Sarah Konare: soil exploration by trees in humid savannas

In this new post, Sarah Konaré, a junior scientist at the University of Korhogo, presents her latest work ‘Spatial heterogeneity in nitrification and soil exploration by trees favour source-sink dynamics in a humid savanna: a modeling approach’, discusses the importance of N cycle on savannas and talks about her future steps in ecology.

Sarah Konaré at the Lamto savanna. Photo: T. Srikanthasamy
Sarah Konaré at the Lamto savanna. Photo: T. Srikanthasamy

1) About the paper

Our paper highlights the importance of the soil exploration by trees and spatial heterogeneity in nitrification fluxes in the functioning of savannas. Savannas are spatially heterogeneous ecosystems characterized by a low soil nutrient concentration but including patches of higher soil fertility such as tree clumps. Savanna trees are able to directly affect their local environment by altering nutrient availability. In the literature, one of the factors explaining the  nutrient enrichment under savanna tree canopy is their ability to access resources by extending horizontally their roots outside their canopy projection. However, up to knowledge, the real influence of horizontal soil exploration has never been fully demonstrated. In the Lamto savanna, a humid nitrogen-limited savanna in Côte d’Ivoire, plants have developed different strategies to increase their nitrogen acquisition. Grasses inhibit nitrification while trees stimulate it, nitrification being the transformation of ammonium into nitrate, two forms of mineral nitrogen. Taken together, the main goal of our paper is to show through a modelling approach how this spatial heterogeneity due to nitrification control by plants and the horizontal soil exploration by trees in the open impact Lamto savanna nitrogen dynamics.

While Lamto savanna grasses limit their soil exploration to locally control their nutrient availability, our study shows that the ability of trees to explore horizontally their surrounding open area beyond their canopy increases their nutrient acquisition. This study also confirms that the horizontal soil exploration by trees in the open contributes to nutrient enrichment under tree canopy because nutrients absorbed by trees are then incorporated to the soil of tree clumps via litterfall and the decay of roots. Grasses appear as a nitrogen source (mainly under ammonium form) for trees as they help to better conserve nitrogen through the inhibition of nitrification. These nitrogen transfers through horizontal fluxes between the open and tree clump patches create source-sink dynamics, which provides new insights of the functioning of savannas as meta-ecosystems. Contrary to horizontal soil exploration that positively influences tree biomass, increasing tree cover tends to decrease tree biomass. This is a rather unexpected result but it is caused by the increase of total nitrogen losses following the increase of nitrate content due to nitrification stimulation by trees. We were wondering about the generalization of our results to other savannas where nitrogen fixation and herbivory are influential. Moreover, little is know about the frequency of plants controlling nitrification in other savannas. Studying this capacity of plants to control nitrification and taking into account herbivores and nitrogen-fixing trees are needed to extrapolate our results in other savannas.

2) About the research

Aerial view showing the patchy structure of the Lamto savanna vegetation. Here, we can see that patches of trees more or less clumped surrounded by open grassy areas. Photo: J. Gignoux
Aerial view showing the patchy structure of the Lamto savanna vegetation. Here, we can see that patches of trees more or less clumped surrounded by open grassy areas. Photo: J. Gignoux

Understanding the factors contributing to savanna functioning is important to predict the stability of savannas in the future especially in the current context of climate change. This study highlights new factors that are influential for the functioning of West African humid savannas. Beyond the importance of spatial heterogeneity in nitrification and soil exploration on savanna nitrogen dynamics, this study also addresses the question of the competition between trees and grasses for nitrogen in savannas. This could be relevant for the management of agroforestry systems. Our results could allow a better understanding of tree-crop-soil interactions and more specifically how the foraging strategies of trees and crops affect their nitrogen acquisition and crop yield, which could be useful for farmers and decision makers to ensure a more sustainable agriculture.

This study leads to new questions about other savannas where the interactions between fire, herbivory and nitrogen fixation impact nitrogen dynamics in different ways. Even if the two-patch model we propose here could be adjusted for these savannas, one of the major challenges is the lack of data about nitrogen cycling in savannas. Collecting data about nitrogen cycling in other savannas is thus needed before considering new studies through modelling approaches.

Moreover, we also know that trees generally have deeper roots than grasses. It could be interesting to take into account both horizontal and vertical soil exploration by tree roots to determine their respective influence on nitrogen dynamics and tree-grass coexistence.

3) About the author

View of the Lamto savanna landscape. Photo: S. Barot
View of the Lamto savanna landscape. Photo: S. Barot

I defended my PhD in Ecology last year and now I am a junior scientist at the University of Korhogo in Côte d’Ivoire. After showing the influence of spatial heterogeneity and horizontal soil exploration on nitrogen fluxes, the next step is to study their effects on the coexistence between trees and grasses. I would like to build on the long term an individual-based model including all mechanisms involved in the functioning of savanna ecosystems to better predict their stability to the increasing CO2 concentrations, nitrogen depositions or changes in the rain regime. Because I was involved in theoretical models during my PhD, I would like to switch towards more practical topics such as the study of nutrient cycling in agroecosystems to improve crop yield.

I really like spending time in the field. For example, one of the best time during my PhD was to visit the Lamto savanna in Côte d’Ivoire and the Kruger national park in South Africa. I think that the best thing about being ecologist is to bring my little contribution to ecosystem conservation by providing a better understanding of ecosystems through my researches. In my spare time, I like cooking and testing new recipes.

Read the article in full here

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