A former University of Minnesota scientist has discovered that even in the plant kingdom, size matters. Only bigger is not necessarily better.

Helene Muller-Landau, who is now at the Smithsonian, looked at the question of why some plants in the same ecosystem have large seeds, such as a cocklebur, and others, such as a common mullein, have small ones. Ecologists have been thinking about this thorny issue for several years.

Muller-Landau, in a study published in Proceedings of the National Academic of Sciences, landed on the answer that environmental variables are the key factor. Her findings are significant because of their potential to predict which plants will survive better when their environment is altered, from climate change, habitat loss and the like.

"We're doing a lot of things that change environmental conditions and change the distribution of stressfulness of regeneration sites," she told me. "This would in turn affect the relative success of some species, and potentially lead to shifts in species composition, extinction of some species, and invasion of others."

Stress factors: changing moisture, temperature
Anyone who has dealt with buckthorn or thistles knows the problem of unwanted species. Changing rainfall and temperatures are two common factors that can stress a plant community, she said.

Big seeds — think about a 20-pound coconut — have the advantage when conditions are tough. Deep shade, low water availability, thick forest duff, and "vertebrates who might chomp your seedling" can be successfully defeated by larger seeds, she said.

"For example, they have the resources to keep growing longer before they actually are able to photosynthesize — so they can push through thicker leaf litter to get some sun, or make a longer root to reach water," Muller-Landau said. "And those resources can be kept in reserve and used to re-sprout if the stem of the initial sprouted seedling is broken by a fallen branch, or eaten by a deer."

The needs of small seeds
Small seeds — the false pimpernel needs 150 million seeds to make a pound — need more tenderness. "When conditions are good, and it doesn't take a lot of resources to get going — a good amount of sunshine, water is available," she said. Small seeds suffer in difficult environments but can still survive because of their large numbers.

In general, plant communities have variations in seed mass of three to five orders of magnitude. That means the biggest seed in an ecosystem is typically quadruple the smallest. Muller-Landau's work, using mathematical modeling, lends support to the idea that plants with variable seed sizes do not necessarily compete — competition that drives some to extinction — because they thrive under different conditions.

Muller-Landau credits fellow U of M scientists Dave Tilman and Claudia Neuhauser for their input. Her work now carries her to the South American jungles, where she is part of an international team quantifying carbon in the forests of the world.