This year’s Harper Prize winner is Rutuja Chitra-Tarak for her paper; The roots of the drought: Hydrology and water uptake strategies mediate forest‐wide demographic response to precipitation. Rutuja received a PhD from the Indian Institute of Science, Bangalore, went onto to do a post-doc at the Smithsonian Environmental Research Center, USA, and is now a post-doctoral researcher at the Los Alamos National Lab in New Mexico, USA.
Q. In your paper, you present a new eco-hydrological framework to investigate how different tree species experience drought. Can you explain your new approach?
Let me begin by giving some background. Trees responses to drought are normally evaluated as a function of variation in rainfall, or top-soil moisture at best. Missing in this scheme is the water availability in the entire rooting zone of the tree. This is important because actual water availability below-ground may be out-of-sync with that present close-to-the ground or from the meteorological drought. Moreover, it is the interaction of many factors that combine to determine actual water-uptake and therefore the experienced drought. These include: trees’ rooting profiles, available water where trees’ roots are, tree hydraulic traits and atmospheric conditions. As a result, species present in the same forest stand may be experiencing a meteorological drought differently. However, our ability to implement this entire scheme of drought experience from bottom-up is marred by data limitations. For example, lack of direct observations of entire rooting or water-uptake profiles for the diversity of trees in a forest, or water availability in the entire rooting zone.
Our new approach implements a scheme of drought experience by using inverse inference on components of water uptake that cannot be directly observed: we used a calibrated hydrological water balance model to estimate 20-yr long temporal water dynamics by depth belowground for the entire rooting zone of a seasonally dry forest of Mudumalai in the Western Ghats biodiversity hotspot in southern India. We then built a simple model of daily water-stress as a function of water-availability by depth, atmospheric water demand and species-specific phenomenological parameters for, what may be termed as, rooting densities and hydraulic restriction by depth. We parameterized this model for species and different stem size classes using 20 year growth census records in the Mudumalai ForestGEO plot. We were essentially asking: to the extent that water-stress determines tree growth, what should be the water-uptake of a species so as to result in the observed growth during drought and non-drought periods? This scheme allowed us to rank species with an index of relative water-uptake depths. We could then evaluate species drought-induced mortality in relation to this index.
Q. What were the challenges associated with this new modelling framework?
One challenge of developing a new and interdisciplinary framework like this, and also an opportunity, was to bring in diverse people together, which I personally enjoyed. We, the close collaborators on this work, learnt principles across disciplines–catchment scale hydrology versus life-history strategies and trade-offs. Exciting exchanges, and we were all richer for it. The new approach was also personally challenging. When I began this project, I was a PhD candidate at the Indian Institute of Science, Bangalore, with a background in ecology and evolution, and there I was learning hydrology at the height of my PhD candidacy, embarking on a risky project. And then I had to learn to explain this interdisciplinary work back to ecologists.
Another challenge with such an interdisciplinary framework is that the data required to execute it is diverse and not readily available. It is hard to find forested catchments that are adequately hydrologically instrumented and also have long-term forest dynamics data at the same location. We further required data on water-uptake depths for model-testing. The framework as it is envisioned can do with less data, because it capitalises on emergent outcomes rather than hard-to-collect intermediate data: for example, instead of missing soil moisture data for the entire rooting zone the framework simulates it such that the catchment level hydrological balance is well calibrated. Or in the absence of observed water-uptake profiles, rooting strategies are estimated such that they best explain observed tree growth through drought. But until this framework is well established, data needs will be a challenge.
Read the full paper online: The roots of the drought: Hydrology and water uptake strategies mediate forest‐wide demographic response to precipitation. You can also find the rest of the shortlisted papers in a special virtual issue on the journal website.