Sorry, tropical rain forests. Grasslands have the most plant species—at least when the area studied is smaller than a few parking spaces.
In a recent study, biologists scoured data from previous papers about plant-rich ecosystems around the world and discovered that grasslands harbor the most plant species in areas less than 540 square feet (50 square meters).

Read the full story from National Geographic: http://news.nationalgeographic.com/news/2012/03/120320-grasslands-rain-forests-species-diversity-environment/