Old timber plantations and secondary forests attain levels of plant diversity and structure similar to primary forests in the West African humid tropics

Abstract

Considering the high rate of primary forest degradation and loss in the tropics, the ability to conserve plant diversity within alternative forest landscape components is critical to biodiversity conservation. This study compares the restoration potential of old forest plantations and secondary forests. We assessed and compared the floristics, plant species diversity, conservation value, and structure of old (42–47 years) timber plantations ofAucoumea klaineana Pierre,Cedrela odorata L.,Tarrietia utilis Sprague, andTerminalia ivorensis A. Chev. and similar-aged secondary forests with nearby primary (old-growth) forests in the moist and wet forest zones of Ghana.We established a systematic sampling set-up of ninety-three 20 m × 20 m plots in total on 11 sites, with smaller nested subplots for saplings and ground vegetation.The floristic composition of the plantation and secondary forest stands were similar to that of the primary forests, with many rare and restricted-range species shared by the three forest types. Approximately 77% and 60% of primary forest plant species also occurred in plantation and secondary forests, respectively. Species diversity, measured by the Shannon-Wiener Diversity Index (H’) and Simpson Index (S), for the primary forest (H’=3.07, S = 0.91) was not statistically different from the plantation (H’=2.85, S = 0.87) or secondary (H’=2.95, S = 0.88) forests. Overall, species richness was higher in the primary and secondary forests compared to the plantations. At the tree stratum (≥10 cm DBH), the assessed diversity indices were significantly different between the primary forest and the plantations. However, such differences did not exist among the saplings (10 cm > DBH ≥ 2 cm) and ground vegetation (<2cm DBH). The plantations and secondary forests were similar to the primary forests for all the structural characteristics assessed. However, basal area and bole volume were significantly higher in the plantations compared with secondary forests. Conservation value (using Genetic Heat Index as an indicator) was highest in one of the C. odorata plantations (W-CO).Our study demonstrates that plantation and secondary forests can develop into structurally complex and floristically diverse self-organized stands similar to primary forests. Passive conversion of plantations to more natural ecosystems is an effective and low-cost forest restoration strategy leading to diverse ecosystems with high conservation value.