Sept 1 (Reuters) - Almost a third of the world"s tree species are at risk of extinction, while hundreds are on the brink of being wiped out, according to a landmark report published by Botanic Gardens Conservation International (BGCI) on Wednesday. According to the State of the World"s Trees report 17,500 tree species - some 30% of the total - are a risk of extinction, while 440 species have fewer than 50 individuals left in the wild. Overall the number of threatened tree species is double the number of threatened mammals, birds, amphibians and reptiles combined, the report said. "This report is a wake up call to everyone around the world that trees need help," BGCI Secretary General Paul Smith said in a statement. Among the most at-risk trees are species including magnolias and dipterocarps - which are commonly found in Southeast Asian rainforests. Oak trees, maple trees and ebonies also face threats, the report said. Trees help support the natural ecosystem and are considered vital for combating global warming and climate change. The extinction of a single tree species could prompt the loss of many others. "Every tree species matters — to the millions of other species that depend on trees, and to people all over the world," Smith added. Thousands of varieties of trees in the world"s top six countries for tree-species diversity are at risk of extinction the report found. The greatest single number is in Brazil, where 1,788 species are at risk. The other five countries are Indonesia, Malaysia, China, Colombia and Venezuela. The top three threats facing tree species are crop production, timber logging and livestock farming, the report said, while climate change and extreme weather are emerging threats. At least 180 tree species are directly threatened by rising seas and severe weather, the report said, especially island species such as magnolias in the Caribbean. Though megadiverse countries see the greatest numbers of varieties at risk of extinction, island tree species are more proportionally at risk. "This is particularly concerning because many islands have species of trees that can be found nowhere else," the report added.