Tree roots in a changing world

Globally, forests cover 4 billion hectares or 30% of the Earth's land surface, and 20%-40% of the forest biomass is made up of roots. Roots play a key role for trees: they take up water and nutrients from the soil, store carbon (C) compounds, and provide physical stabilization. Estimations from temperate forests of Central Europe reveal that C storage in trees accounts for about 110 t Cha(-1), of which 26 t C ha(-1) is in coarse roots and 1.2 t C ha(-1) is in fine roots. Compared with soil C, which is about 65 t C ha(-1) (without roots), the contribution of the root C to the total below-ground C pool is about 42%. Flux of C into soils by plant litter (stemwood excluded) compared with the total soil C pool, however, is relatively small (4.4 t C ha(-1) year(-1)) with the coarse and fine roots each contributing about 20%. Elevated CO2 concentrations and N depositions lead to increased plant biomass, including that of roots. Recent analysis in experiments with elevated CO2 concentrations have shown increases of the forest net primary productivity by about 23%, and, in the case of poplars, an increase of the standing root biomass by about 62%. The turnover of fine roots is also positively influenced by elevated CO2 concentrations and can be increased in poplars by 25%-45%. A recently established international platform for scientists working on woody root processes, COST action E38, allows the exchange of information, ideas, and personnel, and it has the aim to identify knowledge gaps and initiate future collaborations and research activities.