Phosphorus supply influences heathland responses to atmospheric nitrogen deposition

On an upland moor dominated by pioneer Calluna vulgaris and with an understorey of mosses and lichens, experimental plots were treated with factorial combinations of nitrogen (N) at +0 and +20 kg N ha−1 yr−1, and phosphorus (P) at +0 and +5 kg P ha−1 yr−1. Over the 4-year duration of the experiment, the cover of the Calluna canopy increased in density over time as part of normal phenological development. Moss cover increased initially in response to N addition but then remained static; increases in cover in response to P addition became stronger over time, eventually causing reductions in the cover of the dominant Calluna canopy. Lichen cover virtually disappeared within 4 years in plots receiving +20 kg N ha−1 yr−1 and also in separate plots receiving +10 kg N ha−1 yr−1, but this effect was reversed by the addition of P. Phosphorus supply increased the cover of understorey mosses, which in turn caused decreases in the cover of the dominant Calluna canopy.