Forests world over are under threat. About 35 million hectares (Mha) of forests globally have been destroyed due to urbanisation, while urban land has increased from 33.2 to 71.3 Mha between 1992 and 2015. India is witnessing the highest rise in deforestation since the last 30 years, with a stark surge recorded between 2015 and 2020, and has been ranked second after Brazil, with an average deforestation of 668,400 hectares.
Forests cover about 24.62 percent of India and are some of the most biodiverse in the world. They provide a range of important ecosystem services, such as protecting against soil erosion, regulating the water cycle, and serving as a home for a wide variety of plant and animal species. However, they are facing the increasing threat from a range of human induced activities such as illegal logging, mining and land conversion for agriculture and urban development.
Almost 45-64 percent of India’s forest cover will fall under climate hotspots by 2030 and the entire forest by 2050. It is predicted that by 2085, 20 percent of India’s forest cover may experience “catastrophic changes due to adverse impact of climate change.
Why are forests important?
Forests and vegetation have an important role to play in the water cycle. Transpiration from vegetation plays a vital role in the atmospheric component of the hydrological cycle. Approximately 64 percent of global Evapotranspiration (ET) is contributed by transpiration from vegetation.
Evapotranspiration (ET), is the sum of all processes by which water moves from the land surface to the atmosphere via evaporation of water from the soil surface, from the capillary fringes of the groundwater table, from water bodies on land and transpiration through plant canopy when plants take up liquid water from the soil and release water vapour into the air from their leaves.
Forests bring rains
Every tree in the forest performs a unique function - of sucking water out of the ground through its roots and releasing water vapor into the atmosphere through pores in its leaves. Trees in large numbers thus generate huge collections or reservoirs of water in the air that form clouds that travel and bring rains in faraway places. But destruction of more and more trees is increasing the risk of drying up of these aerial rivers and the lands that depend on them for rain.
Forests generate a cooling effect
Forests balance the local climate by producing a cooling effect partly by shading the land, but also by releasing moisture from their leaves through transpiration, which requires energy and is extracted from the surrounding air, thus cooling it. A single tree can transpire hundreds of litres of water in a day.
Deforestation since the 1850s has been found to contribute to global warming by 40 percent, and it is feared that tropical deforestation could add 1.5 degrees Celsius (2.7°Fahrenheit) to global temperatures by 2100. Carbon dioxide emissions can add 10 percent or more to global warming due to deforestation by reducing the quantity of carbon dioxide that the forests normally pull from the atmosphere.
What happens during dry spells?
India receives more than 70 percent of its annual rainfall in the summer monsoon from June to September, while the rainfall is scanty and scattered over the rest of the year. How does the forest system and rainfed agriculture sustain during the rest of the year when there are no rains?
The Indian landmass is known to be one of the global land–atmosphere hotspots further threatened by by high irrigation. Studies have found that ET helps in sustaining the retreating monsoon, but there is very little understanding of how ET is sustained and how it helps in plant productivity after the monsoon and the major cropping season argues this paper titled 'Soil–vegetation moisture capacitor maintains dry season vegetation productivity over India' published in Nature Scientific reports.
The paper discusses the findings of a study that analyses the role of soil moisture and vegetation in supporting forest systems and rainfed agriculture in dry periods in India.
This study finds that:
The role of vegetation in sustaining the moisture supply to the atmosphere is pivotal
For example, the rainfall reaches its peak during the first half of August and starts withdrawing in September in India. The surface soil moisture follows the same pattern as rainfall while ET continues to increase during the monsoon withdrawal phase. However, soil evaporation immediately starts dropping after the peak of the monsoon and follows the same pattern of rainfall and surface soil moisture. Transpiration from vegetation drives the increased ET during retreat of the monsoon.
This high ET during the withdrawal of the monsoon is a major contributor to atmospheric moisture loading over the Indian sub-continent. Hence, the role of vegetation in maintaining water cycle during the monsoon withdrawal is very large.
Vegetation also plays an important role in sustaining the land- atmosphere interactions during drier non rainy periods.
The ability of vegetation to store water during periods of high rainfall and releasing that water to the atmosphere during drier periods is called moisture capacitance.
There are three phases after the peak of monsoon rainfall that affect evapotranspiration:
During the first phase, there is an increase in total ET and transpiration due to the high moisture availability from rainfall and radiation. The average daily ET increases by almost 13 percent during this period despite a 55 percent decline in rainfall from its peak value.
This period thus has a moisture surplus because rainfall in this period is higher than ET. Therefore the increase in ET in this period is because of the high radiation. The rainfall during this period accounts for almost 28 percent of the total annual rainfall, while the ET is approximately 20 percent of the total annual ET. The daily average incoming radiation is 97 percent of its annual daily average. This period lasts for approximately 50 days.
The second phase is the time interval during which ET drops from its peak and rainfall still exceeds ET, the pre-capacitor period, which extends for approximately 50 days and witnesses a drop in both rainfall rate (57 percent) and radiation (18 percent), leading to a decrease in ET (33 percent) from the start of the pre-capacitor period.
However, the decline in ET is slower than that of rainfall. The total ET during the pre-capacitor period accounts for approximately 17 percent of total annual ET while the rainfall is 12 percent of total annual rainfall.
The third phase, i.e., the capacitor period is the phase during which the ET is higher than rainfall. While both rainfall and ET rates decline by approximately 2 percent, ET during the capacitor period accounts for almost 25 percent of the total annual ET, which is mainly because of transpiration (27 percent) of total annual transpiration. Rainfall during this period is only 12 percent of total annual rainfall.
How do ET and the associated plant processes sustain in a situation where water is limited due to less rainfall?
The radiation starts increasing after the monsoon withdrawal, the winds also change direction with the transition of the season. The ET processes during this period are sustained by the soil-vegetation continuum. The soil and vegetation receive moisture during the first and second phases due to the monsoon. This stored moisture is released to the atmosphere during the relatively dry period and supports plant activities.
Hence, the soil moisture and vegetation can act as capacitors in the Indian monsoon water cycle and these capacitor days typically extend for~135 days when spatially averaged over India.
Larger depletions of root zone soil moisture are observed in the Northern and Central zones, while the Western Ghats and Northeast zones, which receive rains during pre-monsoon season experience less depletion of root zone soil moisture. The Western zone with large areas under desert or scarce vegetation also show a lower rate of depletion of root zone soil moisture.
Forests and soil moisture act as buffers to store water during dry periods
The study thus concludes that the water stored by the soil–plant continuum influences the post-monsoon terrestrial water cycle over India. This stored water helps in meeting the crop and vegetation needs for the rest of the year during periods of reduced precipitation. The capacitance of soil moisture –plant continuum maintains ET, productivity, and land–atmosphere feedback by exploiting this water storage capacity.
Thus forests and soil moisture play a crucial role and act as buffers to store water on land even after the decline in monsoon precipitation and then use it during dry periods.
While climate change induced global warming threatens to generate heat waves, bring about monsoonal changes and increase frequency of droughts in India, the need to preserve forests and prevent deforestation is even more urgent in India. Huge destruction of primary forests in the name of infrastructural projects in the country needs to stop.
This is an open access paper under the Creative Commons Attribution 4. International License
