Gone with the wind: canopies of next generation tropical forests will function differently based on today’s understory recruitment

In this post Jarrah Wills discusses his recent paper ‘Next-generation tropical forests: reforestation type affects recruitment of species and functional diversity in a human-dominated landscape

Diverse understory development within forest plantations can provide conservation value in highly modified tropical landscapes, but how many species should be used to establish a framework to encourage recruitment: one species, two species, more? And how does the quality of the understory environment and functional traits of the species matter when it comes to kick-starting understory recruitment? To find answers to these questions we compared species and functional diversity of understories beneath small-scale community-based mixed-species plantations, known as ‘Rainforestation Farming’ that uses mostly native species and aims to provide biodiversity and socio-economic benefits, and exotic monoculture (Swietenia macrophylla) plantations to more natural regenerating selectively logged forest understories across the island of Leyte, Philippines.

In tropical countries the goals and reforestation techniques of smallholder tree-farmers often differ from those of an industrial owner. Smallholders may not prioritize production and timber uniformity over a great diversity of forest products. Consequently, management techniques such as thinning and weed control are not as high priorities, as maximising basal area, and minimising growing deformities is not the goal. In smallholder plantations trees are often selectively harvested with just a few cut down depending on local need. Under these management practices, understories can develop, which may increase the value of these plantations for conservation, by providing habitat and ecosystem services such as nutrient cycling and hydrological flow. Extending plantation rotation times may also provide additional non-timber forest products that extend the socio-economic value of plantations beyond timber alone.

Unique harvesting technique often used in small-scale community-based plantations (Photo credit: Jack Baynes).

We found that exotic mahogany monocultures recruit some understory diversity (averaging 11 species, compared to 19 for mixed-species plantations and 24 for regenerating selectively logged forests) highlighting that re-establishing any tree cover in a highly fragmented landscape can provide conservation value. The family Moraceae maintained similar levels of diversity across forest types, emphasizing their strong dispersal abilities, likely via habitat generalist bird species. Fruit size classes, indicated the strong association these plantations have with local people, with monoculture understories containing the highest number of large fruited domesticated species (e.g. Mangifera indica and Theobroma cacao) and potentially increasing their socio-economic value within the next generation. The functional group that was largely missing in the exotic monocultures was wind-dispersed native tree species, which showed threefold less abundance in monoculture plantations relative to naturally-regenerating selectively logged forest.

The absence of wind-dispersed tree species, such as those from the Dipterocarpaceae family, is significant because they are ecologically dominant and economically important throughout Asia, making up the key framework of the forests in the emergent layers of the canopy. These species are typically highly valued as a timber resource, consequently being subjected to high rates of logging, resulting in a low prevalence and low genetic diversity. Being wind-dispersed they also have relatively shorter dispersal distance especially in highly modified and fragmented landscapes and seed production is often irregular being dependent on unpredictable and sometimes rare mass fruiting events.

Wind-dispersed emergent tree species provide the structural framework in tropical forest ecosystems, but a considerable amount of research and focus has been on animal-dispersed species in tropical forests. Our results show that in addition to other limited functional groups such as large-seeded animal-dispersed species, native wind-dispersed tree species should also be considered in the restoration of tropical forest.

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