was positively associated with soil Mg concentration, marginally with Ca and needle Ca and K concentrations, and negatively with soil NO₃ and needle N, S and Zn concentrations. Generally seed viability, needle Ca and K concentrations were low. Soil Al/P ratio was a better predictor for needle P than soil P concentration indicating that Al inhibits P uptake. Conclusion: We conclude that 1) N eutrophication reduces seed viability, but increases growth, 2) nutrient leaching reduces nutrient uptake, seed viability and subsequently growth, and 3) Al mobilisation reduces P uptake and indirectly growth. N deposition amplifies these mechanisms and therefore inhibits regeneration and survival of juniper populations in western Europe.