Abstract
Defaunation is causing declines of large-seeded animal-dispersed trees in tropical forests worldwide, but whether and how these declines will affect carbon storage across this biome is unclear. Here we show, using a pan-tropical data set, that simulated declines of large-seeded animal-dispersed trees have contrasting effects on aboveground carbon stocks across Earth's tropical forests. In our simulations, African, American and South Asian forests, which have high proportions of animal-dispersed species, consistently show carbon losses (2–12%), but Southeast Asian and Australian forests, where there are more abiotically dispersed species, show little to no carbon losses or marginal gains (±1%). These patterns result primarily from changes in wood volume, and are underlain by consistent relationships in our empirical data (∼2,100 species), wherein, large-seeded animal-dispersed species are larger as adults than small-seeded animal-dispersed species, but are smaller than abiotically dispersed species. Thus, floristic differences and distinct dispersal mode–seed size–adult size combinations can drive contrasting regional responses to defaunation.
Highlights
Defaunation is causing declines of large-seeded animal-dispersed trees in tropical forests worldwide, but whether and how these declines will affect carbon storage across this biome is unclear
Simulated losses of large-seeded, animal-dispersed tree species decreased estimated aboveground carbon stocks by up to 5% under the 50% removal scenario, and by as much as 12% under the 100% removal scenario, that is, complete extirpation of all large-seeded animal-dispersed species in communities (Fig. 1, Supplementary Tables 1 and 2)
Carbon storage was largely unaffected in the individual- and species-based control scenarios, with shifts in carbon stocks ranging between À 0.4% and þ 0.1% at the highest extirpation levels across the 10 sites (Supplementary Fig. 1, Supplementary Tables 1 and 2), suggesting that changes in carbon stocks in the defaunation scenarios were primarily driven by reductions of large-seeded animaldispersed tree species
Summary
Defaunation is causing declines of large-seeded animal-dispersed trees in tropical forests worldwide, but whether and how these declines will affect carbon storage across this biome is unclear. We show, using a pan-tropical data set, that simulated declines of large-seeded animal-dispersed trees have contrasting effects on aboveground carbon stocks across Earth’s tropical forests. We present a pan-tropical assessment of the potential effects of defaunation on aboveground carbon storage by simulating extirpations of large-seeded animal-dispersed species from 10 relatively undisturbed tropical forest tree communities spanning four continents. These sites span a broad range of floristic types and form a gradient in the prevalence of animalmediated seed dispersal from highest in tropical Africa, India and the Americas, to lowest in the more wind-dispersed assemblages of Australia and Southeast Asia. Aboveground carbon stocks of the original and simulated communities were estimated using data on tree diameters and species wood density in a general biomass equation for tropical forest trees[22]
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