Background/Question/Methods 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 28 years the Center for Tropical Forest Science has implemented a standardized system for monitoring the diversity and dynamics of tropical forests. Thirty-four plots of 16-148 hectares have been established in 20 countries across the Americas, Africa, Asia, and Europe. Every tree with a stem diameter ≥1 cm is mapped, measured, identified, and monitored. This international collaboration, involving hundreds of scientists from dozens of institutions, is now monitoring the growth and survival of 3.5 million trees in over 8,200 species – over 15% of all known tropical tree species. These data provide a basis for determining: (i) forces maintaining diversity, and (ii) the response of trees and forest ecosystems to the Earth's changing climate. In this talk, I will discuss some of the key findings of the global network and describe progress with expanding the network to temperate forests.

Results/Conclusions Three main hypotheses have been proposed to explain how so many kinds of tropical tree species can coexist in a small area: (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. Numerous studies across CTFS plots demonstrate that a tree survives better or grows faster where it has fewer neighboring conspecifics. Conspecifics apparently exchange specialist pests more readily when they are closer together. Over periods up to 28 years, many CTFS forests have been very 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. CTFS has embarked on two major initiatives to strengthen our ability to address the dual issues of forest diversity and change: (1) establishment of large plots in temperate forests, and (2) intensified sampling of forest processes within plots, including subannual carbon monitoring, insect monitoring, and assessment of functional traits for all species.