Lush, green rainforest would be many people’s mental image of the tropics. In fact, the largest land area of the tropics is covered in dry forests and savannas. And while the challenges facing rainforests are a frequent topic when discussing the impacts of climate change, the role of the dry tropics has been relatively ignored.
Research plots and remote sensing
Since 2021, a research project funded by the Natural Environment Research Council (UK Research and Innovation), involving the universities of Edinburgh, Leeds and Exeter in the UK and partners across the world, has been working to address this issue.
The project – called SECO (the Spanish and Portuguese for ‘dry’) – is developing the first systematic network of observation plots in the dry tropics to understand how vegetation in these environments is changing. The international team of researchers is also using radar remote sensing to measure how the carbon cycle of the dry tropics has changed over the last 15 years. Bringing together remote sensing with field data will give the first estimates of how much carbon is being absorbed by the dry tropics and how much is released through degradation and deforestation.
There are several reasons why the dry tropics may not be absorbing as much carbon as in the past: increasing numbers of forest fires in some areas; land use changes like increased grazing or agriculture; or climate change, placing increased stress on vegetation due to drought and higher temperatures.
Yet the dry tropics may also be acting as a greater carbon sink than in the past. Higher levels of carbon dioxide (CO2) can make vegetation more resilient to drought. Increasing droughts might also be leading to the depopulation of drier regions of the tropics, allowing the scrub and forest to expand.
SECO draws on data from over 400 plots across the dry tropics to identify the net balance created by these potentially competing factors. These plots span Northern Australia, India, sub-Saharan Africa (from Kenya to South Africa and Angola), and South America (including Bolivia, Peru, Brazil, Colombia, and Mexico).
Not all the plots contributing to the project are new; some were created to support earlier research conducted over many years by both local and overseas scientists. But with SECO, the plot data from multiple countries is being standardised and brought together with remote sensing for the first time, and this is building a comprehensive global picture.
Dr Jose Luís Marcelo Peña first established research plots in Northern Peru’s Marañón Valley in 2013. The valley, on the eastern edge of the Andes, contains one of the most important examples of dry forests of the American tropics.
The SECO project has enabled some of these earlier plots to be resurveyed and supported new ones to be established. Altogether, 15 plots across Peru have now been brought into the SECO project. This has helped to draw in funding from the Peruvian Government, with the data used by the National Park Authority and the Ministry of the Environment as part of Peru’s efforts to meet its Nationally Determined Contributions to the Paris Agreement. This is a set of policy measures to reduce greenhouse gas emissions and adapt to the impacts of climate change.
Hopeful signs
Data from the Marañón Valley is providing some hopeful signs so far.
The number of new trees is greater than the mortality in established trees, and this means the structure and function of the forests are being maintained.
But we need to carry on our observations over a longer period to ensure a full understanding of the dynamics involved.
Each SECO plot is typically half to one hectare in size. Every stem is identified, measured and tagged as part of the initial survey. After two or three years, the survey is repeated to see where trees have died, how much they have grown and whether new trees have established. This means the researchers can estimate whether the overall carbon stored in the plot has gone up or down.
Around half of the mass of wood in a tree is carbon. The SECO team have developed protocols – adapted from rainforest research – to measure the biomass and carbon of the great variety of vegetation found in the dry tropics, often more shrubby than tree-like.
Standardising the data
The University of Leeds has led the creation of these protocols and the standardised systems for gathering and managing data from the research plots.
“Our expertise was developed over many years of research into rainforests, with the same database originally developed for rainforest research – Forestplots.net – now being used to gather and analyse data from the SECO plots in the dry tropics” explained Tim Baker, Professor of Tropical Ecology and Conservation in the University of Leeds School of Geography.
In each of the plots, soil samples are also gathered and sent to University of Leeds researchers for analysis and recording. By looking at the chemical and physical properties of the soil, the researchers hope to understand its role in the dynamics of the dry tropics.
Our original hypothesis was that the soils are a critical factor in determining the land cover: whether you have a savanna - grassland with trees - or a dry forest.
A complex picture
The data so far are painting a more complex picture. A post-doctoral researcher at the University of Pernambuco in Brazil, Dr Renato Vanderlei, has been leading an analysis of the soil data.
“We’ve found far more variability both within and across continents than we were expecting,” he says. “The soils of the dry tropics span a vast range, from the nutrient-poor sands of the Kalahari to places with rich soils and high levels of nutrients”.
Dr Vanderlei has been working in the Catimbau National Park in Brazil for over a decade, looking at how human interactions with the forest are changing the environment and having an impact on forest regeneration. As part of the SECO team, he has created three research plots in the park and is just completing their second survey.
Technical backup
Dr Vanderlei spent a month working with the Forestplots.net team to analyse the soil samples. Each sample is dried and ground in the field before being sent to laboratories at the University of Leeds’ School of Geography. Here they are analysed for a range of soil properties, including nutrient contents (carbon, nitrogen and phosphorus) and texture (whether they are sand, silt
clay and the particle size). The team are also looking at chemical characteristics, such as pH and concentrations of calcium, magnesium, sodium, potassium, aluminium, manganese and phosphorus.
The resulting data are then entered into Forestplots.net to allow soil profiles to be cross-referenced with the vegetation of each plot.
Rachel Gasior manages the School of Geography laboratory and oversees the analysis of the SECO soil samples.
“Most of the analysis we do is relatively straightforward and could be done in-country,” she says. “The advantage of sending all the samples to Leeds is that we can make sure each one goes through standard processes and that the data are consistent. We are also lucky to have more sophisticated equipment that automates some of the analysis.”
Rachel and her colleagues are part of a large technical team working behind the scenes to keep SECO running smoothly and ensure the quality and consistency of the data on which the success of the project depends. Although in-country researchers can enter their own survey data into Forestplots.net, specialists in both Leeds and Brazil manage the database and coordinate between the international teams.
As the database grows, the researchers are developing a better understanding of the challenges facing the dry tropics. They hope to determine under what conditions of climate change the open savannas and dry forests can continue to co-exist into the future, maintaining the unique and ancient biodiversity of these important landscapes, home to more than 1 billion people.