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Carbon Dynamics in Tropical Forest Ecosystems


Abstract and Figures

Tropical forests in the equatorial belt are major carbon sinks and have a major influence on global climate change patterns. These ecosystems account for ~ 40% of the carbon stored in the terrestrial biosphere. However, recent studies on tropical forests suggest that these ecosystems play a disproportionate role in contributing to emissions or mitigating climate. As tropical climatic parameters supports rapid vegetation growth, the area required to capture annual carbon emissions could be reduced by 25% if afforestation efforts were centered in the tropics. The high mean annual temperatures coupled with high precipitation in the tropics can also render the stored carbon highly vulnerable to losses. Further, being an ecosystem located in an area of high human habitation, anthropogenic pressures and the subsequent effects on carbon storage of these systems need to be taken into account before embarking on any developmental interventions. The present review attempts to synthesize the knowledge on carbon storage and the biotic and abiotic drivers affecting carbon balance in the tropical forest systems.
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Retracing 100 Years of
Silvicultural JourneY
Carbon Dynamics in Tropical Forest Ecosystems
S.Viswanathl*, S. SandeeP2
'Director, KSCSTE- Kerala Forest Research Institute (KFRI), Peechi P.O.,Thrissur, Kerala
:Scientist, KSCSTE- Kerala Forest Research Institute (KFRI), Peechi P.O.,Thrissur, Kerala
*e- mail:
Tropical forests are forested landscapes distributed between 23o North and South of the Equator
the tropics of Cancer and Capricorn respectively) with high precipitation levels and strong dry
seasons. Due to huge variations in regional soil and climate, the forest systems of this region are
very diverse and are responsible for nearly half of the total terrestrial gross primary productivity.
These forests cover -60/o of the land surface and cycles around 8o/o of total atmospheric carbon
dioxide annually (Malhi et a1.,1998; Alamg ir et a1.,2016).Consequent to their aerial extend and large
carbon pool in vegetation and soil, tropical forested ecosystems play a major role in the global
carbon cycle (Ashton ef a1.,2012). However, the tropical regions across the globe have also some
of the most rapidly developing areas in terms of population growth, urbanization, land conversion
and resulting carbon emissions (equivalent to between 22 - 37o/o of current fossil fuel emissions)
Houghton, 1991a, b; Soepadm o, 1993; Nightingale et al.,2OO4).Inspite of their importance and
.ole in the global carbon cycle, systematic assessment and knowledge about the carbon pools
and fluxes in tropical forests are lacking (Dixon eta1.,1994;Lal and Kimble,2000; Nightingale ef
st.,2OO4).Though some generalizations can be made about these ecosystems, such highly diverse
and complex systems demand a much more focused attention to have better understanding of the
global carbon budgets. The present paper synthesizes the current knowledge on carbon cycling,
carbon pools and threats (both biotic and abiotic)to carbon balances in tropical forest ecosystems.
currently, tropical forests account for approxim ately 43o/oof the global forest area, most of which is in
tropical America (52o/o),followed by tropical Africa (30olo) and tropical Asia (18olo) (Dixon et al., 1994)'
These ecosystems occur mostly as lowland formations and usually distributed at an elevation of less
than 1000 m above mean sea level.Within the lowlands, a major portion (47o/o) is in the rain forest
ecological zone, 38olo in the moist deciduous zone, and 1 5o/o in dry to very dry zone (FAO, 2007)'
Tropical Forest TYPes
-ropical forests are usually classified based on the amount of precipitation and degree of seasonality'
lhese factors act as the main drivers of productivity, decomposition, and thereby carbon
sequestration. These forest types can be broadly classified as shown in Table 1 .
'. . '',..r '. ' 65
: ':" ""' : ' ' '- ' '
Table 1: Maior forest types in the tropics
Climate /
Forest type
Monthly mean
temperature of 18"C or
higher in the coldest
months. PreciPitation -
2000 mm per year
Very dense and highly productive vegetation arranged in
different strata. Majority of soils in these forests are highly
weathered Ultisols, except in young foothills (lnceptisols) and
volcanic ashes (Andisols). Soils are generally acidic and vary in
fertility depending upon underlying geology, have relatively high
cation exchange capacity, and very susceptible to erosion'
Semi evergreen
Wet periods generallY
longer than dry periods.
Rainfall > 2000 mm Per
year with extended dry
Highly fragmented due to the physical geography and climate'
Soils belong to the highly weathered Oxisols or sometimes
Spodosols, infertile, acidic, have high clay and low cation
exchange caPacitY.
Tropical dry forests
Rain shadow regions of
mountains in and near
mid-latitudes of conver-
Shed their leaves during a dry season during low water i
availability periods. Soils are usually young due to low i
weathering conditions and fall under Inceptisols' i
Above 3,000 m above
sea level. Precipitation
>2,000 mm per year
and lower amounts of
radiation because of
cloud cover.
Vegetation is characterized by Epiphytes, particularly bromeliads'
Soils are highly productive and mostly classified as Inceptisols,
with high soil organic matter contents. Highly erosion-prone and
located in steep slopes.
Found across the
coastlines of tropics.
Mean annual
Temperature: 22
"C. Rainfall: 1000 - 1500
mm per year
Intertidal regions of the tropics. Soils occur in a
saturated state throughout the year'
Carbon Pools in TroPical Forests
For convenience, carbon pools in forest ecosystems can be broken down into different interrelated
components: aboveground biomass, belowground biomass, coarse woody debris, and litter and
.:,,...,.,,.,.,,,,,,,,.,., soilcarbon (to a depth of 1m).Tropicalforests are estimated to contain 553Peta gram (Pg)of carbon
.* - wfiiefr account for 4eo/o of the total carbon in the terrestrial biosphere with 580/o in its vegetation,
410/o in soil and 17o in litter (soepadmo , .19.93;watson.ef ot.,2000)'Ihe ::eic_a1:']1._,:.^L11ti::t:
rapid vegetation growth and the vegetative area required to capture an
600 .
Forest TYPes
TrF - Tropical Forests; TeF - Temperate Forests; BF - Boreal forests
emissions could be reduced by Zilo/oif afforestation efforts were centered in the tropics (schroeder
and Ladd,1991).
Generaily, the above-ground prant carbon density increases with decreasing latitude from
tundra to tropicar rainforest impricating that at rower ratitudes contribution to the carbon
pool wilt be greater from the plant parts (Fishel 1995). Typical plant carbon density ranges
from 40 to 60 Mg c/ha in boreal forests, 60 to 130 Mg c/ha in temperate forests and 120 to
1g4 Mg c/ha in tropical forests (Lal, 2005). The carbon density of an undisturbed tropical rain
forest can be as high as 250 Mg C/ha. The ratio of soil to plant carbon stocks ranges from
0.9 to 1.2for row ratitudes, 1.2 to 3 for mid-ratitudes and 3to17 for high latitudes (Figure 1)'
to cbntain:
Figure 1: Distribution of carbon in soi| ond plant pools in world,s
maior forest zones (source: Prentice 2001)
Above Ground Biomass
Tropicar forests are the rargest repository of above ground carbon containing - 195 Pg of carbon
(pan et ar.,2011; Liu et ar., z0i5). studies by Lewis et ar. (20e9) estimated that nearly 2oo/o of the co,
currentry produced grobaily by industriar emissions and rand conversion is absorbed by the tropical
forest regions through increased productivity. rt was suggested that this increased productivity
may be on account of the increase in atmospheric corlevels' However' under such a scenario trees
wiil eventuaily reach a saturation point and become rimited by some other resource (Phillips et
at.,2ooil).Tlrough several studies have estimated carbon pools in these forest systems' there exist
large differencgs in the varues mainry due to the inconsistencies in the methodologies adopted' A
comparison of the above ground carbon distribution in world's major forest zones show that among
the different tropicar forest types, mangroves are the most carbon-rich storing about 1023 Mg C/
ha(Figure2).About49-98o/oofcarboninthesesystemsisstoredinsoils.lf:l?1:T3:#',I:#'.'--".;:",fl'ffJ:ll lri:l';:.i,'/ :; ;;;',ffi;,;;;;; i s stored i n sol s at depths ra ns i ns -ffi
0.5 - 3 m. Above-ground carbon pools were found to range from 1 59 - 435 Mg C/ha l" .T5,ry,,:.9;
,v'$$stlT:'r1!?::::::l':'-ll*Ti:#il';lli..,lffi ffi.#lffi ffi ffi
amounts of carbon (- 1o74Mg C/ rra]tna1ft9 o .ceanrc *t: !"ry *" n:;
I rrr
0 50 100 150 200 250
Aboveground live + dead carbon (Mg Qha)
BF - Boreal fores! TeF - Temperate fores! TrF - Tropical foresf MF - Mangrove forest
Figure 2: Above ground carbon distribution in world's major forest
zones (source: Donato et a1.,2011)
A meta analysis of 39 diverse forests varying from lowland to montane, dry to wet, and nutrient-
rich to nutrient-poor estimates a total net primary productivity varying from 1.7 to 21.7 Mg C/ha/
year in different tropical forests (Clark et a1.,2001). Above ground biomass values upto 32.3 Mg C/
ha/year has been reported for tropical evergreen forest in southwest Borneo (Paoli and Curran,
2007). In response to elevated CO, levels, many models predict increased forest productivity.
However, recent studies suggest that stem growth rates in tropical forests have actually decreased
in the last 20 years largely due to increased night time temperature, decreased total precipitation,
and increased cloudiness. However, most of these studies are confined to evergreen and semi
evergreen forests and little work has been done in tropicalforest systems such as montane and dry
decid uous forest types.
Below Ground Biomoss
There is no doubt that spatio - temporal variations occur in the belowground biomass allocation
in tropical forests. Measuring belowground biomass is very difficult as roots are embedded in
the soil and difficulties in removing the entire root system without loss. Current estimates of root
masses may be largely underestimated by as much as6o0/o (Robinson, 2OO7), wherein a20 -25
0/o approximation of above ground biomass is considered as root biomass. Studies by Robinson
(2007) predict a root carbon pool of around 268P9, which is 680/o larger than previous estimates.
Accordingly, tropical forests would contain upto 49 Pg more below ground biomass carbon than
previous estimates. Although, it may still be a low-precision estimate (owing to the uncertainties
of b,iome-scale measurements), a global below ground biomass carbon pool of this magnitude
indicgtes stronger terrestrial carbon sinks. Indepth knowledge on carbon pool may provide insights
into the missing carbon sink to a great extend (Espeleta and Clark, 2007).
CoarseWood Debris
Coarse woody debris form a major contributor to carbon fractions in tropical forests' Reports from
various studies indicate that the carbon contribution from the coarse wood debris in tropical forest
systems ranges from 4.5 - 2g Mg c/ha and generally averages 22- 28 Mg C/ha (Table 2)' An average
contributio n of 33o/oof the above ground carbon to the carbon pool with a turnover of about 9 years
can be considered a good approximation for wood debris in tropical forests (Clark et o1.,2002).
Table 2. contribution of wood debris to the carbon pool in tropical forests
Region Forest type Carbon content
(Mg C per ha) Reference
Costa Rica Semi evergreen 22-24 Clarketal.,2002
Brazilian Amazon Semi evergreen 25 -28 Keller et al., 2004
Peruvian upper Amazon Semi evergreen 22-23 Baker et al., 2007
Ecuador Montane forest 4.5 Wilcke et a1.,2004
Soil Carbon
Soil organic carbon (SOC) comprises of both living as well as dead organic substances in soils
and is composed of an infinite number of compounds varying from easily decomposable simple
organic residues to complex recalcitrant products and microbial biomass (Stevenson , 1994; Kogel-
Knabner, 2OO2).SOC is an important component of the carbon pools in tropicalforests and is mainly
confined to the upper layers with maximum root density. A marginal increase of SoC content in soil
by 0.01olo annually could easily offset annual rises in the atmospheric CO2-C by way of soil carbon
sequestration (Lal, 2oo4).Studies from the Indian subcontinent show that soils (upto 30 cm depth)
under montane forest systems could store 45.67 to73.26tonnes carbon/ha which is relatively much
higherthan tropical dryforests (36.04tonnes/ha) (Panwar and Gupta,2013). Mangrove soils, another
major tropical carbon sink has been reported to yield mean carbon densities of 0.038 gCcm 3 in
oceanic systems and 0.061 gCcm-3 in estuarine soils (Donato eta\.,2011). Studies on soil respiration
in semi evergreen and evergreen tropical forests of Brazil and Costa Rica supports this observation
wherein two - thirds of the efflux was produced by the upper 50 cm of soil layer and litter, whereas
contribution from below 1 m depth was less thanTo/o (Schwendenmann and Veldcamp,2005 ).
Although SoC comprises of a continuum of materials which varies in size and decomposability, for
convenience the carbon components in them are broadly grouped into three major pools - active,
slow and passive carbon pools (Tan ef a1.,2007).The active pools represent labile forms of carbon
highly sensitive to alteration with a mean residence time of about 1-5 years. Being vulnerable to
rapid oxidation, this pool exposes the potential for rapid decomposition and thereby accentuating
Co, effluxes to the atmosphere. on the other hand, this pool of carbon plays a pivotal role in fuelling
the soil food web and thereby influences a variety of soil functions and processes from nutrient
cycling to maintaining soil productivity and its quality (Majumder eta1.,2008;Verma eta1.,2010)'
Slow SOC has a mean residence time of about 20-40 years and passive SOC about 200-1500 years.
The stabilized carbon fractions are highly resistant to microbial activity and hardly serve as a 1ellable
indicatorof soilquality (Majumd er eta:.,2008). Studies in moist deciduous forests of Southern Western
Ghats in lndia show that the moist deciduous forests store only 45 - 50 o/o of the soil carbon in the
passive or recalcitrant pools, whereas slow and active pools form 50 - 55o/o (Sandeep 2015). Studies
by Sreekan th et al. (2013) in four typical forest types of Southern Western Ghats have also reported
more than 61olo of SOC as labile fractions indicating the presence of easily mineralizable carbon in
these soils. Furthermore, the positive correlation between activation energy of SOC decomposition
and e,o (rate of change in reaction for a 10 oC rise in temperature - Van't Hoff's exponential) show
that SOC in these forest systems can be highly thermal sensitive and undergo faster decomposition
under conditions of warming (Sandeep, 2015).
Since many climate models predict further soil drying and increased litter fall in tropical forests,
understanding the role of microbes in soil carbon dynamics deserves further attention. Microbes
help in the biochemical cycling of carbon as well as add to the SOC pool either directly or as
byproducts. For example, glomalin, a glycoprotein produced by Arbuscular Mycorrhizal Fungal
(AMF) hyphae, is detected in concentrations of over 60 mg cm-3 from tropicalforest soils (Rilligef o/.,
2001). Carbon dating of glomalin extracted from these soils indicated turnover of several years to
decades, much longer than the turnover of AMF hyphae (turnover of days to weeks). Studies along
a chronosequence spanning millions of years suggest that due to the longer turnover times this
protein accumulates in the soil carbon pool (Rillig et o1.,2001).
Climaticfactorssuch as moistureandtemperature havea positiveinfluenceon carbondecomposition
and are found to affect CO, production even at depths of 2m (Schwendenmann and Veldcamp,
2005). The increases in carbon decomposition and subsequent CO2 production indicate a strong
positive feedback between efflux and ecosystem warming from tropicalforest soils, though further
studies are warranted to verify this.
Carbon Efflux from Tropical Forests
Carbon dioxide evolutionfrom theforestsystems occurs bytwo major reaction pathways- respiration
and decomposition. Respiration occurs when living systems (plants, animals and microbial systems)
release CO, during biochemical processes (e.g. growth and production of chemical defenses).
Decomposition is the transformation of organic detritus in which CO, evolves as the reaction
product. Both these processes are temperature sensitive and positively respond to warming. Except
in regions with poor moisture and aeration, decomposition in the humid tropics tends to be rapid,
limiting accumulation of detritus on the forest floor. Under conditions of restricted aeration, detritus
accumulate such as in peat swamps forming pockets of high carbon storage. However, under such
oxygen stressed environments, microbes resort to anaerobic respiration - a relatively less efficient
method of respiration producing methane as the byproduct.
In tropical forests, respiration rates and carbon dioxide evolution are affected by an array of
interrelated factors such as moisture, temperature, vegetation, substrate quality, ecosystem NPP,
belowground biomass allocations and plant rooting density, ecosystem dynamics of flora and
fauna, soil physico - chemical properties, and disturbance regimes (Rustadef a\.,2A01). Of these
factors, precipitation and temperature are considered the most influential factors as they interact
evergreen forests where NPP is high, and detritus decomposition is not limited by either moisture
ortemperature constraints.In contrast, decomposition of detritus/soilorganic carbon in borealand
tundra ecosystems is often limited by low temperatures and soil aeration (Schlesin ger,1997).
The responses of respiration and decomposition to moisture and temperature contents vary with
extremities of either factor. Schlentner and Van Cleve (1985) theorized that temperature rise has
fittle effect on soil respiration when gravimetric water contents fall below 75o/o and become highly
responsive to temperature variations at water contents between 100-250o/o. Similarly, variations in
moisture content will have little or negligible effect on CO, efflux at temperatures less than 5"C while
it becomes highly responsive at temperatures between 10 - 20"C. Godwin et al., (2017) examined
the interaction of soil moisture content and soil temperature effects on soil CO, effluxes in pine-
grassland forest soils of USA and noted that soil temperature had significant positive correlation
with soil respiration at certain moisture ranges.
Tropical forests experience large scale disturbances in the form of anthropogenic interventions,
wild fires, tree mortality from old age and natural calamities which open up the forest floor. In
large scale disturbances, especially fires, clearance, landslides, or logging, huge amounts of CO,
get emitted to the atmosphere thereby altering the carbon budget of the landscape. Disturbed
sites produce carbon efflux results which deviate from the generally set trends. In the old growth
forests of the Amazon, carbon was released during the wet season and absorbed in the dry season,
a contradiction to the existing model predictions (Saleska et a\.,2003). However, this disconnect was
attributed to available soil moisture and transient effects of recent disturbance on decomposition.
Such disconnects have important implications on carbon budgeting in the most diverse carbon
repositories of the world, tropical forest systems, and may lead to erroneous interpretations and
projections (Saleska eta1.,2003). In a global meta-analysis study, Don ef al. (2011) reported that
land conversion from forest lands to cropland, perennial crops and grasslands caused 25, 30 and
12o/oSOC losses in the tropics. They also pointed that SOC loss from sub-surface soils is also equally
important as surface soils due to land conversion. Losses of soil organic C by land clearing may
result in variations in the composition of plant litter and may have improved rates of soil organic
matter decomposition and accelerated soil erosion (Feller and Beare, 1997).
There have been few studies analyzing land surface with atmosphere interactions in tropical forest
ecosystems. Since 1980s, only three studies have analyzed such interactions at a global scale:
the Anglo-Brazilian Climate Observation Study (ABRACOS; 1990- 95); the Large-scale Biosphere/
Atmosphere Experiment in Amazonia (LBA; 1 996-2003); and the GEWEX Asian Monsoon Experiment
(GAME; since 1996) (Nightingale et a1.,2004). Further, all the studies evaluated the semi evergreen
forests leaving scanty information about other forest types. The major hindrance in modeling
carbon fluxes from tropical forests is their species diversity, structural complexities and varying
productivity attributes. However, a general consensus is that positive correlation exists between
rising temperatures and rate of most if not all biochemical process in tropical plants and soils (Lloyd
and Farquhar,1996).
While the thermal dependence of decomposition and CO, efflux is explicit, there has been diffic{ty-!'n..
relatiorryhtgsr,.ls nOdata,,l"at ,n=t49)collected from multiple studies aro,gndlthewor.!l.a
d.gqglfbing the relationship mathematically. Lloyd and Taylor (1994) evaluated various
a simple linear relationship after observing marked curvilinearity.Van't Hoff's exponential (Q10)
and Arrhenius relationships were observed to be better predictors of soil respiration rates in the
absence of soil moisture limitations. The study found that the best fit curve for the datasets gave a
e10 value of 2.4 and could explain 7Qo/o of the variation in respiration as a function of temperature.
However, the exponential form underestimated respiration rates at low temperatures and
overestimated at high temperatures. Arrhenius equation, an exponential function that accounts
for activation energy of a chemical process assumes a constant activation energy across a range
of temperatures and while more predictive than the simple exponential relationship (R'z=0.74), it
is inadequate, underrating respiration rates at low temperatures and overestimating respiration
rates at high temperatures. Based on the various studies, it can be generally concluded that
the relationship between respiration and temperature cannot be a simple exponential over the
standard range of physiological temperatures and require advanced mathematical models.
Uncertainties in both the estimates of biomass and rates of deforestation contribute to a wide
range of estimates of carbon emissions in the tropics, hence more comprehensive studies may be
needed to get a focused picture how these carbon sinks behave under changing climate scenarios.
Drivers of Carbon Uptake and Release inTropical Forests
Tropical forests occupy a space of high human habitation and natural disturbances. Consequently
evaluating carbon dynamics without considering the disturbances would lead to overestimation of
their storage capacity and underestimation of emissions (Saleska et a1.,2003). Tropical rainforests in
Southeastern Asia, South Asia and Amazon are typically affected from droughts during ENSO (El Nino-
Southern Oscillation) events (Lyon, 2004). Sotta et at. (2007) observed that the effect of drought on
tropical forests depends on the forest type, soil texture, rooting depth and position in the landscape.
Tropicalforest systems experiencing increased water stress during drought usually respond with higher
mortality, thereby releasing more carbon through decay and increase probability of fire (Nepstad etal',
2OO7).Such responses in one of the largest terrestrial carbon sinks assumes significance as ENSO events
are predicted to become more frequent in future due to climate change (Tsonis et o1.,200il. Tropical
forests experience strong trade winds and monsoons and get exposed to large scale periodic wind
eventsthathelpvigorouslyre-sproutand rapid regrowth (Whitmore,1989;Vandermeeref c1.,1998; Eaton
and Lawrence,2006). On the other hand, such events can also be precursors of fire in semi-evergreen or
dry deciduous type forests of the region. In both cases, they drive the successional dynamics of these
forests, hence by implication, the carbon stocks. However, little to no work exists as such on assessing this
dynamism in the development of regional carbon models for predicting future changes.
Fires in tropical forests are more imminent threat and are typically the result of anthropogenic activities
or drought (Bush et a1.,2008). In contrast to other forest zones, fire events in the tropical forest systems
are primarily hgman induced ratherthan naturalforces (Bush etol.,2OO8). Fires enhances plant mortality
(including lianans) and can be detrimentalto even large trees that provide shade, help keep litter moist
and store significant amounts of carbon (Nepstad et a1.,2007). The opening of the canopy dries out the
lltter layer and the dead lianas become ladder fuels for the escaped fire to burn the litter layer and reach
canopy. (Nepstad et al.,2OO7). This in turn impacts the types of plants that can regenerate and colonize
after fire, thereby initiating a vicious degradation cycle. ," . '
Human intervention through forest degradation and deforestation has been the leading cause of
manipulation of carbon cycle in tropicalforests (Houghton,199la; Sampson eta\.,1993). Predictions
show that by 2050, the tropical forests could be a source of atmospheric CO, (Sampson et al., 1993).
Deforestation affects the carbon feedback cycles by enhancing the carbon emissions and albedo
effects (Balaet al.,2OO7).lt should be noted that for every ton of carbon released to the atmosphere
through deforestation an additional 0.6 tons of carbon is released through degradation of the
remaining forest (Houghton,1991b). Current estimates of emissions from deforestation in tropics
vary greatly, hence are difficult to compare due to differences in assumptions, sources and methods.
Developing and integrating multiple variables into new or existing models is quite essential for
determining deforestation effects on carbon fluxes from tropical forests (Nightingale et a1.,2004).
Being in one of the most populated areas on the globe, land conversions are one of the major
threats for these forest systems. lt has been estimated that land use change in these systems could
release around 40-80 Pg C per year over the next 50 years (Nightingale et o1.,2004). Large scale
conversion of forested lands for oil palm plantations in Southeast Asia results in a significant net
reduction in carbon storage (Reijinders and Huijbregts,2008). Oil palm plantations stores less than
36-48 tons C/ha in its above ground biomass which is significantly lower than the tropical primary
forests (235 tons C/ha). Further, using fire for land clearing in these systems carbon releases as much
as 187-199 tons of C/ha (excluding losses of soil carbon). Similarly, a full life cycle analysis of forest
conversions and subsequent cultivation for production of biofuels also raises concerns about the
assertion that such cultivations reduce CO, emissions (Reijinders and Huijbregts, 2008). Though
tropical forests are resilient to many types of environmental changes, given the human footprint
in many of these forests, the expected resiliency may not materialize (Cowling and Shin, 2006).
Tropical forest ecosystems are one of the world's largest terrestrial sinks of carbon. lts location
along the equatorial belt provides it with the needed climatic optima for maximizing growth and
carbon storage. At the same time the high anthropogenic pressures and climate disturbances in
the region forces this ecosystem to negate this gained locational advantages. Though bestowed
with a wide range of forest types, most of the forest ecosystems and their land surface - atmosphere
interactions remain as completely unknown entities. Large uncertainties also exist in the estimated
belowground biomass/ carbon and are often difficult to compare due to differences in assumptions,
sources and methods used. A globally accepted framework to evaluate the carbon dynamics and
its rigorous implementation is advocated to bridge the data gaps and bring forth the importance
of these pristine systems in a rapidly changing climate scenario. A better understanding of the role
of tropical forests in biosphere carbon fluxes and mechanisms controlling forest carbon changes is
criticalfor projecting future atmospheric carbon growth and guiding the design and implementation
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... Tropical forests contain about 553 Pg of carbon, which accounts for 40% of the total carbon in the terrestrial biosphere, with 58% in tropical forest vegetation, 41% in its soil, and 1% in its litter (Soepadmo, 1993). Moreover, nearly 20% of the CO 2 currently produced globally by industrial emissions and land conversion is absorbed by tropical forests (Lewis et al., 2009;Viswanath & Sandeep, 2019). However, it is uncertain whether tropical forests will continue to be carbon sinks or shift to being net carbon sources (Cavaleri et al., 2015), making understanding of their carbon flux and aboveground storage imperative. ...
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Climate change and disturbance make it difficult to project long‐term patterns of carbon sequestration in tropical forests, but large ecosystem experiments in these forests can inform predictions. The Canopy Trimming Experiment (CTE) manipulates two key components of hurricane disturbance, canopy openness and detritus deposition, in a tropical forest in Puerto Rico. We documented how the CTE and a real hurricane affected tree recruitment, biomass, and aboveground carbon storage over 15 years. In the CTE treatments, we trimmed branches, but we did not fell trees. We expected that during the 14‐year period after initial canopy trimming, regrowth of branches and stems and stem recruitment stimulated by increased light and trimmed debris would help restore biomass and carbon loss due to trimming. Compared to control plots, in the trimmed plots recruitment of palms and dicot trees increased markedly after trimming, and stem diameters of standing trees increased. Data showed that recruitment of small trees adds little to aboveground carbon, compared to the amount in large trees. Nevertheless, this response restored pretreatment biomass and carbon in the experimental period. In particular, the experimental additions of trimmed debris on the forest floor seemed to stimulate increase in aboveground carbon. Toward the end of the experimental period, Hurricane Maria (Category 4 hurricane) trimmed and felled some trees but reduced aboveground carbon less in the plots (including untrimmed plots) than experimental trimming had. Thus, it appears that the amount of regrowth recorded after experimental trimming could also restore aboveground carbon in the forest after a severe hurricane in the same time span. However, Hurricane Maria, unlike the trimming treatments, felled large trees, and it may be that with predicted, more frequent severe hurricanes, that the continued loss of large trees would over the long term decrease aboveground carbon stored in this Puerto Rican forest and likewise in other tropical forests affected by cyclonic storms.
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A study was conducted to estimate the organic carbon pool in the soils under different forest types in Himachal Pradesh. Soil organic carbon (SOC) pool was also estimated in all forests subgroup types available in Himachal Pradesh. Maximum pool was in the soils under moist Alpine Scrub (73.26 tonnes/ ha) followed by Himalayan Moist Temperate Forests (55.20 tonnes/ha), Himalayan Dry Temperate Forests (47.61 tonnes/ha) and Sub-alpine Forests (45.67 tonnes/ha) and the least was under Tropical Dry Deciduous Forests (36.04 tonnes/ha). Moist Alpine Forests had maximum mitigation potential (2.03) and the least was in Tropical Dry Deciduous Forests (1.00). Maximum share was occupied by Moist Alpine Scrub (28%) followed by Himalayan Moist Temperate Forests (21%), Himalayan Dry Temperate Forests (19 %), Sub-alpine Forests (18 %) and the least was occupied by Tropical Dry Deciduous Forests (14 %). SOC pool under Moist Alpine Forests was statistically significantly different from the SOC pool under Himalayan Moist Temperate Forests, Himalayan Dry Temperate Forests, Sub-alpine Forests and Tropical Dry Deciduous Forests.
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Soil CO2 efflux (Rs) is a significant source of carbon dioxide from soils to the atmosphere and is a critical component of total ecosystem carbon budgets. Prescribed fire is one of the most prevalent forest management tools employed in the southeastern USA. This study investigated the influence of prescribed fire on Rs rates in old-field pine-grassland forests in north Florida, USA, that had been managed with prescribed fire annually and biennially for over 40 years, or left unburned for approximately the same period. Monthly measurements were taken of Rs, soil temperature (Ts), and soil moisture from August 2009 to May 2011. Results showed that sites managed with annual and biennial dormant season prescribed fire had significantly lower monthly mean Rs rates and estimated annual soil carbon fluxes than sites where fire had been excluded. While Ts explained a significant amount of the temporal variations in Rs, it did not explain the differences in Rs among prescribed fire treatments. Our results provide new insight into the effects of prescribed fire and fire exclusion on soil carbon fluxes, and suggest that future methods to model ecosystem carbon budgets should incorporate not only current vegetative conditions, but also prescribed fire management activities.
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Tropical forests are major contributors to the terrestrial global carbon pool, but this pool is being reduced via deforestation and forest degradation. Relatively few studies have assessed carbon storage in degraded tropical forests. We sampled 37,000 m2 of intact rainforest, degraded rainforest and sclerophyll forest across the greater Wet Tropics bioregion of northeast Australia. We compared aboveground biomass and carbon storage of the three forest types, and the effects of forest structural attributes and environmental factors that influence carbon storage. Some degraded forests were found to store much less aboveground carbon than intact rainforests, whereas others sites had similar carbon storage to primary forest. Sclerophyll forests had lower carbon storage, comparable to the most heavily degraded rainforests. Our findings indicate that under certain situations, degraded forest may store as much carbon as intact rainforests. Strategic rehabilitation of degraded forests could enhance regional carbon storage and have positive benefits for tropical biodiversity.
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Vegetation change plays a critical role in the Earth's carbon (C) budget and its associated radiative forcing in response to anthropogenic and natural climate change. Existing global estimates of aboveground biomass carbon (ABC) based on field survey data provide brief snapshots that are mainly limited to forest ecosystems. Here we use an entirely new remote sensing approach to derive global ABC estimates for both forest and non-forest biomes during the past two decades from satellite passive microwave observations. We estimate a global average ABC of 362 PgC over the period 1998-2002, of which 65% is in forests and 17% in savannahs. Over the period 1993-2012, an estimated '0.07 PgC yr '1 ABC was lost globally, mostly resulting from the loss of tropical forests ('0.26 PgC yr '1) and net gains in mixed forests over boreal and temperate regions (+0.13 PgC yr '1) and tropical savannahs and shrublands (+0.05 PgC yr '1). Interannual ABC patterns are greatly influenced by the strong response of water-limited ecosystems to rainfall variability, particularly savannahs. From 2003 onwards, forest in Russia and China expanded and tropical deforestation declined. Increased ABC associated with wetter conditions in the savannahs of northern Australia and southern Africa reversed global ABC loss, leading to an overall gain, consistent with trends in the global carbon sink reported in recent studies.
Biogeochemistry-winner of a 2014 Textbook Excellence Award (Texty) from the Text and Academic Authors Association-considers how the basic chemical conditions of the Earth, from atmosphere to soil to seawater, have been and are being affected by the existence of life. Human activities in particular, from the rapid consumption of resources to the destruction of the rainforests and the expansion of smog-covered cities, are leading to rapid changes in the basic chemistry of the Earth. This expansive text pulls together the numerous fields of study encompassed by biogeochemistry to analyze the increasing demands of the growing human population on limited resources and the resulting changes in the planet's chemical makeup. The book helps students extrapolate small-scale examples to the global level, and also discusses the instrumentation being used by NASA and its role in studies of global change. With extensive cross-referencing of chapters, figures and tables, and an interdisciplinary coverage of the topic at hand, this updated edition provides an excellent framework for courses examining global change and environmental chemistry, and is also a useful self-study guide.