- Drought has been an important driver that shapes ecosystems over millenia. However, the global area under drought is set to skyrocket to about 40 percent, as the globe warms by 3-4 degrees Celsius.
- The number of heatwaves is also set to increase across the globe. Rather than hotter conditions dominating in drier areas, wetter areas are also succumbing to heatwaves.
- In this commentary, Sandhya Sekar explores the individual effects of drought and heatwaves on forest ecosystems.
- In another story published today, we examine the effect of concurrent drought and heatwaves on forest ecosystems.
About 10 percent of the world’s land surface experienced drought in 2005. The number is set to skyrocket over the twenty-first century to about 40 percent, as the globe warms by 3-4 degrees Celsius. Apart from the extent of drought, the frequency is also set to increase, especially in drier areas.
A recent study has used climate models to show that by 2060-2080, most tropical regions within the 30 degree latitude of the equator may experience extreme temperatures for between 25 and 150 days per year; currently, such temperatures occur very rarely, just once a year.
With drought and heat posing individual threats, there is also the looming threat of frequent ‘double whammies’ of drought and heat: concurrent drought and heatwaves, across India and the globe. India experienced an increased number, and more frequent, concurrent drought and heatwave events between the 1981-2010 duration as compared to 1951-1980.
How will forest areas react to such extreme climates?
When the land dries up
Drought is a naturally occurring phenomenon in which rainfall is significantly below ‘normal’ recorded levels, which have been established through long-term observations. Drought has been an important driver that shapes ecosystems over millenia — the very composition of a forest is determined by its evolutionary history. Dry areas and those with uncertain rainfall support plant species which are adapted to such conditions.
When experiencing drought like conditions, plants sense reduced soil moisture and immediately reduce their net primary production through a variety of mechanisms. The first response in many species is closure of stomata, the tiny openings that allow plants to ‘breathe’ — to take in carbon dioxide and give out oxygen. When stomata close, carbon dioxide uptake is obviously reduced, which can affect photosynthesis. When photosynthesis continues unabated during drought stress, the limited carbon dioxide concentration results in the accumulation of reactive oxygen species, which can cause severe damage to the photosynthetic machinery.
Different types of forests and different species within each forest type react to drought differently. For example, deciduous species shed their leaves during the dry season to reduce water loss through transpiration — since the species are adapted to a prolonged dry spell, their life cycle allows them to ‘escape’ drought. Some other species ‘escape’ by growing rapidly and producing seeds before soil water reduces, while others grow very slowly during the dry season and go into a growth spurt at the first hint of the rains.
Some plants show ‘drought avoidance’ — they maintain higher tissue water content by minimising water loss by reducing transpiration, or increasing water uptake through mechanisms like increased rooting and water conductance.
If with climate change, there is an alteration of distribution of climatic types and disturbances, forest areas across the globe can be affected, especially forest communities at the margins of their most suited habitat.
Droughts and climate related disturbances also influence the survival and spread of insects and pathogens and affect the susceptibility of the forest ecosystems. Global warming can affect species abundances, including those of herbivores; compared to the cooler Paleocene geological era, the Eocene had a greater diversity of herbivores and higher attack rates on abundant tree species. Increased warming would probably increase the diversity of insects in the higher latitudes — many temperate species are likely to encounter unfamiliar pests that were previously restricted to tropical and subtropical areas.
Dry, hot areas are prone to naturally occurring wildfires, which, in turn, lead to soil erosion, loss of biodiversity, carbon emission and other forms of land degradation. Drought can also slow down decomposition, allowing for a buildup of organic matter that can increase fire frequency, besides locking down important nutrients that would be otherwise available to the ecosystem.
Since the middle of the twentieth century, India has experienced some particularly severe droughts. The 1987 drought registered an overall rainfall deficiency of 19 percent, affecting about 60 percent of the cropped area and a population of 285 million. The 2009 drought was even worse, with a rainfall deficiency of 22 percent; it resulted in decrease of food grain production by 16 million tonnes.
During 2014-15 and 2015-16, large parts of the country were affected by drought. The impact of the 2015-16 drought was substantially magnified because it was close on the heels of an extensive drought just a year earlier.
The Indian Meteorological Department (IMD) has 36 meteorological subdivisions which categorise rainfall as excess, normal, deficient or scanty. The possibility of drought arises when the rainfall is deficient or scanty. Data from the IMD has shown that there is an increasing trend in the spatial extent of droughts in central, northeast and west-central India. There is a significant decline in the monsoon rainfall over the Indo-Gangetic plains, and subsequently, the area of plains affected by drought.
The Manual for drought management, released in 2016 by the Ministry of Agriculture & Farmers Welfare, Government of India, attributes India’s high propensity for drought to the seasonality of the Indian summer monsoon. “Almost three-quarters (73 percent) of the total rainfall falls over less than hundred days, between June and September,” states the manual. Even if the country registers normal rains on an average, large variations within the same state — or even within the same district — is common.
However, the factors that influence the monsoon (and in turn drought events) are numerous and complex. Anomalies in the El Nino – Southern Oscillation (ENSO), a recurrent climatic pattern involving changes in the temperature of ocean waters in the Pacific, have been cited as a major influence, but the actual cause-effect link remains unclear. While severe droughts in India have always been accompanied by El-Nino events, not all El-Nino events have resulted in severe droughts.
There are multiple environmental impacts of drought: low water levels in surface reservoirs, loss of forest cover, migration of wildlife and sharpening man-animal conflicts; loss of wetlands and reduced surface water can affect salinity. Increased groundwater usage without adequate recharge can damage aquifers and adversely affect the quality of groundwater, which can in turn affect soil productivity.
Too hot to handle
The number of heatwaves is set to increase across the globe. Broadly speaking, heatwaves are periods of consecutive days when conditions are excessively hotter than normal. A study comparing temperature and precipitation between the periods 1951–1977 and 1978–2004 found that both hot/wet and hot/dry conditions were increasing substantially worldwide. Rather than hotter conditions dominating in drier areas, wetter areas are also succumbing to heatwaves.
Some regions most at risk for extreme heatwaves are north and northeast India, east China, west Africa and southeast United States. Population density is expected to increase in India and west Africa over the 21st century, putting potentially millions of people at risk of exposure to extreme heat. Continued urbanisation will increase the prevalence of urban heat islands, which can raise air temperatures by several degrees celsius.
“Extreme heat is poised to become one of the most significant and directly observable impacts of climate change in the coming decades,” states a 2018 study.
How plants react to heat
The effect of high temperatures on plants can be seen in processes at the molecular level to the whole tree and varies among species. Many physiological processes in trees are affected by extreme heat, most importantly photosynthesis. When there is enough carbon dioxide, the rate of photosynthesis first goes up until about 35 degrees celsius; after which, enzymes start breaking down, which happens at different temperatures for different species.
Heat also affects respiration, transpiration and the production of volatile organic compounds — all critical processes that affect plant survival. At the scale of the whole tree, excessive heat can cause decrease in growth, leaf development and leaf area. Many species are especially vulnerable in the seedling stage, when the soil is exposed to full sun as the tender seedlings emerge. For perennial species, a warm growing season can have a long lasting effect, and cause reduction in growth in subsequent years. When combined with drought, excessive heat can cause death.
Heatwaves have caused significant threats to plant survival at the ecosystem level. The European heat wave in the summer of 2003 resulted in a 30 percent reduction in ecosystem gross primary production, while the 2010 Russian heat wave was the most extreme on record for that area and resulted in an estimated 50 percent reduction in gross primary production. Drought and excessive heat across the western United States in the last decade have caused widespread tree mortality.
Heatwaves in India
Temperatures across India have been increasing in the latter half of the twentieth century, with 2000-2010 being the warmest decade ever recorded for the country. Increase in the frequency, total duration and maximum duration of heatwaves has been observed over central and north-western parts of the country between 1951 and 2010. The heatwave of 1998 had the largest spatial extent during this period.
Increasing trends in heatwaves are more pronounced in northwest India, in some parts of southwest India (Western Ghats) and southeast India (Eastern Ghats). Extreme 10-day heatwaves are prominent in Rajasthan, Western Ghats, Eastern Ghats and in some regions of Telangana, Chhattisgarh, Maharashtra and Madhya Pradesh.
The years with maximum number of heatwave days are found to be preceded by warm ENSO years. Recent studies have linked heatwaves in north and northwest India to anomalous cooling over the Atlantic and Pacific.
When drought and heatwaves co-occur
Many studies predict frequent droughts in India between 2020 and 2049, induced mostly by failure of the monsoon. The temperatures in northern India are set to break all records due to a combination of global warming and urban heat island effect, putting millions at risk. The area under concurrent drought and heatwaves has increased all over the country, especially in central and northeast India.
As per the IPCC 2012 report, the physiological stress on forests is projected to aggravate through warmer temperatures, longer periods of drought and more frequent extreme events.
A recent study has found that the area under concurrent drought and heatwaves in more in recent years (1981-2000) compared to an earlier time period (1951-1980). Moreover, there is an increase in frequency of concurrent heatwaves and drought.
How will India’s forests react to a drought/heatwave double whammy? Read more: When drought and extreme heat strike forests at the same time.
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