Enrichment of Logging Gaps with High-Value Timber Species: How Far Fertilizer, Biochar and Mammal Predation Affect Performances of Cylicodiscus gabunensis Harms Seedlings

Abstract

Many commercial species are light-demanding and regenerate with difficulty in natural forest, which compromises the sustainability of logging. Okan, Cylicodiscus gabunensis Harms is one of the most exploited species in Central Africa and its regeneration is deficient in evergreen forest. In forest concessions, the enrichment of logging gaps with commercial species has already been tested but only for a few species. Mixed results have been obtained because the ability of seedlings to emerge from competing vegetation depends on the species, the environment and the silvicultural techniques adopted. This paper aims to determine the performance of C. gabunensis when planted in felling gaps. The impact of fertilization and biochar application on the performance of the seedlings was examined, as well as the role of predation played by large mammals. In 30 gaps, whose light levels were quantified, we planted nine seedlings and applied three treatments (fertilizer and biochar, fertilizer, control). The performance of the seedlings (survival, mammal damage and growth) was followed for 18 months. In another 30 gaps, the preferential consumption of C. gabunensis seedlings was quantified using camera traps. Seedlings had moderate and highly variable growth (1.84 cm to 2.50 cm in height and 0.201 mm to 0.267 mm in basal diameter per month, all treatments combined). Gap size and initial fertilization significantly boosted growth in diameter and survival rate. Elephants preferentially sought out C. gabunensis seedlings and after 18 months they destroyed 35% of the plants. Enrichment of logging gaps with C. gabunensis should therefore be limited to the largest gaps in forests with low elephant densities. Initial fertilization is recommended but not allowed under the sustainable management certification guidelines. We suggest that these standards should be adapted to maximize the chances of success.

Contrasted spatial, demographic and genetic structures of a light-demanding African timber species, Cylicodiscus gabunensis Harms – Implications for a sustainable management of its populations

“…The legal minimum cutting diameters (LMCD) for C. in Cameroon (C) and Gabon (G) are also shown to envision the potential impact of selective logging on the reproduction potential. Makemba et al, 2022b). Notably, site A experiences higher levels of irradiance (Philippon et al, 2019;Seiwa, 1998;Tang & Dubayah, 2017) and the absence of elephants (Brittain et al, 2020), potentially promoting favorable conditions for seedling growth.…”

“…The LMCD of C. gabunensis varies from 60 cm in Cameroon to 70 cm in Gabon (Ndonda Makemba et al, 2019). Its bark, which has medicinal properties, is highly prized by elephants which severely damage the base of the trees and consume the seedlings (Ndonda Makemba et al, 2022b).…”

“…2) and variable elephant densities. A recent study investigating how mammal predation affects performances of C. gabunensis seedlings in logging gaps pointed out important damages (destruction of 35 % of the seedlings) by the African forest elephant (Loxodonta cyclotis) (Ndonda Makemba et al, 2022b), which is rare in site A but moderately to very abundant in sites B and C (Table 1). All three locations have an equatorial climate with a consistent temperature averaging 24-25 • C throughout the year.…”

“…Hence, additional research should be conducted on the impact of logging on natural regeneration in different forest types to assess whether canopy opening effectively foster seedling recruitment. It must also be taken into account that the skidding trail network leading to felling gaps are used by elephants (Scalbert et al, 2022), which selectively forage on C. gabunensis seedlings (Ndonda Makemba et al, 2022b). Therefore, one might enrich logging gaps or other open areas like log yards (Ndonda Makemba, 2023) to compensate for reduction in reproductive trees but such measures may be effective only in areas with low elephant densities (Ndonda Makemba et al, 2022b).…”