In Insights we discover the story behind and beyond a recent publication in Functional Ecology. What inspired the authors to do the research, how did the project develop and what implications might their results have on the scientific community and on society?
This week, Sean Gleason from the Water Management and Systems Research Unit, USDA-ARS, Fort Collins, USA will illustrate his recent paper on predicting shrub and tree growth from plant traits (read the free plain language summary here.)
What is your field of expertise and why do you study it?
Like most large projects, the expertise for this study came from many different people with quite diverse backgrounds. The idea for the study was developed in Mark Westoby’s Lab at Macquarie University, Sydney, Australia, and was funded by an ARC Discovery Grant awarded to Mark. All co-authors worked closely together for two years developing the ideas, collecting the data, and interpreting the results. There were several different hypotheses at the start of the project, with each of us interested in different aspects of the study. As such, it was truly a group collaboration, drawing on the expertise and efforts of all.
For my small part, I am a plant physiologist and soil scientist. My research focuses on the evolution and diversity of water transport systems in vascular plant species. My primary aim is to provide an evolutionary perspective to the improvement of drought tolerance in crop species, and to provide a better understanding of the link between plant performance and plant physiology. What fascinates me most about plant physiology (and plant biology more generally) is that through measurement and experiment we are able to peer into the murky depths of an evolutionary legacy stretching back at least 400 million years… and provide an interpretation and context to the diversity of form and function that has arisen from these depths. Our job has all the making of a Jules Verne novel, no?
There are a lot of papers on using plant traits to predict plant performance. What makes your paper different?
Generally speaking, prior studies that have examined the link between traits and growth have focused on single sites, single species, or a more narrow range of traits (e.g., leaf or wood traits). Our study extends these efforts to a much broader range of species and habitats, and in so doing, greatly extends the level of inference, e.g., to woody dicots on the Australian continent. In addition to this, we present a new way to measure and think about growth (the extension and thickening of terminal branches) and show that this metric of growth is strongly aligned with height and the hydraulic functioning of species.
For your research you visited three sites across Australia. Was the selection to come to these three sites difficult? Which site did you especially like, and why?
Sites were carefully chosen by Don Butler in 2009, which were then part of larger group of sites that were included in a previous research effort, also in the Westoby lab at Macquarie. We settled on using the three sites described in our study because they represented a wide range of soil and atmospheric aridity. I suppose my favourite site was the topical site in Girringun National Park, Queensland. The site is located in a secluded corner of the park, situated on a high plateau (670 m above sea level), resulting in a quiet pleasant year-round temperature. The forest there is a wonderful transition from the savannahs to the west and rainforests to the east, and is an absolute tangle of tropical plants and animals.
How did your ideas for this research evolve during the project? If you ran into any difficulties, how did you overcome them?
The threads of several hypotheses were interwoven in the fabric of this project. The first thread examined a possible tradeoff between xylem density and growth rate, e.g. that high-density species should exhibit slower growth than low-density species. The second research thread postulated that xylem- and leaf-specific hydraulic conductivity would be the primary correlates with plant size, access to direct radiation, and growth. A third thread examined the possibility that leaf level traits would be strong predictors of growth in small plants, but not in large plants, e.g, that the drivers of growth would be size-dependant. Each of these threads matured in parallel with the others, and were refined and re-evaluated as the study progressed.
Yes, there were many difficulties. Access to the canopies of large plants growing in fantastically remote locations was one of them. After much deliberation, we decided to rent towable elevated lifts (“cherrypickers”) and take them to each our sites. Another difficulty was how to mark individual branches such that extension and diameter growth could be measured after one year. We were not sure what would work best, so we marked the initial measuring points using paint pens, flagging, and loosely fastened “twist ties”. None of these methods worked every time and at every site, but in nearly every case at least one method survived and allowed for an accurate measurement of growth.
You found a strong correlation between plant growth and plant height in an evolutionary context. Can you explain this a bit more?
I will give my personal view on this topic. We assume in our analyses that the variation in trait values represents intrinsic differences across species. In principle, natural selection could “tune” these values to any number that is biologically possible. There is no a-priori reason why any two traits must exhibit correlation. For example, natural selection could select for tall and slow growing plants in one case, but short stature and slow growth in another case. The same argument could be applied to all traits measured in this study, i.e., orthogonal relationships could exist among all traits. However, it is very clear that across present day species, this is not what natural selection has done. Rather, tall species exhibit strong tendency to have faster rates of growth, more conductive wood (greater capacity to transport water), and “leafier” branches than shorter species. It is my view that the alignment of these traits has been engendered by natural selection acting within the strong light gradients existing in forests. Tall plants have access to a much larger proportion of light than do small species living in the shaded understory (light extinction is exponential). All else being equal, greater light interception will drive higher rates of shoot-level evaporation (thus, the requirement for more conductive wood), net CO2 assimilation, and therefore growth. However, even if this interpretation is wrong, the important point is that correlations among these traits is not likely an evolutionary accident, but rather reflect a range of ecological strategies among the species.
Which new knowledge gaps did your research expose?
I think the relationships among growth, plant size, water transport, and photosynthesis are now fairly well understood. For me, the next important step for science should be to identify the genes that code for these traits as well as their regulation. A better understanding of the genetic underpinnings of these physiological traits will be necessary for the development of better performing crop species as well as understanding the impacts of climate change on vegetation communities.
In your conclusions section you highlight an opportunity to better study trait-trait relationships by focussing on species that did not match with your model predictions. Could you elaborate a bit more on what steps need to be taken?
One of the limitations of our study, and studies like ours, is that there is often much collinearity among traits. With the exception of height, the other traits in this study did not explain much independent variation in growth. However, if we want to know the true effect of a single trait on growth, we would need to allow this trait to vary whilst keeping the effects of all other traits constant. However, natural selection has resulted in such tight correlation among traits that we cannot separate them in this way, i.e., natural selection has confounded their independent effects. The point we make in the paper is that not all species were well-predicted by the model. For example, some small species did have relatively fast growth and some tall species did have relatively slow growth. Therefore, these species with large residual errors (not well predicted by the model) may represent opportunities to understand when height is required for fast growth and when it is not.
As a final question, a bit more of a personal one; how do you fill your free time?
I know it’s not very exciting for most people, but I suppose science is my hobby as well as my job, and so everything outside works seems to be an extension of my primary obsession. I like to tinker with programming and electronics. I like to spend time with my son in the outdoors. I like to read stories about the fantastic and the seemingly impossible.