Biomass, productivity, and nutrient-use efficiency of aboveground vegetation in four Rocky Mountain coniferous forests

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Abstract
Aboveground biomass, annual production, and internal nitrogen and phosphorus dynamics of vegetation were compared among a 90-year-old Pinuscontorta Loudon forest, a 120-year-old Piceaglauca (Moench) Voss–P. contorta forest, a 350-year-old Piceaengelmannii Parry ex Engelm.–Abieslasiocarpa (Hook.) Nutt. forest, and a 13-year-old P. contorta stand in the Rocky Mountains of southwestern Alberta. Aboveground biomass of vegetation ranged from 109 to 203 t•ha−1, while aboveground net primary productivity ranged from 4.4 to 5.3 t•ha−1•year−1 in the mature forests. Approximately 30% of the N and 20–40% of the P in ground vegetation were reabsorbed during senescence; 40–50% of the N and 50–80% of the P were reabsorbed from senescing tree foliage. Annual uptake of nutrients (production minus reabsorption) was between 1.8 and 2.2 g•m−2•year−1 for N and 0.2–0.4 g•m−2•year−1 for P. Efficiency of nutrient use (milligrams of new biomass produced per milligram of nutrient taken up in 1 year) ranged from 249 to 262 for N and 1604 to 2355 for P in the mature forests, and 72 and 642, respectively, in the young pine stand. Both N and P were used very efficiently in the pine forest and relatively inefficiently in the spruce–pine forest, reflecting differences in the inherent nutrient-use efficiency of these tree species. In the spruce–fir forest, N was used less efficiently and P more efficiently than in other forests, in response to lower phosphorus availability in this forest. Differences in nutrient-use efficiency of vegetation were related to differences in the amount of biomass produced per unit amount of N or P taken up, and not to differences in efficiency of N or P reabsorption.