Is it a tree or a plant?--It's eucalyptus.

• Position: Ecology

Australia's giant eucalyptus trees are the tallest flowering plants on Earth, yet their unique relationship with fire makes them a huge puzzle for ecologists. Now, the first global assessment of these giants, published in New Phytologist, seeks to end a century of debate over the species' classification, a debate which may determine their future.

Gigantic trees are as rare as they are awe inspiring. Of the 100,000 global tree species, only 50 reach more than 70 meters in height. While many of these giants thrive in Pacific North America, Borneo, and similar habitats, 13 are eucalypts that live in southern and eastern Australia. The tallest flowering plant to call Australia home is Eucalyptus regnans, with temperate eastern Victoria and Tasmania being home to the six tallest recorded species of the genus. One Tasmanian tree was measured at 99.6 meters, while a tree in Victoria achieved the historic record of 114.3 meters.

"The conifers of North America are often thought of as the largest trees on Earth, yet the flowering eucalyptus trees of Australia reach comparable heights," points out David Bowman, a professor from the University of Tasmania's School of Plant Science. "This is surprising, as Australia is the world's driest inhabited continent. Yet, this apparent paradox may explain the evolutionary advantage of gigantism in trees."

Wildfires are common in dry and arid environments such as Australia, but Eucalyptus regnans has evolved a unique dependency on fire for regeneration. While fire may kill the trees, their seeds are protected in aerial capsules, which then are released onto the scorched earth. The fire not only clears away potential seed predators and any forest canopy which may block sunlight, it releases nutrients in the soil...