The westward movement of some tree types in the eastern United States puts plant responses to climate change in a new perspective. Credit: Karen Bleier/AFP/Getty Images

Ecologists have long predicted that climate change will send plants and animals uphill and towards the poles in search of familiar temperatures. Such movements have increasingly been documented around the world. But a study now shows that some tree species in the eastern United States are going their own way and moving west, not north, as the climate changes.

Songlin Fei, a forest ecologist at Purdue University in West Lafayette, Indiana, and his colleagues tracked the shifting distributions of 86 types of tree (species and genus level) using data collected by the US Forest Service’s Forest Inventory and Analysis programme during two periods: between 1980 and 1995 and in 2015 for most states. They found more species heading west than north, probably partly because of changing precipitation patterns, the team reported on 17 May in Science Advances1. “That was a huge surprise for us,” says Fei.

This study indicates that changes in moisture availability owing to climate change probably exert stronger pressures on tree distributions in the near-term than temperature changes, he says.

The team measured shifts in the centres of abundance for the 86 types of tree and found that over the past 30 years or so, 34% showed statistically significant poleward shifts at an average rate of 11 kilometres per decade. Forty-seven per cent made statistically significant westward shifts at an even faster rate — 15.4 kilometres per decade. Hardly any types of tree moved south or east.

A new direction

A closer look at the data revealed that most of the trees that shifted west were angiosperms, or flowering trees. Northbound trees were usually gymnosperms, which are mostly conifers in North America.

Increased precipitation in the central United States could be one explanation for the angiosperms’ westward movement, say the study's authors. The increase in moisture is still subtle enough that only the more drought-tolerant and faster-growing flowering trees, with their more-efficient and robust vascular systems, can take advantage for now.

In addition, nearly all of the wind-pollinated trees, which includes most of the gymnosperms and some of the angiosperms, shifted north. This suggests that these trees find it easier to change their distribution than those that depend on range-limited animal pollinators.

Teasing out the true explanations for these shifts is complicated by the fact that the eastern United States hosts a complex and dynamic forest inhabited by people. Many researchers say that this forest is still in the process of growing back from large-scale clearing before the 1920s2. The changing distribution of tree type could be in part owing to regrowth and the natural succession of species through an area, combined with human management such as the suppression of fires.

Uncertain future

Tree physiologist Leander Love-Anderegg at the University of Washington in Seattle says that the study did well to acknowledge these potentially confounding variables. “They point out that in the eastern US it is a really tricky question to pull out climate-related changes in forests, from forests getting older and the effects of fire suppression,” he says.

Whether the mechanisms are perfectly understood or not, knowing movement trends helps forest managers, Love-Anderegg says. “We live in an era of very rapid ecological change. In order to avoid some of the more drastic and negative consequences of that change — like massive forest fires and massive beetle outbreaks — we all have an interest in trying to predict change before it occurs.”

What is certain is that the forests of today will look different 10, 20 or 30 years from now. “If you think of these species as members of a family, the question is, will some families break apart, or will they travel together?” says Fei. “We might be talking about these families breaking apart.”