Height matters more than size for dispersing seeds

Height matters more than size in the plant world when it comes to dispersing seeds

Plant height rather than seed mass is a better predictor of how far seeds will be dispersed, a study has shown.

The authors said they were surprised by the findings because it was assumed that lighter seeds would be carried further by the wind, water or animals.

However, the team found that plants of greater height - rather than those of reduced seed size - recorded greater dispersal distances.

The findings will be published in the Journal of Ecology.

"In terms of maximising dispersal distances, it seems it is better that species increase their height rather than decrease their seed size to gain an advantage over other species," explained co-author Fiona Thomson, who was based at the University of New South Wales, Sydney.

"Tall species also tend to have the advantage of being able to compete more for light, and large-seeded species tend to have an advantage of low levels of seedling mortality."

Dr Thomson, now a researcher for Landcare Research, New Zealand, said there had been an assumption that seed size was the main determining factor of dispersal distances - with smaller seeds covering greater differences than larger ones.

"For example, if you are using wind or water for dispersal, heavy seeds may drop or sink faster than lighter seeds; therefore, they are going travel shorter distances," she told BBC News.

"Also, animals may take small or light seeds greater distances because they are easier to carry. So we initially thought seed mass would be really important for the dispersal distances..

On the other hand, she added, the team thought that that plant height would have little influence on how far species dispersed except for plants that used the wind to disperse seeds.

She explained: "We actually found that plant height was a better predictor of how far seeds dispersed than seed mass, this result was pretty surprising."

The team also found that plant height also was also a surprising factor for species whose seeds were dispersed by animals.

"I'm still not sure why we found this pattern, but it is definitely a line of investigation for the future," Dr Thomson observed.

Foxy business

The findings are based on data collected for 380 plant species from 97 families.

"I searched the scientific literature for any studies that gave the mean and the maximum distances that seeds were dispersed for individual species," she said.

"We didn't specifically choose the species, rather it was where other scientists had chosen to study the dispersal distances for a plant species and we were then able to then use that information."

During their research, the team came across a range of innovative ways used by scientists to understand the mechanics of how plants scattered seeds.

For example, one study used a dummy fox that was wheeled through vegetation to see what seeds became attached to the fur, and where they fell off.

Dr Thomson said she decided to carry out the research because plants had evolved a variety of amazing ways to move seeds around - from hooks to attach to passing animals, to seeds with wings to catch a ride on a gust of wind.

However, she added that there was still a great deal more to learn about how far the majority of plant species disperse their seeds.

"This work helps us understand how far plants can move, and the characteristics or traits which are associated with long versus short dispersal distances," she explained.

"Understanding the distances plant species move is important for managing invasive species because it will help us predict how far invasive species can spread if the conditions are suitable for their establishment.

"Also, as our climate changes its critical that we understand if plant species will be able to move to new areas where the climate is suitable for them, or whether they will be left behind in unsuitable environments because they can't move far enough, fast enough."