In this new post, David Bartholomew presents his last work ‘Differential nutrient limitation and tree height control leaf physiology, supporting niche partitioning in tropical dipterocarp forests’, shares the difficulties of working in tropical forests and invites everyone to help any ecologists seeking for help.
About the paper
Dipterocarp trees are often canopy emergents with their crowns metres above those of other canopy trees (credit: David Bartholomew)
In the rainforests of north Borneo in South-East Asia exist the world’s tallest tropical trees. These are the dipterocarps that can grow up to 100m tall. These giant trees, however, are restricted to nutrient-rich soils, with shorter species of dipterocarp growing on nutrient-poor soils. In the Sepilok Forest Reserve, a gradient of soil nutrient availability exists, driving dramatic changes in forest structure and species composition over less than 5km. Here, dipterocarp species are so highly specialised that no species is abundant at both ends of the spectrum. Species are either specialised to nutrient-rich or nutrient-poor soils. But what drives this turnover of dipterocarp species? And how do these species differentiate their niche and avoid competitive exclusion?
David Bartholomew standing next to a giant 80m dipterocarp tree (credit: David Bartholomew)
In order to answer these questions, we studied the leaf physiology of dipterocarps across four forest types along the Sepilok soil nutrient availability gradient. In Sepilok, the most nutrient-rich forests are alluvial forests, followed by mudstone and sandstone forests, with Kerangas forest being the most nutrient poor. In our study, we measured traits on both the giant, >60m tall trees, as well as the shorter understory trees of the same species. This allowed us to understand whether differences in soil nutrient availability affect ontogenetic development. We also measured a mix of specialist species that are restricted to one forest type, and more generalist species that survive across two forest types. This was done in order to test whether greater trait variation supports wider niches. We selected a range of leaf traits for our study, including maximum photosynthetic capacity, leaf respiration in the dark, leaf morphology, minimum stomatal conductance, and leaf nutrient concentrations. Overall, we found that nutrients and tree height are both important controls on dipterocarp leaf physiology. Leaf traits were the most acquisitive (e.g. high photosynthetic capacity) in the nutrient-rich alluvial forest, and the most conservative in the nutrient-poor Kerangas forest. Photosynthetic capacity and respiration were both limited by nutrients in these forests, but the main limiting nutrient varied by forest type: nitrogen and phosphorus limited the nutrient rich forests; cations limited the nutrient poor forests. Generalists were also found to have greater trait variation, suggesting adjustment of leaf traits—particularly leaf mass per area (LMA)—is important for a wider niche. Changes in leaf thickness and LMA also allowed species to respond to changes in the environment as they grew taller. In general, dipterocarp species were found to be highly specialised and to divide up several axes of environmental niche space, facilitating coexistence of a high number of species.
The nutrient-poor Kerangas forest only supports shorter trees, but still has a unique and diverse tree community (credit: David Bartholomew) Shorea smithiana – a generalist species that is able to grow in both mudstone and sandstone forest types (credit: David Bartholomew)
The field of functional ecology has been growing over recent decades with many studies investigating the controls on functional traits. Our paper, though, presents for the first time an extensive look at how nutrient availability, tree height, and niche breadth interact to affect trait expression in Bornean forests. Our results provide a more detailed insight into how species differentiate their niche to coexist and avoid competition. Our results are important for forest conservation and restoration practitioners as we provide evidence that careful species-site matching is needed to allow these specialist dipterocarp species to grow.
About the research
Our team of field assistants who helped collect branches from the giant dipterocarp trees (credit: David Bartholomew)
Our research helps contribute to understanding how tropical forests can support high species richness. This question has been a longstanding conundrum in ecology, with a range of theories proposed. We provide support towards the hypothesis that species differentiate their environmental niche in order to coexist. The functional ecology field is advancing rapidly and we can now link tree function and diversity patterns. New mechanisms that can explain patterns of diversity are emerging each year. Research that investigates new potential environmental niche axes could help us to explain even more of the diversity of tropical forests.
Measuring photosynthesis on cut branches using a Li-cor portable photosynthesis system (credit: David Bartholomew)
For this research, I was fortunate enough to work with the charismatic dipterocarp family—the world’s tallest tropical trees. Whilst I was in awe of these giants, their height presented us with several challenges. Accessing branches that were up to 70m above the forest floor required highly skilled tree climbers. Nevertheless, even for these skilled climbers it was still a major challenge to install ropes and to climb several each day. As a result of their immense size, it meant sampling was restricted to 3–5 trees per day and almost five months were needed to measure just over 200 individuals. Working in a tropical forest for this length of time was physically and emotionally demanding as heavy equipment had to be transported in and out of the forest each day. A constant attack from leaches, orangutans, and spiky plants made it even harder. Thankfully, however, a great team spirit helped us to all stay motivated and to get through the long fieldwork campaign.
About the author
My passion for the natural world started at a young age, growing up with grandparents who loved wildlife and the outdoors. It was my first trip to the rainforests of Borneo though that confirmed I wanted to work with tropical conservation. I couldn’t believe the diversity I found in these forests. At the same time, however, I saw their destruction and I knew I wanted to dedicate my life to saving these amazing ecosystems.
My PhD focused on understanding diversity patterns in lowland tropical rainforests. I have recently started a postdoc position at Umeå University, Sweden, where I now study the impacts of climate change on tropical Andean cloud forests. I am intrigued to find out whether montane ecosystems show similar patterns to those found in lowland forests. I am also trying to apply my research to a forest restoration charity that I co-founded called the Little Environmental Action Foundation (LEAF). We are trying to use forest restoration to save threatened species in some of the world’s biodiversity hotspots.
I feel privileged to have grown up in a developed country with access to excellent educational opportunities. From working in developing countries for my research, I am increasingly aware of the inequalities regarding access to education. Whenever possible, I always try to help people who have not had the same opportunities in life and to break down the barriers people from these countries face. I always try to learn the local language where I work and try not to impose my western culture on them. I encourage other scientists to break down the barriers that people from less developed countries face and to reveal the fantastic potential of scientists from these countries.
Enjoyed the blogpost? Read the research here.