Madelon Case – Harper Prize Shortlist

Throughout May, we are featuring all the articles that were shortlisted for the Harper Prize 2020. The Harper Prize is an annual award for the best early career research paper published in Journal of Ecology.

Madelon’s article ‘Root‐niche separation between savanna trees and grasses is greater on sandier soils‘ was one of those shortlisted for this year’s award.

About me

As a plant ecologist working in grassland, savanna, and shrubland ecosystems, I am keenly interested in understanding vegetation dynamics in the face of global change, and using this knowledge to inform the management of ecosystems into the future. In 2020 I completed a Ph.D. in Ecology and Evolutionary Biology in Carla Staver’s lab at Yale University. Prior to that I studied ecology as an undergraduate at Princeton University and spent a year in Vietnam as a Luce Scholar developing environmental education programmes.

I am now a postdoctoral scholar at the University of Oregon working with Lauren Hallett, as well as Kirk Davies and Chad Boyd at the Eastern Oregon Agricultural Research Center, on understanding annual grass invasion and rangeland resilience across the Northern Great Basin. I grew up in Oregon and first got hooked on plant ecology through a summer research programme in the sagebrush steppe in Idaho as a teenager, so my current work has given me a chance to reflect on what got me here, and I would be delighted to tell my 15-year-old self that this has turned out to be just as much fun as I imagined.

Shortlisted Article

Trees in savannas live surrounded by grasses, and savanna ecologists have long debated to what extent trees use deeper soil water than the grasses they compete with. I became fascinated by this question in the process of trying to explain some broadly observed patterns in savanna vegetation: that tree densities and rates of woody encroachment (increases in woody biomass and cover) are often higher on sandy soils. What if root-niche separation was not the same everywhere? Specifically, what if soil texture was shaping the rooting strategies of trees and grasses, and allowing trees to perform better on some soils than others?

Although the question of tree vs. grass rooting niches is an old one, it had mostly been examined at one site at a time and with varying methods, limiting our ability to make broader generalisations about variation across environments. For this paper, I measured tree and grass rooting depths at eight savanna sites in Kruger National Park, South Africa, across a rainfall gradient and a soil texture contrast, to gain a robust comparative understanding of variation in tree-grass niche separation. I used two different stable isotope methods for a comprehensive picture of rooting strategies, examining active water uptake (using water isotopes) as well as the distribution of fine roots (using carbon isotopes).

This research found soil texture to be a key determinant of root-niche separation between trees and grasses in savannas, with trees on sandy soils consistently using deeper water than surrounding grasses.

Trees on sandy soils consistently used deeper water and diverged more from grasses in how they distributed their roots, while trees on clayey soils overlapped more with the shallow root distributions and water uptake of grasses. In short, these niche differences should help trees perform better in escaping competition with grass on sandy soils. These findings help explain patterns in savanna vegetation that previously lacked a clear mechanistic basis, illuminating the belowground interactions that shape aboveground differences in savanna tree dynamics. This is also critical information for predicting the future of savannas under global change, as trees on sandy soils will be better able to take advantage of an expected shift to less frequent, larger rainstorms.

Madelon measured tree and grass rooting depths in the South African savanna of Kruger National Park. Photo: Madelon Case

This work was part of my Ph.D. dissertation at Yale University, as part of a broader examination of savanna vegetation dynamics and responses to rainfall variability. Out of my dissertation chapters, this one may have been my favourite, partly because I enjoyed the different hands-on skills it required: from navigating the savanna landscape and collecting plant and soil samples in the field in South Africa; to handling liquid nitrogen and a vacuum system to extract the water from those samples in an isotope lab at Kansas State University. The senior co-authors on this paper (Jesse Nippert, Ricardo Holdo, and my Ph.D. advisor Carla Staver) were excellent mentors and collaborators. I am also enormously grateful to have been able to do the fieldwork for this project in Kruger National Park, and for the support of the Scientific Services community there.

We announced the winners of the Harper Prize 2020 at the end of April, find out more on the blog.

You can also read all 8 shortlisted papers in our new Harper Prize 2020 Virtual Issue. These articles are free to read for a limited time!

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