Winners and losers: tropical forest tree seedling survival across a West African forest–savanna transition

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.

Woody Plant Encroachment: Causes and Consequences

“…Saplings have better developed root systems, are taller, and have higher leaf area and carbohydrate reserves than seedlings. Unfavorable climate conditions and competition will affect their growth rates, but not necessarily their survival (Cardoso et al 2016). Belowground competition grasses can slow sapling growth particularly during periods of higher than average rainfall (February et al 2013) or if mineral availability is increased (Vadigi and Ward 2012) and can also be amplified by browsing (Vadigi and Ward 2014).…”

“…Among tree species of tropical Africa, seedlings that allocated resources preferentially to growth and resource-capture traits (e.g., height, leaf area, root-shoot ratios) survived better in ecotones between forests and savannas where fire frequency was low; species that allocated preferentially to carbohydrate storage in leaves and roots had better survivorship in fireprone savannas (Cardoso et al 2016). However, larger saplings survived better than smaller saplings, irrespective of allocation traits in either plant community.…”

Many shades of green: the dynamic tropical forest–savannah transition zones

“…For example, Geiger et al [85] showed that in a Brazilian savannah -gallery forest transition not burned for 35 years, only five forest species were successfully expanding into the adjacent grassy vegetation environment. Cardoso et al [123] showed that only a few forest species are sufficiently drought-and fire-resistant to be able to survive the establishment phase in an African savannah environment.…”

Understanding the ecology of tree-seedling growth in dry tropical environment: a management perspective

“…Tree seedling shows greater mortality during dry season as compared to wet season, which may be attributed to its competition with grasses for nutrient resources (Chirara et al 1999). On the contrary, under limited or lower resource conditions, grass competition has been reported to have a less negative or even a positive role on the establishment of tree seedling during dry season (Vieira and Scariot 2006a, b;Cardoso et al 2016). Therefore, field experiments are needed to elucidate these conflicting observations and to understand how grass competition determines the tree recruitment in dry tropical environment under the effect of other set of environmental drivers.…”

“…It may have serious ecological consequences, particularly under changing climate. Studies suggest that tree-seedling survival and establishment in dry tropical ecosystems are constrained mainly by water availability (Khurana and Singh 2001;Kraaij and Ward 2006;Cardoso et al 2016), nutrient availability (Bardgett and Wardle 2003;Vadigi and Ward 2013), shade (Gerhardt 1996;Khurana and Singh 2001), and grass competition (Griscom et al 2014). Therefore, tropical tree species may have considerably poor productivity under resource-limited conditions (e.g., inadequate water, light, and soil nutrients) and under grass competition.…”