Tropical plants like to grow tall, while temperate zone plants are dwarfs in comparison.
This global pattern to plant height has been discovered for the first time, after scientists reviewed the size and locations of more than 7000 species.
Species growing at the equator are around 30 times taller on average than those at high latitudes, they found.
Their analysis also shows that rainfall has a bigger influence on plant height than temperature or soil fertility.
Finding such a clear global trend in plant height surprised the researchers who conducted the analysis.
“It might seem obvious that plants are taller in the tropics, after all tropical rainforests are clearly taller than Arctic tundra,” says Angela Moles of The University of New South Wales in Sydney, Australia, who led the review.
“However, there are plenty of tropical ecosystems that are dominated by short plants, such as savannas, and plenty of high-latitude ecosystems that are dominated by very tall plants, such as boreal forests.”
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Mature willow trees, just 2cm tall, grow in Greenland. The trees produce the rounded leaves.
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What’s more, she points out, the tallest plants in the world do not grow in the tropics.
“The tallest plant species on earth is the coast redwood, from California, which grows over 100m tall and the tallest flowering plant is mountain ash, which grows in southern Australia.”
Moles explains that until now, no-one had quantified the differences in plant height around the world.
“The lack of information is kind of surprising,” she says. “Plant height is such an important trait.”
In particular, the differences in the heights of plants in each ecosystem affects the variety and type of animal living there.
So to evaluate any trend, Moles and colleagues from Australia and the US collated data on 7084 plant species, including their locations and maximum height reached, from research already published in databases and scientific journals. In all, the team collated over 32,000 records of plant height around the globe.
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The world plant order
Low latitude plants grow tall, high latitude ones grow short
The average plant height at the equator is 7.8m, while north of Stockholm, plants average just 27cm
Few dwarf species live in the tropics
Precipitation in the wettest month of the year best predicts how tall plants grow
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They found that plant species at the equator are, on average, 29 times taller than those growing between a latitude of 60 degrees and 70 degrees North, and 31 times taller than those between a latitude of 45 and 60 degrees South. Plants within each region grew an average of 7.8m, and 27cm and 25cm respectively.
However, the researchers were surprised to find that there is not a smooth decrease in plant height moving further from the tropics.
“There is a zone at the edge of the tropics where plant height suddenly drops,” says Moles. This two-fold decrease in height suggests plants switch growing strategies in temperate zones.
“We’re not sure why. But it may be to do with the deserts around this latitude,” Moles explains.
They were also surprised to discover that the best predictor of plant height was not net primary productivity, a measure of how much plants grow in a given location, which is affected by temperature, rainfall and soil quality.
Nor did low temperatures, which can limit the growth of large trees by freezing the water inside their trunks, have a strong influence.
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The view from 65m up a mountain ash tree, the tallest flowering plant species in the world.
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Instead, the single best predictor of plant height was how much it rained during the wettest month of the year.
The final surprise related to short rather than tall plants.
“We thought that very cold or very dry ecosystems would lack tall plants, but there would be short plants just about everywhere,” says Moles.
“Instead there turns out to be a remarkable scarcity of very short plants in very warm, wet, productive environments like rainforests.”
“We hadn’t predicted this, but in hindsight, it seems likely that in these highly productive ecosystems it is just so dark at ground level that there aren’t many species that can actually make a living on the forest floor.”