Is Your Green Thumb Turning Brown?

Climate change could force your garden to turn over a new leaf

As crocuses poke their purple noses out of the cool ground, and faint green leaves appear on the trees, a gardener's fancy turns to planting. But climate change has begun to flummox gardeners. Plants that once would have been killed by harsh winters now thrive in the milder temperatures, while plants that would have thrived in cool summers now wither from drought and heat. Similar trends are likely to disrupt plant species in the world's woods, fields, pastures, and forests, according to a new study from the Nees Institute for Biodiversity of Plants at the University of Bonn.

When deciding which vegetables and flowers to plant, serious gardeners have long turned to the U.S. Department of Agriculture's Plant Hardiness Zone Map, a color-coded map of North America denoting 11 climate zones that support different species. The most recent USDA map was completed in 1990, although an update is in the works. However, an unofficial 2006 update from the National Arbor Day Foundation revealed that the zones on the 1990 map have shifted northward because of our warming climate. That change is expected to continue.

For example, the best science shows that in a few decades, Chicago's climate will likely resemble that of southern Illinois, says Katharine Hayhoe, an atmospheric scientist at Texas Tech University in Lubbock, who has helped put together several influential reports on the regional impacts of climate change. Such shifts in plant ecosystems will affect far more than vegetables and flowers in the garden. They will affect many of the world's roughly 300,000 species of land plants that provide us with food, clothing, shelter, and medicine. For example, a U.S. Forest Service Tree Atlas shows tree species shifting significantly northward. "Zones that trees are suited for are moving faster than the trees," Hayhoe says.

To gauge how climate change could alter plant biodiversity, researchers at the Nees Institute first divided the world into a grid of 1,032 regions, which included all the major ecosystems. Then they built a computer model to examine which environmental factors, such as temperature, elevation, precipitation, and humidity, determine the number of species of plants that can live in a geographic region. They gathered data from scientific publications on the number of plant species in each region—a measure ecologists call "species richness." Then they experimented with their model to see which environmental factors best accounted for the current number of plant species in different regions.

The most important factors, it turned out, were temperature and the availability of water. Neither variable on its own explained today's species richness entirely, but together they accounted for about 70 percent of  it. "If there's enough water, a warmer temperature is good for species richness. If there's not enough water, a warmer temperature is bad for species richness," says team member Jan Henning Sommer, an ecologist at the Nees Institute.

To project how climate change would alter species richness, Sommer and five colleagues then focused on two scenarios developed by the Intergovernmental Panel on Climate Change: a low-emission scenario, in which we shift to renewable energy as the century progresses and hold overall climate warming to 2° C, and a high-emission scenario, in which we keep growing the fossil-fuel-dependent economy and the climate warms by 4° C. They projected the future climate in each of the 1,032 regions under each scenario, and calculated each region's future species richness, assuming that the region would support about the same number of species a century from now as a region with a similar climate does today.

The results were striking. In cool northern areas the number of species is expected to increase, permanently altering today's ecosystems, though the warmer climate could potentially benefit agriculture, according to Sommer. Meanwhile, tropical areas such as the Amazonian rainforest are projected to become hotter, drier, and less able to support the biodiversity they host today, the authors reported in the March 24 online edition of the Proceedings of the Royal Society B.  "If climate changes as expected, many species there will be in serious trouble," Sommer says.

What's more, quickly phasing out fossil fuels could make a huge difference, the results show. In the low-emission scenario, although northern regions on average would gain species and tropical regions lose them, overall plant biodiversity would remain about the same. In the high-emission scenario, regions would host about 10 percent fewer plant species, and everyone would lose.

Sommer cautions that their model projects only the capacity of a region to host biodiversity, not which types of plants will live, die out, or move. “We just make an alert and provide a risk map,” Sommer says. What happens next is up to the rest of us.


USDA Hardiness Zone Finder

"New Hardiness Zone Map reflects warmer climate," press release from Arbor Day Foundation,; 2006 Arbor Day Foundation Hardiness Zone Map:

Jan Henning Sommer, Holger Kreft, Gerold Kier, Walter Jetz, Jens Mutke and Wilhelm Barthlott. "Projected impacts of climate change on regional capacities for global plant species richness." Proceedings of the Royal Society B, published online 24 March 2010 doi: 10.1098/rspb.2010.0120.

Holger Kreft and Walter Jetz, "Global patterns and determinants of vascular plant diversity," Proceedings of the National Academy of Sciences, Vol. 104, pp. 5925-5930 (2007).

U.S. Forest Service Climate Change Atlas:

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