Annual and monthly variations in litter macronutrients of three subalpine forests in western China

Macronutrients (N, P, K, Ca, Mg, and S) in litter of three primarily spruce (Picea purpurea Masters) (SF), fir (Abies faxoniana Rehder & E. H. Wilson) (FF), and birch (Betula platyphylla Sukaczev) (BF) subalpine forests in western China were measured to understand the monthly variations in litter nutrient concentrations and annual and monthly nutrient returns via litterfall. Nutrient concentration in litter showed the rank order of Ca > N > Mg > K > S > P. Monthly variations in nutrient concentrations were greater in leaf litter (LL) than other litter components. The highest and lowest concentrations of N, P, K, and S in LL were found in the growing season and the nongrowing season, respectively, but Ca and Mg were the opposite. Nutrient returns via litterfall showed a marked monthly pattern with a major peak in October and one or two small peaks in February and/or May, varying with the element and stand type, but no marked monthly variations in nutrient returns via woody litter, reproductive litter, except in May for the BF, and moss litter. Not only litter production but also nutrient concentration controlled the annual nutrient return and the monthly nutrient return pattern. The monthly patterns of the nutrient concentration and return were of ecological importance for nutrient cycling and plant growth in the subalpine forest ecosystems.