Jungles can't stand the heat

Bristol University scientist discovers rainforest collapse at time of Carboniferous global warming. Could the Amazon go the same way? Ted Nield at the BA finds out.

Geoscientist Online 8 September 2008

Dr Howard Falcon Lang of the University of Bristol has some alarming news for rainforests – loggers may not be their biggest problem. New studies of the way in which the world’s first true rainforests responded to global warming are suggesting that they can’t take the heat.

Dr Falcon-Lang and US colleagues have been studying a 300-million-year-old forest in the underground workings of a coalmine, in Illinois, USA. The forests in the lower part of the succession are composed of a bizarre mixture of extinct plants: abundant club mosses, more than 40 metres high, towering over a sub-canopy of tree ferns, intermixed with shrubs and tree-sized horsetails.

The fossil forest was preserved after a major earthquake 300 million years ago. The quake caused the whole region to drop below sea level, burying the forest quickly in mud, preserving it forever. Dr Howard Falcon-Lang said: "It was an amazing experience. We drove down the mine in an armoured vehicle, until we were a hundred metres below the surface. The fossil forest was rooted on top of the coal seam, so where the coal had been mined away the fossilised forest was visible in the ceiling of the mine.”

“We walked for miles and miles along pitch-black passages with the fossil forest just above our heads. We were able to make a map of the forest by the light of our miners’ lamps.” The fossil forest is the largest ever found, covering over 10,000 hectares, an area 10km square. The fossils preserve a unique snapshot of what tropical rainforests were like 300 million years ago.

“As there is nothing like it around today, before our work we knew very little about the ecological preferences and community structure of these ancient plants. This spectacular discovery allows us to track how the species make-up of the forest changed across the landscape, and how that species make-up is affected by subtle differences in the local environment.”

Falcon-Lang’s original study reconstructed a Carboniferous rainforest at the largest spatial scale ever attempted. The fossils showed that the Earth's first rainforests were highly diverse and how the kinds of tree species changed across the ancient landscape. The forest dated from 306.5 million years ago, in the late Carboniferous (Pennsylvanian) Period, when most of the world's coal resources were formed.

Subsequent studies on this same succession have now revealed several more forests, preserved in sequence from the late Moscovian to the Kasimovian stages. In all, Falcon-Lang has now discovered six successively preserved rainforests. He told reporters at the BA in Liverpool: “The most interesting thing about this succession is that it straddles a time when the Earth flipped from an icehouse state, like today, to a greenhouse state.”

The disturbing news is that the great stands of horsetails and lycopsids which thrived in the Carboniferous icehouse, never made it through the transition to greenhouse. Instead they gave way to smaller, scrubby vegetation more characterised by smaller plants like ferns.

“Of course, we don’t know for certain that modern rainforest plants will respond in exactly the same way” said Falcon-Lang. However, with a five-year grant from the Natural Environment Research Council to explore these forests in greater detail, he hopes he may be able to refine his ancient analogues.

Further reading

Ecological gradients within a Pennsylvanian mire forest, by William A. DiMichele, Howard J. Falcon-Lang, W. John Nelson, Scott D. Elrick and Philip R. Ames. Geology, May 2007; v. 35; no. 5; p. 415–418; doi: 10.1130/G23472A