Effects of competition and indirect facilitation by shrubs on Quercus robur saplings

摘要

Indirect facilitation by shrubs has been suggested as a cost-effective way of regenerating oaks in forests of conservation interest. In this study, we tested whether shrubs can enhance growth in pedunculate oak (Quercus robur) by suppressing herbaceous competitors. We studied interactions between young oaks, shrubs, and/or herbaceous vegetation in an open-field experiment, in southern Sweden, over the first 3 years after planting. Oak saplings were grown in four competition treatments: no competing vegetation; with herbaceous vegetation; with shrubs; and with both herbaceous vegetation and shrubs. Competition from shrubs and herbaceous vegetation both reduced stem diameter and biomass accumulation, but they affected biomass partitioning differently. Saplings grown with competition from shrubs partitioned biomass primarily into height growth, while those saplings exposed to competition from herbaceous vegetation invested a relatively higher proportion in root growth. Competition between shrubs and herbaceous vegetation reduced the above-ground biomass of the herbaceous vegetation, resulting in an indirect facilitative effect for the oaks during the first 2 years after planting. However, during the third year, shrubs had a negative effect on biomass accumulation. In summary, results from this study suggest that shrubs indirectly facilitate biomass accumulation of oak saplings by suppressing herbaceous vegetation, possibly by reducing competition for below-ground resources. However, owing to the relatively short duration of positive net outcome for the oak, we recommend that a longer-term assessment of the interaction between oak regeneration and neighboring shrubs be made before the outcome of this study is applied to practical forestry.