Growth & Development

Embracing the variability within a single plant species: how and why we need to study it

Scientists present 28 recommendations for studying plant intraspecific trait variation.

Some species or individuals within a species are better adapted to certain environments than others, so they have a better chance of survival. This within-species variability is the “hidden biodiversity” on Earth but can come at a physiological cost (e.g., plants may invest in more, smaller seeds or in fewer, larger seeds). There are many traits used to compare how individuals within a plant species grow differently in contrasting ecosystems (e.g. leaf area could be larger under shaded compared to sunny conditions) that can reveal local adaptations.

Annals of Botany has recently released a Special Issue on Intraspecific Variation in Plant Functional Traits to synthesise how within-species variability can contribute to a better understanding of plants across these multiple scales, from gene expression and within-plant variation to species interactions to population, community, and ecosystem dynamics. The guest editors of the special issue, Drs Andrea Westerband (Macquarie University), Kasey Barton (University of Hawai‘i at Mānoa), and Jennifer Funk (University of California, Davis) have written a comprehensive review on intraspecific variability, highlighted recent breakthroughs, and listed 28 recommendations how future studies should be carried out. 

In an online interview with BotanyOne, Westerband shared the motivation behind writing this review.

“Over the last five or ten years there’s been a real surge in interest among community and trait-based ecological studies highlighting the extent and sources of intraspecific variation (ITV) in plant functional traits,” Westerband writes. 

“There have also been a number of reviews calling for these studies to shift away from using species means and to instead more explicitly consider the role of ITV for community assembly and species interactions. The motivation for this review, and the special issue in general, was essentially to highlight the various ways that ITV influences higher order processes such as community dynamics but also invasion and ecosystem dynamics.” 

“In my own work, for example, I consider how climate-driven ITV can influence invasion success across the Hawaiian Islands. In this system, environmental heterogeneity is high across a very small land area, and it’s important to understand how this heterogeneity drives species interactions. “

Westerband’s colleagues all study plant functional traits in their research (e.g., seedling traits linked to ecological strategies, leaf nitrogen content linked to litter decomposition, trait plasticity in response to salinity, tree traits in response to drought).

“The other key impetus for this review was to bring more attention to particularly underrepresented functional traits within this literature strand, and to make some new suggestions for future studies. Whereas evolutionary and population biologists often consider variation in reproductive traits and whole-plant traits, trait-based and community ecologists focus very heavily on leaf traits. And even among leaf traits, specific leaf area (SLA) is by far the most popular trait that has been examined. As a result of these tendencies, it’s quite difficult to quantify the true extent of ITV in plants. In sum, there really is a pressing need to broaden the scope of our focus within this literature strand and we felt this was a good opportunity to do.”

Leaf shape diversity of brambles (Rubus sp.). Source: J. Kiss

Westerband and colleagues reviewed the underlying mechanisms of within-species variability, the extent of trait variability across ecosystems, the effects on higher-order processes, and the future directions in this field. Whilst writing this review, the authors have come across a few recent studies and emerging topics that were really surprising. 

“The role of epigenetics in driving intraspecific trait variation is a growing area of research that is very novel, and we highlight the study by Puy and colleagues (2020) as an example,: Westerband writes.

“The authors found that Arabidopsis thaliana populations displayed considerable variation in SLA but also plant growth in response to waterlogging and fertilization treatments when they applied experimental demethylation treatments. What’s particularly interesting to me is the idea that phenotypic plasticity (and therefore ITV) can be maintained over generations, known as transgenerational effects. This process is likely to be much more common in plants than we think, and warrants further investigation.”

“The other study that stood out to me was that of Yang et al. (2020), which described how models that predict plant performance (in this case, growth) from traits were much weaker when evaluated at species level than they were when evaluated at the individual-level. They also determined that the selection of focal traits strongly influenced the study conclusions. Together, I think this is an important study because it shows how our interpretation of the effects of ITV largely depends on the data structure but also the selection of traits.” 

Plant functional traits have been and continuously are used in ecological studies so writing a comprehensive review is an ambitious project.  

“One of the key challenges in writing this paper was to acknowledge the substantial body of work that describes intraspecific variation in plant phenotypes but does not consider plant functional traits,” Westerband explains. 

“In other words, in this review we focused on studies from the community ecology and trait-based ecology literatures where interest in ITV has been surging over the last several years. But we are well aware that evolutionary biologists and population biologists/demographers explicitly consider intraspecific variation, and so we introduce the paper by first exploring the rich history of examining intraspecific variation in plants. Along the same line, designing a conceptual figure that brought together these fields of study was also challenging, and we went through several iterations of the figure to ensure that it was sufficiently synthetic and broad in scope.”

The main drivers (black text) and consequences (blue text) of within-species variability from local environments to communities and ecosystems. Source: Westerband et al., 2021

Westerband and colleagues made 28 recommendations for experimental designs, data collection and analysis for future studies to quantify within-species variability. Trait databases are growing (e.g.  TRY, GRooT) but in order to compare variability reported of a species across the world, it is crucial to establish uniform research protocols.

“We emphasize, as others have before us, that there are several scenarios where analyzing species means rather individual replicates or populations means can produce misleading or erroneous conclusions. Therefore, caution should be taken when using species means, as they omit any potentially meaningful intraspecific variation. While it’s clear that the research questions should dictate the type of dataset used in the analysis, more and more researchers are accessing published datasets via online repositories, and many of these datasets report species means. Thus, we run the very real risk of being unable to draw meaningful conclusions at the level of individuals, which is the level of organization at which species interactions occur. This problem can easily be rectified by reporting population means or individual replicates instead of species means, and by quantifying the sources of variation in one’s dataset. 

“We also suggest more studies consider using nested study designs, and that more attention be paid to underrepresented traits, e.g. anatomy, defense, hydraulics, and traits that integrate across multiple functions. We summarize a number of key suggestions in Table 1.” 

This review is a fantastic synthesis from the driving forces of within-species variability to its importance to whole ecosystems functioning and provides a great resource to any plant ecologists. Look out for the Annals of Botany‘s Special Issue that showcases the newest research efforts embracing plant intraspecific variability.