Botany at the Boiling River

Botany at the Boiling River: How Tropical Plants Respond to Extreme Heat

Words and photos by Riley Fortier

Tucked away in a peculiar sandstone outcrop in the middle of the Amazon, a river rushes through dense vegetation, carving out paths in the soft rock. It cascades over small falls and rapids molded by time, and the water is cool, refreshing, and home to abundant frog and invertebrate life. The river suddenly swells in size, but no major tributaries are in sight. The influx of water comes from below and gives this river its name – the Boiling River. 

Like blood coursing through our veins, water courses through the Earth’s crust, cooked by hot rock and magma far below our feet. Occasionally, this water escapes through cracks and crevasses, and often produces spectacular natural features we know as hot springs. These geothermal phenomena are usually affiliated with volcanoes, where subterranean magma is relatively shallow and escape routes through Earth’s crust are aplenty. The Amazon (excluding the Andes) is distinctly not volcanic, yet the Boiling River is full of geothermal activity, making it one of the most unique hot springs in the world. 

As hot water joins the Boiling River, the temperature jumps from 20 to 40 degrees C in a matter of seconds. It slowly begins to cool, but subsequent influxes boost the temperature even more, up to 70, then 80, then nearly 100 degrees. Falling in at this point would ensure instant death. The heat from the river bathes the surrounding forest, and even standing next to the river can be exhausting. Upon our first visit to the Boiling River, it was clear that this was a forest like no other. How can the plants here withstand such extreme heat? Can all plants withstand the heat? Can some do it better than others? The Boiling River provided an unprecedented opportunity to study the effects of elevated temperatures on tropical plants.

Walking through any forest in the Amazon can be an overwhelming experience. To make sense of the staggering diversity of life in this expansive jungle requires a lifetime investment for a biologist. With enough practice, one can begin to find patterns of how things are organized across space and time. For plants, we know that certain species like wetter places while others like it dry. Some trees thrive in well-drained soils while others need “wet feet” year-round. It has been hard to fully understand what role temperature has in influencing trees in the Amazon, especially in the context of climate change. We can find cooler temperatures, but how do we find hotter temperatures? Warming experiments typically can’t cover the area needed to study how an entire community responds to the heat. That is, until we realized the Boiling River is a warming experiment.

The first thing we needed to do to show that the Boiling River is the world’s largest tropical warming experiment, and for that we needed temperature data, and lots of it. We put out around a dozen temperature loggers by the river, starting from upstream of hot water inputs, slowly making our way downstream into hotter and hotter territory. The resulting data gave us something awesome – a heat map of air temperatures along the river, and proof that the forest here could very well be the hottest rainforest in the world. We finally had evidence that the Boiling River creates essentially what is a giant warming experiment, so now we could study how plants respond to the heat. We specifically wanted to know how the tree community responded to such extreme temperatures. To answer that, we took an extensive inventory of the trees along the river, from the cool upstream forest to the hottest areas we could access.

Our results were stark – with every 1 degree C increase in mean annual temperature, there was an 11% reduction in overall tree diversity. That means if temperature in the Amazon increases by 2 ˚C by the end of the century (a widely accepted prediction), we could see tree diversity slashed by 20%. One fifth of all species could become extinct due to temperature increases alone. This ignores other changes in climate, ongoing habitat destruction, pollution, invasive species, and all other human-induced stressors.

We also saw changes in the composition of trees – that is, the assemblage of species growing in cool versus hot forest. Aside from lower overall diversity in higher temperatures, the species growing there are ones that are associated with hotter forests regionally. Why some species are able to withstand hotter temperatures than others is still poorly understood, but the fact is that there are “thermophiles”, species that can function in hot temperatures, and “thermophobes”, ones that stay on the cooler end of the spectrum. In the future, if we’re able to identify which species are thermophiles and which are thermophobes, it may be possible to predict which species we will lose with climate change, and which will remain. The implications for all of the animals that rely on thermophobic plants are obvious. Losing so many species could result in widespread ecosystem collapse and biotic attrition. Our work at the Boiling River is a call for concern about how trees in the Amazon will fare as temperatures increase and raises yet another flag about the state of the natural world. Our conclusions are rather bleak, but such is the nature of studying climate change and its impacts on planet Earth.

Tags: Amazon, Boiling River, botany, climate change, Rio Hirviente, temperature, tree composition