Swapping needles for flat leaves allowed members of one conifer family to diversify and compete with flowering plants in the wet tropics, according to Australian researchers.
Conifers, or gymnosperms, have been around for approximately 300 million years, and were dominant until flowering plants, known as angiosperms, exploded onto the scene around 65 million years ago.
A new study led by Dr Ed Biffin from the University of Adelaide’s Australian Centre for Evolutionary Biology and Biodiversity, has found the arrival of angiosperms restricted most conifer species to marginal habitats and alpine areas in the northern hemisphere.
However, one distinctively different southern hemisphere family of conifers known as the Podocarpacae (plum pines) was able to adapt.
“By evolving flattened leaves to increase the surface area available for light capture podocarps have become one of the most successful conifer groups,” says study co-author Professor Andrew Lowe.
“Today their highest diversity is found in tropical rainforests where they compete very effectively with flowering plants.”
Tracing back through time
To work out when the transition from needle leaves to flattened leaves began, the researchers used a combination of fossil and molecular evidence to map the evolution of 120 podocarp species.
The results of their research, which are published in Proceedings of the Royal Society B, show the transition began between 150 and 90 million years ago, with their rate of evolution and diversification plateauing around the time flowering plants first appeared.
“At that time, around sixty million years ago, the flattened podocarp leaf form started to evolve and then underwent a steady rate of evolution and diversification which has continued until today,” says Lowe.
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The success of angiosperms is due to efficient water pumping mechanisms, as well as fruit and flowers to attract pollinators. Being able to harvest light in low light conditions also allowed them to germinate and grow in forest floors, rather than wait for a light-filled gap.
Podocarps appear to have adopted a similar strategy.
“The podocarps’ broad leaf-like structure, which may be formed from one or more flattened needles or from a flattened petiole (stalk), has allowed them to mimic one of the most successful adaptations of flowering plants,” he says.
Previous research by Timothy Brodribb of the University of Tasmania, another of the study’s co-authors, has shown that podocarps have also evolved good water transportation mechanisms that facilitated broadening of the leaves.
End of the evolutionary line?
As to why other conifers, with the exception of Pinus krempfii a Vietnamese pine, and Agathis or kauri pines, which are both predominantly tropical, didn’t make the same adaptation, Lowe speculates that it may be because they are a very old group.
He says it is possible that they reached the limit of their evolutionary potential in some areas of morphological adaptation.
Lowe says the study may also help shed some light on the date of origin of modern rainforests, a subject that is still under intense debate.
“The Podocarpacae evolved because of selection pressures placed on them by the evolution of rainforests, either around the same time or shortly after,” he says. “This work will help focus on the timing of rainforest evolution.”