Nutrient Dynamics of Aboveground Detritus in Lodgepole Pine (Pinus contorta ssp. latifolia) Ecosystems, Southeastern Wyoming

Abstract

Storage and fluxes on N, P, Ca, Mg, and K in aboveground detritus were measured in six contrasting lodgepole pine (Pinus contorta ssp. latifolia) stands in southeastern Wyoming. Litterfall was predominantly leaves (67—80%) in 80—100 yr old stands, while woody litter was more important in an older stand (240 yr old). Leaf litter nutrient concentrations were very low compared with other pine forests, particularly for N (0.40% dry mass). Dry mass loss from decomposing leaf litter was slow (15%/yr in first 2 yr), and summer rates did not differ significantly from winter rates beneath the insulating snowpack. Significant amounts of N, P, and Ca were added to decomposing leaves during the first winter, and N and Ca addition continued for 2 yr. Potassium and magnesium were rapidly lost from decomposing leaves. Rates of mass and nutrient loss from decomposing bark, twigs, and cones were comparable to those observed in other studies of temperate—zone forests. Mass loss from decaying bole wood appeared to be exponential through 40 yr, with an average decay coefficient (k) of 0.016, which is comparable to that in other cold temperate forests. Nitrogen content of decaying boles doubled between 30 and 55 yr following tree death, while smaller additions of P, Ca, K, and Mg also were noted. Relatively large accumulations of organic matter and nutrients were observed in the forest floor, leading to very high steady—state residence times for dry mass (mean = 18 yr), N (54 yr), P (39 yr), Ca (35 yr), Mg (21 yr), and K (18 yr). Deadfall contributed by the present forest generation was a minor component of the aboveground detritus except in an old—age stand and in a dense, self—thinning forest site. In contrast, dead wood inherited from the previous forest generation (killed by fire) was a major detrital storage component, exceeding forest floor mass by several—fold in 80—100 yr old stands. High nutrient immobilization in the dead wood led to storage values which were similar to those of the forest floor in these stands.