Climate change is shifting the forests of America in an unexpected direction. All over the world, global warming is causing ecosystems to move away from the equator or to higher altitudes, in search of favorable climatic conditions. However, in the eastern United States, even more tree species have shifted westward than north.
Dr Songlin Fei of Purdue University examined an extensive database on the locations of 86 species over the past 30 years. Of these, 62 percent were found to be moving north, averaging around 20 kilometers (12 miles) a decade. This entirely expected shift was overshadowed by a more surprising one. In the same sample, 73 percent were moving west, at slightly faster rates, with most change happening at the leading edge.
There were patterns to the movements. “Most angiosperms [flowering plants, such as oaks] shifted westward and most gymnosperms [non-flowering seed producers such as conifers] shifted poleward,” Fei and co-authors write in Science Advances. As expected, the changes were more noticeable for saplings than established trees.
The movement appears to be driven by changes in rainfall. Over the period the paper considers, temperatures in the eastern United States rose 0.16ºC (0.29ºF), but there have also been big shifts in rainfall, with an increase of more than 150 millimeters (6 inches) per year in the central US, and a major drop in much of the south-east.
The paper notes: “Our results indicate that changes in moisture availability have stronger near-term impacts on vegetation dynamics than changes in temperature.”
The flight of ecosystems to higher altitudes runs into an obvious problem when there is no more mountain to climb. Poleward movements can run into obstacles, be they oceans or human modifications to the land. The westward movement of tree species could be blocked by roads or farms, but it faces another problem as well.
The paper notes that if different tree species respond to changing rainfall patterns differently, we could see forests change as species grow apart. For ecosystems that depend on the inter-relationship between different tree types, the authors fear this could end up “putting the resilience and sustainability of various forest ecosystems in question.”
Of course, the climate of North America has changed before, and pollen trapped in sediment reveals trees have had ranges very different from those they now occupy. However, past changes have been much slower, allowing ecosystems to adapt in ways that are unlikely to occur in the face of faster-paced change.
Eventually, perhaps, the forests of Appalachia will, like Birnham Wood come to Dunsinane or the Ents at Isengurd, occupy the cities of the Great Plains. By then, however, far more dramatic consequences of global warming will be visible.