Seeing the Forest for the Trees: SIGEO Is Revolutionizing Understanding of Forests

Tropical deforestation continues at around 13 million hectares per year. This and other land-use change in the tropics contributes greenhouse gases to the atmosphere, reduces the ability of forest to regulate climates, and threatens many species that are known only from tropical rain forests. Over the past 30 years the Smithsonian Institution in partnership with a global network of research institutions has implemented a standardized system for monitoring the diversity and dynamics of forests. Forty-seven plots of 4-148 hectares have been established in 21 countries across the Americas, Africa, Asia, and Europe. Every tree with a stem diameter ≥1 cm is mapped, identified, and repeatedly measured. This international collaboration, involving hundreds of scientists from dozens of institutions, is now monitoring the growth and survival of 4 million trees in over 8,500 species – over 15% of all known tree species. These data provide a basis for determining the forces maintaining diversity, and the response of forests to the Earth’s changing climate. Three main hypotheses that have been proposed to explain how so many kinds of tropical tree species can coexist in a small area are being tested with the data: (i) that tree survival and reproduction do not depend on neighbor identity, but that diversity represents a balance between speciation and random extinction, (ii) that species are highly specialized to different microhabitats, and (iii) that each tree species is kept rare by specialized pests and pathogens. Long-term observations reveal that most forests have been highly dynamic. Some change is directly attributable to rare long-periodicity disturbance events (e.g., drought or fire), and some change is directional and may be a response to global climate change.