Abstract
Forest encroachment into savanna is occurring at an unprecedented rate across tropical Africa, leading to a loss of valuable savanna habitat. One of the first stages of forest encroachment is the establishment of tree seedlings at the forest–savanna transition. This study examines the demographic bottleneck in the seedlings of five species of tropical forest pioneer trees in a forest–savanna transition zone in West Africa. Five species of tropical pioneer forest tree seedlings were planted in savanna, mixed/transition, and forest vegetation types and grown for 12 months, during which time fire occurred in the area. We examined seedling survival rates, height, and stem diameter before and after fire; and seedling biomass and starch allocation patterns after fire. Seedling survival rates were significantly affected by fire, drought, and vegetation type. Seedlings that preferentially allocated more resources to increasing root and leaf starch (starch storage helps recovery from fire) survived better in savanna environments (frequently burnt), while seedlings that allocated more resources to growth and resource‐capture traits (height, the number of leaves, stem diameter, specific leaf area, specific root length, root‐to‐shoot ratio) survived better in mixed/transition and forest environments. Larger (taller with a greater stem diameter) seedlings survived burning better than smaller seedlings. However, larger seedlings survived better than smaller ones even in the absence of fire. Bombax buonopozense was the forest species that survived best in the savanna environment, likely as a result of increased access to light allowing greater investment in belowground starch storage capacity and therefore a greater ability to cope with fire. Synthesis: Forest pioneer tree species survived best through fire and drought in the savanna compared to the other two vegetation types. This was likely a result of the open‐canopied savanna providing greater access to light, thereby releasing seedlings from light limitation and enabling them to make and store more starch. Fire can be used as a management tool for controlling forest encroachment into savanna as it significantly affects seedling survival. However, if rainfall increases as a result of global change factors, encroachment may be more difficult to control as seedling survival ostensibly increases when the pressure of drought is lifted. We propose B. buonopozense as an indicator species for forest encroachment into savanna in West African forest–savanna transitions.
Highlights
Tropical forests are generally defined as a continuous closed-canopy tree layer with a discontinuous herbaceous understory
The balance of forest and savanna is dynamic through time and space, the loss of savanna as a result of forest encroachment has been occurring at an unprecedented rate in the recent years
N. diderrichii showed the sharpest decrease in survival of all species (64% of mortality, Tables 2 and S2), with survival dropping to zero following the fire (Fig. 1, Table 2)
Summary
Tropical forests are generally defined as a continuous closed-canopy tree layer with a discontinuous herbaceous understory. Forest encroachment has already been reported in Southern (Parr et al 2012), Western (Fairhead and Leach 1996), Eastern (Belsky and Amundson 1992), and Central Africa (Mitchard et al 2009). In some sites, it is proceeding as rapidly as 50 m per century (Schwartz et al 1996), and in fire-protected areas, there can be a complete transformation from savanna to forest in just 15 years (Jeffery et al 2014). The loss of savanna habitat as a result of forest encroachment is concerning as savannas are “globally extensive, provide critical ecosystem services, and influence the earth-atmosphere system [providing] significant environmental, economic, and cultural value to the world” (Parr et al 2014)
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