Forest restoration with Betula ssp. and Populus ssp. nurse crops increases productivity and soil fertility

• We compare forest restoration of target tree species with and without nurse crops.
• We study effects of pioneer nurse crops on productivity and nutrient cycling.
• 2.5 times larger pools of exchangeable base cations in top soils under nurse crops.
• 100 Mg/ha additional biomass available in nurse crop systems after 20 years.
• Nutrient costs of birch wood low compared to several other tree species of Europe.

The rapid re-establishment of forests following large disturbances is being seen as one option to increase the contribution of forests to climate change mitigation. The temporary inclusion of pioneer trees as nurse crops on disturbed sites can facilitate the establishment of target tree species and may additionally benefit productivity and soil fertility. In this study we compared productivity and nutrient cycling between stands of oak target species (Quercus robur and Quercus petraea) that were established with and without widely spaced Betula ssp. or Populus ssp. nurse crops. Simulation results for a full rotation of oaks (180 years) indicated that both types of forests, with and without nurse crops, have a comparable total productivity. However, stands with nurse crops supplied 59–96 Mg ha−1 harvestable biomass after 20 years, whereas the first harvest of biomass from stands without nurse crops would occur at least 30 years later. Nutrient element costs associated with the removal of Betula ssp. wood were low compared to Populus ssp. Also, nurse crop stands had up to 2.5 times larger pools of exchangeable base cations in top mineral soils (0–30 cm) compared to mono-specific oak stands. The high soil cation pools may have resulted from reduced leaching under nurse crops or the increased recycling of cations, also from deeper soil depth, via litter fall and fine-root turnover. Our results show that forest reestablishment with pioneer tree species may be a suitable tool for the rapid recovery of forest productivity and mitigation potential following disturbances while simultaneously helping to maintain or increase soil fertility.