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A unique assemblage of giant fossil trees has been found in 300-million-year-old rocks in Newfoundland, Canada, by Dr Howard Falcon-Lang of Bristol University's Earth Sciences Department. The fossilised trees represent the oldest upland forests ever documented. They were more than 45m in height and were the ancestors of present-day conifers.

Because vegetation growing at high-altitudes is rarely preserved as fossils, the formation of upland forests has long been the subject of great controversy. Knowing when this happened is highly important because forests accelerate the rate at which rock is weathered, which in turn removes huge amounts of carbon dioxide from the atmosphere. This causes global cooling " a reverse of the greenhouse effect.

Dr Falcon Lang said: 'Models of the Earth's climatic evolution have long indicated that a dramatic cooling event occurred in the Early Carboniferous Period, the age immediately preceding the formation of the Newfoundland rocks. It seems likely that the evolution of upland forests during Carboniferous times accelerated the rate that the mountains were weathering, removing carbon dioxide from the atmosphere, and leading to the observed global cooling.'

The fossil trees were alive at a time when North America and Europe lay together on the equator and were covered by steamy tropical rainforests " the remains of which occur today as vast coal deposits. A huge Himalayan-scale mountain belt is known to have stretched across this ancient tropical zone from France to Texas. The Newfoundland rocks were deposited in a tiny basin right in the heart of this mountain belt.

The greening of upland environments exerted an enormous impact on the global carbon cycle and climate. Knowing the timing of when they formed helps understand the huge contribution trees in upland areas make to the Earth's climate.

The work of Dr Falcon-Lang and his Masters student, Arden Bashforth from the Memorial University of Newfoundland, will be reported in the May issue of GEOLOGY.