- Ecologists have long debated why some extremely different habitats like grasslands and forests co-exist in the same climatic conditions.
- A team of researchers tried to investigate this question in the Nilgiris, where shola forests and grasslands coexist in the high altitudes.
- They found that Shola forest seedlings are unable to survive in grasslands because of frost in the winter. Thus, a clear boundary is maintained between grasslands and forests.
- However, exotic invasives such as wattle are able to survive the winter frost, making grasslands vulnerable to further invasion.
Across the globe, naturalists have recorded distinct habitats that they call forest-grassland mosaics, where both ecosystems seem to co-exist. In the highlands of the Western Ghats forests and grasslands stand next to each other. The change from one ecosystem to the other is not gradual; a dense rainforest does not slowly open up shifting into a grassland habitat as the climate changes. The change is abrupt; one habitat suddenly giving way to the other while the climatic conditions remain same.
Why does this happen? “The one climate-one biome view predicts the presence of only one biome (e.g. forest or grassland) in any given climatic condition,” explained ecologist Atul Joshi from the National Centre for Biological Sciences, in Bengaluru. “However, there are few ecosystems across the globe where two different plant communities (e.g. evergreen forest and grassland) co-occur within a same landscape/climatic envelope and often with abrupt transition between them. Shola-grassland mosaics are among few such unique ecosystems,” said Joshi.
In a study published last month in the Journal of Ecology, the researcher and his colleagues Jayshree Ratnam and Mahesh Sankaran, tried to answer these questions through a series of experiments in the shola-grassland mosaic of the Nilgiris.
Between 1200 and 2600 metres in the Western Ghats, rainforests give way to a mix of open grasslands interspersed with clumps of stunted evergreen forests called sholas. Joshi points out that several of these forest-grassland mosaics occurred in regions where the climatic conditions seem suitable for forests not grasslands.
“The obvious question then is, what maintains these mosaic ecosystems,” said Joshi. How then did two extremely different biomes, co-exist in the same climatic conditions? Why didn’t shola forest take over grasslands?
Whodunnit – humans, fire or frost?
As the researchers note in their paper, there are several studies that theorise how grasslands are formed. Two common hypotheses are that either these grasslands are man-made, formed because humans cut down forests or that they are maintained because of periodic wildfires burning down forests. Another hypothesis is that frost in winter which is more prevalent in open areas kills off tree seedlings, helping maintain open grasslands.
Humans, fire and frost all three are all present in the Nilgiris. Indeed, as Joshi documented in a previous study, colonial era British naturalists believed that the grasslands were the result of the Toda tribe setting fire to the forests to create meadows and their grazing buffalos prevented tree seedlings from taking hold. The researchers could safely dismiss this theory as a study from 1993 showed that the grasslands had existed for at least 20,000 years.
That leaves fire and frost.
Although wildfires did occur in the Nilgiri grasslands, the authors note in the study that there was no evidence that natural fires were large enough and frequent enough to burn down forests and maintain grasslands for thousands of years. However, frost was another matter. The average minimum winter temperatures in the Nilgiris is around zero degrees Celsius. While it was warmer in the shola forests, the researchers recorded several nights of below freezing temperatures in the open grasslands.
Frost is no barrier for wattle
To add to the complexity of these mosaics, the Nilgiris are today overrun by exotic invasive trees like Wattle and Eucalyptus. While shola forests have not been able to take over these grasslands, the exotic species have spread. Could frost explain why?
This is important because studies have shown that tropical tree species cannot tolerate freezing temperatures. The researchers hypothesised that for evergreen shola forests to take over the grasslands, seedlings that had germinated in spring and summer would have to survive the winter. Joshi planted seedlings of five common shola forest species and one exotic species (Wattle) in the adjacent grasslands. Some seedlings were placed in the open, exposed to the winter temperatures. The others were placed in temperature-controlled plots that were not exposed to freezing temperatures.
At the end of the 60-day experiment, Joshi checked on the seedlings. “All five native species incurred high mortality due to frost and freezing temperatures in this experiment,” he said. In warmer temperatures, shola seedlings showed approximately 40 percent chance of survival, but in the exposed frost, the likelihood of survival was around 10 percent, showing that frost indeed stopped shola trees from settling on grasslands, maintaining this dual habitat.
Worryingly the experiment revealed that exotic wattle was much more likely to survive in both warm temperatures (approximately 90 percent chance) and exposed grasslands (a little less than 40 percent). “Our results show that relatively greater frost tolerance of Acacia than native shola species enables Acacia’s spread in grasslands. We have not encountered an invasion/spread by Acacia in shola forests, which is expected as Acacia is a light demanding species,” Joshi explained. This may explain why much of exotic species spread happens on grasslands in the Western Ghats and not shola forests.
But does this mean it is ice and not fire that has kept grasslands intact in the Nilgiris?
“Our results and chrono-sequence of events show that, fire, if any, plays a negligible role in limiting native tree establishment in grasslands of our study system,” Joshi said. “In our study system, frost, which occurs during the winter, incurs very high mortality to native tree seedlings in grasslands. Fire, if any, occurs after winter by when most seedlings had already died due to frost,” he added.
But it is the future, of warming climates that the study really warns against. With warmer temperatures and less frost, shola forests may be able to spread into the grasslands. But considering the exotic wattle can survive better in either conditions, a shola takeover may not really be in the offing. Either way though, it is the grassland that might be lost.
Joshi, A. A., Ratnam, J., & Sankaran, M. Frost maintains forests and grasslands as alternate states in a montane tropical forest‐grassland mosaic; but alien tree invasion and warming can disrupt this balance. Journal of Ecology. https://doi.org/10.1111/1365-2745.13239.
Banner image: Ground frost in the Nilgiris. Photo by Aasif Iqbal J./Flickr.