When drought and extreme heat strike forests at the same time

The frequency of extreme weather events is going up around the world, and droughts and heatwaves are no exception. When the globe warms by 3-4 degrees – a possibility during this century under a ‘business as usual’ scenario – about 40 percent of the world’s land surface is predicted to experience drought. The tropics are all set to experience high temperatures as the new normal.

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.

A recent study has found that the area under concurrent drought and heatwaves is more in recent years (1981-2000) compared to an earlier time period (1951-1980) in India. Moreover, there is an increase in frequency of concurrent heatwaves and drought.

“We divided 60 years of data into two halves taking 1951-1980 as a base period. At each grid, we computed the differences in number of concurrent droughts and heatwaves for the period 1981 to 2010 with respect to the base period. These results indicate that simultaneous occurrence of droughts and heatwaves have increased,” said Shailza Sharma, an author of the study.

The study found that concurrence of extreme drought and longer, more severe heatwaves is increasing in Gujarat, Central India and Peninsular India.

The response of vegetation to a combination of drought and stress is complex, ranging from short-lived local mortality events to regional-scale forest die-offs. A variety of forest types have shown mortality in the face of concurrent heat and drought: dry savannas which are adapted to seasonal rainfall, coniferous forests with a Mediterranean climate to tropical rainforests.

Forest response to combined drought and heat

Trees typically handle high temperatures for short durations when there is sufficient water; they use water to cool leaves by evapotranspiration and cool the stems as water passes through. Drought conditions when temperatures are already high impede the trees’ cooling mechanisms and can cause cell damage and oxidative stress. Dry conditions increase transpiration; with an already dry soil, the trees must deal with both dry atmosphere and dry soil. They need to remain hydrated to avoid a “whole-system transport failure” which would ultimately lead to death of the tree.

At the local forest scale, mortality due to drought is influenced by life history and tolerances of individual species to drought. Because trees are long lived, they can re-allocate resources and change their hydraulic architecture in response to drought. This particular adaptation makes it difficult to interpret or predict the response of trees to drought; a severe drought can manifest as tree mortality after a gap of many years.

However, drought-induced mortality is not restricted to dry forests where water is scarce. After a severe El Nino event in 1997-98, more than a quarter (26 percent) of the trees in the rainforests of Borneo died out. Mortality in tropical wet forests seems to be triggered by short but extreme seasonal drought. The prevalence of climate induced tree mortality in wet forests implies that the global rise in temperature may be pushing species over the edge.

Fires would probably negatively affect forests that have not historically evolved with fire, said Ankila Hiremath from the Ashoka Trust for Research in Ecology and the Environment (ATREE). Such forests would have species that are not adapted to withstand heat-caused damage, she said.

In temperate forests, drought induces death of deciduous species rather than the evergreen conifers. A well-documented, large scale heat and drought event in the summer of 2003 in Europe caused a 30 percent decrease in gross primary production, making the region a net source of carbon dioxide rather than a sink as it had been in previous years. Some forests showed growth reductions of 50 percent. A study of 18 tree species during this heat and drought event reported that most species had decreased chlorophyll and increased tocopherol and xanthophyll pigments, both of which would increase protection of the leaf photosynthesis machinery against excessive light energy.

“It is possible that there could be a shift in forest type,” said Hiremath. “Severe multi-year drought could potentially lead to mortality of species that are not tolerant of such conditions. In such situations fire could exacerbate the effects by causing additional tree mortality. This could open up the forest, give way to more savanna like conditions. But then it also depends on what you mean by prolonged drought.”

“Apart from concurrent drought and heat, a severe drought followed by a heatwave can have a more severe effect,” said Jagdish Krishnaswamy, an ecologist studying forest hydrology from ATREE.

A study examining remote sensing vegetation data from 47 protected areas, all biodiversity hotspots above 1000 m above mean sea level in five continental regions in Africa, Central America, South America, South Asia and Southeast Asia found ‘browning’ or drying up of forests across all the tropical mountain regions. Such browning has been reported for large areas of the northern hemisphere and tropical regions. The main reason behind the forest browning is combined moisture stress: the joint effects of rising temperatures and decreasing precipitation.

India’s fire-prone forests

“Typically, droughts are associated with a greater occurrence of fire. Fires need fuel and an ignition source. Hot and dry conditions can lead to more dry biomass being available to burn, so one would expect a higher probability of fire occurrence,” said Hiremath.

An analysis of fires in India between 2004 and 2011 released by the Forest Survey of India (FSI) shows that the periods with maximum fires varied among different areas in India: northeast India had maximum fires between the first week of March and third week of April, southern India between the first week of February and first week of April and north and central India between the last week of February and third week of June — fires coincided with the dry season in all these areas. Also, fires during this period correlated with scanty premonsoon rainfall.

The most fires were in 2009-10, in (erstwhile) Andhra Pradesh, Assam, Chhattisgarh, Jharkhand, Madhya Pradesh, Maharashtra, Manipur, Odisha, Uttarakhand, Tripura and Mizoram. The high number of fires coincided with a monsoon failure and resultant drought due to cyclone Aila.

The droughts in 2004, 2004, 2009, 2012 and 2016 correspond with the total number of fires in these years, especially in peninsular India. In the northern states, the correlation between fires and drought is not so evident, probably because of the practice of crop residue burning, which is also picked up by satellites.

“After a period of drought, the fuel load will first increase,” said Jagmohan Sharma, a researcher and the additional principal chief conservator of forests (Utilisation) at the Karnataka Forest Department. “If the monsoon fails in successive years, productivity will go down and lesser material will be available for burning. If fires occur continuously and productivity keeps going down, it will result in desertification.”

The maximum number of fires occurred in tropical semi evergreen forests, followed by tropical moist deciduous, tropical dry deciduous, subtropical broadleaved hill forests and subtropical pine forests. The highest fire radiant power (FRP), which denotes the quantity of fuel burnt during fires, shows that the most intense fires occur in tropical moist deciduous forests, implying the forest type is very vulnerable. Tropical dry deciduous forests had a higher number of fires, but at a lower FRP, showing that there was lesser fuel available in dry forests.

“Many dry forests are infested with the invasive weed Lantana camara, which adds to the threat of fire in interesting ways,” said Sharma. “Lantana increases the fuel load because the species is opportunistic, out-competes native species and proliferates almost without assistance from any other agent. The plant can sometimes climb trees, which is an acquired behaviour. It causes fire that is higher than what would be if Lantana wasn’t there.”

High chances of future fires

A whopping 55 percent of India’s forests are prone to annual fires, predominantly the deciduous type in the peninsula and the pine forests of the Western Himalayas, reports a 2013 study.

The FSI mapped the country into 2.5 inch by 2.5 inch grids, calculated the vulnerability to fire and found that about 15 percent of the area of the country is vulnerable to forest fire. Of the 348 vulnerable districts, the maximum number of highly vulnerable districts are in central India, especially Madhya Pradesh, Maharashtra, Chhattisgarh and Odisha.

On correlating areas vulnerable to fires with other socioeconomic parameters like poverty, the analysis found that more than half of the districts vulnerable to fire have a large percentage of the population (31-47 percent) living below poverty line, increasing their dependence on forests for livelihoods. People venturing into forests to collect various items in turn makes the forest areas more vulnerable to fire.

Jagmohan Sharma agrees. “Most fires are human induced in our country,” he said. “When there is an extended period of drought, more humans and domestic cattle move into the forest for collecting various materials because the drought adds an economic pressure. If it is already dry, it takes lesser ignition force for the fire to start.”

“The reasons behind humans setting fire to forests is varied,” added Sharma. “For example, some villagers may set fire to forests hoping to stimulate growth of grass for their cattle.”

The 2017 Indian State of Forest Report recognises fire as a tool in scientific forest management. “People living around forests light intentional fires to promote grass growth, clear areas for shifting cultivation and clear the forest floor for collection of NTFPs (non-timber forest produce). Fires are also used to flush out wildlife for hunting, and encroach forest land,” states the report. Unintentional fires are also common — cigarette or beedi butts can cause havoc if not noticed at an early stage.

Will India’s forests remain the same, with climate change?

‘Vulnerability’ is a term used to describe the susceptibility of a forest to the adverse effects of climate change, including climate variability and extremes. An idea of the vulnerability of different forest types can inform management in the face of climate change — a good place to start when analysing the effect of a drastic climatic event like concurrent drought and heat. A 2017 study based on the Indian State of Forest Report 2013 has categorised forests into different vulnerability classes — low, medium, high and very high — based on four indicators, including biological richness and level of disturbance.

At the national level, 40 percent of the forest grids show high or very high vulnerability. These grids were concentrated in the dry deciduous forests, especially in peninsular and central India. Plantation forests showed more vulnerability than natural forests.

The study also went a step further and used models to predict future climate and vegetation type in India’s forest areas in the short term (2030s) and long term (2080s). With this information, they were able to predict a change in vegetation type in almost half (46 percent) of India’s forest grid cells by 2030; the number goes up to 49-54 percent in the long term. Most of the grids prone to vegetation change are located in west and central India and in the interior peninsula — tropical dry deciduous forests and plantations.

“In these grids, it was seen that the present vegetation cannot sustain itself under future climate,” said Jagmohan Sharma from Karnataka Forest Department, one of the study authors. “But, will forests get a chance to make a transition into another vegetation type? Ideally, there should not be any non-climatic factors affecting the transition.”

“Since this is never the case, typically, the existing forest goes, a new forest tries to establish itself, it is unable to do so, and there is degradation,” he added. “We also don’t know at what speed climate changes would occur. How would a forest respond? We don’t know.”

Read an accompanying commentary on forest response to drought and heat — Hot and dry: What plants do when pushed to their very limits.

CITATIONS:

Sharma, S., & Mujumdar, P. (2017). Increasing frequency and spatial extent of concurrent meteorological droughts and heatwaves in India. Scientific reports, 7(1), 15582.

Teskey, R., Wertin, T., Bauweraerts, I., Ameye, M., McGuire, M. A., & Steppe, K. (2015). Responses of tree species to heat waves and extreme heat events. Plant, cell & environment, 38(9), 1699-1712.

Banner image: Burnt trees in Mudumalai forest after a forest fire. Photo by samueldotj/Wikimedia Commons.