Large old trees are rapidly declining in many parts of the world, which has big implications for ecosystems and biodiversity. The new study, published in the journal Science, finds that the loss of large old trees in many areas around the world poses a threat to ecosystem integrity.

Large, old trees play critical ecological roles, providing up to 30 percent of all vertebrate species with nesting spots or other shelter. In addition to shelter, they act as a source of food, attract pollinators, store large amounts of carbon, and play a crucial role in local hydrology. Even after death, large trees persist for decades or centuries, when they become standing dead wood or logs. Younger, smaller versions simply can’t replicate the critical ecological roles played by large old trees.

The loss of these trees is a recognized concern in ecosystems worldwide. In intensively grazed landscapes such as California, Costa Rica, and Spain, these trees are predicted to disappear within 90 to 180 years. In southeastern Australian, millions of hectares of grazing lands are seeing a significant decline. Within the next 50 to 100 years, they’re projected to support less than 1.3 percent of the historical densities of large old trees. Chronic livestock overgrazing, excessive nutrients from fertilization, and removal of trees to clear land are all contributing to these trends.

But the trend isn’t limited to areas used for livestock. Forests around the world are seeing declines in large old trees as well. In Yosemite National Park, the density of the largest trees declined by 24 percent between the 1930s and 1990s. In Brazil, fragmented rainforests are likely to lose half of their originally large trees in the first tree decades after isolation. One of the planet’s tallest trees—the mountain ash, a species of eucalyptus—is predicted to decline from 5.1 trees per hectare in 1997 to 0.6 trees per hectare by 2070. Logging, increasingly severe wildfires, and insects such as the western pine beetle are taking their toll on these forest giants.

Large old trees are keystone structures in many different environments. Yet like many other large species such as elephants, tigers and whales, they are facing an uncertain future. More research is necessary to better understand the key threats these trees face, and devise strategies to protect them.

Science, 2012. DOI: 10.1126/science.1231070 (About DOIs).