How to Live on a Hot, Dry Rock: Different Solutions to the Same Problem

After colonising the dry land some 470 million years ago, plants have spread throughout the world, and now they can be found on all continents: from the Antarctic pearlwort in the Antarctic Peninsula to the Rhododendrons of the Himalayan ecosystems. Despite this undisputed success, one can imagine that not all places are easy to live in, and, for instance, there are some environments where conditions are so adverse that scientists have decided to call them “extreme environments“.

A great example of this kind of environments are inselbergs, which are enormous granite outcrops that protrude in various places all over the world. In these places, plants establish themselves almost directly in the rock or very shallow soils. When growing in such a substrate, plants face highly stressful conditions, such as immense temperature changes throughout the day (from 18 °C at night to 60 °C at midday) and little water availability. Also, because they are very ancient rocks –about 540 million years old– almost the nutrients they could offer are already gone. As a result, inselbergs bring together all the elements necessary to make plant life extremely challenging. Yet, against all odds, these towering monoliths are home to a great richness of species, including several that can only be found in these places. The question is: how do they manage to live in such a stressful environment?

Ecologists expect species will tend to have the same characteristics in ecosystems with such strong environmental constraints. In the case of inselbergs, species will necessarily possess characteristics that allow them to tolerate the stress that comes from both high temperatures and low availability of nutrients and water in the soil; otherwise, they would not be able to live there! However, is it possible that in this environment, with so many different species, each and every one of them really behaves in the exact same way? Such questions were addressed by Colombian researcher Lina Aragón in her Master’s thesis, which was recently published in the Annals of Botany.

Aragón study takes us to Bojonawi Natural Reserve, a private protected area on the Northwest border of Colombia, where it is possible to find some outcrops of the Guiana Shield –one of the oldest geological formations on Earth. She first knew this place during the last year of her undergrad studies at Universidad de los Andes. Her mentor and supervisor, Dr Eloisa Lasso, was looking for a student to come as a field assistant for one of her projects and, in an unexpected turn of events, Aragón was the only one available! Two days later, they were embarking on a journey whose scenario, in Aragón’s words, could only be described as “breathtaking”: big rock outcrops between the river and the tropical forest with a mesmerising view of the savanna on the horizon.

However, in an interview with Botany One, she confessed that the plants living in the inselbergs were her weakness. She said: “I could not believe they could live in a rock that by noon was 60 ºC. I could not understand how they grew on a surface without soil, withstanding high radiation, and more than 4 months without water. After coming back, I knew I would do something there.” In fact, she did return to Bojonawi and, for her Master’s research, she and her team characterised the morphological and physiological characteristics of three dominant and endemic species from different families: Acanthella sprucei, Mandevilla lancifolia, and Tabebuia orinocensis.

Specifically, they measured 22 characteristics related to water regulation, leaf structure, and photosynthesis, including leaf area and thickness, stomata size and density, and maximum photosynthetic capacity. This thorough analysis allowed them to better understand the strategies these plants exploit water, carbon and light. Still, such analysis in the conditions of Bojonawi Natural Reserve was anything but easy, as it involved transporting two suitcases worth thousands of dollars to a location around 700 km away from Colombia’s capital, lengthy conversations with airport staff explaining why they were transporting such things in the first place and even bringing a massive and noisy electric charger that could sustain the measurements in the scorching weather of the site. Even so, the efforts of Aragón and his team paid off, as this detailed analysis helped to better understand how these plants adapted to these adverse conditions.

As expected for these environments, all three species exhibited morphological characteristics commonly associated with stress tolerance, such as small, thick leaves with high dry matter content. Such leaves are optimal for stressful environments because their reduced size prevents them from being too exposed to sunlight and losing excessive amounts of water through transpiration. Moreover, this kind of leaf is associated with resource storage, a strategy that one would think is very useful in environments where nutrients are extremely scarce.

However, when physiological characteristics come into the picture, things get interesting as they allow us to see that, despite the apparent morphological similarity of the plants, there are very important differences in their physiology. In other words, while the structure of the leaves of these shrubs is quite similar, the way they use water, carbon and light is not! 

On the one hand, they found that A. sprucei had a quite risky strategy regarding water use when compared to the other two species, as it had the highest stomatal density, and water could quickly escape from them. Stomata are tiny hole-like structures that allow carbon dioxide to enter and oxygen to leave. But it’s not just oxygen that exits, water does too. This outflow of water through the stomata generates the necessary force for water circulation in plants. These characteristics imply that A. sprucei can transport a lot of water very easily. However, having so many stomata means that the chances of water loss are even higher, making it easier for plants to dehydrate!

Still, M. lancifolia showed a strategy related to a higher carbon acquisition, as its leaves were relatively thinner and lighter than the other ones but had higher photosynthetic capacity, meaning that they were designed to capture carbon rather than store it. Contrastingly, A. sprucei leaves were thicker and had less carbon assimilation capacity, suggesting they were specialised in storing carbon rather than capturing it.

These results are consistent with what the authors found for photosynthesis traits, as A. sprucei has a low light compensation point compared to other species, meaning that it requires less light for photosynthesis to generate enough energy to maintain its metabolism and start storing carbon. In contrast, the other species, particularly T. orinocensis, require more light to have a positive carbon balance.

Altogether, Aragón and colleagues’ research highlights the importance of assessing both the morphology and physiology of plants to get a more complete picture of the strategies they use to occupy certain environments and the mechanisms that make those strategies possible. As Aragón told us in our interview, “Some time ago, these species passed the environmental filter imposed by the edaphic and climatic conditions of Inselbergs. After that, they have been using the available and limited resources differently to ensure their co-occurrence.”

Another exciting result of this research is that easily measured anatomical traits, such as stomatal density and size (which were measured by taking prints using nail polish!), can provide vital information about how plants use carbon and water. For instance, they found to be highly correlated with other traits that requires sophisticated equipment, such as carbon assimilation. As a result, its inclusion in future studies promises valuable insights into a plant’s physiology without the need for costly or time-consuming tests –an aspect that would be particularly important in remote sites, such as Bojonawi Natural Reserve. More importantly, this study indicates that different plant species can employ distinct ecological strategies to overcome the same challenges even when growing side by side in the harshest conditions. Aragon’s fascination with extreme environments has not been extinguished. Now a PhD student at the University of Miami, she is studying the páramo, an open ecosystem that arises at the top of the mountains of the American tropics, above the forests, where low temperatures and high radiation are the rule rather than the exception. We hope that in the future, Aragon will continue to surprise us with new discoveries about how plants manage to establish themselves in the most unexpected places.

READ THE ARTICLE:
Aragón, L., Messier, J., Atuesta-Escobar, N., & Lasso, E. (2023). Tropical shrubs living in an extreme environment show convergent ecological strategies but divergent ecophysiological strategies. Annals of Botany, 131(3), 491-502. https://doi.org/10.1093/aob/mcad002

Carlos A. Ordóñez-Parra

Carlos (he/him) is a Colombian seed ecologist currently doing his PhD at Universidade Federal de Minas Gerais (Belo Horizonte, Brazil) and working as a Science Editor at Botany One. You can follow him on Twitter at @caordonezparra.

Spanish and Portuguese Translation by Carlos A. Ordóñez-Parra.

Cover Photo by Lina Aragón.