Abstract
The increase in severity of droughts associated with greater mortality
and reduced vegetation growth is one of the main threats to tropical
forests. Drought resilience of tropical forests is affected by multiple
biotic and abiotic factors varying at different scales. Identifying
those factors can help understanding the resilience to ongoing and
future climate change. Altitude leads to high climate variation and to
different forest formations, principally moist or dry tropical forests
with contrasted vegetation structure. Each tropical forest can show
distinct responses to droughts. Locally, topography is also a key factor
controlling biotic and abiotic factors related to drought resilience in
each forest type. Both dry tropical forests and ridges (steeper and
drier habitats) are more sensitive to droughts than moist tropical
forest and valleys (flatter and wetter habitats). The most important
biotic factors are leaf economic and hydraulic plant traits, and
vegetation structure. The most important abiotic factors are soil
nutrients, water availability and microclimate. Here we show that
topography has key roles controlling biotic and abiotic factors in each
forest type. Our synthesis highlights that gradients of altitude and
topography are essential to understand tropical forest’s resilience to
future drought events. We described important factors related to drought
resilience, however many important knowledge gaps remain. Filling those
gaps will help improve future
practices and studies about mitigation capacity, conservation, and
restoration of tropical ecosystems.
Keywords: Climate change, El Niño, mortality, growth, recovery,
resistance
Introduction
Drought is a natural phenomenon characterized by abnormally low
precipitations and high temperatures, which strongly affect plants at
different scales, from individuals, to populations and communities, and
thus ecosystems worldwide (Dai 2011, Fauset et al. 2012). Drought events
are predicted to modify species composition and abundance, as well as
ecosystem functioning and dynamics (Boeck et al. 2017), especially in
tropical forests (Reichstein et al. 2013) due to high exposure to El
Niño events (IPCC, 2018). Nevertheless, resilience to droughts in
tropical forests remains poorly understood (Xu et al. 2013, Meir et al.
2015). Identifying biotic and abiotic factors related to mortality and
low growth induced by droughts would allow us to better understand and
predict forest resilience and dynamics.
The interest in studying drought effects in tropical forests has
increased more than four times during the last 20 years (Fig. 1). This
is likely because tropical forests are among the most vulnerable
ecosystems to climate change (Bellard et al. 2014). Indeed, tropical
forests were strongly affected by the two most severe droughts ever
recorded, in 2015/2016 (Otto et al. 2015, Kogan and Guo 2017) and
1998/1999 (Slik 2004). The former and most recent one led to a high net
carbon loss, even higher than the 2010 drought in the Amazonian forest
(Liu et al. 2017). Tropical forests provide key ecosystem services –
such as water supply, carbon sequestration, pollination and climate
control – to urban and rural areas (Joly et al. 2014, Silva et al.
2021). Although our understanding of drought effects worldwide is
growing, the impacts of droughts in tropical forests are still poorly
understood, which limits our ability to model forest responses to future
climate scenarios.
Repeated occurrences of strong droughts can select for drought-tolerant
species due to higher mortality of vulnerable species (Aguirre-Gutiérrez
et al. 2020). Thus, it has been predicted that trees of tropical forests
will become smaller with denser wood (Phillips et al. 2010), due to high
mortality rates of canopy trees and shade-tolerant species (Fauset et
al. 2012). There is a field of evidence that tropical forests can change
to a drier ecosystem due to the interactions between recurrent fires and
climate change. This process drives communities through a savannization
(Sansevero et al. 2020) and/or a secondarization process (Barlow and
Peres 2008), thus decreasing carbon sequestration and enhancing climate
change effects, because dry ecosystems sequester less
CO2 than moist forests (Taylor et al. 2017). Therefore,
repeated drought events associated with high deforestation and recovery
can drive tropical forests into a different type of forest following the
future climate scenarios. This process can lead to changes in forest
composition, vegetation structure, traits and ecosystem services
(Aguirre-Gutiérrez et al. 2020). Predictions of which plant species are
most vulnerable to drought effects, and where they occur, are needed to
model consequences of climate change and to determine the best
mitigation strategies.
Climate changes impact forests at multiple spatial scales, and this is
the interaction of processes across scales that may determine forest
resilience (Reyer et al. 2015). For instance, drought effects are
influenced by gradients varying at global, regional and local scales
(O’Brien et al. 2017). Thus, looking at multiple spatial scales may
offer the best way to predict forest resilience because plant mortality
and reduced growth can occur locally with potentially negative effects,
but they may smooth out at larger spatial scales (Reyer et al. 2015).
The aim of this study was to synthesize
biotic and abiotic factors, mostly
controlled by topography, that can be used to predict drought-induced
mortality in tropical forests. We expect that this review will help
identify knowledge gaps in drought resilience research and provide a
framework for future studies.
What is drought resilience and how do we quantify it?
Resilience is the capacity of a biome, an ecosystem or a species to both
resist and recover from a disturbance (Oliver et al. 2015). Resistance
is the ability of a system to maintain its properties and functions
during a disturbance (Mariotte et al. 2013) whereas recovery is the
ability of a system to return to initial conditions (Verbesselt et al.
2016). Droughts are associated with vegetation mortality and reduced
growth (Adams et al. 2009, Verbesselt et al. 2016, Greenwood et al.
2017, Meir et al. 2018). Thus, quantifying mortality and growth to
estimate drought resilience is an important approach to measure
vulnerability and drought resilience of plant species (Redmond et al.
2018). Evidences suggest a growth-survival trade-off, showing for
example, that survival rates are negatively correlated to growth rates
(Wright et al. 2010, O’Brien et al. 2017). Another pattern can be found
when recovery rates are measured after a normal condition, for example,
after a La Niña event. In this case, a resistance-recovery trade-off is
expected, with sites or species showing greater resistance rate during
drought but lower recovery rate after a rainy period (Gazol et al.
2017). Furthermore, during the recovery period, new individuals can be
recruited, mainly because of the available niches resulting from large
tree mortality (Redmond et al. 2018). Tree resistance and recovery to
droughts will likely determine the long-term trajectory of tropical
forests. Therefore, resilience is an important component to determine
community and ecosystem sensitivity to disturbances and to predict
shifts in forest carbon stocks caused by climate changes
(Sánchez-Salguero et al. 2018).
Drought timing and severity, as well as forest type, are important
factors to take into account when estimating the resistance and
necessary time for the recovery. For instance, in 2017, savannas and
grasslands had already recovered from the 2015/2016 drought but tropical
forests did not because tropical trees have slower growth and
recruitment rates (Wigneron et al. 2020). Drought legacy, i.e.,the time of recovery after a drought, can take longer according to the
drought magnitude (Huang et al. 2018, Kannenberg et al. 2020). For
instance, tropical forests in Costa Rica recovered from the 1997 drought
within two years (Silva et al. 2013) and the Puerto Rican forests
recovered from the 2015 drought within 1 year (Schwartz et al. 2019,
2020). Quantifying the local magnitude of droughts can be difficult due
to spatial variation in precipitation and temperature. An El Niño event
does not mean the same dry condition because some forests can be more
exposed to such conditions depending on the latitude and the topography.
Drought resilience in different types of forests
Tropical forests include dry and moist forests (Olson et al. 2001). Dry
tropical forests (also called seasonally dry tropical forest) range from
tall forest on moister regions to shrublands on the driest regions and
the vegetation is mostly deciduous during the dry season (Pennington et
al. 2009). Moist tropical forests are characterized by the presence of
evergreen plant species in high-altitude moister areas of the tropics
but also by the presence of deciduous species in areas with seasonal
climatic regime (WCMC 1992). Therefore, this classification (dryvs. moist tropical forest) is mainly due to different climatic
conditions, which are driven by altitude, latitude and longitude (Fig.
2).
Distinct patterns of drought resilience likely exist among different
forest types because each type of forest will impact differently soil
nutrients and water availability, vegetation structure and functional
traits (Fig. 2). For instance, African tropical forests are more
resistant to climatic extremes than Amazonian and Asian forests due to
differences in their vegetation structure (Bennett et al. 2021).
Furthermore, El Niño impacts vary latitudinally in the Atlantic Forest,
with tropical areas being more impacted by El Niño events than
subtropical areas (Rodrigues et al. 2011). Eastern and southern regions
of the Amazon Forest experienced stronger drought impacts than the
north-western region (Van Schaik et al. 2018).
Tropical dry forests are less
resistant to droughts (Allen et al. 2017). Furthermore, tropical forest
communities in West Africa that normally experience higher seasonal
water deficit, and that became drier through time, tended to become more
homogeneous in functional, taxonomic and phylogenetic diversity
(Aguirre-Gutiérrez et al. 2020). Some areas of tropical forest are
secondary forest patches recovering from land use, timber extraction or
natural disturbances (Rüger et al. 2020). Pioneer species occurring in
those secondary forests present high mortality during drought periods
(Rocha et al. 2020), but high recovery rates after a drought event
(Poorter et al. 2016, Gazol et al. 2017). Those findings highlight that
each forest type of tropical forest can show different patterns of
growth and mortality and this regulates how much carbon each forest type
will store under the future climate.
Drought resilience across topographic gradients
Locally, topography is also a source of environmental heterogeneity
within each forest type (Jucker, Bongalov, et al. 2018, Nettesheim et
al. 2018). Abiotic factors strongly vary across topographic gradients,
such as water availability, soil nutrients and microclimate (Fig. 2,
Fig. 3, Fyllas et al., 2017). Topographic variation of tropical forests
can be distinguished into two main habitats: valleys (flatter and wetter
habitats) and ridges (steeper and drier habitats). Biotic factors, such
as species traits and vegetation structure, are also driven by
topography (Fig. 2, Fig. 3, Jucker et al., 2018a; Rodrigues et al.,
2019). Topography is thus an important factor to take into account to
understand drought impacts in plant communities due to the role of those
factors in survival and growth of plant species (O’Brien et al. 2017,
Hollunder et al. 2021).
Species occurring in different topographical habitats can show distinct
patterns of mortality and growth rate. Studies in tropical forests have
shown that woody plant species occurring in valleys can be more
resistant than the ones occurring in ridges (Table 1). Valleys can act
as refuges, providing nutrients and water during droughts (Costa et al.
2020, Hollunder et al. 2021). Furthermore, shade availability in valleys
can reduce drought impacts and improve the performance of plant species
during drought periods (Holmgren et al. 2012). Species occurring in dry
habitats are living under their microclimate limits and under water
stress, and a small change in precipitation and temperature can be
physiologically stressful (Allen et al. 2017, Aguirre-Gutiérrez et al.
2020, Cartwright et al. 2020). In turn, species from dry habitats can
show high recovery rates during a post-drought period (Schwartz et al.
2019), following the resistance-recovery trade-off. However, it has been
suggested that dry habitats can also show strong drought legacies
(Anderegg et al. 2015, Allen et al. 2017).
Other studies have shown that valleys can be less resistant than the
ones occurring in ridges (Table 1). Species from valleys do not have
traits related to drought tolerance, while species occurring in ridges
have traits related to survival during periods of water-stress (Allen et
al. 2017, Gessler et al. 2017). Such distinct evidences indicate that
there is no general pattern of drought impacts for all tropical forests,
though most of the studies found stronger resistance in wetter habitats.
Biotic and abiotic factors controlled by topographic gradients are also
strong drivers of tree growth and mortality, as well as of forest
resilience, and can thus contribute to help elucidating such patterns
(Fig. 2).
Abiotic factors