A climate response function explaining most of the variation of the forest floor needle mass and the needle decomposition in pine forests across Europe

The forest floor needle mass and the decomposition rates of pine needle litter in a European climate transect were studied in order to estimate the impact of climate change on forest soil carbon sequestration. Eight pine forests preserved from fire were selected along a climatic latitudinal gradient from 40° to 60° N, from Spain and Portugal to Sweden. The forest floor (Oi and Oe layers) was sorted into five categories of increasing decomposition level according to morphological criteria. The needle mass loss in each category was determined using a linear mass density method. The needle decomposition rate was calculated from the needle fall (NF), the mass of each category and its mass loss. For each site, the remaining mass vs. the calculated time was best fitted by an asymptotic model which indicates that the organic matter should be made up of two fractions: a decomposable one and a recalcitrant one. NF was correlated with actual evapotranspiration (AET) whereas the decomposition parameters (decomposition rate of the decomposable fraction, first year mass loss, forest floor needle mass, age of the most-decomposed category) were related to a combined response function to climate (CRF) based on the van’t Hoff law for temperature and the water deficit (DEF) for moisture. Scenarios with temperature increases, without and with DEF increases, were applied to predict forest floor needle mass changes. C would be lost from the forest floor if only temperature increases and this loss would increase from south to north. If more droughts occur, the forest floor would then tend to sequester C according to the level of the DEF and the latitude of the site. For example, a site in Portugal which is presently the most active site of the transect in terms of decomposition because of its present favourable warm Atlantic climate would react with a large range of responses, losing carbon under an unchanged precipitation regime and sequestering up to 3 times its present stock of carbon under drier conditions.Section Editor: C. Neill