Study challenges belief on growth-reproduction tradeoff

Released on 10/30/2007, at 2:00 AM
Office of University Communications
University of Nebraska–Lincoln
Lincoln, Neb., October 30th, 2007 —

A long-held assumption among biologists is that plants face a tradeoff between growth and reproduction. If they grow a lot, they have few resources left for reproduction, and vice versa.

But new research based on a 13-year study indicates that's not necessarily the case, at least for some species of oak trees. In a paper published in the Oct. 15-19 online version of the Proceedings of the National Academy of Sciences, a team led by Johannes M. "Jean"Knops of the University of Nebraska-Lincoln showed that growth and reproduction are both controlled by rainfall and competition for resources between growth and reproduction is negligible.

In the study conducted at the University of California's Hastings Natural History Reservation in Monterey County, Calif., Knops and his colleagues, Walter Koenig of the Hastings Reservation and William Carmen of Mill Valley, Calif., collected growth and reproduction data on 239 mature oak trees from five species intermingled in the same geographic area from 1994 through 2006. They used devices called dendrometers fitted around the main trunks of the trees to measure growth and counted acorns to measure reproduction.

Three of the species that Knops and his colleagues studied (valley oak, blue oak and coast live oak) are pollinated and produce acorns in the same year. But the other two species (canyon live oak and California black oak) have two-year reproduction cycles and don't produce acorns until the year after pollination. It was the two-year species that enabled them to determine that growth and reproduction don't have a direct negative correlation.

As expected, the scientists found a negative correlation between growth and reproduction with each of the one-year species. But in species where growth and reproduction occur in the same year, it's impossible to determine if there is an actual tradeoff between the two factors.

With the two-year species, however, Knops and his colleagues found a positive correlation between growth and reproduction in the same year (i.e., more growth, more acorns, and vice versa). But they found a negative correlation (i.e., more growth, fewer acorns, and vice versa) between growth in the year of pollination and reproduction in the following year, when acorns were produced.

Their conclusion was that rainfall controlled both growth and reproduction, but independently of each other.

"In life history studies, one of the key assumptions is that there's a tradeoff between growth and reproduction, which most studies pretty much take for granted," Knops said. "But in the last five years or so, more and more people have questioned that assumption for various reasons. When we started looking at our data, the numbers didn't add up (to support the tradeoff hypothesis). When we looked at it a little more precisely, it was pretty clear that both growth and reproduction are controlled by rainfall, but the negative correlation between growth and reproduction isn't causal. It's just an artifact of all these weather variables.

"The key thing is we had two species of oak trees who produce acorns in two years, so all the pollination happened the year before. That's why we could separate out the weather during pollination from that actual growth. For most species, you really can't do that."

Knops said it's not surprising that there isn't a direct correlation between growth and reproduction, since reproduction for the oaks in question involves only about 5 to 10 percent of the total annual biomass investment. Still, he said, the trees can't produce an unlimited number of acorns and the study showed strong negative correlations in acorn crops from year-to-year, so something must be controlling that.

The trees' never-ending battle against squirrels and other herbivores is one of the prime suspects, Knops said.

"Squirrels eat acorns," he said. "These trees live hundreds of years and they make acorns to produce new trees, not to get fat squirrels. So if they produce regular seed crops, you get a buildup of squirrel population. But if they fluctuate the crop, there are some years where they're loaded with acorns and squirrels can't eat them all, and in other years there are no seeds, and the squirrel population goes down.

"It looks like the evolution of this acorn productivity pattern is not directly controlled by a tradeoff within the tree for resources, but more this evolutionary game with animals that eat the acorns. It's a much more complicated, much more interesting story that way."

CONTACT: Johannes M. "Jean" Knops, Assoc. Professor, Biological Sciences, (402) 310-3904