Oren, R., Werk, K. S., Schulze, E. D., Meyer, J., Schneider, B. U., & Schramel, P. (1988).
Performance of two Picea abies (L.) Karst. stands at different stages of decline. VI. Nutrient concentration.
Oecologia, 77(2), 151-162. doi:10.1007/BF00379181.