Ricard Arasa-Gisbert: Forest loss and treeless matrices cause the functional impoverishment of sapling communities in old-growth forest patches across tropical regions

Shortlisted for the Southwood Prize 2022

This post is also available in Spanish here.

Ricard Arasa-Gisbert discusses his and research colleagues’ article on forest loss and the functional impoverishment of sapling communities in tropical regions.

Tropical forests

Tropical forests are recognised as the ultimate exponents of biodiversity. They are home to animals and plants of all shapes and colours, and their ecosystem services (water cycle, nutrients, carbon sequestration) sustain millions of people and regulate the global climate.

Unfortunately, due to deforestation and fragmentation, an increasing area of tropical forests are already considered human-modified landscapes. Surprisingly, we still do not completely know how biodiversity is affected in these new ecosystems.

A fragmented landscape in Los Tuxtlas region © Ricard Arasa-Gisbert

An interesting component of biodiversity is functional composition and functional diversity. There are species whose functional traits (e.g. seed size, wood density) may be favoured by anthropogenic disturbances (winner species), while others may be disadvantaged (loser species). Knowing the balance of winners vs. losers in the regenerating plant community (i.e. the sapling community) can be very important to know what forests of the future will look like, and, if they will be able to provide the same ecosystem functions and services as they do now.

Under this premise, we evaluated the effect of forest cover (proxy for forest loss), treeless matrices and the number of patches (proxy for forest fragmentation) on the functional composition and diversity of saplings in 60 tropical rainforest patches in three regions of Mexico.

Sapling community in the tropical understory of a sampled patch © Ricard Arasa-Gisbert

The great adventure in the Mexican Tropics

For this, I had to travel for 6 months through the lush tropical rainforests of south-eastern Mexico. Try to imagine the following: a 26-year-old Mediterranean man who has never seen the rainforest before and who has been in Mexico for only 5 months, sent into the depths of the Mexican rainforest.

It is impossible to summarise the whole experience here, but I think it has been one of the most difficult and, at the same time, most rewarding experiences I have ever had in my life. In the Selva Lacandona I slept in a wooden room with tarantulas, bats and fireflies crawling through. The fieldwork was exhausting, with continuous rain, dodging the numerous lianas that grow in the forest floor and always alert for wasps and snakes.

During the fieldwork, Gilberto and I had to cross a non-abandoned railroad in Northern Chiapas © Ricard Arasa-Gisbert

After a long day of fieldwork, you arrive at home and there is no toilet, bath or shower. There was no internet or phone signal either. In Los Tuxtlas, Veracruz, I had to ride a motorbike (for the first time!) on the steep dirt roads, as it was impossible to get there by car. In Northern Chiapas we had to ride a horse to reach an inhospitable forest patch (we called it ‘The Lost Word’) and we crossed, on several occasions, active train tracks.

Gilberto Jamangapé (back) and I (front) riding a horse to reach a forest patch called ‘The Lost World’ © Ricard Arasa-Gisbert

But it was worth it. I was able to enjoy a firefly show at Loma Bonita every night, something like I had never seen before. Also, a clueless armadillo in the forests of Veracruz, which was 2 metres away from us and had not noticed us, or a group of howler monkeys that welcomed us in a fragment of 5 ha by urinating on us.

A male mantled howler monkey (Alouatta palliata) aggressively greets us in a 5 ha fragment of Northern Chiapas © Ricard Arasa-Gisbert

And, of course, the imposing starry sky that accompanied us on the clear nights. But what surprised me most is how disconnected we are from nature. There you can feel the vibe of the wild, of the ancestral, of what we were. My field guides, Gilberto Jamangapé in Chiapas and Santiago Sinaca in Veracruz, have always accompanied me on this journey. Without them, this work would not have been possible.

So, what did we find?

After sampling more than 23000 individuals of more than 350 species and extracting their functional values, we found that the functional composition and diversity of saplings are decreasing in landscapes with less forest cover and more open matrices, especially in the two most deforested regions (Northern Chiapas and Los Tuxtlas).

Fragmentation had no effect in Selva Lacandona and Los Tuxtlas, but did have a negative effect in Northern Chiapas. Human disturbances also caused the decline of species with conservative traits, i.e. those traits typical of mature forest species, such as canopy trees, with large seeds and fruits, high wood density and low specific leaf area, thus corroborating our initial suspicions. The loss of such species (i.e. loser species) could cause a domino effect, jeopardising some key ecosystem services, such as carbon storage or the provision of food for many animal species.

A recently logged forest patch in Northern Chiapas. This place is called, ironically, ‘The heaven’ © Ricard Arasa-Gisbert

Our study was the first to assess the effect of some landscape variables on the tropical sapling community in different regions. Previously, assessments in fragmented landscapes have mostly been done using patch size and isolation – variables derived from The Theory of Island Biogeography. However, we now know that everything that happens around forest patches also matters, such as matrix (non-habitat zone in the landscape) composition.

This study demonstrates that in order to preserve the original sapling community, the following steps must be taken:

  • First, preventing deforestation. Effects were only evident in regions with <20% regional forest cover; in the Selva Lacandona (~40% forest cover) no effects of human disturbances were detected
  • Second, increasing forest cover, to reach at least the target of 40% forest cover in those regions
  • Finally, avoid open areas in the matrix. This can be solved by using agroforestry systems or live fences; both proposals favour the movement of fauna between forest patches, which increases the exchange of seeds between forest patches, thus increasing the functional diversity of sapling communities within them.

About the author

I am currently a postdoctoral researcher at the Faculty of Sciences of the UNAM. I continue exploring the human impact on saplings, now from a global perspective. I also work with primate communities.

Photograph of the author, Ricard Arasa-Gisbert © Ricard Arasa-Gisbert

I had a passion for nature and ecology practically since I was born. I spent my childhood summers with my grandfather in a house on the Costa Brava, on the Mediterranean coast, among pine trees, oaks and many, many insects. There I learned the value of each of the elements that make up the ecosystem and the need to conserve them.

Later, my father, an avid reader, bought me a lot of books on ecology, which led me to develop a special interest in tropical forests. From then on, that was my dream, which I was lucky enough (not without a lot of effort, even with two PhD rejections) to achieve.

I recommend those who are thinking of pursuing a PhD to take an introspective journey: Do I like the topic? Do I feel a good connection with my advisor? Do I like to read and write? If so, go ahead, we must pursue our dreams despite the difficulties that seem to loom on the horizon. It can all be done.

A Deppe’s squirrel (Sciureus deppei) staring at me in Los Tuxtlas © Ricard Arasa-Gisbert

Finally, something about me (it’s not all about ecology in life!). In my free time I enjoy hiking, taking photographs, spending time with friends in the wonderful Mexico City, playing videogames and spending time with Bart, my dog.

Read the full article, “Forest loss and treeless matrices cause the functional impoverishment of sapling communities in old-growth forest patches across tropical regions” in Journal of Applied Ecology.

Find the other early career researchers and their articles that have been shortlisted for the 2022 Southwood Prize here!

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