Clouds, curtains and carbon: Assessing the impacts of climate change on cloud forest metabolism

Climate change represents a major threat to biodiversity across the planet. Dramatic changes to Earth’s climate are increasing the frequency and intensity of extreme weather phenomena, including record temperatures and more intense drought and flooding events. I recently spent two months in Peru’s montane cloud forest to try to understand how climate change may impact one of the world’s key biodiversity hotspots.

At the top of the eastern slope of the Andean mountains exists a highly unique and biodiverse forest ecosystem. This is the tropical montane cloud forest. At 3000m above sea level, the forests are immersed in the cloud for much of the day, providing a constant supply of fog to the forest. Much of this water can be intercepted by the forest here as visibly shown by the vast abundance and diversity of epiphytes (plants that live on other plants) in the forest. Everywhere there are mosses, orchids, bromeliads, ferns and lichens living on the trunks and branches of trees, all making use of the fog to grow and survive. However, fog is not the only source of water here. As the mountains force the clouds to rise, water begins to condense and rain begins to fall. The enormous volume of water that is transpired in the lowland Amazon forest provides a vast volume of rain to these forests. However, climate change threatens both sources of water. How these changes may affect the ecosystem remains unknown.

In southern Peru, an experiment aims to reveal how reductions in fog formation and rainfall could affect the Andean cloud forests. Located inside the Wayqecha Biological Station, a giant curtain has been installed to exclude the incoming fog from a small patch of forest. Meanwhile, a nearby roof excludes rainfall from another patch of forest. The roof installed at 2-3m above the ground prevents most of the rainfall from reaching the soil and therefore simulates drought conditions. By comparing the trees found in these experimental plots with those found in adjacent undisturbed forest, we can begin to understand how changes to fog formation and rainfall may impact this unique biodiversity hotspot.

The cloud curtain excludes fog from the forest, simulating conditions that may occur if fog begins to form at higher elevations. Photo copyright: David Bartholomew

From January to March 2022, I visited the experiment to understand how changes to the climate may affect the metabolism of the trees. Over 2 months, I measured the photosynthetic capacity, respiration, carbohydrate storage and morphology of leaves from 165 trees. Each day, a tree climber climbed the trees in the morning to harvest a branch that I would then use to measure its physiology. From these branches, I first took leaf and branch samples to measure the concentration of starch and sugar (collectively known as non-structural carbohydrates) to understand the extent of long-term energy storage in the trees. These samples have to be microwaved in the forest to destroy any enzymes that may break down the starch before it is analysed later in a lab. After collecting these samples, the remaining branches were transferred to the lab where I used an infra-red gas analyser to measure photosynthesis and respiration. These analysers allow the concentration of carbon dioxide and water to be measured in real-time, giving an estimate of the photosynthetic or respiratory rate of the leaf inside. By manipulating the carbon dioxide and light concentrations inside the gas analyser, it is possible to reveal the capacity of the leaf to do photosynthesis. After using the gas analyser, the samples were scanned, dried and weighed to get the ratio of leaf mass to leaf area, whilst other morphological metrics, such as leaf thickness, were also measured. These morphological traits provide an insight into whether the trees are adapted to a fast-acquisitive strategy or are adapted to a slow-conservative growth strategy. Finally, samples were set aside for analysis of leaf nutrient concentrations, helping to reveal whether changes in climate affect nutrient uptake.

A roof has been installed in the forest to exclude rainfall from reaching the soil (known as throughfall). The panels are placed 2-3m above the ground to ensure that the environment of the leaves is not affected. Photo copyright: David Bartholomew

Altogether, this wide range of physiological traits will reveal how changes to fog formation and rainfall may affect the metabolism of trees in these forests. Meanwhile, testing for differences between species will reveal whether some species have a greater capacity to adapt than others. Understanding these dynamics is important to understand how effectively the trees can take up carbon from the atmosphere and use it to grow, survive and reproduce. Maintenance of metabolism even under climate change may be critical if the trees are to overcome the stress of climate change. Further analysis of the data will reveal whether the trees have the potential to survive climate change or not.

Measuring the photosynthesis and respiration of cut branches using the Licor LI-6400XT Portable Photosynthesis System – an infrared gas analyser that can measure real-time photosynthesis of the leaf. Photo copyright: David Bartholomew

The montane cloud forests are rich in epiphytic life including many species of orchids, bromeliads, ferns, mosses and lichens. Photo copyright: David Bartholomew

The forests of Wayqecha are also rich in non-epiphytic life. Photo copyright: David Bartholomew

A big thank you to my amazing team of assistants who made the research possible. Photo copyright: David Bartholomew

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