Chemical Composition and Decomposition of Silver Birch Leaf Litter Produced under Elevated CO2 and O3

Abstract

Two field-growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO2 and O3 over three growing seasons (1999–2001). In each year, the nutrients and cell wall chemistry of naturally abscised leaf litter were analyzed in order to determine the possible CO2- and O3-induced changes in the litter quality. Also CO2 and O3 effects on the early leaf litter decomposition dynamics (i.e. decomposition before the lignin decay has started) were studied with litter-bag experiments (Incubation 1 with 1999 leaf litter, Incubation 2 with 2000 leaf litter, and Incubation 3 with 2001 leaf litter) in a nearby silver birch forest. Elevated CO2 decreased N, S, C:P and α-cellulose concentrations, but increased P, hemicellulose and lignin+polyphenolic concentrations, C:N and lignin+polyphenolic:N in both clones. CO2 enrichment decreased the subsequent decomposition of leaves of clone 4 transiently (in Incubations 1 and 2), whereas elevated CO2 effects on the subsequent leaf decomposition of clone 80 were inconsistent. In contrast to CO2, O3 decreased P concentrations and increased C:P, but both of these trends were visible in elevated O3 treatment only. O3-induced decreases in Mn, Zn and B concentrations were observed also, but O3 effects on the cell wall chemistry of leaf litter were minor. Some O3-induced changes either became more consistent in leaf litter collected during 2001 (decrease in B concentrations) or appeared only in this litter lot (decrease in N concentrations, decrease in decomposition at the end of Incubation 3). In conclusion, in northern birch forests elevated CO2 and O3 levels have the potential to affect leaf litter quality, but consistent CO2 and O3 effects on the decomposition process remain to be validated.