Monique Weemstra: how do trees modify their roots to adapt to their location?

We welcome 2021 with a new post by Monique Weemstra, a postdoc at the University of Michigan. Here she talks about her latest research looking at how trees can modulate their root traits to account for environmental gradients and the importance of working with people enjoying ecology as much as you do.

About the paper

This study is a part of the ECOPICS project: a French–Mexican research collaboration that examines various aspects of belowground ecosystem services along a 1000m elevation gradient in a temperate (French Alps) and in a tropical (Pico de Orizaba, Mexico) system; we study soil fauna, microbial, edaphic and plant community responses to, and effects on ecosystem functioning. This specific paper zooms in on plant responses at the species level along the gradient in the French Alps: how do root functional traits differ between plants of the same species growing along this gradient, and are there general patterns in this trait variation between species?

It turns out that there are different ways roots adjust to their environment: species differed in which, how and to what extent root traits varied along the gradient. For example, among our 11 study species, we found an increase, decrease, a U-shaped, a bell-shaped and no significant pattern in root diameter across elevations. The local soil environment also seemed an important driver of root trait variation: surprisingly, conspecific plants at different elevations had more similar root traits than conspecifics in different replicate plots at the same altitude.

The team on its way to the highest sampling plot (top of the upper ridge)
The team on its way to the highest sampling plot (top of the upper ridge)

Our study implies that we need more in-depth studies to learn how plants deal with their environment. In the first place, along natural environmental gradients, many variables change in different directions and to different degrees at the same time, and species may not be equally sensitive to these changes, e.g., trees may be more susceptible to temperature changes than grasses. In turn, these different limiting factors along the gradient and between species may select for different root trait expressions which likely contributes to the idiosyncratic patterns we observed – e.g., temperature may limit root growth, while nutrient limitations may alter root diameter.

Secondly, we probably only showed the tip of the iceberg regarding the belowground strategies of plants, and there are likely many more that we did not quantify. For example, plants can also enhance root biomass or regulate root exudation to increase resource uptake and/or availability, and they can simultaneously change aboveground traits. Such traits are relatively little quantified but may prove crucial plant responses to a changing world.

Finding the right grass species: Monique Weemstra, Nereyda Cruz-Maldonado (Ph.D. student, AMPA, France), Dr. Leonor Jiménez (INECOL, Mexico)
Finding the right grass species: Monique Weemstra, Nereyda Cruz-Maldonado (Ph.D. student, AMPA, France), Dr. Leonor Jiménez (INECOL, Mexico)

These results lead to some interesting next questions to address, for instance 1) which are the main environmental drivers of intraspecific root trait variation and do these differ between species? 2) Which other – less commonly measured – root traits vary, and how, in response to environmental gradients? 3) And how do such belowground responses interact with aboveground responses, and how do these interactions the plant as a whole? Based on our work, answering these questions will improve our understanding of how plants cope with environmental change.

About the research

Our camera man/field and lab assistant/driver/cook/my husband looking for a good spot to film: Hans Reijnen
Our camera man/field and lab assistant/driver/cook/my husband looking for a good spot to film: Hans Reijnen

Our root data were collected by a great team of ~10 students and (international) researchers during an awesome four-week field campaign in the French Alps. We sampled roots (and leaves) along the 1400 – 2400 m gradient. For small species, this meant digging up the whole plant, but for trees and shrubs, we had to follow the big roots from the base of the stem until we reached the absorptive roots (the most distal roots that take up resources) to make sure we were collecting roots of the right individual – this was a challenging part; on the rocky soils of mountains, roots can grow in very unexpected directions: after tracing a tree’s big root, it would suddenly grow very deep to avoid a big boulder where we could no longer access them and had to start again (see the photo).

Scanning roots in the lab/bedroom/laundry room: Monique Weemstra
Scanning roots in the lab/bedroom/laundry room: Monique Weemstra

After collecting the roots, they were processed in the multifunctional lab/bedroom/rec room/laundry room in our chalet. The main challenge here was to select the absorptive roots; selection criteria can be root order, thickness, sturdiness, etc., but these criteria may differ between species and are partly subjective. For example, for grasses, third-order roots (i.e., less distal roots) were thicker and sturdier than the lower order roots (e.g., the root tips), and could thus be considered transport (so, non-absorptive) roots; however, for some grass species, these roots were densely covered by root hairs which play a large role in resource uptake, but which cannot be removed and analysed separately from the main root – the question then arises: do we include these roots in our sample or not? (Yes, we did.) To make sure we compared ‘functionally similar’ roots for the same species along the gradient, we had to make very detailed descriptions and drawings of our species-specific selection criteria and only one person (me…) selected the absorptive roots to avoid observer bias.

About the Author

Monique Weemstra

I am a plant ecologist interested in roots and how they vary and interact with their (a)biotic soil environment, and with aboveground plant parts. For this project, I did a Postdoc in Montpellier, France and got to work with some of the experts (and great colleagues) in plant (root) ecology and to spend several weeks in the Alps and in Mexico with them to collect data in these spectacular mountain systems. Since January 2020, I am happy to work as a Postdoc at the University of Michigan where my work on roots brought me to the beautiful Michigan forests. Despite covid-19, I was and am lucky enough to meet and share some good times with my colleagues here. In this position, I study how, and which leaf and root trait combinations best explain tree demography in temperate forests.

My advice based on working in the Ecopics project is to try to find collaborators you enjoy working with – they can make root washing fun, lift you up when manuscripts get rejected, teach you about roots, or earth worms or alpine growth forms, share your joy when you are selected for an oral talk, and make you appreciate the beauty of your field work environment even more! Although this is partly a matter of luck, I recommend to actively search for a good team that matches your personality: do not only sign up for a project because of its scientific opportunities, but also use whatever influence you have to surround yourself with great people! Our field work campaigns in France and Mexico – where we carried out a parallel study with our colleagues at INECOL – really were some highlights of my professional and personal life, to a large extent due to the great group of colleagues I worked with. My search for a nice lab and great PI’s, as well as joining some new research consortia (online for now…), also definitely paid off this year to get me through these crazy pandemic times.

Read the article in full here.

PS. For some beautiful images of the French Alps and some more background of this study, please check out our video:

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