I study very species-rich plant communities-tropical rain forests. I particularly want to know the answer to a classic question in ecology: how can so many tree species coexist in tropical forests? To try to find out, in 1980 a colleague, Robin Foster, and I set up a permanent forest plot on Barro Colorado Island (BCI) in the Panama Canal. Our plot was huge by 1980 standards, measuring 1 kilometer by half a kilometer (roughly the size of the UCLA campus). We tagged, mapped, measured, and identified >240,000 trees and saplings. Every five years we repeat this census to record growth, survival and new recruitment. Tropical forests truly do have more tree species: the BCI plot has >300 tree species, half the number in all of North America north of Mexico-an area 44 million times larger! My collaborators and I now have a network of 24 large forest plots set up on the BCI model in 16 tropical countries. These plots contain >6,000 tree species, about 10% of the world’s known tree species. So, what have we found out so far? Two examples: (1) Most tropical tree species are extremely rare. In the global network, >50% of all tree species together make up <1% of all individuals. This is worrisome because rare species are more vulnerable to extinction under likely climate change scenarios. (2) Another worrisome finding is that tropical tree growth rates are slowing down worldwide. The best explanation is rising nighttime temperatures which increase nighttime respiration, reducing net carbon gain from daytime photosynthesis. We are also getting some still tentative but surprising answers to the question of why tropical forests have so many tree species.