Wind-dispersed emergent trees missing from the canopies of next generation tropical forests

Congratulations Jarrah Wills on the discovery of these important findings concerning restoration of tropical forests in the Philippines.

Read more at the Journal of Applied Ecology Early View http://onlinelibrary.wiley.com/doi/10.1111/1365-2664.12770/full

Summary

1.In tropical countries where little natural forest remains, such as the Philippines, small-holder monocultures, and mixed-species plantations potentially provide conservation values by providing habitat for local fauna and recruiting diverse understorey species. However, little information exists as to whether monocultures offer a framework to kick-start understorey species recruitment and how this compares to mixtures.

2.In this study, species and trait diversity were measured within the understories of three different forest types on the Island of Leyte, Philippines: mixed-species forests (known as ‘Rainforestation Farming’), Swietenia macrophylla monocultures and regenerating selectively logged forests. All plants less than two meters in height were identified, and dispersal type, fruit type, seed and fruit size were extracted from literature and online databases.

3.We found that overall seedling richness and diversity were lower within the monoculture forests compared with the regenerating selectively logged forests, with the Rainforestation forests showing intermediate seedling diversity, including trait diversity. Monoculture understories had a higher proportion of large fruited domesticated species that are likely dispersed by people and significantly lower wind-dispersed native seedlings than the other forest types. Higher understorey diversity was generally negatively correlated with soil nutrients and positively correlated with increased leaf area index, i.e. more canopy cover.

4.Our results confirm that mixed-species plantations and regenerating selectively logged forests recruit higher species diversity, but we also found evidence that monocultures can recruit diverse species in the understorey. However, monoculture understories were depauperate of native wind-dispersed traits that are often important emergent species in tropical rainforests.

5.Synthesis and applications. Overall, our results show that just having trees in a cleared landscape provides conservation value, but if monocultures are used as less costly and technically simpler solutions for initiating recruitment then wind-dispersed native species (e.g. species from the Dipterocarpaceae family) in addition to other limited functional traits (e.g. large-seeded species) should be planted to enhance long-term term survival of ecologically significant native tree populations.

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