Benefits of using tree-mixes rather than monocultures in carbon plantings

Background/Question/Methods

Increasingly governments and the private sector are using planted forests to offset carbon emissions.  Few studies, however, examine how tree diversity affects C-sequestration and storage in these plantings.  Here we consider arguments for planting tree species mixes rather than monocultures in C-offset projects.  First, using above-ground biomass as a proxy for C-sequestration/storage, we use meta-analysis to synthesize studies comparing mixtures to monoculture plantings.  We conducted three separate meta-analyses designed to address the following questions:  (1) On average, are tree mixtures more productive than their species grown in monoculture?  (2) Do tree mixtures produce at least as much above-ground biomass as their most productive species grown in monoculture?  (3) Is there evidence that the addition of individual species or key functional groups increase above-ground biomass?  We next examined additional motivations for incorporating tree richness into planted forests including the likely role diversity plays in forming multiple C-pools, its contribution to C-pool stability, and its contribution to additional ecosystem services. 

Results/Conclusions

We found that tree mixtures are more productive than their average monoculture. Most importantly for the design of C-plantings, mixtures also produced equal and sometimes more biomass than their most productive species grown in monoculture. This suggests that diverse C-plantings sequester/store at least as much carbon as monocultures and can potentially outperform monoculture plantings.  We also found that nitrogen-fixing trees increase biomass, and by extension, C-sequestration/storage in plantings.  Our findings highlight the benefit of a two-pronged strategy for designing C-plantings including:  (1) increased tree species richness and (2) the addition of species that contribute to C-sequestration/storage as well as to other target processes and functions.