Bark harvesting in indigenous forests impacts forest birds

A ring-barked black stintwood in Gomo forest. Picture: Jessica Leaver

The unregulated harvesting of timber, bark and poles from indigenous forests in the Eastern Cape is having a negative impact on bird diversity in these vulnerable forests, a new study from Stellenbosch University shows.

The Eastern Cape harbours 46% of South Africa’s limited remaining natural forest cover, including many of the country’s most threatened forest types, and forms part of the Maputuland-Pondoland-Albany hotspot for biodiversity. However, while there has been an increase in forest cover in these areas, unregulated bark harvesting is leading  to degradation of the forests.

In a study published in the journal Forest Ecology and Management, PhD student, Jessica Leaver, and fellow researchers, investigated the impact of timber, pole and bark harvesting on bird diversity in five Eastern Cape forests: Mqaba, Manubi, Ntlaboya, Gomo and Pirie.

But instead of looking only at the number of species, they focused on the number of ecological roles filled by birds within the forest. “Species that perform more diverse and specialised roles in a forest are more likely to be more sensitive to disturbance,” Ms Leaver says.

Professor Michael Cherry, a behavioural ecologist from the department of botany and zoology at Stellenbosch University and co-author, says the persistence of bird diversity is critical for the functioning and regeneration of these forest systems, some of them at least five million years old.

“Many different species of birds play very important roles in these forests, such as with the dispersal of seeds and pollination. When these birds and the functions they perform start to disappear, the overall health and resilience of the forest is affected,” he says.

Overall, 64 bird species were recorded across the five forests: 23 species at Gomo; 42 species at Mqaba; 51 species at Manubi; 44 at Ntlaboya; and 38 at Pirie. This is the first time that a baseline study of this nature on these forests has been published.

Ms Leaver’s work shows that 29% of trees from which bark has been removed in these forests have been ring-barked. This means the bark has been removed from around the entire circumference of a tree, leading to its slow but inevitable death.

Insect-eating birds such as the Southern Black Flycatcher (Melaenornis pammelaina), the Blue-mantled Crested Flycatcher (Trochocercus cyanomelas), and the African Dusky Flycatcher (Muscicapa adusta) may be negatively affected because, as more sunlight penetrates the canopy due to more trees dying, there may be fewer insects on the forest floor.

Three species of seed-eaters, the Forest Canary (Crithagra scotops), the Green Twinspot (Mandingoa nitidula) and the Lemon Dove (Aplopelia larvata), are also negatively affected by bark harvesting. Again, more sunlight penetrating the canopy cover leads to more foliage plants and less grass on the forest floor, thereby reducing the availability of seeds.

Pole harvesting, i.e. the selective removal of sub-canopy trees , also negatively affected forest specialist species such as the Dark-backed Weaver (Ploceus bicolor), the Grey Cuckooshrike (Coracina caesia), the Olive Bushshrike (Chlorophoneus olivaceus), the Yellow-streaked Greenbul (Phyllastrephus flavostriatus) and the Yellow-throated Woodland Warbler (Phylloscopus ruficapilla) – forest birds which are mostly found in the tree canopy.

Conversely, generalist species such as the Cape White-eye (Zosterops capensis) and the Black-headed Oriole (Oriolus larvatus) were found to be positively affected by pole harvesting. Generalist species are more easily able to take advantage of habitat changes.

A 2017 study by Professor Cherry’s group showed that half of South Africa’s forest-dependent bird species had experienced range declines since 1992, with declines most prominent in the Eastern Cape, despite an increase in forest cover in this region over the same period.

“It now seems likely that habitat degradation arising from informal harvesting is negatively affecting the number of ecological roles filled by forest birds in this region, and contributing to their decline,” Professor Cherry says.