Interview with the author: Seed and pollen dispersal distances in two African legume timber trees and their reproductive potential under selective logging

Seed and pollen dispersal, which are critical processes that influence the regeneration of trees, are likely affected by human activities like logging. However, determining how far pollen and seed move is notoriously difficult in the field. Thankfully, modern genetic techniques have provided us new approaches to estimating pollen and seed dispersal, which alleviates some of this issue. Using genetic markers, Dr. Olivier Hardy and colleagues assessed how the movement of pollen and seed in two African timber species is affected by selective logging, where one or two trees per hectare is removed on a 25-30 year cycle. Read below for a behind the scenes interview with Olivier.

Link to the study:

Inflorescence of Distemonanthus benthamianus. Photograph by Xander van der Burgt

What led to your interest in this topic / what was the motivation for this study? 
Tropical forests are fascinating but threatened ecosystems. Logging is usually perceived as one of their major threat by the public but selective logging might also be a solution when adequate management plans allows the regeneration of logged tree species, while the revenue generated prevents land conversion. However, forest regeneration is complex because many biotic and abiotic factors can be involved in the process. The extent of seed and pollen dispersal is one of the factors for which few data exist, and it can vary among species or locations. Fundamental research has developed methodologies based on genetic markers to quantify seed and pollen dispersal, so it is a great satisfaction to apply these methods for generating data that should help improving the management of tropical forests.

What difficulties did you run into along the way? 
The study required sampling exhaustively all adult trees of our focal species over several square kilometres in very remote sites of Central African forests, where visibility can be limited to a few tens of meters. This is the kind of field work that would have been very difficult to achieve for a small team of researchers. The collaboration with logging companies, FSC-certified, was a great advantage because we could rely on their field technicians who are trained to conduct similar inventories for planning logging activities.

What is the biggest or most surprising finding from this study? 
First that pollen dispersal distances were so different between two insect-pollinated tree species from the same family but bearing different flower types. It suggests that different flower types could attract different guilds of pollinators with contrasted flight abilities, but for now we ignore who are the pollinators. Second that about 25% of the seeds of the wind-dispersed species were transported over >500 m while their pods do not seem to be efficient gliders and, indeed, the majority falls within <100 m. It suggests that storm winds could play a crucial role in the dissemination of wind-dispersed canopy trees.

Moving forward, what are the next steps for this research?
First, we need to identify the pollinators and seed dispersers. This is challenging, in particular for pollinators because flowers open in the canopy, 20 to 40 meters above the ground and we must invent devices to observe or capture pollinators. Sporty, adventurous and creative PhD or MSc students are welcome! Second, we need to replicate these studies on numerous tree species with different contrasted reproductive characters to be able to derive some generalizations about seed and pollen dispersal according to species traits and environmental conditions. Third, we need to translate these results into recommendations for the sustainable management of tropical forests in collaboration with the forestry sector, and disseminate the messages to policy makers. Fourth, beyond seed and pollen dispersal, many other key processes affect the regeneration cycle and must be considered as well.

What would your message be for students about to start their first research projects in this topic? 
Tropical biology is fascinating for those attracted by fundamental research on biotic interactions, and it is full of opportunities for those inclined towards applied research of societal importance. Some students might be afraid by the remote and little equipped field conditions, and this is obviously constraining. But most students who discovered tropical Africa for the first time returned enthusiastic by their experiences, and some decided to pursue scientific research there.

Describe the significance of this research for the general scientific community in one sentence.
Seed and pollen dispersal investigations are relevant to assess the sustainability of selective logging in tropical forests.

Describe the significance of this research for your scientific community in one sentence.
Seed and pollen dispersal appears to be non-limiting for the natural regeneration of two African tree species under selective logging, while the minimal cutting diameter should be defined by accounting of the relationship between reproductive success and tree size

Olivier J. Hardy, Boris Delaide, Hélène Hainaut, Jean‐François Gillet, Pauline Gillet, Esra Kaymak, Nina Vankerckhove, Jérôme Duminil, & Jean‐Louis Doucet (2019). Seed and pollen dispersal distances in two African legume timber trees and their reproductive potential under selective logging. Molecular Ecology, 28(12), 3119-3134.

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