Mon, Aug 02, 2021:On Demand
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.
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.