Forests, often referred to as the “lungs of the planet,” play a crucial role in regulating the Earth’s climate and sequestering carbon dioxide. To understand the effects of climate change and formulate mitigation measures, it is important to know how climate change affects the biogeochemical cycles of forest, especially carbon and water cycles that sustain life on our planet. In terrestrial ecosystems, as plants fix carbon dioxide from the atmosphere through photosynthesis, plants transpire water vapor through stomata. While carbon and water fluxes remain tightly coupled, the study of the eco-physiological indicators of their efficiency is limited in the Indian context. Water use efficiency (WUE) is one such indicator of the water use strategy of different ecosystems. Transpiration and photosynthesis are coupled processes and the ratio of photosynthesis and transpiration is defined as WUE at leaf level. At the ecosystem level, WUE is defined as the ratio of gross primary productivity (GPP) to evapotranspiration (ET). WUE is crucial to understand how different forest ecosystems respond to water availability.
In the present study, WUE of major forest formations of India was calculated from 2003 to 2018 as the ratio of Moderate Resolution Spectroradiometer (MODIS) Gross Primary Productivity (GPP, MOD17A2H) to evapotranspiration (ET, MOD16A). The spatial distribution of WUE was mapped and the inter-annual (Fig. 1) and monthly variations (Fig. 2) were analysed. The mean annual WUE of forests ranged from 1.78–2.02 gC kgH2O-1. The highest WUE was observed during the dry season. It was observed that tropical thorn forests had the highest WUE (3.52 ± 1.08 gC kgH2O-1) and moist alpine scrub had the lowest WUE (1.05 ± 0.17gC kgH2O-1) among the forest type groups of India (Fig. 3). WUE of forests showed an increasing trend with latitude and decreasing trend with elevation. The study also examined the impact of various bio-meteorological drivers on WUE and their significance in regulating these effects. The bio-meteorological drivers were able to explain 65% of the variability in WUE. Temperature was identified as the most important driver in influencing the WUE of forests. Based on the findings of the study, it can be expected that the global increase in temperature would negatively affect the WUE of the major forest formations of India.
Overall this study presented the first investigation of the variation of WUE in major forest type groups of India, to give an insight into how forest ecosystems respond to various driving factors especially climate. A better understanding of WUE will help to improve ecosystem management for mitigation as well as adaption to global climate change. The tropical wet evergreen forest and subtropical pine forest showed an increasing trend in WUE. This might be an indication that these ecosystems could be getting drier, as drier ecosystems are known to have higher WUE. Further investigations are suggested in these ecosystems to understand the underlying cause and mechanism for such change in WUE. It is recommended to establish a network of eddy covariance flux towers, if possible, one flux tower in each forest type-group to understand the WUE and the underlying mechanisms. Availability of this kind of data will be helpful in the validation of such remote sensing-based studies.
For further detailed insights, please refer to the following publication mentioned in the References.
References:
Nandy, S., Saranya, M., & Srinet, R. (2022). Spatio-temporal variability of water use efficiency and its drivers in major forest formations in India. Remote Sensing of Environment, 269, 112791. https://doi.org/10.1016/j.rse.2021.112791