A global study of more than 7,000 plant species has revealed what biologists expected but no one had proved - plants are taller in the tropics than those in temperate zones away from the equator. What nobody realised was that species growing at the equator are about 30 times taller on average than those at high latitudes
"Any ecologist would have guessed that plants would generally be taller in the tropics - but nobody had ever actually quantified the latitudinal gradient in plant height at the global scale before," says Angela Moles of the University of New South Wales in Sydney, Australia, who led the study published in the latest issue of the Journal of Ecology.
"It might seem obvious that plants are taller in the tropics, after all tropical rainforests are clearly taller than Arctic tundra. 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."
Dr Moles and her colleagues from Australia and the US collated data on 7,042 plant species, including their locations and maximum height reached, from information gathered from databases and scientific journals. The team collated over 32,000 records of plant height and logged plants from 222 locations ranging from 74 degrees north to 54 degrees south.
The study reveals that the average maximum height of species growing within 15 degrees of the equator (7.8m) was 29 times greater than the height of species between 60 and 75 degrees north of the equator (27cm), and 31 times greater than the height of species between 45 and 60 degrees south of the equator (25cm).
However, the tallest plants in the world do not grow in the tropics, according to Dr Moles. "The tallest living plant species on earth is the Californian coast redwood (Sequoia sempervirens), which grows over 100 metres and the tallest flowering plant is Mountain Ash (Eucalyptus regnans), which grows in southern Australia."
The study also surprised researchers by revealing that rainfall has a bigger influence on plant height than temperature or soil fertility. "We thought that the best predictor of plant height would be net primary productivity - a measure of annual plant growth in a year in a given location - which is affected by temperature, rainfall, and soil fertility," says Dr Moles.
"We also thought that the lowest temperatures might be important in determining maximum plant heights - because when it gets really cold, the water inside the trunks of large trees is susceptible to freezing. However, we were wrong on both counts. It turned out that the single best predictor of plant height was how much it rained in the wettest month of the year."
The researchers were intrigued to find a sudden decrease in plant height at the edge of the tropics. The size of the drop in plant height - 2.4-fold - was substantial and comparable to the 7-fold step in seed mass at the edge of the tropics reported two years ago by Dr Moles and colleagues as part of the landmark World Herbivory Project. Together, these findings suggest that there might be a sudden switch in ecological strategy at the edge of the tropics.
"We're not sure why. But it may be to do with the deserts around this latitude," Dr Moles explains. "We saw the same sort of thing with seed mass in an earlier study. That is, it looks like there is a fairly sudden switch in plant strategy - from tall, large-seeded species to shorter, smaller-seeded species - as you move from the tropics to the temperate zone."
"We had thought that very cold, or very dry ecosystems would lack tall plants, but that there would be short plants just about everywhere. One of the really surprising things was that the maximum height of plants in different parts of the world was only loosely related to climate. Instead, there is a remarkable scarcity of very short plants in very warm, wet, productive environments like rainforests.
"We hadn't predicted this. It seems likely that in these highly-productive ecosystems it's just so dark at ground level that there aren't many species that can actually make a living on the forest floor."
Having quantified global patterns in plant height the study has important implications for understanding how environmental variables shape these patterns and for developing our understanding how plants and the animals that depend on them for food will be affected by climate change.
Dr Moles is a member of the UNSW Evolution & Ecology Research Centre, within the School of Biological Earth and Environmental Sciences.
Media contacts: Dr Angela Moles, +612 9385 8302 or Dan Gaffney, UNSW Media, 0411 156 015