Not all trees can make a forest: Tree species composition and competition control forest encroachment in a tropical savanna

Abstract

1. Forest encroachment into savannas is a widespread phenomenon, the rate of which may depend on soil conditions, species composition or changes in stand structure. As savanna specialist trees are replaced by generalist species, rates of stand development may increase. Because generalists can persist in forests, they are likely to grow more quickly and survive longer in dense stands, compared to savanna specialists. Furthermore, the faster growth rates of generalists may allow them to overtop and outcompete savanna specialists, causing rapid species turnover.2. We measured growth and survival of 6,147 individuals of 112 species of savanna and generalist tree species over a period of 10 years in an ecological reserve in Assis, São Paulo State, Brazil. We modelled growth and mortality as a function of soil texture and nutrients, tree size, competitive neighbourhood, and membership in savanna or generalist (species which can persist in forests and savannas) functional groups.3. Tree growth and survival was strongly influenced by competition, as estimated by the basal area of trees taller than a focal tree. At the stand level, savanna species are unable to contribute basal area growth in closed stands, while generalist species continue to increase in basal area even at high stand basal area. This phenomenon is driven by differences in growth and mortality. Generalists grew faster than savanna species, both in height and diameter. This difference in growth rates led to savanna species becoming suppressed more rapidly than generalists. When suppressed, savanna species were more than twice as likely to die than were generalists. Soils had inconsistent and mostly weak effects which were difficult to separate from gradients of stand structure. Synthesis.We demonstrate that the presence of generalist trees accelerates the rates of basal area accumulation due to their greater growth rates and tolerance of shading. Generalists outcompete savanna trees by growing faster in the open and overtopping savanna specialists. Due to the slow growth and high mortality of savanna species in the shade, they are unable to form closed-canopy stands.Accounting for differences among functional types and development of vegetation structure is critical for modelling forest encroachment.

Canopy plant composition and structure of Cape subtropical dune thicket are predicted by the levels of fire exposure

“…Our results on compositional differences along a fire exposure gradient are broadly consistent with those from other systems where fire-sensitive and fire-dependent systems coexist in the same landscape. The degree of fire exposure is an important influence in the composition of vegetation in forest-fynbos systems in the CFR ( Manders, 1990 ; Geldenhuys, 1994 ) and in the tropical forest-savanna systems of Africa, Brazil and Australia ( Charles-Dominique et al, 2015 ; Hoffmann et al, 2009 ; Murphy & Bowman, 2012 ; Flake et al, 2021a ; Flake et al, 2021b ; Becket et al, 2022 ). In both systems, regular fires maintain the more open, fire-dependent ecosystems (fynbos, savanna) by preventing invasion of fire-sensitive forest species ( Geldenhuys, 1994 ; Hoffmann et al, 2009 ).…”

“…Several studies have investigated the influence and importance of fire on species composition and structure of savanna-forest mosaics of Africa, Australia and Brazil ( Charles-Dominique et al, 2015 ; Hoffmann et al, 2009 ; Murphy & Bowman, 2012 ; Flake et al, 2021a ; Flake et al, 2021b ; Becket et al, 2022 ). In these systems, regular fires promote savanna tree and grass abundance and suppress forest trees, which are mostly killed by fire; however, when the fire frequency increases, savanna trees can also be suppressed, thereby favoring grasslands.…”

“…In these systems, regular fires promote savanna tree and grass abundance and suppress forest trees, which are mostly killed by fire; however, when the fire frequency increases, savanna trees can also be suppressed, thereby favoring grasslands. Accordingly, if fire is absent for too long, forest tree abundance will increase, and savanna tree and grass cover will decrease ( Hoffmann et al, 2009 ; Flake et al, 2021b ; Becket et al, 2022 ). Less is known about the influence of fire on species composition and structure of closed-canopy vegetation types that rarely burn, such as Afrotemperate forest ( Watson & Cameron, 2001 ; Giddey, Baard & Kraaij, 2022a ) and thicket ( Cowling, 1984 ; Vlok, Euston-Brown & Cowling, 2003 ; Cowling & Potts, 2015 ); however, some information on species-specific responses to fire is available, which leads to the question of how fire exposure shapes community composition in these closed communities.…”

Intra‐seasonal and daily variations in nectar availability affect bee assemblage in a monodominant afforested Brazilian Cerrado

“…Furthermore, the increase in the reproductive success of exotic trees might be added to their high capacity to grow and survive in nonforested habitats (e.g. Abreu et al 2021;Flake et al 2022). However, further analyses are still needed to assess the effect of a highrewarding exotic tree species on other plant species in afforested habitats worldwide.…”

“…2021; Flake et al . 2022). However, further analyses are still needed to assess the effect of a high‐rewarding exotic tree species on other plant species in afforested habitats worldwide.…”