By Lance Olsen

There will be winners and losers as the world's plant and animal species attempt their responses to a changing climate. For example, because C3 plants (e.g., trees) and C4 plants (e.g., grasses) do the work of photosynthesis differently, elevated levels of atmospheric CO2 can favor C3 plants at the expense of C4s. 

This selective favoritism comes independent of the rising global temperatures that are also produced by elevated levels of atmospheric CO2.  And so does another ecologically important impact: plants grown under elevated CO2 have less nutritional value for herbivores all the way from insects to mammals. So, while its differential impact on C3s and C4s will be re-arranging the botanical face of the planet, the increased atmospheric CO2 content will also be affecting the animal side of the plant-animal kingdom. And it will be having these effects even without hiking the temps by a single degree
But, whether on the plains or in the mountains, heat sill matters to plant life and all animals that depend on plants, and we are headed into a centuries-long period of heightened heat.

Hot Plains
For a hint of the future of life on the plains,  take a look at the shortgrass steppe, and a species of grass called blue grama. I've lately been saying that blue grama could be credited as the grass that won the West.

It helped secure a place for the great bison herds of the shortgrass steppe, and then became a crucial species in support of domesticated cattle. Because of its recent commercial importance, blue grama been a fairly well-studied species, and the shortgrass steppe a fairly well-studied ecoystem.

This grass has a wide distribution -- all the way from the southern Canadian plains to central Mexico. According to researchers who've studied this grass in long-term projects, blue grama has been in decline.  The documented decline has been enough to create some nervousness about the future of the western cattle industry.

So, what's behind its downhill trend?

Blue grama is a C4 grass, which automatically raises the question of whether it's documented decline stems from nothing more that rising concentrations of CO2. However, this C4 grass is something of a rarity for not being put at competitive disadvantage under elevated CO2 levels.

So something else is afoot here. In a word, it's heat.

Some years ago, Science published an article by Colorado State University researchers who tested an hypothesis that warmer overnight temperatures were behind blue grama's decline.  Their hypothesis was supported by the evidence. But the researchers subsequently tested an alternative hypothesis - that  longer growing seasons were behind the decline. The evidence supported the second hypothesis too.
Support for both hypotheses doesn't mean that each is competitive with the other. Instead, the warmer overnight temps and longer growing seasons share their origins in rising temperatures, and Colorado State's scientists have calculated that blue grama declines by 30 percent per each degree C of warming.

Important as blue grama is, a bigger story lies in its relationships with other species. So, I invite you to think grasshoppers, which are another hallmark species of the shortgrass ecosystem.

Just about everything eats grasshoppers. Birds may be the most widely recognized 'hopper-eaters, but anyone who has fished the streams and rivers crossing the plains knows that fish eat them too. So do a variety of familiar mammals including shrews and mice, coyotes and foxes.  While none of these 'hopper-eaters depends on 'hoppers alone, the insect does give something a leg up to a wide range of species.

Enter, the finned grasshopper. This species is widely distributed within the shortgrass ecosystem. In areas where its own eating habits have been studied, its stomach contents are about 90 percent blue grama.

The message in the interaction of this grass and this grasshopper  is that any significant  decline in the blue grama population would will hold important implications for the finned grasshopper, and because many species make lunch of the 'hopper, a decline of blue grama will have reverberating impact throughout North America's shortgrass system.

These concerns are by no means confined to the shortgrass steppe. Whether we look at grasslands to the east of west of blue grama country, the story is basically the same.
When we look east of the shortgrass steppe,  we find similar concerns for  the tallgrass system. In March of 2007, Colorado State researcher Alan Knapp reported evidence that changes associated with rising global temperatures imperil the tall grasses, to the advantage of other plants. "One of the first patterns that became evident was of reduced productivity among varieties of grasses that had previously grown very tall," Knapp said. "This allowed other plant species that previously had been shadowed by these tall grasses to flourish."

As in the shortgrass system,  changes of the tallgrass system have both commercial and ecological implications for the plains.  According to a Colorado State news release, Knapp says, "If you are a rancher, anything that reduces the total biomass produced by these grasslands would be an economic disadvantage," he said. "From an ecological perspective, the consequences of changes in the composition of the species of grasses and other plants will require further study."

If we look west of the shortgrass steppe, we see similar trends. For example, in early February, 2007, Science published results of a study by K.B. Suttle and colleagues, who reported that heating brought simplification of the food web in  Californian grasslands, and that the simplification brought "... deep reductions in consumer abundance after 5 years."

Hot Mountains
Any division of plains and mountains is doomed to be a bit arbitrary,  if only because the grasslands - and their vulnerabilities -- extend far up into the mountains. Still, the differences are plain enough, including that the mountains host a greater variety and overall density of trees.

Over the past few decades, the logging industry has said that forests can come back after logging. For our part, we environmentalists have been saying that forests can rebound from fire. But it's been evident for some time that trees face temperature-driven change as great as that for the grasses, and recent evidence persuades me that the claims of logging execs and conservation activists will be endangered.

In October of 2005, the Proceedings of the National Academy of Sciences published "Regional vegetation die-off in response to global-change-type drought, " by Breshears, et al. This team had access to evidence of earlier drought-caused dieoff of pines in the American Southwest, but found evidence that the droughts and dieoffs of yesterday differ from the more recent droughts that coincide with rising heat in the forest.
In comparing a drought in the opening years of the 21st Century to earlier droughts, Breshears' team determined that the earlier droughts had actually been more severe droughts than the recent one.  But the recent one killed many more trees. And, while the historic droughts had largely killed trees over a hundred years old, the recent one killed them across age and size classes.

The recent drought was simply hotter than earlier ones, and the team identified heat as the "trigger" for the deaths of millions of Southwest U.S. pines. And the risk to forest rebound shows up with their evidence that high heat can kill across age-size classes.

As I say when conducting seminars on climate change in the Northern Rockies/Plains region, I'm as capable as anyone of putting two and two together and coming up with something other than four. But my reading of Breshears et al leads to suspect that, if a forested region should happen to come under extremes of heat on a repeat basis in the decades and centuries ahead, we face risk that forests struck down by drought, fire, heat, or insects will never be able to recover.

Enter, Gerald Meehl and Claudia Tebaldi. In  August, 2004, Science published their article, "More Intense, More Frequent, and Longer Lasting Heat Waves in the 21st Century."  I pair this paper with the one by Breshears, et al when running seminars for climate and wildlife researchers, grad students, and environmentalists. My message is that prior confidence about forests' capacity for recovery may soon meet a harsh reality that few are talking about today.