Mixture-effects on tree biomass production and soil organic C quality in a temperate plantation forest

Forest restoration and afforestation on degraded lands are receiving tremendous research efforts globally as a climate change mitigation option. There is a growing interest in mixed species plantation to ensure sustainable ecosystem services and biodiversity. However, successful mixture of achieving these potential benefits is rare. We studied the polyculture of two pioneer fast growing species (i.e. B. pendula,  and A. glutinosa- of which A. glutinosa is N-fixing) and one shed tolerant species with slow juvenile growth (i.e. F. sylvatica) to examine the effects of species mixture on biomass production and quality of soil organic C stock following the replacement series approach. Standing woody biomass in polyculture demonstrated no over-yielding, presumably due to concurrent impacts of suppression of F sylvatica by two fast growing species and competitive reduction benefits in A. glutinosa. Similarly, standing fine root biomass production and turnover showed no significant mixture effect.  Although the quantity of soil organic C stock was unaffected by tree mixture, the vertical distribution of biodegradable C fractions was differed between mono and polyculture stands,  most probably due to slow decay rate of mixed litter. We found that species mixture decreased soil C lability in the upper soil layers, and increased recalcitrant C  in deep soil (>40 cm) that has enormous potential for long-term sequestration. We concluded that contrasting growth responses can result in no biomass over-yielding in polyculture stands but the mixed litter can affect soil C quality.

Key words: Mixture effects, Tree polyculture, biomass, over-yielding, C quality, recalcitrant C