Abstract
Climate change is one of the main challenges that human societies are currently facing. Given that forests represent major natural carbon sinks in terrestrial ecosystems, administrations worldwide are launching broad-scale programs to promote forests, including stands of non-native trees. Yet, non-native trees may have profound impacts on the functions and services of forest ecosystems, including the carbon cycle, as they may differ widely from native trees in structural and functional characteristics. Also, the allocation of carbon between above- and belowground compartments may vary between native and non-native forests and affect the vulnerability of the carbon stocks to disturbances. We conducted a global meta-analysis to compare carbon stocks and fluxes among co-occurring forests dominated by native and non-native trees, while accounting for the effects of climate, tree life stage, and stand type. We compiled 1678 case studies from 250 papers, with quantitative data for carbon cycle-related variables from co-occurring forests dominated by native and non-native trees. We included 170 non-native species from 42 families, spanning 55 countries from all continents except Antarctica. Non-native forests showed higher overall carbon stock due to higher aboveground tree biomass. However, the belowground carbon stock, particularly soil organic carbon, was greater in forests dominated by native trees. Among fluxes, carbon uptake rate was higher in non-native forests, while carbon loss rate and carbon lability did not differ between native and non-native forests. Differences in carbon stocks and fluxes between native and non-native trees were greater at early life stages (i.e. seedling and juvenile). Overall, non-native forests had greater carbon stocks and fluxes than native forests when both were natural/naturalised or planted; however, native natural forests had greater values for the carbon cycle-related variables than plantations of non-native trees. Our findings indicate that promoting non-native forests may increase carbon stocks in the aboveground compartment at the expense of belowground carbon stocks. This may have far-reaching implications on the durability and vulnerability of carbon to disturbances. Forestry policies aimed at improving long-term carbon sequestration and storage should conserve and promote native forests.