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

Two field-growing silver birch (Betula pendula Roth) clones (clone 4 and 80) were exposed to elevated CO^sub 2^ and O^sub 3^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 CO^sub 2^- and O^sub 3^-induced changes in the litter quality. Also CO^sub 2^ and O^sub 3^ 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 CO^sub 2^ 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. CO^sub 2^ enrichment decreased the subsequent decomposition of leaves of clone 4 transiently (in Incubations 1 and 2), whereas elevated CO^sub 2^ effects on the subsequent leaf decomposition of clone 80 were inconsistent. In contrast to CO^sub 2^, O^sub 3^ decreased P concentrations and increased C:P, but both of these trends were visible in elevated O^sub 3^ treatment only. O^sub 3^-induced decreases in Mn, Zn and B concentrations were observed also, but O^sub 3^ effects on the cell wall chemistry of leaf litter were minor. Some O^sub 3^-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 CO^sub 2^ and O^sub 3^ levels have the potential to affect leaf litter quality, but consistent CO^sub 2^ and O^sub 3^ effects on the decomposition process remain to be validated.[PUBLICATION ABSTRACT]