Download How do drought events impact the ecosystem carbon balance?

Global Change Biology
Andrea Bohny, 2017
How do drought events impact the ecosystem carbon
balance?
Date: 03.04.2017
Supervisor: Susanne Burri
Introduction
The consequences of climate changes and their impacts on global climate affect humankind in a large
scale. With increasing greenhouse gas emissions, global climate and thus, ecosystems are influenced
in a negative way. The prediction and understanding of the consequences of climatic changes on
ecosystems will need huge effort by global change scientists worldwide since consequences of climate
change are of global character (Allen et al., 2009). Global climate is also depending on the ecosystem
carbon balance, which is known as the balance of carbon exchange between atmosphere and
biosphere. The carbon balance is influenced by carbon sources (add carbon in various forms to a
reservoir) and sinks (removal of carbon from a reservoir) within the reservoirs and the movement of
carbon among them, the so-called carbon cycle (Canadell, 2006). The current changes in climate lead
to a transformation in those sources and sinks (Cramer et al., 2001) and thus, danger for biosphere and
humankind are expected and already recorded (Brunner et al., 2015). An increase in carbon sources
(through deforestation, wildfires, permafrost thawing and respiration, and a decrease in carbon sinks
(forests, oceans and photosynthesis (Canadell, 2006)) will lead to an elevated atmospheric CO2
concentration (Lei et al., 2016), to a change in the vegetation and thus, to huge changes in the structure
of an ecosystem (Cramer et al., 2001).
The Net Ecosystem Carbon Balance (NECB, gCm-2y-1) is mostly determined by the Net ecosystem
production (NEP, gCm-2y-1, eq. 1) which is the difference between the gross photosynthesis (Pg, gCm2 -1
y ) and the total ecosystem respiration (Re, gCm-2y-1):
𝑁𝑁𝑁𝑁𝑁𝑁 = 𝑃𝑃𝑔𝑔 − 𝑅𝑅𝑒𝑒
[1]
The most variation in ecosystems carbon balance is due to disturbances, drought for example, and the
subsequent recovery of them since numerous processes including carbon do take place (Lambers et al.,
2008).
Frank et al. defined drought as an extreme climate event, which has ‘massive impacts on harvests,
economies and human health, as well as on the carbon balance of terrestrial ecosystems‘ (Frank et al.,
2015) and which will be of importance in the future. Since drought – and water – is intensively linked
with the terrestrial carbon cycle, of which we are a part of, a fundamental knowledge about droughts
and their impacts on nature and humankind is necessary and urgent to acquire (Xu et al., 2016). Stateof-the-art climate models project stronger, longer-lasting and more frequent droughts (Frank et al.,
2015).
Questions
An insight of the impact of drought on an ecosystems carbon balance will be given in the following,
with special regard to forest ecosystems. The following questions will be addressed:
- What happens to the ecosystem carbon balance in response to drought?
- What happens to trees, when they are exposed to drought?
Results
The carbon balance of an ecosystem is affected through drought by the modification of carbon uptake
and carbon release. In forests, the uptake is mostly driven by photosynthesis while the release is driven
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Global Change Biology
Andrea Bohny, 2017
by the ecosystems respiration (v.d.Molen et al., 2011). Besides the modification, drought reduces
carbon fixation, leads to a decline in terrestrial carbon sink (Xu et al., 2016) and leads to higher CO2
fluxes to the atmosphere (Schlesinger et al., 2016).
Forests
One well known ecosystem is the forest, which cover 30% of the world’s land surface (Allen et al.,
2009). They are expected to exhibit large effects of climate extremes, due to their large carbon pools
and fluxes, above- and underground. But the impacts depend in large part on the nature of the drought.
The more severe and frequent a drought is, the more fragile the forest becomes. Direct (reduced
growth and mortality) and indirect (insect outbreaks, pathogens and wildfires) impacts affect a forest
stronger, if it could not well enough recover from consequences of the last drought.
One important direct impact of prolonged drought is the drying of the soil. In a dry soil nutrient and
water uptake are reduced and thus, photosynthetic activity is reduced as well since they are directly
correlated (Schlesinger et al., 2016). Forest die-back, mostly regionally occurring, lead to huge loss of
carbon since biologically fixed carbon is released (Allen et al., 2009). The major reasons for tree
mortality are hydraulic failure due to cavitation and carbon starvation as a result of increased carbon
loss during drought (v.d.Molen et al., 2011). With forest spread all over the globe, drought can affect
the carbon balance regionally, nationally and globally. An extended controlling system of carbon
fluxes, climate changes as well as forests and their integration is of profoundly need.
V.d.Molen et al. (2011) showed a lot of tree responses during, while and after a drought. First, trees
reduce their stomatal conductivity to prevent the loss of water through the stomata, meaning a
decrease in the photosynthetic rate. This results in a reduced gross primary production (GPP) and a
lower total ecosystem respiration (TER). The reduction in GPP depends on physiological, such as
reduced stomatal conductance, and structural adaptions, such as reduced leaf area or adapted root
systems, of the trees.
With ongoing drought, the soil keeps continuing losing water until the point where the soil and the
surrounding air water supply cannot meet the trees water demand. The result is a transportation of
aspirated air through the xylem, instead of water. This leads to a large decrease in water conductivity
of the trees since the water flow is disturbed. Does the conductivity sink under a certain threshold,
cavitation takes place: the water can no longer flow through the trees since the waters’ connectivity is
interrupted. Lastly the trees collapse completely and die.
Fortunately, trees and plants in general, have some strategies to extend their lives. With adaptions in
more efficient light and energy use (increased light absorption and photosynthesis, decreased
respiration), elevated water use efficiency (adapted stomatal conductivity), faster carbon allocation
from carbon reserves to carbon using parts, improved rooting systems (deeper roots with more
mycorrhizal activity), increased tolerance to fires (thicker and more endure leaves) and better
strategies for regeneration (faster seed dispersal and higher recovery rates) trees are able to handle
drought better.
Conclusion
Drought associated with climate change is a function of frequency, duration and severity (Allen et al.,
2009) and is expected to be increased and strengthened in those traits. If a drought hits an ecosystem,
for example a forest, changes in its carbon balance are observed since the structure, biogeography and
the composition of an ecosystem are changed due to different processes, such as tree death and insect
outbreaks. The most influences are due to the ecosystems net productivity (NEP). The productivity is
linked with a lot of carbon producing and consuming processes. Since forests are huge carbon sinks, a
climate event, such as drought, can turn it into a carbon source, because of the above-stated processes.
Since forests are a huge sink of atmospheric and anthropologic carbon emissions and can mitigate
global warming, their protection and sustainable use will be, and already is, one of the most important
responsibilities of humankind. Important will be the severity by which a forest is hit, since their
survival depend on the trees survival strategies. It is unknown whether those will be able to prevent
forest-die back in big scale.
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Global Change Biology
Andrea Bohny, 2017
References
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