A Question of Time: Understanding Factors Driving Monodominance in Gilbertiodendron dewevrei Forest

Hall et al. just had their paper entitled ‘Resource acquisition strategies facilitate Gilbertiodendron dewevrei monodominance in African lowland forests‘ published in Journal of Ecology. In this blog post, Jefferson Hall explains the history of the research carried out in Gilbertiodendron dewevrei Forest and shares his own experience working on the topic for the last few decades in Central Africa.

I first set foot in monodominant Gilbertiodendron dewevrei forest almost 35 years ago when, as a third year Peace Corps Volunteer, I visited the Ituri forest research site of Terese Hart. Terese had completed her PhD studying a tropical conundrum: If tropical forests are celebrated for their incredible diversity, how is it that large areas – totaling many thousands of square kilometers in some regions – could be dominated by a single tree species representing over 60 percent of the stems and basal area? In her work Terese was following in the footsteps of Belgian pioneers – Louis and Gérard, among others. 

Jefferson Hall (center), Terese Hart (immediately to Hall’s left), Hart’s field assistants, technicians, other employees and family members at Terese Hart’s home in Epulu, Zaire (present day Democratic Republic of Congo) circa 1986. (photo credit: JS Hall).

A big part of her research at the time was to look at the potential of Gilbertiodendron for partitioning the forest based on soil fertility, following the idea that its relationship with ectomycorrhizae might afford it an advantage in acquiring and cycling nutrients. Terese also looked at forest structure and disturbance and initiated a fantastic long-term study following seedlings. In a testament to her academic acumen (and as a reminder to us all that research is worth publishing even if you are not able to reject the null hypotheses!) she published two foundational papers for people studying the monodominance phenomenon – one in The American Naturalist and another in Trends in Ecology and Evolution. Terese was also warm and welcoming, so much so that I soon joined Terese and her husband in the Ituri as the project leader of a conservation project with the goal of establishing a protected area. While the paths we choose in life no doubt have many origins, it is fair to say that I would not have continued in tropical biology and conservation were it not for Terese and her husband.    

Due to her work, the Ituri forest became a hub of sorts for studies of monodominance. An undergraduate researcher also looked at soil fertility under Gilbertiodendron and mixed species forest… to no avail. The Harts established CTFS (Center for Tropical Forest Science) plots in both mixed species and Gilbertiodendronforest to better understand diversity and dynamics. This work was followed by Sylvia Torti who also published in The American Naturalistwith her PhD advisors Phyllis (Lissy) Coley and the late Tom Kursar. Torti’s work brought some order to the long list of potential traits leading to monodominant Gilbertiodendron forest, concluding that “a number of traits in adult trees… significantly modify the understory environment, making it difficult for other species to regenerate there.” An interesting thread of her research that resurfaced as important in our study was that she found significantly lower levels of both nitrate (NO3) and ammonium (NH4+) in monodominant as compared to mixed species forest soil. 

The last decade and a half has seen many more studies of Gilbertiodendron forest. Publishing in Ecology and Evolution, Elizabeth Kearsley and colleagues made a convincing argument that G. dewevrei‘s distribution is, at least in part, controlled by its poor ability to regulate water use, an explanation for the common observation that it is found adjacent to water courses and swampy clearings in many areas. Kearsley and colleagues, however, also found indirect evidence of the role of ectomycorrhizae in helping drive monodominance. Yet, this evidence might suggest that soil is indeed important and begs the question of how this aligns with other evidence. Indeed, based on their work in Cameroon, Kelvin Peh and colleagues boldly titled an article in PLOS ONESoil Does Not Explain Monodominance in a Central African Tropical Forest”. Peh and colleagues made a further attempt at advancing more coherent models explaining monodominance in tropical forests writ large by publishing models in the Journal of Ecology, where some traits underpinned others. But it was clear the story did not end there.

Following my own work as a practicing conservationist in the Ituri Forest and beyond, I ended up in the Dzanga-Sangha Special Reserve, Central African Republic (CAR) studying the regeneration ecology of African mahogany of the genus Entandrophragma. With Gilbertiodendron still on my mind in the late 1990s, I teamed up with David Harris. We were not satisfied with the work done on soils and thought it worth testing hypotheses at a different scale and site. Kristin Saltonstall and Mark Ashton joined Dave, Vincent Medjibe and I in the field at different times and, although the Gilbertiodendron question was not our priority, it was always the “elephant in the room” or a big question…in a forest full of elephants! We collected soil across a 8,400 km2 landscape in Congo and CAR and wondered why, if a common “trait” helping to explain Gilbertiodendron monodominance was its shade tolerance and ability to survive a decade or more in deep shade (thank you Terese), we observed seedlings growing quite well in full sunlight adjacent to roads and in gaps. If it grows so slowly recruiting from the understory, what happens in the large gaps reported by Terese Hart? Thus, we tested the ability of its seedlings to grow in a variety of light levels, including full sunlight, and concluded that it grew as well or better than most everything else!

Different life phases of Gilbertiodendron dewevrei in Central African Republic. From lower left moving clockwise: germinating seed, seedling in shade, seedlings along road, tree in gap created during logging. Note how seedlings in full sunlight create shade suggesting their ability to modify the light environment for smaller seeded species germinating and establishing at the same time. Also note field assistant in very lower left-hand corner of image with tree (far right). (photo credits: D. Harris, used with permission).

Some ideas need to mature and need further inspiration. Ben Turner provided the inspiration and cogent thinking regarding ectomycorrhizae and further analyses that might help tease out indicators to address its potential importance. So here we are today, some 35 years after I first wandered in forest of and wondered about this incredible tree species. We build on Gérard, Letouzey, Hart, Torti, Peh, Kearsley, their colleagues and many others who have reflected on the question. In our recent paper published in Journal of Ecology, we propose a model whereby resource acquisition and use strategies lead to and underpin other traits resulting in the monodominance of Gilbertiodendron dewevrei. We believe that our framework may help better organize thinking about monodominance in other tropical and temperate forests. Only time will tell!

Jefferson S. Hall, ForestGeo, Smithsonian Tropical Research Institute, Panama.

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