Restoring forest structure, biodiversity and ecosystem function in the West African humid tropics: secondary versus plantation forests
Authors:   University of Helsinki, Faculty of Agriculture and Forestry, Department of Forest Sciences, Brown Hugh C. A. 
Published: October 2024

Deforestation and forest degradation in the tropics has resulted in the depletion of vital forest resources and services, the near eradication of suitable habitats for forest fauna and flora, and the impoverishment of human populations reliant on forest ecosystems. The rapid and concerning pace of deforestation in tropical regions calls for urgent and pragmatic steps to tackle the root causes and rehabilitate or restore degraded and deforested landscapes. The aim of the study was to evaluate the effectiveness of old, unmanaged forest plantations compared to similar-aged secondary forests in restoring forest stand structure, floristics and diversity of vascular plants, and important ecological functions with reference to neighbouring primary forests. In addition, timber value was estimated and compared among the three forest types. The research was conducted across 11 sites within Ghana's moist and wet climatic/forest zones. Systematic random sampling of 93 plots each measuring 20m × 20m with nested subplots measuring 5m x 5m for saplings and 2m x 2m for ground vegetation was undertaken. Forty-two years after establishment and/or abandonment, both the plantation and secondary forests showed structural attributes comparable to those of the primary forests. Nevertheless, the plantation recorded much higher bole volume and basal area compared to the secondary forests. The secondary, plantation and primary forests exhibited considerable overlap in terms of floral composition, with the presence of several rare and restricted-range species. A significant proportion of primary forest vascular plant species, namely 60% and 77%, were identified in the secondary and plantation forests, respectively. The diversity of plant species, as quantified by the Shannon-Wiener Diversity Index (H') and Simpson Index (S), showed no significant variation between primary (H'=3.07, S = 0.91) and secondary (H'=2.95, S = 0.87) or plantation (H'=2.85, S = 0.87) forests. Generally, the primary and secondary forests exhibited higher species richness than the plantations. The mean above-ground carbon stocks of the plantations (159.7 ± 14.3 Mg ha-1 ) was found to be similar to that of the primary forests (173.0 ± 25.1 Mg ha-1 ), but both were much higher than the secondary forests (103.4 ± 12.0 Mg ha-1 ). Soil pH levels in the wet sites were much lower, ranging from 4.2 to 4.6, compared to moist sites, which had pH levels ranging from 4.6 to 5.4. Soil physicochemical properties, carbon stocks, fertility, microbial activity, and litter decomposition measurements across the different forest types within the climatic zones were similar. Nevertheless, significant differences were observed between climatic zones. Contrary to results of earlier tropical studies, we observed higher litter decomposition rates in the moist compared to the wet zone, which experiences higher annual rainfall, especially for the recalcitrant carbon fraction of the litter. Relatedly, soil microbial biomass and microbial population were significantly greater in the moist compared to the wet zone. Mean soil carbon stocks (0 - 50 cm) was significantly higher in the wet (106.8 Mg ha-1 ) compared to the moist (56.9 Mg ha-1 ), with mean site values ranging from 51.16 Mg ha-1 to 122.84 Mg ha-1 . The mean timber stumpage value of plantations was $8577 per hectare, compared to primary and secondary forests, which were $3112 and $1870 per hectare, respectively. Tropical forest plantations established on long rotations under low-intensity management regimes, and secondary forests can evolve into forest systems that exhibit structural complexity, floristic diversity, ecological functionality, and self-sustainability, akin to primary forests. Such forest plantations and secondary forests constitute viable pathways for the restoration of deforested landscapes and climate change mitigation, while potentially providing landowners with moderate financial returns through selective timber harvesting.