Warming and ozone exposure effects on silver birch (Betula pendula Roth) leaf litter quality, microbial growth and decomposition

Abstract

Climate warming is expected to accelerate decomposition in boreal forests, but the concomitant effects of tropospheric ozone (O3), a phytotoxic greenhouse gas, alone and in combination with warming, are poorly understood. We studied how these two climatic factors affect leaf litter decomposition of two silver birch genotypes. We used field exposure for growing saplings and native and reciprocal transplant experiments for litter incubation to disentangle environmental and litter quality mediated effects of temperature and O3 on litter mass loss. We analysed litter C% and N% and microbial biomass (using qPCR) in fresh litter and after 217 and 257 days of incubation. Warming decreased fresh litter C% and N% and bacterial DNA, whereas elevated O3 increased N% and bacterial and fungal DNA, equally for both genotypes. In contrast, most effects on microbes during litter incubation varied between the two genotypes. Warming effects on microbes were mainly environmental, but despite having effects on litter quality and microbial growth, warming and O3 both had only weak or no effects on litter mass loss. Litter quality and microbial growth in northern birch stands are likely to change due to warming and O3 exposures, but effects on litter decomposition rate may remain weak.