Tree seedling recruitment dynamics in forest-savanna transitions

2020

English

Tropical forests and savannas make up between 15% to 20% of the earth’s terrestrial surface (Grace et al. 2006) and account for over 60% of terrestrial productivity (Beer et al. 2010). Tropical forests and savannas represent distinct biomes both of which are crucial to the earth’s land-atmosphere feedbacks, other ecosystem services (Bonan 2008, Furley et al. 1992) and support many sociocultural and economic livelihoods (Grace et al. 2006). Tropical forests refer to those tropical vegetation types dominated by woody species with a canopy cover sufficiently high to reduce (or eliminate) C4 grasses (Torello-Raventos et al. 2013, Veenendaal et al. 2015). This definition includes a wide range of canopy cover types, but generally with a canopy area index ≥ 0.7, tree height ≥ 12 m and fractional herb and grass cover ≤ 0.1 (Torello-Raventos et al. 2013, Veenendaal et al. 2015). At times, “grassy” are also distinguished, e.g. forests in South East Asia, (Torello-Raventos et al. 2013). The term woodland (sometimes, with a prefix, e.g. “tall” or “closed”) is used to distinguish tropical vegetation formations with almost closed canopies but often comprising species that may not be regarded as true forest species, the latter being defined as more shade-tolerant and fire-sensitive (see for various views e.g. Torello-Raventos et al. 2013, Ratnam et al. 2011). Tropical savannas, on the other hand, are vegetation types characterized by the coexistence of herbaceous vegetation with woody species (Torrelo-Raventos et al. 2013, Scholes & Archer 1997). The relative dominance of trees and grasses varies considerably across savanna types (Ratnam et al. 2011, Torrelo-Raventos et al. 2013, Veenendaal et al. 2015). Generally, canopy area index is < 0.7 (i.e. not closed-canopy) with fractional herb and grass cover > 0.1, although woody plant height may be as tall as forest vegetation (Torrelo-Raventos et al. 2013).

Issifu, H.