Tropical forests are one of the largest terrestrial carbon sinks, and so play an important role in global climate regulation.
Carbon dioxide absorbed by trees in these forests is converted to woody biomass, effectively taking the carbon out of circulation for the life of the tree. In this way, tropical forests store around 40% of the world’s carbon.
The carbon storage capacity of a forest depends on its structure and composition, taking into account factors such as the age, size, species and growth rate of trees.
The composition and structure of tropical forests is, in turn, dependent on the fruit-eating vertebrates that live there. These animals serve as important seed dispersal agents; however, hunting, habitat fragmentation and other human interventions is reducing their numbers.
Declining numbers of seed dispersers
Declines of large frugivores have been shown to reduce the abundance of large-seeded species by up to 60%, and change the forest composition to favour the smaller-seeded species, which are dispersed by smaller vertebrates and abiotically (for example, by the wind). It follows that there must be some implications for carbon storage capacity, but what are they?
Investigating the impact on carbon storage
A study published recently in Nature Communications suggests that forests with a high proportion of large-seeded species (that rely on dispersal by large animals) will have reduced capacity to store carbon as these large animals are removed, while forests that rely on smaller animals or abiotic agents to disperse seeds experience lower or no loss of storage capacity.
Researchers simulated the loss of large animals by modelling the removal of large-seeded animal-dispersed tree species in data from 10 tropical forest plot networks across four continents, then estimating carbon storage using tree stem diameter and wood density data.
Contrasting results
They found that African, American and South Asian forests consistently showed carbon losses on removal of the large-seeded species due to replacement by small-seeded species which on average store less carbon.
However, in South-east Asian and Australian forests, which comprise more wind-dispersed species than the other regions, there were little to no losses or marginal gains in above-ground carbon storage capacity. In these forests, large-seeded species will be replaced by higher proportions of wind dispersed species which on average store a greater amount of carbon.
With more than three-quarters of the world’s tropical forests in the Americas, Africa and South Asia, defaunation in these regions will have a marked impact on the overall aboveground carbon storage capacity of tropical forests.
CSIRO scientists in Atherton, who have maintained 20 permanent rainforest plots in North Queensland since 1971, contributed data for the study. As more data on tropical forest tree community dynamics and carbon budgets of defaunated forests become available, we will have a better understanding of the size and timescale of changes to carbon storage.