Daniel Zuleta: Do small-scale changes in topography affect functional trait variability in an aseasonal Amazon forest?

In our newest post Daniel Zuleta—a postdoc researcher at Smithsonian ForestGEO—presents his last work ‘Interspecific and intraspecific variation of tree branch, leaf, and stomatal traits in relation to topography in an aseasonal Amazon forest’. He presents the huge Amacayacu Forest Dynamics Plot, findings about the major impact that tree size exerts on tree functional traits, and how he changed his interests from forest plantations industry to natural forest science.

About the paper

Topography is a key factor structuring soil water availability and forest composition and functioning. At small spatial scales (<1 km2), our previous work in an aseasonal upland (terra firme) forest in the Amazon has shown significant variation in both tree species distribution and drought-induced tree mortality across small ridges and valleys, generated by an elevational gradient of only 22 m. Therefore, in this paper we aimed to identify the underlying functional traits driving such changes across topography while controlling for a highly documented source of trait variability within species—tree size. 

We measured 18 branch, leaf, and stomatal traits on 1,077 trees of 72 dominant species to explore the sources of trait variability among and within species in the 25-ha Amacayacu Forest Dynamics Plot (FDP). Surprisingly, we found large trait variability across trees within species (i.e., intraspecific) which was related to trees’ topographic location for leaf traits and tree size for branch and stomatal traits. This unprecedented sampling effort also allowed us to test the extent to which these traits covariate with each other at different levels. We identified promising correlations (coordination) across branch (Huber value, wood density), leaf (leaf area, leaf electrolyte leakage), and stomatal (stomatal size) traits that were generally maintained at both the species-level (mean species value) and tree-level (one point per tree). However, most bivariate trait correlations changed depending on the species and size classes, highlighting the diversity of functional relationships in this tropical forest.

Some of the hundred thousand trees monitored in the 25-ha Amacayacu Forest Dynamics Plot (credit: Sebastian Ramirez)

This paper demonstrates the importance of accounting for intraspecific trait variation when testing trait-environment relationships and suggests tree size as a critical source of variability to be included in mechanistic models aiming to predict forest dynamics. Beyond this paper, the implications of our research in the Amacayacu FDP transcend Amazonian forests by providing baseline observations on the influence of small-scale changes in topography on forest composition, structure, and dynamics, while also accounting for other sources of variation, such as tree ontogeny and soil nutrients. The next steps include quantifying physiological traits, functional rooting depths, and water table dynamics to comprehensively understand trees’ vulnerability to climatic drivers (e.g., droughts) and their implications for forest composition and ecosystem services.

About the research

This research would not have been possible without the support of dozens of people from multiple institutions, including members of the Palmeras indigenous community (Amazonas, Colombia), students at Universidad Nacional de Colombia, and staff members of the Amacayacu National Park. 

Staff members of the Amacayacu National Park (Amazonas, Colombia) (credit: Maria José Bermudez Jurado)

Given that the research was conceived at Alvaro Duque’s Lab in Medellín, Colombia—1,300 km from the Amacayacu FDP—our first challenge was bringing the materials to the plot. This genuine team effort involved land, air, and water transportation. Two forest engineers, five students, six indigenous co-workers, and two biologists worked in the field and lab for almost six months in 2018. Every day, we located and climbed 10–12 trees, collected a branch, processed the leaves, and packed the samples to send them by boat to the closest town—Leticia, Amazonas—where a biologist dried and weighed the leaf samples. At the same time, two other students took soil moisture measurements across the plot and collected leaf samples from most of the 1,232 woody species monitored at this site.

You might wonder how we identified the target trees in a forest with such tree diversity—we didn’t! Many other botanists collected the vouchers and identified the trees in the Herbario Amazónico Colombiano (COAH) of the Instituto Amazónico de Investigaciones Científicas (SINCHI) previously. The Amacayacu FDP is a long-term ForestGEO plot—established by Dr. Dairon Cárdenas (1957–2022) and Prof. Alvaro Duque in 2006—in which hundreds of students, co-workers, and national and international scientists have contributed time and energy to tag, map, identify, and measure each of the 125,000 trees in its 25-ha plot. By relying on the work of dedicated researchers before us, we were able to use tree coordinates, taxonomic ID, and diameter obtained from these field campaigns to select and locate the trees in the field. This research is just one of the many examples in which multi-institutional initiatives like this build local capacity and foster the development of local scientists. 

About the author

Dr. Daniel Zuleta standing on a big dead tree in the Yasuní FDP (Ecuador) during the ForestGEO Annual Mortality and Damage census in 2019 (credit: Salomé Ríos)

My early contact with nature, my parents’ guide, and the fortune of having inspiring professors influenced me to pursue my passion in forest ecology. I grew up in a small village located in the Colombian coffee region, up in the Andean mountains. In my hometown, I was frequently in contact with natural forests and the rural landscape was dominated by coffee and plantain plantations. There was also an increasing area of planted Pinus and Eucalyptus forests—an industry in which my parents shared a particular interest as it became the leading employer in the region. Having established my passion for mathematics and forests, I decided early on that this was the career I wanted to pursue. I moved to Medellín when I was 17 to start my undergrad studies in forestry engineering, aiming to return to my hometown and work on these forest plantations which focused on paper production. However, my focus quickly changed at university. For me, the possibility of explaining the complexity of ecological systems through mathematical models was fascinating, even more so when these models had broad applicability in improving the welfare of societies. Then, after attending Prof. Alvaro Duque’s courses on forest biometry and forest inventories, I started to work with him and I became fascinated with the many unsolved questions in forest ecology. My parents still ask me if I will consider a position in the forestry industry at some point.

Growing up in a Spanish-speaking developing country in the middle of an armed conflict makes for harsh prospects with regards to progressing in any scientific career. I could overcome most of these issues thanks to the support of my parents and tutors and, of course, luck. In particular, Prof. Alvaro Duque inspired me over all these years. He gave me his trust to run the Amacayacu FDP and collaborate with scientists in the ForestGEO network—an opportunity I consider an inflection point in my career.

As an early career researcher with an eight-year-old daughter, the main barrier I currently see is job instability and funding inequalities to further field-based forest science, especially in developing countries. I aim to work hard to transmit my passion for forest dynamics and pass on the opportunities I have had to the next generation of forest ecologists.

Enjoyed the blogpost? Read the research here!

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