The effect of diversity on carbon storage in plantation forests: A meta-analysis

Emily Warner, Nick Brown and Andy Hector, Department of Plant Sciences, University of Oxford, Oxford, United Kingdom, Susan C. Cook-Patton, Global Lands, The Nature Conservancy, Arlington, VA, Owen Lewis, Department of Zoology, University of Oxford, Oxford, United Kingdom, Julia Koricheva, Department of Biological Sciences, Royal Holloway University of London, Egham, United Kingdom, Nico Eisenhauer and Olga Ferlian, Institute of Biology, Leipzig University, Leipzig, Germany, Nico Eisenhauer and Olga Ferlian, German Centre for Integrative Biodiversity Research, Leipzig, Germany, Dominique Gravel, Départment de Biologie, University of Sherbrooke, Sherbrooke, QC, Canada, Jefferson Hall and Carolina Mayoral, Smithsonian Tropical Research Institute, Balboa, Panama, Hervé Jactel and Céline Meredieu, INRAE Centre de Nouvelle-Aquitaine-Bordeaux, Cestas, France, Carolina Mayoral, Birmingham Institute of Forest Research, Birmingham, United Kingdom, Christian Messier, Sciences Biologiques, Université du Québec à Montréal, Alain Paquette, Centre d'étude de la forêt (CEF), Montreal, QC, Canada, William Parker, Ontario Forest Research Institute, Sault-Sainte-Marie, ON, Canada, Catherine Potvin, Biology, McGill University, Montreal, QC, Canada, Peter B. Reich, Department of Forest Resources, University of Minnesota, St. Paul, MN

Background/Question/Methods
There is growing global interest in forest establishment for carbon accumulation. Most new forests globally are established as monoculture plantations. Experimental evidence in non-forest ecosystems and theory suggest that increasing diversity can increase productivity and therefore carbon storage, alongside other benefits. However, we lack an understanding of the impact of plantation diversification on carbon accumulation.

We compiled a database of studies assessing carbon accumulation in monoculture versus diverse plantations, pulling empirical data from the primary literature and the Tree Diversity Network. We conducted a meta-analysis addressing the following questions: (i) do diverse plantations accumulate more aboveground carbon than the most productive monoculture plantations (i.e., transgressive overyielding); (ii) do diverse plantations accumulate more carbon than the average of monocultures within a study site; (iii) do diverse plantations accumulate more carbon than commercial species monocultures; (iv) what is the effect of species richness on carbon accumulation; (v) what drives differences in carbon storage between diverse and monoculture plantations?

Results/Conclusions
After reviewing >11,000 publications and compiling data from the Tree Diversity Network, we identified 59 observations (from 17 independent sites). Studies came from North and Central America, Europe, East Asia, and Australia. Studies considering diverse plantations with more than two species were less well represented.

We found (i) no evidence of transgressive overyielding (standardised mean difference 0.60 [95% CI −0.10, 1.30], k = 57). However, (ii) diverse plantations had higher aboveground carbon than the average of monoculture plantations (1.34 [0.73, 1.95], k = 56). Similarly, (iii) diverse plantations accumulate more aboveground carbon than commercial species monocultures (1.23 [0.56, 1.89], k = 20). We found that (iv) four species plantations had higher carbon accumulation than both the most productive monoculture (2.80 [1.69, 3.91], k = 9) and average of monocultures (6.37 [3.56, 9.18], k = 8). In contrast, two species plantations only had higher carbon accumulation than the average of monocultures (0.98 [0.56, 1.39], k = 43). Increasing diversity beyond four species did not show clear benefits for carbon sequestration. Finally, (v) we found that diverse plantations enriched by a nitrogen-fixing species outperformed diverse plantations without nitrogen fixers.

Given the evidence that diverse plantations can accumulate similar or greater amounts of aboveground carbon than monoculture plantations, this meta-analysis provides support for the diversification of plantation forests. Our results suggest that four species mixtures may achieve the highest levels of carbon storage and that including a nitrogen fixer enhances carbon accumulation.