If you can't remember what you did last Thursday, this may make you feel even worse. Vegetation in the Russian Arctic has been shown to retain a memory of how cold it was during the last Ice Age, 10,000 years ago. Even though the world is much warmer now, tree species growing at high latitudes are still operating on the basis that the cold has yet to fully leave.
Scientists have been puzzled that Siberian larch is the dominant tree in parts of eastern Russia where the climate should be more suited to pine and spruce. To resolve this mystery a team from the Alfred Wegener Institute for Polar and Marine Research examined pollen trapped in the sediments at the bottom of El'gygytgyn Lake through the period 3.5 to 2.1 million years ago. This represents the only high latitude continuous record of vegetation covering several ice ages and the periods between.
The pollen reveals the tree species around the lake over that period, which the researchers compared with climatic records for the era. "The comparison of our data shows that the vegetation needs thousands of years to adapt from a cold to a warmer period," said Professor Ulrike Herzschuh in a statement.
After a particularly cold ice age, it took warm climate plants thousands of years longer to recolonize the area than when the glacial had been comparatively mild.
Although a few individual organisms live more than 10,000 years this so-called “memory” is not the result of individual trees that make Methuselah look like a mayfly. Instead, Herzschuh and her co-authors of a paper in Nature Communications attribute the delay to long migration distances and the time it takes for permafrost to thaw. Siberian larch flourish where permafrost is found close to the surface, which can persist for millennia in a moderately warm climate.
Near river beds the permafrost is often more deeply buried, allowing different trees to flourish. Stefan Kruse/AWI
“Our results imply that the current widespread larch ecosystem actually represents a transitional vegetation type reflecting severe last glacial conditions rather than contemporaneous Holocene interglacial climate,” the paper reports.
Past climate proved more important than influences such as the presence or absence of large herbivores, although fire frequency also appears to play a part.
The findings force a complete rethink of the interaction between forests and climate. “Vegetation-climate lags are generally thought to have lasted no more than a few centuries,” the paper points out. However, evidence has been found suggesting such lags are not restricted to the Russian Arctic, but may be occurring in North and South American forests today.
After a cold glacial period, the permafrost takes a long time to thaw to depths great enough to allow pines. Herzschuh et al/Nature Communications
The effect does not seem to apply in reverse, with the authors observing, “In contrast, no effects from the preceding interglacial on glacial vegetation are detected.”
"The new findings are also relevant for predictions of future developments in the Arctic," Professor Martin Melles, of the University of Cologne said: "In the future, they should be taken into consideration when we draft new climate models to improve our prognoses."