The interplay between climate and bedrock type determines litter decomposition in temperate forest ecosystems

At regional scale, it is dominantly considered in the European literature that litter decomposition is higher in soils from calcareous than siliceous bedrocks because of higher pH in the former forest ecosystems. We tested the alternative hypothesis that this should rather be due to differences in physical characteristics of the bedrock, with likely higher decomposition on calcareous than siliceous bedrocks in wet climates because of more favourable texture in the former and nutrient leaching in the latter, and the opposite in dry climates because of higher drought stress in calcareous soils. We assessed, four consecutive years with contrasting climates, the litter decomposition of a unified litter of Abies alba needles with the litterbag method in 70 forest sites located on both bedrocks and in wet oceanic and dry continental climates of the European Alps and the Pyrenees. Average and annual climate data were analysed with principal component analyses and the effects of bedrock type, average and annual climate drought stresses on litter decomposition were analysed, separately in the Alps and the Pyrenees, with analyses of variance. We found, in both mountain ranges, a highly significant bedrock type by average climate drought stress interaction on litter decomposition, due to a strong decrease in litter decomposition from wet oceanic to dry continental sites on calcareous bedrocks only. Although litter decomposition did not change over all years in siliceous sites with increasing climate drought stress, it increased during wet years in the dry continental sites only, in particular in the Pyrenees where interannual climate variability was higher. Together our results strongly support the physical hypothesis and not the chemical hypothesis. We argue that the chemical hypothesis was proposed based on studies only conducted in low elevation wet temperate sites comparing mull humus types from deciduous forests on calcareous soils to mor and moder humus types from evergreen forests on highly sandy siliceous bedrocks. Our study conducted on a wider range of climate and bedrock conditions bring strong evidence that litter decomposition is rather dependent on the physical characteristics of the bedrocks.