By studying the reproductive habits of a mast-seeding high alpine grass, a Carleton College biologist has found that an increase in temperature causes the grass to flower more often, providing more food for its insect predators and threatening its potential for survival. His research may have implications for more common mast-seeding species, such as pine, spruce, birch and beech trees.
Mark McKone, associate professor and chair of the biology department at Carleton College, published his study of five masting Chionochloa species as the cover story in the August 1998 issue of the journal Global Change Biology. The article, titled "Effect of climate change on mast-seeding species: frequency of mass flowering and escape from specialist insect seed predators," details his research on the seed production of the Chionochloa, a waist-high, snow-tussock grass that is common on the mountainsides of New Zealand.
McKone, who also directs Carleton's 800-acre Cowling Arboretum and McKnight Prairie, began his research in 1985, when he was a postdoctoral research fellow at the University of Canterbury, Christchurch, New Zealand. He was assisted by Dave Kelly of the University of Canterbury and William G. Lee of Manaaki Whenua Landcare Research in Dunedin, New Zealand.
The trio began by studying the Chionochloa's reproductive patterns, looking at how seed production varies within a mast-seeding species. Mast-seeding refers to plants that show a large variation in the amount of flowers or fruits they produce each year, and that are synchronized in seed production. These particular grasses presented dramatic behavior in flowering-heavy one year, light the next year and not at all in other years.
By studying 22 years' worth of weather data available for the region, the researchers tested the hypothesis that heavy flowering in a given year was induced by an unusually warm summer. A strong correlation between temperature and flowering became apparent. "It was obviously a temperature phenomenon," McKone said. "It struck me that the temperature of the Earth is set to go up a degree or two due to greenhouse effects...so it's definitely going to change how these plants act."
Logically, in the years when the Chionochloa didn't flower, it didn't reproduce. So why didn't this particular grass evolve a pattern of yearly reproduction, like that of many other perennials?
According to McKone, one leading theory is that of predator satiation. "There are three different insect species that feed on the seed of the Chionochloa, in effect killing it," he said.
McKone found that the insect predator population followed the same pattern as flowering in the Chionochloa. During heavy flowering or mast years, the food resource for the insects was so abundant that the predators couldn't keep up-they became satiated, or full. Thus, the Chionochloa was able to "stay ahead" of the seed-killing insects and reproduce.
In non-mast years, the insects had few resources and so populations remained low. "It's really a hide-and-seek game. The insects winter in the soil, popping up each spring to look for food. Sometimes, it's 'oops, no food now.' Other times it's a heyday," McKone said.
The threat from global warming comes when an increase in temperature ultimately triggers flowering in the Chionochloa every year. Initially, this appears to be good-year after year of heavy flowering would allow for more seed production.
On the contrary, McKone said. "It's the worst possible thing. If it's a good flowering year every year, the insects keep increasing in population-the grasses are required to use all their resources for flower production and predator satiation is short-circuited. The plants can't ever escape their predators and seed production could be eventually devastated."
McKone's research proves that variability in flowering is key to Chionochloa's survival. Flowering must be isolated in time for the Chionochloa to keep its predators at bay.
According to McKone, much time has been spent worrying about global warming with regard to plant distribution. This study suggests that plant reproduction will be affected by global warming via an ecological interaction. "This has been a fairly unusual conclusion," McKone said. "I think there are probably a lot more interactions between species that will be affected by global warming-this is just one of them."
This theory also can be applied to trees such as beech, pine, spruce and birch, common to northern temperate zones like Minnesota. Like the Chionochloa, these trees are mast-seeding, temperature is their flowering cue and they have specialized predators.
In order to survive global warming, McKone noted, these species will be required to migrate to higher altitudes. Ironically, due to the higher temperatures caused by global warming "they'll be unable to set seed because populations of their seed predators will be very high. Just at the time migration is essential, seed production will go down."