"The realization that climate warming is overriding local drivers of tree growth is upsetting," Jordi Voltas of the University of Lleida, Spain, told environmentalresearchweb. "Events of increasing synchrony are usually interpreted in ecology as triggers of populations' extinction."

The local drivers of forest tree growth that climate change is over-riding include soil fertility, competition among trees, differences among species in the way they cope with environmental stressors, stand management, and the influence of pests and diseases.

Forests can reveal harm from climate change through widespread forest mortality, insect outbreaks, and increasing recurrence of forest fires. "Here we report on a non-visible early warning signal that can be traced back in time through the analysis of tree rings," said Voltas. "Trends of increasing synchrony in tree growth can be useful to establish climatic thresholds for tree survival and anticipate forest decline events locally and regionally."

According to Voltas, a handful of recent studies have reported on enhanced synchronous tree growth across small geographical scales, e.g. within mountain ranges, attributed to recent warming, especially in the Mediterranean. "We wanted to check if this phenomenon was a local one or rather it spreads over large – subcontinental – regions," he said. "To this purpose we selected two very contrasting terrestrial ecosystems: the extremely cold taiga forests of continental central Siberia and the warm, drought-constrained forest of Mediterranean Spain."

The researchers used 93 ring-width chronologies from six species of conifer: 45 chronologies from central Siberia and 48 from forests in Spain. A novel mixed model framework revealed the changes in spatial synchrony over time. "We needed to develop a new methodological framework capable of dealing with large tree-ring datasets going back in time to several decades or even centuries," said Voltas.

The team found an increasing influence of climate on the spatial variability of tree growth over the last 120 years, bringing an "unprecedented" coherence with time to ring-width records over a wide geographical scale. By the early 21st century, the synchrony in growth patterns between forests around 1000 km from each other was about the same as that between trees within the same forest stand.

"Climate warming is triggering a more synchronous forest growth, but … the particular mechanisms behind this response are regionally dependent," said Voltas. "In particular, they are related to increased drought stress in late spring in Spain and to higher variability of temperature extremes in summer in Siberia. On top of this, the enhanced synchronous forest growth in both regions is also related to an earlier start of wood formation, which we demonstrate is induced by a warmer climate."

Next the researchers, who reported the study in PNAS, would like to study other forest ecosystems to determine the relevance of such increasing synchrony trends worldwide.

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