Now researchers in the US believe that they have found an explanation for this varied response. Their studies of two Arctic and alpine-dwelling plants in North America indicate that at the southern end of the plants' ranges, where temperatures are warmer, faster vegetation growth may be compensating for the decreased survival and propagation rates. But there's a snag – at some point it's likely that further temperature rises will cause plant growth to slow, removing this compensation effect.

"We show why many species have – to date – defied the expectation of shifting ranges as a result of climate change, but instead have maintained stable ranges, especially at their southern boundaries," Dan Doak from the University of Wyoming, US, told environmentalresearchweb. "However, our results also show that the ecological mechanisms behind such stability are unlikely to continue, and are likely to swiftly change to rapid declines of southerly populations once climate warms past a 'tipping point' for each species."

Doak and colleague William Morris of Duke University studied the moss campion (Silene acaulis) and alpine bistort (Polygonum viviparum) at four sites in western North America, from Toolik Lake in northern Alaska to Niwot Ridge in Colorado via the Wrangell Mountains in south-central Alaska and Banff National Park in Alberta, Canada. The pair measured the survival, size and reproductive output of the plants from 2001 to 2006.

"Predictions of how species ranges will shift northwards with climate change have been borne out for some species, but not for many others," said Doak. "However, these predictions take a simple view of what warming will do to individuals. We find that studying the same two plant species from arctic Alaska to Colorado, and across many years, provides a more complicated answer to warming effects."

Doak and Morris predict that once the temperature of a species' tipping point – where its growth rate starts to drop with increasing warmth – is routinely exceeded, populations and ranges that have appeared stable to date will rapidly decline.

"Our results not only show why we have seen very different results of climate change on different species to date, but also that past performance is no guarantee of future stability in the face of continued warming," said Doak. "Knowledge of how and why these ecological effects of climate change occur will help us to better anticipate future climate change effects and thus prepare for them."

The researchers were originally interested in how fixed or variable the life history patterns – birth rates, death rates, and longevity – of plants really were. "Delving into this basic ecological question made us realize that very, very few studies of either plants or animals have sought to measure these simple aspects of a species' ecology across their entire geographic ranges," said Doak.

And the team also realized that exploring these patterns was "the best way to try to understand the ecological mechanisms that would generate range limits – or changes in range limits with climate change".

Now the researchers are continuing their fieldwork and have extended their measurements to include sites in New Mexico, closer to the southern range limit of the two study species. "As in any study of climate effects, it is critical to have many years of data, since each year has different weather patterns," said Doak. "In addition, we have added the use of micro-temperature loggers so that we can better tie the performance of plants to the climate occurring right where they are, rather than using general regional climate to predict what whole sets of plants do."

The researchers reported their work in Nature.