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Investigating the Dry Tropics Brazilian Style

In May 2022 SENSE student Lucy Wells travelled to Brazil for field work looking at the dry tropical areas. Lucy is a student at the University of Edinburgh and is supervised by Dr Casey Ryan. Here Lucy reports on her visit to Brazil

Dry tropical ecosystems are broadly made up of two main biomes – dry forests and savannas. These are the largest tropical land cover type, globally covering more area than rainforests, yet receiving far less research attention. In addition, they have unique and ancient biodiversity and are home to more than 1 billion people. Consequently, they are incredibly important ecosystems.

My PhD project is focused around using earth observation data to investigate the vegetation structure in these biomes, to try and understand their distribution. This is a challenging question because dry forests and savannas occupy the same climatic space, and so their distribution is controlled by different things, including soil type and fire.

The first half of my PhD looks at the Brazilian dry tropics, particularly the Caatinga, which is an area of dry forest located in the North-East of Brazil. During May-June 2022 I was able to visit the Caatinga, to help with some fieldwork that was taking place, and to visit different areas of the Caatinga to understand its vegetation.

Map here:

The Caatinga is generally characterised by the presence of relatively hort trees, which have leaves in the wet season (December-June), but are lost in the dry months of the year (July-November). The trees commonly have spines, and there are often cacti present. However, as I learnt, there are lots of variations on this theme!

I visited three sites with a team of Brazilian scientists, who were resampling trees in plots that had previously been established. Tree plots are an area of forest (in this case, a rectangle 100 metres * 50 metres), where every tree is recorded. At each plot, the diameter of every tree is measured, its species recorded, and other measurements taken such as the tree height, health of the canopy and overall status of the tree, for example, whether it has multiple stems, is alive or dead. Ideally, the plot is recensused a few years later, to understand how these measurements change over time. This recensus is what my team were doing, five years after the plots were set up in 2017.

This information has a wide range of uses, for example understanding which tree species are present where, tree mortality and recruitment, and competition between species. I work within the SECO project, which is a research project aiming to generate estimates of the carbon fluxes of the dry tropics. Carbon fluxes in dry tropical ecosystems are important to understand, as they currently both mitigate and exacerbate climate change, but the processes are poorly understood. The plot data provides information how much carbon is stored as biomass by the trees on these sites, and is used in conjunction with radar remote sensing to understand the carbon dynamics of dry tropical ecosystems.

Plot 1 – Serra das Almas. This plot is in a nature reserve at quite high elevation, which means there is more rain than typical dry forest. The trees are therefore relatively tall here.
Plot 2 – Lagoa Grande. This plot was more what I was expecting, small trees and lots of spiky plants! Although it rained the days that we visited, it was the end of the wet season and you can see that the trees (not the bushy undergrowth) are losing their leaves.
Plot 3 – Serra Talhada. This plot was interesting to visit because it is in an area of farmland, and so experiences some disturbance from livestock.

These plots are part of a large network of plots which are spread across the worlds tropical forests, collated by hosts data from over 6000 plots across the tropical biomes, where the trees are measured using standardised techniques, allowing them to be compared.  The network also aims to promote equitable science via collaborative networks throughout the tropics. I’ve included some links to resources about ethical and equitable tropical forest science at the bottom of the article.

I was also able to visit another collaborator, Desiree Ramos, in Brasilia, which is located in an area of cerrado. The cerrado is the Brazilian area of savanna, the difference from dry forest being the grassy understory underneath the trees. This grassy layer provides fuel for the fires which characterise the savanna ecosystem. Desiree and I visited several areas of cerrado around Brasilia, and she demonstrated her exciting work with phenocams, which take daily photographs of the vegetation, providing information on the timing of things like flowering, and when the leaves become green.

Cerrado, Brasilia – much more grass underneath the trees!
Cross-section of a tree stump in the cerrado. The star-shape around the tree is the bark, which is really thick to protect the tree from the natural fires that occur in this ecosystem.

Overall, the trip was a great experience, and has greatly improved understanding of these ecosystems which I can now apply to my research – I’ve got lots of new ideas! As I’d never worked on dry tropical ecology prior to my PhD it was great to learn from knowledgeable Brazilian people on the ground, and establish links with scientists there.