Factors contributing to the superior growth and N nutrition of 11-year-old lodgepole pine compared with Sitka spruce on a N-poor cedar-hemlock cutover

We tested several hypotheses to explain the superior growth and nitrogen (N) status of lodgepole pine (Pinus contorta Dougl. ex Laws. var. contorta) compared with Sitka spruce (Picea sitchensis (Bong.) Carrière) on a N-poor site by comparing N distribution, N retranslocation, rooting distribution, and mycorrhizal fungi in plots of 11-year-old trees on a cedar-hemlock cutover. Aboveground N content was nine times greater in pine than in spruce of the same age, and thus, we conclude that pine acquired more N during the 11 years than spruce. Greater N acquisition by pine was not related to rooting depth, as both species rooted primarily in the residual forest floor. There were differences in mycorrhizal fungal associates: a high proportion of pine roots were infected with Suillus-like fungi. Pine produced more aboveground biomass per unit N (388 compared with 292 g·g-1 in spruce) and distributed more N to young foliage. Nitrogen retranslocation efficiency (based on foliar N contents in July and October) was higher in pine (50-52%) than in spruce (24-36%). These characteristics all appear to contribute to pineʹs abilities to both acquire more N and use it more efficiently and, thus, outperform spruce on this N-poor site.