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

Background and aims Climate warming is expected to accelerate decomposition in boreal forests, but the concomitant effects of tropospheric ozone (O-3), 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. Methods 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 O-3 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. Results Warming decreased fresh litter C% and N% and bacterial DNA, whereas elevated O-3 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 O-3 both had only weak or no effects on litter mass loss. Conclusions Litter quality and microbial growth in northern birch stands are likely to change due to warming and O-3 exposures, but effects on litter decomposition rate may remain weak.Peer reviewe

Intrapopulation Genotypic Variation of Foliar Secondary Chemistry during Leaf Senescence and Litter Decomposition in Silver Birch (Betula pendula)

Abundant secondary metabolites, such as condensed tannins, and their interpopulation genotypic variation can remain through plant leaf senescence and affect litter decomposition. Whether the intrapopulation genotypic variation of a more diverse assortment of secondary metabolites equally persists through leaf senescence and litter decomposition is not well understood. We analyzed concentrations of intracellular phenolics, epicuticular flavonoid aglycones, epicuticular triterpenoids, condensed tannins, and lignin in green leaves, senescent leaves and partly decomposed litter of silver birch, Betula pendula. Broad-sense heritability (H-2) and coefficient of genotypic variation (CVG) were estimated for metabolites in senescent leaves and litter using 19 genotypes selected from a B. pendula population in southern Finland. We found that most of the secondary metabolites remained through senescence and decomposition and that their persistence was related to their chemical properties. Intrapopulation H-2 and CVG for intracellular phenolics, epicuticular flavonoid aglycones and condensed tannins were high and remarkably, increased from senescent leaves to decomposed litter. The rank of genotypes in metabolite concentrations was persistent through litter decomposition. Lignin was an exception, however, with a diminishing genotypic variation during decomposition, and the concentrations of lignin and condensed tannins had a negative genotypic correlation in the senescent leaves. Our results show that secondary metabolites and their intrapopulation genotypic variation can for the most part remain through leaf senescence and early decomposition, which is a prerequisite for initial litter quality to predict variation in litter decomposition rates. Persistent genotypic variation also opens an avenue for selection to impact litter decomposition in B. pendula populations through acting on their green foliage secondary chemistry. The negative genotypic correlations and diminishing heritability of lignin concentrations may, however, counteract this process.Peer reviewe

Genotype x Herbivore Effect on Leaf Litter Decomposition in Betula Pendula Saplings : Ecological and Evolutionary Consequences and the Role of Secondary Metabolites

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Intrapopulation genotypic variation in leaf litter chemistry does not control microbial abundance and litter mass loss in silver birch, Betula pendula

Background and aims Differences among plant genotypes can influence ecosystem functioning such as the rate of litter decomposition. Little is known, however, of the strength of genotypic links between litter quality, microbial abundance and litter decomposition within plant populations, or the likelihood that these processes are driven by natural selection. Methods We used 19 Betula pendula genotypes randomly selected from a local population in south-eastern Finland to establish a long-term, 35-month litter decomposition trial on forest ground. We analysed the effect of litter quality (N, phenolics and triterpenoids) of senescent leaves and decomposed litter on microbial abundance and litter mass loss. Results We found that while litter quality and mass loss both had significant genotypic variation, the genotypic variation among silver birch trees in the quantity of bacterial and fungal DNA was marginal. In addition, although the quantity of bacterial DNA at individual tree level was negatively associated with most secondary metabolites of litter and positively with litter N, litter chemistry was not genotypically linked to litter mass loss. Conclusions Contrary to our expectations, these results suggest that natural selection may have limited influence on overall microbial DNA and litter decomposition rate in B. pendula populations by reworking the genetically controlled foliage chemistry of these populations.Peer reviewe