Climate change advances spring more every year

15/10/2015

In one of the most fascinating strategies of the flora, the trees of the zones of temperate and humid climates, lose their leaves when the fall arrives. The shorter duration of the day, with the decrease in solar radiation (photoperiod), the decrease in temperatures and the occasional freezing of the soil, make keeping the leaves a waste of energy. The beech, oak, linden, chestnut, ash ... will return to green with spring and its increasingly longer and warmer days. This direct relationship between temperature and budding of the leaves of deciduous species has led many scientists to argue that global warming is advancing in the spring.

To put figures to these predictions, a group of researchers from several countries, including Spain, have studied this phenomenon in the continental forests of Europe. They analyzed sprouting data of seven large tree species present in 1.245 locations in a strip running from the North Sea to the Adriatic and from Belgium to Bosnia-Herzegovina.

Their analysis has been supported by data collected from 1980 by the Pan-European Phenological Project, which records the periodic biological phenomena related to time (such as the return of the swallows or the flowering of almonds and cherry trees). They verified that all the analyzed species, and in all the sites with data, have 30 years ahead of the outbreak of their leaves.

"The output of the leaves has been advanced six or seven days from 1980," says the director of the Global Ecology Unit of the Center for Ecological Research and Forestry Applications, CSIC, and co-author of the study, Josep Peñuelas. The impact of this phenomenon is enormous. On the one hand, the advancement of the outbreak causes the leaves to fix more carbon, balancing the excess emissions. But, on the other, "it produces chain effects in all the ecosystems that end up influencing how the whole planet works"adds Peñuelas.

However, for the Catalan researcher, the research, published in the journal Nature, discovers another phenomenon that is even more intriguing: the rate of advancement of sprouting is slowing down, but without stopping. Thus, between 1980 and 1994, the output of the sheets was advanced by an average of 4 days for each extra degree of temperature increase. But, since 1999, the ratio has dropped to 2,3 days per degree, that is, a reduction of 40%.

The braking is not the same in all species. The horse chestnut or false chestnut (Aesculus hippo), for example, has softened its budding advance to two days per degree. At the opposite end, the leaves of the common beech (Fagus sylvatica) maintain almost the same accelerated rate of outbreak. So the leaves come out every time before, but in recent times, those rushes have softened.

The researchers then studied the reason for this slowdown. They handled several hypotheses, such as a progressive adaptation of deciduous trees to the greater variability of spring temperatures or a kind of physical limit that the leaves would have at the time of sprouting related to the photoperiod or amount of solar radiation. It is as if the trees knew that they can not go too far into winter, lest a late frost should end up with its first green stems.

"We have observed that the leaves of European trees do not sprout as soon as thought, because they need to accumulate a certain number of cold nights to wake up from the state of winter dormancy"says Peñuelas. The cold is the part of the equation that was missing to explain the anticipated arrival of spring. The deciduous species need a good dose of cold before the heat arrives and the extra amount of sunshine that announces the end of winter.

But climate change is not only causing more heat in summer, it is also softening the temperatures of autumn and winter. Without going any further, the AEMET has already announced that the new station that now begins will be particularly soft and humid. This will make the trees take longer to reach the cold quota they need, as if it were harder to realize that it is time to take out the leaves.

For Peñuelas "It's like the trees are going crazy". And with their madness, they drive the rest of the ecosystem crazy and all the animal or vegetable species that make their lives depending on when the leaves come out of the trees.

Source: Agriculturers.com