Effects of nitrogen and phosphorus fertilization on the growth and nutrition of hybrid poplars on Vancouver Island

The effects of nitrogen (N) and phosphorus (P) rates, P source, and method of P application were tested on growth and nutrition of four clones of hybrid poplar (Populus trichocarpa Torr. & Gray x P. deltoides Bartr. ex Marsh or P. trichocarpa x P. maximowiczii A. Henry) in a plantation on Vancouver Island. Treatments were applied shortly after planting. Nitrogen (0, 500 kg N ha(-1)) was supplied as ammonium sulfate (AeS); half of the AS was added at the start of the second growing season. Phosphorus (0, 100, 200 kg P ha(-1) ) was supplied as triple super phosphate (TSP), rock phosphate (RP), or diammonium phosphate (DAP) and banded or broadcast. Over four growing seasons, stem volumes increased with AS and P additions. The effects of AS and P each were greatest when the other was also added. Volume was greater when P was applied at 100 kg ha(-1) than in the P control and did not significantly increase further at 200 kg P ha(-1). Phosphorus additions were more effective when added as TSP or DAP than when added as RP, but only in P100 and when banded. Banding increased volume when P was applied at 200 kg ha(-1), but increases were significant only for DAP fertilized trees. Roto-tilling associated with the banding treatment also increased volume when AS and P were not added. Rankings of clones with respect to stem volume varied with N supply and changed over time.