Why are there differences in the growth of a dozen tree species in an Indian forest? Their rooting depth and ability to overcome exceptional droughts. And the most resilient are not the ones we may have imagined…
The members of the Franco-Indian Water Science Research Unit (Cefirse) would have been unable to demonstrate this without data from long-term observations: trees share vertical access to water in soil… “Every species therefore implements a different strategy: remain on the surface, plunge its roots to a depth of up to 30 metres, etc.”, states Laurent Ruiz, agricultural hydrologist (1). To achieve these results, Rutuja Chitra-Tarak, doctoral student at the Indian Institute of Science (2), used a traditional hydrological model. But she got it to work backwards!
A “niche” for every species
The idea came from an observation: when studying data on the growth of 7,677 trees, collected over 20 years by the BVET Observation Service (3) in Mudumalai National Park?in the state of Tamil Nadu, southern India, no logical pattern seemed to explain how each species responded to climate. “With models, more often than not the physiological parameters characteristic of the functioning of plants are imposed, and their growth is simulated in response to weather conditions, explains Laurent Ruiz. Here, we ran the model many times with a wide range of values for these parameters, particularly those which define the depth and efficacy of their root system. And we found that only certain combinations of parameters explained the growth dynamics observed. And these combinations differed for the 12 most represented species. Conclusion? Variations between these species could be explained by the differences in the relative depth at which they access water”. Researchers realised that these depths varied dramatically: from 2 to 3 metres for the most superficial to nearly 30 metres for the deepest. “It’s because every species has found its place, its “hydrological niche”, that they can all coexist”, insists Jean Riotte, co-head of Cefirse, who participated in research.
The roots of drought
The results extend beyond this initial discovery. Researchers had access to another set of data, independent from the first one: data on tree mortality. By comparing information on the depth of roots, prolonged dry spells, mortality, etc., the hammer dropped: the deeper a tree goes to draw water, the more vulnerable it is to extreme drought. To explain this counter-intuitive result, Laurent Ruiz recalls the two major strategies which govern life: the “acquisitive” strategy which consists of tapping into easily accessible yet intermittent resources as effectively as possible – as is the case of trees with superficial roots such as teak, for example. These species have become accustomed to competition and have built up a strong resilience to stress. The “conservative” strategy entails the ability to rely on less easily accessible yet permanent resources. While developing a powerful root system - reaching depths of tens of metres – is an ordeal, it allows them to avoid competition for surface water. These species find it hard however to withstand deprivation… “15 years ago the forest experienced an unusually long drought, which resulted in the temporary depletion of deep reserves. Our results show that the rise in mortality selectively targeted deeply rooted species. This is the first time a link has been established between sharing of “hydrological niches” and demographics on the scale of a forest community”, points out Laurent Ruiz. Should these extreme events increase under the effect of climate change, deeply rooted trees could disappear, thereby reducing biodiversity.
1. R. Chitra-Tarak, L. Ruiz, H. S. Dattaraja, M. S. Mohan Kumar, J. Riotte, H. S. Suresh, S. M. McMahon and R. Sukumar, The roots of the drought: Hydrology and water uptake strategies mediate forest-wide demographic response to precipitation,Journal of Ecology, 30 janvier 2018
2. The Cefirse International Joint Laboratory was created in 2001 by the Indian Institute of Science and IRD
3. SO BVET : Tropical experimental water catchment area observation system