Now researchers from China and Kenya believe they may have found an explanation for this anomaly. Their study of plants on the Tibetan Plateau indicated that a lack of winter chilling as climate warms may actually be retarding plants' growth in the spring.

"Increasing temperatures in winter appear to have a delaying effect on winter-dormant plants," Jianchu Xu of the Kunming Institute of Botany of the Chinese Academy of Sciences, and World Agroforestry Centre East-Asia Program in China told environmentalresearchweb. "This effect, though theoretically expected, has rarely been observed, and adds a new aspect to our understanding of the effects of warming on winter dormancy."

Xu and colleagues from the Kunming Institute of Botany, the World Agroforestry Centre East-Asia Program in China and the World Agroforestry Centre in Nairobi, Kenya, used satellite data to analyse vegetation growth on the plateau, along with temperature data from the China Meteorological Administration for 1982–2006. For both steppe and meadow vegetation, spring growth became earlier over the first part of this time period but gradually latened from the mid-1990s onwards.

According to Xu, most previous studies have focused on the warming effect that is currently stronger in most places – earlier growth in spring due to spring warming. "This also happens on the Tibetan Plateau, but the statistical approach that we used was able to detect an additional temperature effect, which works in the opposite direction – delaying the breaking of winter dormancy because of temperature increases in winter," he explained. "Because the Tibetan Plateau is warming fast and the vegetation is very sensitive to temperature cues, this effect has grown stronger there than the advancing effect of spring warming."

The researchers believe that this effect happens in most winter-dormant plants. As a result, adding the effect of winter warming to studies in other regions may help to explain "more of the variation in spring phenology dates than we have understood so far".

Xu says there is great uncertainty about how plants will respond to future climate change in the Tibetan Plateau, which is seeing greater-than-average warming.

"It seems certain that there will be appreciable changes in the timing of the beginning of the growing season," said Xu. "In the first phase, the 'spring warming' phase, spring warming dominates and phenology advances. In the second phase, the 'equilibrium' phase, the advancing effects of spring warming and the delaying effect of winter warming cancel each other out. In the third phase, the 'winter warming' phase, the delaying effect becomes stronger."

Xu reckons that these three phases happen along a continuum of warming, which will ultimately lead to a fourth phase, in which chilling requirements may not be fulfilled in most years. "The result will likely be very irregular phenology, which may pose serious problems for plant species," he said. "So we would rather see the current balance as an unstable equilibrium, which may tumble into a less-than-desirable state if warming continues."

The researchers reported their work in PNAS.