Landmark plant studies find new patterns of successful trait combinations and species competition

Scientists are gaining a better understanding of how plant traits such as plant height, seed size and leaf area influence the way species evolve, survive, grow and reproduce, potentially assisting efforts to stem biodiversity loss in the face of increasing human pressure on our ecosystems.

Two papers published in Nature have analysed data from different datasets. Some of the data came from the same global archive of plant traits – the TRY database, which is part of Future Earth. The communal database includes 5.6 million trait records for 100,000 of the world's roughly 300,000 plant species.

A study led by Sandra Díaz of Argentina’s National Scientific and Technical Research Council (CONICET) and Córdoba National University published in Nature outlines the “playing field” on which plants are located with respect to their leaf, stem, seed and whole plant traits. 

An independent but complementary Nature study led by Georges Kunstler from both France’s National Research Institute of Science and Technology for Environment and Agriculture (Irstea) and Macquarie University, Department Biological Sciences, Sydney, reveals how plant traits can be used to predict competition between species. 

“These studies…help us identify which species are particularly novel and may be most worthy of the resources society devotes to conservation,” says Jonathan Levine from ETH Zurich and author of a Nature News and Views article summarizing both papers.

What are functional traits?

Ecologists have used a number of different approaches to understand and categorize the overwhelming diversity of organisms on the planet – from emphasizing the uniqueness of each species to treating all plant species as exchangeable units in mathematical or statistical models.

“All these approaches have been useful for particular purposes, but they have strong limitations in the face of the present intellectual and practical challenges posed by fast-changing ecosystems,” says Díaz who is also a member of the Future Earth Science Committee.

“For example, we know that species, very often, are not equivalent in their responses to environmental factors or their effects on ecosystems.”
“On the other hand we cannot really treat each species as unique, not only because there is not enough computational power, but, importantly, because it is the essence of ecology to try to identify mechanistic rules and general patterns underpinning the gorgeous variety of life we see on the planet.”

What is now called the ‘trait approach’ has actually been around for many centuries. It is based on the premise that a small number of key traits such as plant height, seed size or leaf area determine the way plants function – such as how they reproduce, acquire water and sequester carbon. The agenda to develop an overview of plant ecological strategies via getting trait data together worldwide began in the 1990s.

“Prior work has shown that as one leaf trait changes from one plant to the next, so too do other leaf traits, suggesting that it is the trait combinations (rather than individual traits) that define the plant strategy,” Levine says.

Creating a global spectrum of plant form and function

According to Díaz, their work is building on the pioneering and inspiring work on traits conducted throughout the 20th century.

“In our case, we wanted something that could be applicable in principle to all vascular plants in the world, so we went for traits that encapsulate the essence of what Darwin called “the struggle for existence”, that is their growth, survival and reproduction, and that are available for large number of species.”

They looked at six basic traits shared by all vascular plants: plant height, stem density, seed mass, leaf area, leaf mass to area ratio and nitrogen concentration found in the leaves. Using the TRY database, they mapped 45,000 plant species based on these six basic traits, creating a six dimensional “global spectrum within which we can position any plant—from star anise to sunflower—based on its traits,” authors say.

Their analysis revealed that, as plants have evolved, some combinations of these six plant traits have been repeatedly selected, indicating that they are most fundamental to survival, growth and reproduction.

“Three quarters of the variation of this trait hyperspace is concentrated in a two-dimensional ‘global spectrum of plant form and function’,” Diaz says.

“One major dimension within this plane reflects the size of whole plants and their parts; the other represents the construction costs and quality of photosynthetic leaf area, running from cheap, fragile, high-productivity leaves to ‘conservative’ leaves that are expensive to construct, physically robust and are more resistant to drought or herbivory [consumption by animals].”

According to Díaz, crossing these findings with information on abundance and status of different plant species, as well as what climate and land use drivers are currently occurring, “will enable us to get a deeper understanding on how plants are changing in the face of human pressures – what trait combinations are becoming increasingly common and which ones are fading fast.”

Díaz’s findings provide “essential information for studies that aim to understand how competition and natural selection have shaped the diversity of plant life around us,” Levine says. 

Three traits to rule them all?

Ecologists have long sought an approach that might allow competition to be predicted in a general way across the tens of thousands of different tree species worldwide.

Most trait-based studies have explored the link between traits and individual plant performance, but few studies have explored the link between traits and competitive interactions between plants.

“No one before Kunstler et al. has used plant traits to understand competition between so many trees over global spatial scales,” Levine says.

Classical ecological theory says that species that have similar traits should compete more intensely than species with different traits.

Using growth data from more than 3 million trees in over 140,000 forest plots across the world, Kunstler and colleagues investigated how three traits — wood density, leaf mass to area ratio and maximum height — impacted the way neighboring trees influenced a given tree’s growth.

“A key feature of the study was to dissect the portmanteau idea of “competitiveness” into components — the ability to tolerate others, the impact on others, and the potential growth rate in absence of competition — and then to look at those in terms of traits rather than in terms of species,” said Mark Westoby, co-author of the study and Professor at Macquarie University.

They found that no matter what forests in the world they were analyzing, these three traits affected competition in predictable ways.

For example, trees with greater wood density are more able to tolerate competition (and are competitive themselves).

“Finding such consistent effect of traits was surprising, as these biomes host strikingly different vegetation,” Kunstler said.
“Our study also contradicted the classical ecological theory according to which the similarity of traits between species is the major determinant of their competition.”

Diversity of species matters a great deal to the ability of forest to tolerate climate change impacts. “Our study provides tools to help to describe and integrate this diversity in predictive models,” Kunstler says.

While the study examines which traits enable species to be better competitors, “the traits explaining how tree species coexist with one another largely remains a mystery,” Levine says.

Kunstler is hoping that these results will inform future studies to explore how traits influence tree response to climate conditions in addition to competition.

Taken together, the Díaz and Kunstler studies extend our appreciation for the diversity of form and function found among plant species worldwide. 

“In so doing, they help us identify which species are particularly novel and maybe most worthy of the resources society devotes to conservation,” Levine says.

References

Díaz, S. et al (2016): The global spectrum of plant form and function. In: Nature 529, 167–171

Kunstler, G. et al (2016): Plant functional traits have globally consistent effects on competition. In: Nature 529, 204–207

Levine, J. M. (2016): Ecology: A trail map for trait-based studies. In: Nature 529,163–164