Inserting Tropical Dry Forests Into the Discussion on Biome Transitions in the Tropics

Abstract

Tropical moist forests and savannas are iconic biomes. There is, however, a third principal biome in the lowland tropics that is less well known: tropical dry forest. Discussions on responses of vegetation in the tropics to climate and land-use change often focus on shifts between forests and savannas, but ignore dry forests. Tropical dry forests are distinct from moist forests in their seasonal drought stress and consequent deciduousness and differ from savannas in rarely experiencing fire. These factors lead tropical dry forests to have unique ecosystem function. Here, we discuss the underlying environmental drivers of transitions among tropical dry forests, moist forests and savannas, and demonstrate how incorporating tropical dry forests into our understanding of tropical biome transitions is critical to understanding the future of tropical vegetation under global environmental change.

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PERSPECTIVE
published: 24 July 2018
doi: 10.3389/fevo.2018.00104
Frontiers in Ecology and Evolution | www.frontiersin.org 1July 2018 | Volume 6 | Article 104
Edited by:
Colin Osborne,
University of Sheffield,
United Kingdom
Reviewed by:
Imma Oliveras,
University of Oxford, United Kingdom
Liam Jude Langan,
Senckenberg Biodiversität und Klima
Forschungszentrum (SBiK-F),
Germany
*Correspondence:
Kyle G. Dexter
kyle.dexter@ed.ac.uk
Specialty section:
This article was submitted to
Biogeography and Macroecology,
a section of the journal
Frontiers in Ecology and Evolution
Received: 01 May 2018
Accepted: 29 June 2018
Published: 24 July 2018
Citation:
Dexter KG, Pennington RT,
Oliveira-Filho AT, Bueno ML, Silva de
Miranda PL and Neves DM (2018)
Inserting Tropical Dry Forests Into the
Discussion on Biome Transitions in the
Tropics. Front. Ecol. Evol. 6:104.
doi: 10.3389/fevo.2018.00104
Inserting Tropical Dry Forests Into
the Discussion on Biome Transitions
in the Tropics
Kyle G. Dexter 1,2
*, R. Toby Pennington 2,3 , Ary T. Oliveira-Filho 4, Marcelo L. Bueno 5,
Pedro L. Silva de Miranda 1and Danilo M. Neves 4,6
1School of GeoSciences, University of Edinburgh, Edinburgh, United Kingdom, 2Royal Botanic Garden Edinburgh,
Edinburgh, United Kingdom, 3Geography, University of Exeter, Exeter, United Kingdom, 4Departamento de Botânica,
Universidade Federal de Minas Gerais, Belo Horizonte, Brazil, 5Laboratório de Ecologia e Evolução de Plantas, Universidade
Federal de Viçosa, Viçosa, Brazil, 6Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, AZ,
United States
Tropical moist forests and savannas are iconic biomes. There is, however, a third
principal biome in the lowland tropics that is less well known: tropical dry forest.
Discussions on responses of vegetation in the tropics to climate and land-use change
often focus on shifts between forests and savannas, but ignore dry forests. Tropical dry
forests are distinct from moist forests in their seasonal drought stress and consequent
deciduousness and differ from savannas in rarely experiencing fire. These factors lead
tropical dry forests to have unique ecosystem function. Here, we discuss the underlying
environmental drivers of transitions among tropical dry forests, moist forests and
savannas, and demonstrate how incorporating tropical dry forests into our understanding
of tropical biome transitions is critical to understanding the future of tropical vegetation
under global environmental change.
Keywords: tropical dry forest, tropical moist forest,savanna, biomes, fire, soil fertility, water stress, deciduousness
INTRODUCTION
Predicting vegetation change in the tropics depends on understanding the drivers of transitions
among major vegetation types, or biomes. Climatic factors, such as mean annual precipitation
(MAP) and its seasonality are of obvious importance, but edaphic factors, and disturbance via
fire, humans and herbivores also play key roles. Recent large-scale studies across the tropics have
focused on transitions between forest and savanna (Hirota et al., 2011; Staver et al., 2011; Oliveras
and Malhi, 2016; Xu et al., 2016; Langan et al., 2017). While there is value to simplifying vegetation
concepts in the tropics, we believe the simplification used by these authors in defining “forest”
goes perhaps one step too far. There are two principal kinds of forest in the lowland tropics, moist
forests and dry forests. With very few exceptions (e.g., Hirota et al., 2010; Lehmann et al., 2011),
studies of biome transitions in the tropics have either failed to distinguish them, or have completely
ignored dry forests, focusing solely on moist forests when using the term “forest.” The aim of
this perspective is to discuss biome transitions in the tropics and their underlying drivers, while
including dry forests in the discussion. We focus on transitions among savanna, moist forest and
dry forest, the three biomes in the lowland tropics with a substantial tree component.
Tropical moist forest and savanna are relatively well understood at a global scale compared to
tropical dry forests (Pennington et al., 2018). Moist forest is tall, multi-stratal, and with a closed
canopy. Tropical moist forests include tropical rain forests, as well as forests with lower rainfall
where soil moisture is maintained throughout the year, via edaphic factors such as proximity to

Dexter et al. Tropical Dry Forests and Biome Transitions
rivers, or water recycling, allowing most trees to be evergreen
(Guan et al., 2015). The understory is often dominated by
saplings of taller-statured tree species, although small tree and
shrub species are present. Terrestrial forbs and grasses are
a minor component of diversity and biomass. Savanna is a
more open environment, where tree species are present, but
individuals do not form a closed canopy. There is a significant
understory grass component, which is flammable, and fires are
common. Tree species that occur in savannas are adapted to these
recurring fires (Simon and Pennington, 2012), and regular fires
are necessary for the maintenance of savanna biodiversity (Parr
et al., 2014; Durigan and Ratter, 2016; Abreu et al., 2017). Some
savannas in the paleotropics (e.g., miombo woodlands in Africa,
deciduous dipterocarp forests in southeast Asia) are generally
referred to as dry forests, but we consider them as savannas given
that they have a grassy understory and experience regular fire
(Ratnam et al., 2011; Dexter et al., 2015; Pennington et al., 2018).
Tropical dry forests vary greatly in structure, from tall, closed
canopy forest to short scrub vegetation, occasionally not forming
a closed canopy, especially in drier areas (Pennington et al.,
2000). They are distinct from savanna in not having a significant
grass component and not experiencing regular fires (Murphy and
Lugo, 1986; Gentry, 1995). In fact, regular fires would be lethal
for many of the characteristic life forms and taxa of tropical
dry forest (e.g., cacti; Mooney et al., 1995). This is not to say
that dry forests never experience fires. Even moist forests can
experience fire under extreme drought conditions (Aragão et al.,
2016). Rather, damaging fire is sufficiently rare in dry forests
such that fire-intolerant species can persist in the landscape
as metapopulations (Hanski, 1998). The exact threshold of fire
return interval or intensity involved in the tropical dry forest -
savanna transition is poorly understood and likely to vary with
the broader environmental context (e.g., soil fertility and annual
precipitation; Hoffmann et al., 2012; Murphy and Bowman,
2012), and we suggest that this should be a priority for further
study. Tropical dry forest is distinct from moist forest in its
seasonal drought stress, which leads many tree species to lose
their leaves in the dry season (Reich and Borchert, 1984; Murphy
and Lugo, 1986). The combination of seasonal drought stress
and lack of fire leads to ecosystem function in dry forests that is
markedly different from savannas or moist forests, which justifies
their distinction as a unique biome.
BIOMES IN LOWLAND TROPICAL SOUTH
AMERICA
We focus this review on continental lowland tropical South
America (LTSA), where we have conducted most of our research.
In a recent study (Silva de Miranda et al., in press), we
used an unsupervised classification, or hierarchical clustering,
of sites based on their tree species composition (see inset in
Figure 1), followed by interpretation of the resulting cluster
using site information on vegetation physiognomy (savanna vs.
forest) and leaf flush regime (evergreen vs. semideciduous vs.
deciduous) to delimit and map biomes across LTSA east of the
Andes (Figure 1). Moist forests fell in two major groups in the
cluster and occurred in two large geographic blocks, one in the
Amazon basin and another along the Atlantic coast of Brazil.
Semideciduous forests did not form a distinct group in the
cluster and were instead mixed with evergreen, moist forest sites.
Semideciduous forests are often found in drier regions, where
they occur along rivers, lake margins and submontane areas
with orographic precipitation. Savanna formed a single group in
the cluster and was most prevalent in central Brazil, in an area
commonly termed the Cerrado.
Tropical dry forest also formed a single group in the
cluster, which was comprised almost entirely of forest sites
with deciduous phenology. In LTSA, the largest block of dry
forest occurs in the Caatinga region of northeast Brazil. The
Caatinga has been referred to as a biome (Hirota et al., 2010;
Instituto Brasileiro de Geografia e Estatística, 2012), although
as a region it contains non-dry forest habitat (e.g., patches
of savanna). Further, tropical dry forest is found outside of
the Caatinga, in patches throughout the Cerrado, in an area
spanning the Pantanal and Chiquitania in Bolivia (Figure 1),
and scattered more widely across the Neotropics (DRYFLOR,
2016). While Silva de Miranda et al. (in press) broadly assessed
climatic overlaps amongst biomes, they did not focus on the
environmental drivers of transitions between individual biomes.
That is the goal of the present manuscript.
TRANSITIONS BETWEEN TROPICAL
SAVANNA AND DRY FORESTS
A common view of dry forests in the tropics is that they
are transitional between savannas and moist forests along
precipitation gradients (e.g., Whittaker, 1970; Malhi et al., 2009).
If, however, we examine how the sites featured in Figure 1 are
distributed over variation in MAP (Figure 2), a more complex
picture emerges. Moist forests do occur under wetter conditions
than savanna, but dry forests are largely found under drier
conditions. Below 1,000 mm MAP, savanna quickly disappears
and dry forest becomes the only tree-dominated vegetation type.
The largest area of these arid dry forests is found in the Caatinga
region of northeast Brazil. As discussed above, a key distinction
between savanna and dry forest is the regularity of fire, and in
these dry conditions, there is not sufficient biomass build-up to
sustain regular fires (Van Der Werf et al., 2008). In particular,
this reflects the relative lack of grasses. The tree species that are
present are able to tolerate severe drought, but they do not invest
in adaptations for fire, such as thick bark or underground stems,
characteristic of savanna species (Simon and Pennington, 2012).
There is also extensive occurrence of the dry forest biome
under the same precipitation conditions as savanna. These
are dry forests found in the Cerrado region and around the
Pantanal and Chiquitania regions of Brazil and Bolivia. Within
the Cerrado, dry forests are known to occur on and around
calcareous outcrops, where soils have higher phosphorus and
base cation concentrations (Ratter et al., 1978; Furley and
Ratter, 1988; Oliveira-Filho and Ratter, 2002; Neves et al.,
2015). On these soils, trees can grow more quickly, have better
chances of escaping the “fire trap” and are more likely to
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Dexter et al. Tropical Dry Forests and Biome Transitions
FIGURE 1 | Map of biomes across lowland tropical South America (after Silva de Miranda et al., in press). Inset shows results of unsupervised clustering of 3,331
georeferenced sites based on tree species composition. The biomes of the major groups in the cluster were determined based on vegetation physiognomy and leaf
flush regime. Semideciduous forests were placed in the moist forest biome based on similarities in tree species composition evident in the cluster by Silva de Miranda
et al. (in press), but are distinguished here. We exclude sites south of 23◦S latitude or above 1,000 m elevation.
form a closed canopy (Hoffmann et al., 2009). There can be
positive feedbacks between tree growth, grass exclusion and fire
mitigation that leads to a forested vegetation (Hoffmann et al.,
2012; Silva et al., 2013; Pausas and Dantas, 2017). Calcareous
outcrops in the Cerrado also have poorly developed, shallow
soils, and vegetation occurring on them may experience greater
drought stress than surrounding vegetation, thus making them
similar to the arid dry forests, which lack fire because of
insufficient fuel build-up. However, it is likely that soil fertility
is relevant for the presence of dry forests around calcareous
outcrops as a different vegetation, cerrado rupestre, which is
floristically related to savanna vegetation, is found on non-
calcareous outcrops in the Cerrado (Ribeiro and Walter, 1998).
Whichever factor is more important (soils with high fertility
or low water-holding capacity), it is evident that the same
drought-tolerant, fire-intolerant tree species and lineages that
dominate vegetation in the arid Caatinga are also found in dry
forest patches in the moister Cerrado (Prado and Gibbs, 1993;
Neves et al., 2015; DRYFLOR, 2016; Silva de Miranda et al.,
in press).
Dry forest and savanna vegetation also intermingle in the
Chiquitania region of Bolivia, but here dry forest predominates
and savanna occurs in patches, which may be because soils in the
Chiquitania are more fertile on average than in the Cerrado (Silva
de Miranda et al., in press). Some of the savannas that are present
in the Chiquitania region may represent dry forest that has been
degraded by logging or anthropogenic fire (Devisscher et al.,
2016), highlighting that human land-use patterns can readily
drive transitions between dry forest and savanna. However, “old-
growth savannas” that would exist independent of anthropogenic
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Dexter et al. Tropical Dry Forests and Biome Transitions
FIGURE 2 | Frequency distribution of sites in Figure 1 over a gradient in mean annual precipitation. For heuristic purposes, semideciduous and moist forests are
distinguished although they may represent the same biome.
influence are also clearly present in Bolivia (Power et al., 2016;
Veldman, 2016).
If fire is excluded from savanna vegetation, it first converts
to a formation with a higher percentage of tree canopy cover
termed “cerradão” (Durigan and Ratter, 2006, 2016), which
translates from the Portuguese as “big cerrado” and is generally
considered as a forest. Cerradão shares some tree species with
tropical dry forest and comparatively few with semideciduous
and evergreen moist forests (Bueno et al., 2018), even though
the latter are present near savanna vegetation along river
courses and lake margins that have year-round water availability
(Ribeiro and Walter, 1998). Tree species from semideciduous
and evergreen forests may be less likely to immigrate into
cerradão than typical dry forest tree species because they are
not adapted to seasonal drought. While cerradão is initially
comprised of fire-adapted tree species from the cerrado, dry
forest tree species colonize this environment if propagules are
available, fire remains absent and soils are sufficiently fertile.
These dry forest tree species may eventually outcompete cerrado
tree species, since they do not invest in fire defense (Ratajczak
et al., 2017), and in the prolonged absence of fire, cerradão
may transition to a dry forest if there are positive feedback
cycles between forest vegetation, lack of fire and soil fertility
(Silva et al., 2013; Pellegrini et al., 2014, 2018). However, if
the underlying soils remain poor and/or if there are high
aluminum concentrations in the soil that do not attenuate
over time, then cerrado tree species, which are adapted to
infertile, aluminum-rich soils may continue to dominate the
vegetation.
Overall, the savanna-dry forest transition is distinct from
the savanna-moist forest transition in two key ways: (1) the
contrasting role of water availability (lowest in dry forest,
intermediate in savanna and highest in moist forest) and (2) the
potentially critical importance of soil fertility for the savanna-
dry forest transition (savanna and moist forests are similar in
generally having infertile, acidic soils).
TRANSITIONS BETWEEN MOIST FOREST
AND DRY FOREST
Both of these biomes are forest, but they function in distinct
ways. Dry forests are found in areas with marked precipitation
seasonality, which leads most species to lose their leaves
during the dry season and has significant implications for
nutrient cycling (Reich and Borchert, 1984; Murphy and Lugo,
1986). Many moist forests also experience seasonality in water
availability (e.g., in the southern and eastern Amazon), but the
dry season is three months or less and subsurface water remains
available to trees (Guan et al., 2015). The systems also differ in the
rate at which they accrue and cycle carbon, with trees in moist
forests growing more quickly and storing more total carbon
(Murphy and Lugo, 1986; Poorter et al., 2017). There are often
differences in soil fertility, with dry forests occurring on more
fertile soils, which facilitates their ability to shed their leaves as
they can readily afford to grow new ones. However, high rainfall
in moist forests results in nutrient leaching, and this correlation
between soil fertility and biome identity may be due to overriding
climatic factors (Webb, 1968; Hall and Swaine, 1976).
In LTSA, there are multiple areas of contact between the
moist and dry forest biomes (Figure 1). One transition zone is in
northeastern Brazil, where evergreen Atlantic forest on the coast
transitions to dry forest in the arid Caatinga. In between the two
lies a band of semideciduous forests. Another transition zone is
found in the Chiquitania region of eastern Bolivia and adjacent
areas of Brazil, where there is a gradual transition over 200+km
of geographic distance, largely covered by semideciduous forest.
We suggest that transitions between dry forest and moist forest
are primarily mediated by water availability and that intermediate
states are possible in zones of intermediate water availability
(Oliveira-Filho and Fontes, 2000; Oliveira-Filho et al., 2006). This
contrasts with transitions between savanna and moist forest that
can be more abrupt and may represent alternative stable states
(although see Lloyd and Veenendaal, 2016).
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Dexter et al. Tropical Dry Forests and Biome Transitions
SEMIDECIDUOUS FORESTS
Previous studies have variously grouped semideciduous forests
with the dry forest biome (Murphy and Lugo, 1986; Pennington
et al., 2000) or the moist forest biome (DRYFLOR, 2016; Silva de
Miranda et al., in press). In fact, as discussed above, these forests
may be transitional between the two. Semideciduous forests in
LTSA have few endemic tree species and instead contain tree
species associated with the dry or moist forest biomes (Oliveira-
Filho and Fontes, 2000). As moist forests contain many more
tree species than dry forests (Esquivel-Muelbert et al., 2017), we
suggest that they may contribute more species to semideciduous
forests simply via mass effects (Shmida and Wilson, 1985), and
this may be why they group with moist forests in clustering
analyses based on presence versus absence of tree species (as
in Silva de Miranda et al., in press). If abundance information
were to be taken into account (e.g., via inventory plot data), we
hypothesize that semideciduous forests may cluster with moist
or dry forest based on the proportion of individuals belonging
to moist versus dry forest tree species. It is clear that future
comparative studies across dry, moist and semideciduous forests
are needed to understand their origins and how they compare
in terms of ecosystem function. Their geographically variable
species composition and lack of endemic species suggests that
semideciduous forests may have been independently and recently
assembled in different ecotonal areas.
BIOME TRANSITIONS TO DRY FOREST
OUTSIDE THE NEOTROPICS
As in South America, moist forest—savanna transitions have
been studied extensively on other continents, but transitions to
dry forest have received less attention. This is partly because it is
unclear where dry forest exists outside of the Neotropics (Lock,
2006; Dexter et al., 2015; Pennington et al., 2018). In a recent
study, Linder (2014) delimited and described the main “floras”
of Africa, which are large-scale units of vegetation that have
a distinct evolutionary and biogeographic history and differ in
their present-day plant taxonomic composition. Linder did not
assign the term “biome” to these vegetation units, although his
“floras” correspond to several previously defined biomes. There
is a “savanna flora,” which readily corresponds to the savanna
biome, and a “lowland forest flora” that largely corresponds to
the moist forest biome. Linder postulated an “arid flora” that
is most evident in the Horn of Africa (Somalia, Ethiopia and
northern Kenya), but is also present in arid regions of Angola
and Namibia. The distribution of this “arid flora” largely overlaps
the distribution of the “succulent biome” in Africa, as proposed
by Schrire et al. (2005). The “arid flora” or “succulent biome” is
similar to dry forest in arid regions of the Neotropics in that there
is not adequate water availability to allow for sufficient biomass
build-up to sustain regular fires. Thus, as in the Neotropics,
water availability may be one environmental factor that underlies
transitions between savanna and dry forest in Africa.
Soil fertility is another significant factor that has been shown
to underlie savanna-dry forest transitions in the Neotropics.
An important question for future research in Africa is whether,
amongst its great expanses of savanna, there is distinct vegetation
that does not regularly burn, is found on more fertile soils and
shows greater floristic similarity with the “arid flora” of Linder
(2014) than it does with surrounding savanna vegetation. It
may be that in Africa, a higher abundance of large herbivores,
including elephants, favors grasses over trees, leading to a more
open savanna vegetation with more frequent fires, even in areas
of higher soil fertility (Charles-Dominique et al., 2016; Pellegrini
et al., 2017). If dry forests are not found on fertile soils in
more mesic areas of Africa, there may not be moist forest-dry
forest transitions on this continent, because the areas mapped as
belonging to the “arid flora” or “succulent biome” are completely
separated from moist forest regions by large areas of savanna
(Schrire et al., 2005; Linder, 2014).
In the tropical regions of continental Asia and in Malesia,
moist forest is the predominant vegetation type, although drier
forest formations are present (e.g., deciduous forests in the
Western Ghats and dry dipterocarp forests in Indochina). As
we have discussed elsewhere (Dexter et al., 2015; Pennington
et al., 2018), the majority of these drier forest formations have
a significant grassy component in the understory, burn regularly
and may be better considered as savannas (Ratnam et al., 2011).
The succulent biome of Schrire et al. (2005) is mapped as present
in arid regions of northwest India and extending across the coast
of Pakistan and Iran to the Arabian Peninsula. Thus, an arid
form of the dry forest biome may occur in Asia, as in Africa, and
water availability may underlie transitions between vegetation in
seasonally dry areas that regularly burns (what we term savanna)
and that does not regularly burn (what we term dry forest).
As with Africa, future research in Asia should assess if there
are vegetation formations in seasonally dry, yet not arid, areas
that: (1) are found on fertile soils, (2) do not regularly burn
and (3) show greater floristic similarity with arid areas than
with surrounding vegetation that does regularly burn. This will
help determine if soil fertility is also important in understanding
savanna-dry forest transitions in Asia, as it is in the Neotropics.
CONCLUSIONS
The aim of this perspective has been to bring the tropical
dry forest biome into discussions of biome transitions in the
tropics. Previous studies of tropical biome transitions have largely
focused on forest-savanna transitions, with all forests being
considered as a single biome. In fact, there are many kinds of
forests in the tropics, some of which are distinct from each other
in species composition and ecosystem function and represent
different biomes (i.e., dry vs. moist forest) and others which are
more difficult to classify (e.g., semideciduous forests, cerradão).
Water availability is a key factor underlying tropical biome
transitions. While forests are often thought to occur under wetter
conditions than savannas, tropical dry forest is actually more
prevalent in areas of lower water availability (<1,000 mm MAP).
Meanwhile, soil fertility, which has received limited attention in
studies of biome transitions, is also critical in the Neotropics,
and merits future research on other continents. More generally,
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Dexter et al. Tropical Dry Forests and Biome Transitions
recognizing tropical dry forest as a distinct biome within the
tropics should improve the accuracy of modeling studies that
aim to predict the future of tropical vegetation and ecosystem
function under global environmental change.
AUTHOR CONTRIBUTIONS
KD and RP wrote the first draft of the manuscript and
contributed to revising the manuscript. AO-F, MB, PS and DN
contributed to revising and improving the manuscript.
FUNDING
KD, RP, and DN thank the Natural Environment Research
Council (UK) for funding via NE/I028122/1. KD thanks
the Leverhulme Trust for supporting him during the time
this study was completed via an International Academic
Fellowship. PS thanks the Coordenação de Aperfeiçoamento
de Pessoal de Nível Superior–Brazil (CAPES) for support via
the Science without Borders Programme (grant 99999.013197/
2013-04).
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Conflict of Interest Statement: The authors declare that the research was
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