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Recovery of bird communities in Afrotropical rainforests after anthropogenic disturbances

Public examination of a doctoral dissertation in the field of Biology
Doctoral candidate MSc Pirita Latja
Date and venue: 22.9.2017 at 12 noon, N100, Natura, Joensuu campus
Language of the dissertation: English
Language of the public examination: Finnish

ABSTRACT

Anthropogenic pressure on tropical rainforests presents one of the most alarming threats to the Earth’s biodiversity of our time. Because of extensive tropical forest loss, an increasing proportion of biodiversity depends on regenerating forests for survival. However, rates of tropical forest recovery after disturbances are highly variable, and while some areas recover naturally, some areas need assistance in the form of restoration. With the ever-growing proportion of degraded and secondary forests across tropical landscapes, it is important to understand how forests regenerate after disturbances, and how they are able to maintain viable populations and support emerging communities.

The aim of this dissertation was to study the recovery of bird communities in differently regenerating rainforests in Kibale National Park (KNP), Uganda, after human-induced disturbances. The objectives were to explore how bird community compositions and functional groups (classified by forest dependence or feeding guild of birds) recover in naturally regenerating and actively restored forests, and to examine how they are related to the vegetation structure of the differently regenerating forests.

Birds were sampled by point counting in naturally regenerating clear-cuts of former timber plantations and selectively logged forests, in actively restored abandoned agricultural and grasslands, and in adjacent primary forests. To assess how the community compositions and the functional groups of birds are related to the vegetation structure, datasets of vegetation structure and tree species diversity collected from the same sampling areas were used.

The findings of my dissertation indicate that the recovery of bird communities in naturally regenerating forests takes long after human-induced disturbances. In naturally regenerating forests, the bird community compositions of clear-cuts of former timber plantations and selectively logged forests differed from those in adjacent primary forests after 19 and 43 years of natural regeneration, respectively. Furthermore, the communities of clear-cuts differed from those in selectively logged forests. The forest regeneration in the naturally regenerating forests of KNP has been slow or arrested due to the persistence of a shrub Acanthus pubescens in logging gaps. This arrested succession and the slow recovery of some characteristics of the forest (e.g., tree species composition and/or the number of large trees) most likely contribute to the slow recovery of bird communities.

The results of my dissertation show, for the first time in the Afrotropics, that bird communities can start recovering rapidly after human perturbations with the help of active restoration of rainforests. Although only the first 16 years of active restoration were covered in this dissertation, the gradual change in bird community compositions along the restoration gradient offers hope that bird community recovery, and therefore possibly also the recovery of other taxa, can be helped by restoration. The results indicate that if the rate of recovery would continue equally rapidly also after the studied time period, the time needed for full recovery would be approximately 20 years. However, this estimation is most likely a best-case scenario, as natural succession processes in tropical forests are not necessarily linear. It remains to be seen how bird community recovery continues to proceed during the coming decades. As for now, it seems that active restoration of rainforests promotes rapid initial bird community recovery.

The formation of emerging bird communities in both naturally regenerating and actively restored forests was strongly related to the increasing complexity of vegetation structure towards older forests. My dissertation identified tree basal area and tree species diversity as the best predictors of bird communities in these forests. Additionally, gap cover in the naturally regenerating forests, and the cover of the elephant grass Pennisetum purpureum in the actively restored forests, predicted the bird community compositions. In actively restored areas, P. purpureum initially necessitated restoration actions by suppressing the natural forest regeneration. Fortunately, the restoration actions seem to be sufficient to allow decreasing cover of P. purpureum through time, start of forest regeneration and the recovery of bird communities.

The bird functional groups showed consistent responses to natural regeneration and active restoration of rainforests. In naturally regenerating forests, even four decades of natural regeneration has not allowed full recovery of forest specialist birds as their communities in the selectively logged forests differed from those in primary forests. In actively restored forests, the abundance of the most vulnerable bird functional groups, that is, forest specialists, frugivores and understory insectivores, responded positively to the decreasing landscape openness associated with the reducing cover of P. purpureum, and to the increasing tree basal area and canopy closure towards older restoration forests. In particular, it was promising that the abundance of frugivores increased towards older restoration forests, since the recovery of regenerating forests could be accelerated by seed dispersal provided by frugivores.

While the responses of birds to vegetation structure were clear in naturally regenerating and actively restored forests, they were variable in primary forests; for example, forest specialists were more abundant in gaps. This could be related to the small gap dynamics of the primary forests, where the tree community generates best in small gaps, and naturally occurring treefall gaps provide suitable microhabitats for feeding for forest birds.

In conclusion, the timescale might be long for rainforest bird community recovery after human-induced disturbances. It seems that the recovery might be slow in naturally regenerating forests if the forest regeneration is arrested, and this highlights the need to preserve primary forests as habitats for birds. However, the active restoration of rainforests can promote rapid initial bird community recovery. This is of high significance, because an increasing proportion of tropical biodiversity depends on the regrowth of forests for survival.

The doctoral dissertation of MSc Pirita Latja, entitled Recovery of bird communities in Afrotropical rainforests after anthropogenic disturbances will be examined at the Faculty of Science and Forestry. The opponent in the public examination will be Docent Hannu Pöysä, Natural Resources Institute Finland (Luke) and the custos will be Heikki Roininen, University of Eastern Finland.

Photo available for download at https://kuvapankki.uef.fi/A/UEF+kuvahakemisto/11932?encoding=UTF-8