7 - Biogeography Flashcards
define ecosystem function
the rates at which an ecosystem processes substances (like carbon, water, energy), exchanging these with the atmosphere, how it develops over time and how its entities interact with one another
define physiology
the processes within an individual which contribute to its functioning, also ecosystems as a whole
define physiognomy
the morphological features of an organism that contribute to its appearance and functioning within the ecosystem
define plant ecology
the study of the interaction between plants and their environments
outline the theories of succession and climax from Clements and Tansley
clements - succession was a developmental process that led to a climax community, which was determined by the regional climate. He also believed that all other types of vegetation were either successional stages or arrested successional stages. Clements described the climax community as an indicator of the climatic conditions that created It
tansley - Disagreed with Clements’ view of succession, believing that a variety of environmental factors could lead to different types of climax formations in a given region. He introduced the concept of the ecosystem into biology and was a pioneer of the science of ecology in Britain
define system
the sum of the components that interact with each other to produce the coupled behaviour
define cycling
the flow of a substance in and out of the system
define resource limitation
the concept of the ability of a function to operate being limited by some resource (ie N supply may influence photosynthesis)
define phenotype
the visible appearance of an organism and results from the interaction of genetics with the environment
define niche
the environmental limits within which a species can survive and grow.
define fundamental niche
the potential niche in the absence of competition from other species
define realised niche
the niche that is observed and includes competition as a contraint
define genotype
the set of genes in an organism
define ecotype
a subset of a species and represents a population genetically adapted to a particular environment, eg. lowland vs montane
define scale
can be spatial or temporal. discussion of all ecological concepts requires specifying scales of relevance
outline abiotic and biotic components
abiotic - those that are purely physical and chemical
biotic - those made of the living organisms. when these die their organic remains become abiotic and decompose into different organic and inorganic forms
outline soil
composed of organic and mineral components.
organic component is derived from living organisms and contains carbon
mineral soil is largely derived from the underlying rock, but organic material can be broken down so much that the products are no longer considered organic, they contain no carbon, and are therefore minerals, or nutrients if can be taken up by plants
define decomposition
the breaking down of organic material into simpler forms. as this happens part of the carbon in the organic material is released as CO2, which is classed as heterotrophic respiration
define mineralisation
production of inorganic from organic material
define functional type
group of species that share some functional characteristic
define trait
some feature of an organism that can be observed and is related to one or more of its functions
Define resilience of a system
its ability to maintain a certain level of functionality when subject to perturbation
what is the simplest form of radiation and its characteristics
plane wave
angular frequency (w) and wavenumber (k)
4 types of ecosystem services
provisioning
regulating
supporting
cultural
why is carbon ideal for life
abundant
reactive (but not too much)
forms sturdy structures
captures and releases energy
outline photosynthesis
occurs in chloroplasts, and produces sugars that can be used for growth or broken down to release energy anywhere in the plant. The net balance in the leaves is used for growth. Oxygen is produced and sustains aerobic life such as us
what is the enzyme responsible for producing carbohydrates from CO2 during photosynthesis
Rubisco
what powers the cycling of molecules in photosynthesis
light
why is Rubisco considered a common and important enzyme in leaves
very abundant and uses a lot of leaf’s nitrogen
what factors affect rate of photosynthesis
CO2 concentration, light, temperature, nitrogen availability
What is photorespiration
a process where Rubisco fixes O₂ instead of CO₂, wasting energy.
How much energy can photorespiration waste
Up to 30% of the energy fixed in photosynthesis.
Why might plants use photorespiration despite its inefficiency
It may act as a safety valve to prevent damage from excess light by avoiding radical formation
What are the two possible pathways for Rubisco, and how do they differ?
Rubisco ideally fixes CO₂ to make sugars (photosynthesis),
but under high light and low CO₂, it may fix O₂ instead, triggering photorespiration.
Though inefficient, photorespiration helps prevent cellular damage by using up excess energy and reducing reactive oxygen species (ROS)
What molecules can Rubisco bind to when it combines with RuBP?
Either CO₂ or O₂
Which molecule does Rubisco have a greater affinity for: CO₂ or O₂?
CO₂.
What condition causes Rubisco to bind with O₂ more often?
: Low CO₂ concentration, such as when stomata are nearly closed
What is the result when Rubisco binds with O₂ instead of CO₂?
Photorespiration occurs.
How does temperature affect Rubisco’s binding to O₂?
Higher temperatures increase the rate of O₂ fixation.
What kind of reaction is the Rubisco-CO₂/O₂ interaction?
A competitive reaction — Rubisco can only bind one molecule (CO₂ or O₂) at a time.
How can photorespiration be reduced?
By increasing CO₂ concentration so Rubisco is more likely to bind with CO₂.
What is the primary function of stomata in plants
To allow CO₂ into the leaf for photosynthesis
What is the unavoidable consequence of stomata opening for CO₂ uptake?
Water loss through transpiration
What trade-off do stomata have to manage?
Balancing CO₂ intake for photosynthesis with minimizing water loss.
What concept does the stomatal trade-off illustrate in plant science?
A common trade-off in plant ecophysiology
What two factors influence stomatal behavior?
Genetic constraints and physiological flexibility
How quickly can stomata respond to environmental changes?
They are dynamic and can respond on the timescale of minutes
What is the ancestral photosynthetic pathway from which others have evolved?
The C3 pathway
What are the two evolved types of leaf physiology that help plants cope with dry or hot environments?
CAM and C4 photosynthesis.
What type of environment is CAM photosynthesis adapted to?
Dry environments (e.g., deserts and epiphytic habitats).
Name two types of plants that use CAM photosynthesis
Cacti and succulents.
Besides deserts, where else are CAM plants commonly found
In the canopies of tropical rainforests (e.g., orchids, bromeliads
What environmental condition is C4 photosynthesis especially suited to?
Hot environments.
Q: Which evolved first: CAM or C4 photosynthesis
CAM photosynthesis evolved before C4.
What problem occurs in C3 plants when stomata are closed to conserve water?
CO₂ levels in the leaf drop, increasing photorespiration.
When do CAM plants open their stomata?
At night
How do CAM plants reduce photorespiration under water-limited conditions?
They fix CO₂ at night as malic acid, then release it during the day for photosynthesis while stomata are closed.
What is the advantage of CAM plants fixing CO₂ at night?
reduces water loss through transpiration and minimizes photorespiration.
What type of separation is used in CAM photosynthesis to reduce water loss?
Temporal separation of CO₂ fixation (at night) and photosynthesis (during the day).
Approximately what percentage of all land plant species use CAM photosynthesis?
About 6% (roughly 16,000 species).
Are CAM plants a major contributor to global productivity?
No, they represent a small portion of global productivity but are important ecologically in specific environments.
what type of pathway aer large areas of tropical grassland
C4, eliminating problem of photorespiration
In how many plant families has C4 photosynthesis evolved independently
In how many plant families has C4 photosynthesis evolved independently
When did C4 photosynthesis first evolve
Around 30 million years ago
When did C4 plants diversify significantly?
About 5–7 million years ago, during the Miocene.
What percentage of all plant species are C4 plants?
Around 5%.
Despite being only 5% of plant species, how much do C4 plants contribute to global gross primary productivity (GPP)?
About 30% of global GPP.
What does the high contribution of C4 plants to global GPP show about their ecological role?
It highlights their incredible importance in Earth’s ecosystems today, especially in warm, open environments.
Why did C4 photosynthesis likely evolve?
As a response to falling atmospheric CO₂ levels, which increased photorespiration in C3 plants.
How do C4 plants reduce photorespiration?
By spatially separating the C4 and C3 processes within the leaf
What is the cost of the C4 mechanism?
: It requires more energy to pump CO₂ into special high-CO₂ cells.
Under what conditions do C4 plants grow especially well?
In sunny, hot, and dry environments where photorespiration is likely to be high.
How do C4 plants compare to C3 plants in hot, dry conditions?
C4 plants out-compete C3 plants because they avoid photorespiration more efficiently.
Name some tropical crops that use C4 photosynthesis
Sorghum, maize, and sugar cane.
What popular alcoholic drink comes from the sugar products of C4 photosynthesis?
Rum (from fermented sugar cane).
What type of separation do C4 plants use to reduce photorespiration?
Spatial separation — C4 and C3 steps occur in different cells
What type of separation do CAM plants use to reduce water loss?
Temporal separation — CO₂ uptake at night, sugar production during the day.
Under what conditions do C4 plants outcompete C3 plants
In hot, sunny, and dry environments.
Under what conditions do C3 plants perform best
Cool, moist, and moderate-light conditions
Which pathway uses more energy: C3 or C4
C4 (due to active CO₂ pumping).
What common problem do both C4 and CAM pathways help solve?
Reducing photorespiration caused by Rubisco binding to O₂
What does it mean that plants adjust photosynthesis for co-limitation
Plants balance physical and biochemical limitations so that neither one solely limits carbon fixation
What are the two main limitations to photosynthesis
1) CO₂ supply through stomata
2) Biochemical potential (light, enzymes, phosphate)
When does CO₂ supply become the limiting factor in photosynthesis?
When stomatal conductance is low (e.g., due to water stress or closed stomata).
When do biochemical limitations dominate photosynthesis?
At high CO₂ concentrations inside the leaf.
How do stomata help maintain co-limitation?
By adjusting their opening to keep internal CO₂ levels at a point where CO₂ supply and biochemical capacity are equally limiting.
What is the significance of the “point of inflection” in the photosynthesis curve?
It is where CO₂ limitation and biochemical limitation are equal — representing optimal stomatal behavior
Over time, how do plants maintain co-limitation?
By adjusting the biochemical machinery (e.g., enzyme levels, pigments) to match environmental conditions.
What are these longer-term adjustments of photosynthesis to the environment called?
Acclimation responses.
What is the goal of photosynthetic acclimation
To minimise environmental limitations and maximize photosynthetic efficiency under prevailing conditions
Why is understanding acclimation important in plant ecophysiology?
It explains how plants adapt physiologically to changing environments over days or weeks.
As leaves mature, how do they adjust to their light environment?
Through permanent morphological responses, like changes in structure and function.
How are maximum stomatal conductance and photosynthetic rate affected by light availability during leaf development?
They are lower in leaves grown under low light
What structural difference is seen between leaves in sun and shade?
Leaves develop distinct sun leaf or shade leaf morphology.
What is the adaptive advantage of shade leaf structure
Shade leaves function efficiently at low light and have low compensation points, avoiding excessive respiration
Why is it beneficial for lower canopy leaves to have a low compensation point?
It prevents them from respiring more carbon than they fix, maintaining energy efficiency in the whole canopy
What is the compensation point in leaf physiology?
The light level at which photosynthesis = respiration (net carbon gain = 0).
How is whole-canopy photosynthesis optimised in forests?
Leaves develop traits suited to their light levels, and nutrients are distributed efficiently across the canopy
How do species traits influence their position in a canopy or successional stage?
Species with efficient low-light use dominate shade layers or early succession, while high-light species dominate canopy tops or late succession.
How does photosynthesis generally respond to rising atmospheric CO₂
It increases, especially in C3 plants and under high temperatures, because higher CO₂ reduces photorespiration.
Why might rising CO₂ make C3 plants more competitive in some regions
Because reduced photorespiration under high CO₂ may allow C3 plants to outcompete C4 species where C4 currently dominates.
What ecological consequence could result from C3 plants expanding into C4-dominated regions?
Shifts in plant distributions and ecosystem changes, with potential knock-on effects on biodiversity and function
How does high CO₂ affect stomata
It causes stomatal closure, reducing water loss and improving water use efficiency.
Why does CO₂ supply become relatively less limiting under elevated CO₂
Because more CO₂ is available, so plants don’t need to open stomata as wide or as often
What is water use efficiency (WUE) and how is it affected by high CO₂?
WUE is the ratio of carbon fixed to water lost — it increases under high CO₂ due to reduced transpiration.
What happens to the long-term growth response of plants under elevated CO₂?
It may be less than expected because plants acclimate, reallocating resources to other functions (e.g., nutrient uptake).
What is an acclimative response to high CO₂ in plants
physiological adjustment where investment shifts away from photosynthesis toward other needs like nutrient acquisition.
Why is understanding species-level differences in CO₂ response important
Because predicting ecosystem changes requires knowledge of diverse, species-specific responses.
What non-photosynthetic benefit of high CO₂ may be just as important as its effect on carbon fixation
Improved water conservation through reduced stomatal conductance
What does GPP stand for in ecosystem ecology
Gross Primary Production – the total amount of carbon fixed by photosynthesis at the canopy or ecosystem level.
What three immediate factors influence GPP at the canopy level?
Light, temperature, and nitrogen availability
What are the two main factors that cause annual GPP to vary between ecosystems
Leaf area and length of the growing season
What controls the amount of leaf area and duration of the photosynthetic season
Water and nutrient availability, climate, and disturbance (e.g., fire, grazing, human activity)
Why does growing season length matter for GPP?
A longer season means more time for photosynthesis, increasing total carbon fixation
How can disturbances impact GPP?
They can reduce leaf area or shorten the growing season, lowering overall GPP.
At the ecosystem level, what do soil nutrient and water supply primarily control
The amount of leaf area, not the photosynthetic rate per unit area.
Why is leaf area important for ecosystem productivity
Because it controls how much light is absorbed for photosynthesis
What is the basic equation for ecosystem production
Production = Light absorbed × Light use efficiency
What controls how much light is absorbed in an ecosystem?
The leaf area, which is influenced by soil water and nutrient availability
is light use efficiency highly variable across ecosystems?
No — it tends to be relatively constant
Which soil nutrient can be a limiting factor in tropical ecosystems
Phosphorus (P)
Why is phosphorus a common limiting nutrient in tropical soils
Because tropical soils are often old and heavily weathered, leading to low P availability
What is NPP and what does it represent?
Net Primary Production – the portion of GPP left after subtracting autotrophic respiration (Ra); represents plant growth.
What does Ra stand for in ecosystem carbon accounting?
Autotrophic respiration – carbon used by plants for maintenance and metabolic activity
What is the formula linking GPP, NPP, and Ra?
NPP = GPP – Ra
What does NEE stand for
Net Ecosystem Exchange – the net carbon flux between the ecosystem and the atmosphere.
What additional component does NEE account for that NPP does not?
Heterotrophic respiration (Rh) – carbon released by non-plants like fungi, bacteria, and animals
What does a negative NEE value indicate
ecosystem is acting as a carbon sink (taking in more CO₂ than it releases).
What is the full formula linking GPP, Ra, Rh, and NEE
NEE = GPP – (Ra + Rh)
Net ecosystem exchange = Gross Primary Production -
(Autotrophic Respiration + Heterotrophic Respiration)
What are the primary factors controlling the spatial distribution of NPP
Leaf area and length of the photosynthetic (growing) season
What determines the amount of leaf area in an ecosystem
: Water and nutrient availability
How does the growing season affect NPP
A longer photosynthetic season allows more time for carbon fixation and thus higher NPP.
What does NPP represent
Net Primary Production – the carbon available for plant growth after subtracting autotrophic respiration from GPP
Why is leaf area such a strong predictor of NPP?
Because it controls light absorption, which drives photosynthesis at the ecosystem scale.
What are the two major anthropogenic (human-caused) sources of CO₂ in the global carbon budget?
Fossil fuel emissions (grey area) and land-use change (e.g., deforestation; brown area).
What are the three main carbon sinks that remove CO₂ from the atmosphere?
The atmosphere, ocean sink, and land sink .
What does GPP stand for, and how is it related to the land sink?
Gross Primary Production – part of the land sink, it reflects carbon fixed by plants through photosynthesis, minus respiration and turnover.
What is the “missing” or “inferred” land sink?
The part of the carbon budget not directly measured but inferred by subtracting atmospheric and ocean uptake from total emissions
Why is there a “budget imbalance” in the global carbon budget?
Because the sum of known emissions does not perfectly match the sum of known sinks, due to uncertainties, especially in the land sink
What is Net Ecosystem Exchange (NEE) in this context
The net carbon flux between the land ecosystem and atmosphere, contributing to the land sink term.
Which component of the carbon budget is best known and most accurately measured?
Atmospheric CO₂ from direct observations
Which sink has higher uncertainty: ocean or land?
The land sink has higher uncertainty due to variable biological processes and lack of complete data
What is the current trend shown in fossil fuel emissions
sharp increase since the 1950s, contributing the largest share to total CO₂ emissions
What is the significance of understanding the “missing land sink”?
It is critical for predicting future climate change, land feedbacks, and for designing climate mitigation strategies
what does Keenan et al 2018 say is the dominant driver of the land sink
dominant driver of the sink is CO2 fertilization of photosynthesis.
Why does carbon uptake by plants increase as atmospheric CO₂ increases?
Because CO₂ is the main substrate for photosynthesis, so more CO₂ enhances carbon fixation (CO₂ fertilization effect).
How can warming lead to increased carbon losses from ecosystems
By increasing respiration from plants and soils, which releases more CO₂ into the atmosphere.
What instrument is commonly used to measure short-term carbon fluxes
infrared gas analyzers (IRGAs) – they measure CO₂ exchange between plants and the atmosphere
What do process-based models of the carbon cycle typically rely on?
Photosynthesis models that simulate how CO₂ uptake responds to environmental changes
What is the main driver of increased land carbon sinks in most current models?
Enhanced photosynthesis due to rising atmospheric CO₂
What do models often struggle to predict accurately
Carbon losses from warming-induced respiration, especially in soils
Where is most terrestrial plant production concentrated globally?
In tropical rainforests
Why might the tropics hold the “missing” or additional land carbon sink?
Because they have high plant productivity, and CO₂ fertilization may enhance photosynthesis even at high temperatures.
How does higher atmospheric CO₂ affect photorespiration and photosynthesis temperature optimum?
It reduces photorespiration and raises the optimum temperature for photosynthesis
Why is the tropics’ role in the carbon budget complex
Because they are both a potential sink (via forests) and a major source (via deforestation).
Where must any significant land carbon sink be located to balance the budget?
in the remaining natural (undisturbed) tropical lands, not deforested areas.
What limits our understanding of the tropical land sink?
Lack of direct measurements, high variability, and uncertainty in how tropical ecosystems respond to CO₂ and climate.
How does the carbon stored in global biomass compare to the pre-industrial atmosphere?
Biomass contains about the same amount of carbon as the pre-industrial atmosphere
How does the carbon stored in biomass compare to that in the oceans?
Biomass holds much less carbon than the oceans, but is still ecologically and climatically significant
Where is most of the Earth’s biomass carbon stored?
In tropical ecosystems, especially rainforests.
Why are tropical regions particularly important for atmospheric carbon dynamics?
: Because large changes in tropical biomass (e.g., deforestation or regrowth) can significantly impact atmospheric CO₂ levels.
What does the similarity in carbon content between biomass and the atmosphere imply?
That land-use changes, especially in the tropics, can substantially alter atmospheric CO₂ concentrations
How much carbon is stored in the atmosphere (current, not pre-industrial)?
About 880 GtC
How much carbon is stored in terrestrial biomass (plants and animals)?
About 450–500 GtC
How much carbon is stored in soils (organic + inorganic)?
About 1,500–2,400 GtC
Which reservoirs are part of the fast (active) carbon cycle?
The atmosphere, terrestrial biomass, surface ocean, and soils.
Through what structure does CO₂ enter the leaf
Through the stomata
What do stomata also control besides CO₂ uptake?
Water loss through transpiration
What is the typical ratio of internal to external CO₂ concentration in leaves?
About 70:100 — or 70% of the external CO₂ level
What does the 70:100 ratio imply about the CO₂ gradient across the leaf surface?
There is a 30% drop in CO₂ concentration from outside air to the chloroplast
How does the ratio help us estimate CO₂ concentration in the chloroplast?
By multiplying the ambient (external) CO₂ concentration by 0.7.
What was the pre-industrial atmospheric CO₂ concentration?
280 ppm
If external CO₂ increases, what happens to the CO₂ concentration in chloroplasts (assuming the ratio holds)?
It also increases, potentially boosting photosynthesis
Where in the cell is CO₂ actually fixed into sugar?
In the chloroplasts, by the enzyme Rubisco
Why does increasing chloroplast CO₂ reduce photorespiration
Because Rubisco is more likely to bind CO₂ instead of O₂, improving photosynthetic efficiency
What does an A/Ci curve show?
The net rate of photosynthesis (A) as a function of internal leaf CO₂ concentration (Ci)
What does the variable A represent in the A/Ci curve
Net photosynthetic rate – the rate of CO₂ assimilation by the leaf, net of mitochondrial respiration
What does Ci stand for in the A/Ci curve?
The internal (chloroplast) CO₂ concentration in the leaf
What happens to the rate of photosynthesis when CO₂ is low?
: It is limited by CO₂ availability and Rubisco activity, so increases nearly linearly with CO₂.
What controls the slope of the A/Ci curve at low CO₂ levels?
The concentration of Rubisco (enzyme capacity).
What limits photosynthesis at high internal CO₂ concentrations?
The regeneration of RuBP, which depends on light-driven reactions of the Calvin cycle.
What ultimately limits photosynthesis when CO₂ is no longer limiting?
The availability of light energy and chlorophyll, which drive the Calvin cycle.
Why does the A/Ci curve level off at high CO₂?
Because the system becomes light-limited – RuBP regeneration can’t keep up.
How is glucose produced in photosynthesis
By Rubisco fixing CO₂ into RuBP, leading to the eventual formation of 6-carbon sugars like glucose
How can the A/Ci curve be used to predict future responses to elevated atmospheric CO₂?
By showing how increased Ci (from rising CO₂) affects photosynthetic rate, especially under different light conditions
provide a Summary of the A/Ci Graph in Words
At low Ci, photosynthesis is Rubisco-limited — increasing CO₂ boosts photosynthesis strongly.
At high Ci, photosynthesis becomes RuBP-regeneration-limited — so further CO₂ increases have little effect.
This plateau is controlled by light availability, chlorophyll content, and energy supply from the light reactions.
What proportion of atmospheric CO₂ typically reaches the chloroplast due to stomatal regulation?
About 70% of atmospheric CO₂.
How is the CO₂ gradient across the stomata represented in an A/Ci graph?
By a diagonal line from atmospheric CO₂ to 70% of that value on the x-axis (internal CO₂)
What does the intersection of the CO₂ gradient line with the A/Ci curve show?
The net rate of photosynthesis at that atmospheric CO₂ level
If atmospheric CO₂ rises and the stomatal gradient remains at 30%, what does the A/Ci curve suggest will happen to photosynthesis?
Net photosynthesis increases by nearly 50%.
What global-scale phenomenon might this increase in photosynthesis help explain?
The “missing” terrestrial carbon sink
How large is the estimated “missing” land carbon sink?
About 3 PgC (petagrams of carbon) per year
How much carbon is fixed globally by photosynthesis each year?
About 120 PgC yr⁻¹.
Why can’t we assume a direct relationship between CO₂ rise and increased long-term carbon storage?
Because of plant acclimation, photosynthate allocation, and ecological processes like competition and mortality.
What is acclimation in this context
Plants adjusting their photosynthetic machinery over time in response to elevated CO₂ or growth conditions
What is allocation in plant physiology?
The way plants distribute carbon (photosynthates) to different functions — leaves, roots, storage, reproduction, etc
Why is allocation important when considering the carbon sink?
Because only carbon allocated to long-lived biomass contributes to lasting carbon storage.
What ecological processes can offset gains from increased photosynthesis
Competition, disturbance, mortality, and nutrient limitations.
What is a major limitation of many current models or approaches to estimating the land sink?
They often underestimate the role of acclimation, allocation, and ecological processes.
What is the projected atmospheric CO₂ concentration by 2100 under high-emission scenarios?
About 800 ppm.
If the internal CO₂ gradient remains constant, what is the expected increase in net photosynthesis at 800 ppm
About 50% higher than current rates
What assumption does this prediction rely on?
That there is no acclimation and no ecological limitations to photosynthesis
Why might the land carbon sink not continue to increase linearly with CO₂ emissions
Because of acclimation, nutrient limitations, and ecological disruptions like tree mortality and drought
How could climate change reduce sink capacity even with more CO₂?
By increasing tree mortality, leading to decomposition and heterotrophic respiration, which releases CO₂.
What is heterotrophic respiration
CO₂ release from non-plant organisms (e.g., microbes and fungi) decomposing dead biomass.
What is the danger of relying solely on CO₂ fertilization to mitigate emissions?
It overestimates sink strength and ignores feedbacks that may reduce carbon storage in ecosystems
What is the Calvin cycle?
The biochemical pathway in chloroplasts where CO₂ is fixed into sugars using ATP and NADPH from the light reactions
What key enzyme initiates carbon fixation in the Calvin cycle
Rubisco – it fixes CO₂ to RuBP to start sugar production.
How does acclimation affect Rubisco concentrations under high CO₂?
Rubisco concentrations may decline, as less enzyme is needed when CO₂ is abundant
Why might plants reduce investment in Calvin cycle enzymes under high CO₂?
To reallocate resources (e.g., nitrogen) to other functions like nutrient uptake or root growth
What is meant by photosynthetic down-regulation?
A reduction in photosynthetic capacity, due to lower investment in Calvin cycle machinery when it’s no longer limiting growth.
What is meant by up-regulation in roots during acclimation?
Increased investment in root systems or nutrient acquisition processes, often in response to new limiting factors.
Why does acclimation challenge the assumption of a linear CO₂-photosynthesis relationship?
Because over time, plants adjust, and photosynthesis becomes less responsive to CO₂ enrichment.
What type of ecosystems did Yude Pan et al. (2011) identify as a major global carbon sink?
Intact forests, especially tropical rainforests.
What proportion of total global carbon emissions were intact forests estimated to absorb?
Around 26% of total emissions.
(Pan et al, 2011)
How much of fossil fuel emissions were intact forests estimated to absorb?
About 30%.
What method was used to calculate this forest sink? (Yude et al, 2011)
Remeasurement of forest plots around the world — comparing carbon stocks over time
Which regions contributed most to this forest carbon sink?
Tropical rainforests, particularly in South America, Central Africa, and Southeast Asia.
How could intact forests compensate for the carbon lost through deforestation?
Through natural regrowth and continued carbon uptake in undisturbed areas
outline Pan et al 2011
compiled data from forest inventory plots globally.
They found that intact forests were removing roughly 2.4 PgC/year from the atmosphere — about 26% of total emissions.
This uptake was not just in reforested or managed land, but in natural, undisturbed forests, especially tropical ones.
This sink was large enough to compensate for the net emissions from deforestation in many regions.
The findings showed that forests are vital for climate regulation, and that their preservation is essential for maintaining this carbon sink.
How many forest inventory plots were included in the Yude Pan et al. (2011) study?
Between 25 and 321 plots (depending on the region and analysis)
How is tree biomass typically estimated from forest plots
Using tree diameter measurements, which are converted to carbon mass via allometric equations.
Over what time period were inventory data averaged in the Pan et al 2011 study?
Over three-year intervals
What fraction of the Earth’s forest area was actually sampled in the Pan et al study?
About 0.3 millionths of a percent — an extremely small fraction.
Why does such a small sample raise concerns about generalising the results?
Because global forests are highly variable, and such a small sample may not be representative
Q: What did Brienen et al. (2015) observe about tree productivity in tropical forests
Productivity (growth) increased over recent decades, likely due to CO₂ fertilization
What trend did Brienen et al 2015 observe in tree mortality?
Higher temperatures, drought stress, and competition for light.
What happens when more trees grow faster in a closed canopy forest
It can lead to increased competition, crowding, and light limitation, resulting in higher mortality
What is the key message of the Brienen et al. 2015 study?
The tropical forest carbon sink is shaped by both physiological responses (e.g. growth) and ecological processes (e.g. mortality, competition)
write a flowchart of Brienen et al 2015
[Atmospheric CO₂ ↑]
↓
[Photosynthesis ↑] → [Individual Tree Growth ↑]
↓
[Forest Biomass ↑] → [Competition ↑] + [Heat/Drought Stress ↑]
↓
[Tree Mortality ↑]
↓
[Net Biomass Gain ↓ or Saturates]
↓
[Tropical Forest Carbon Sink Weakens Over Time]
What apparent contradiction exists in current carbon budget data (Brienen et al 2015)
Observed forest mortality and biomass loss seem to contradict the atmospheric signal of a strong land sink
Q: What do atmospheric inversion models compare?
They compare observed atmospheric CO₂ with modeled concentrations based on emissions and transport
Q: What causes a mismatch between observed and modeled atmospheric CO₂?
A: The presence of an unknown sink or source not included in the model.
Q: What did early inversion studies reveal about the carbon sink?
A: That a large land carbon sink must exist in the Northern Hemisphere.
Q: Why do modeled concentrations without a land sink fail to match observations?
A: Because the model assumes the land is carbon neutral, which doesn’t reflect reality.
Q: Where are most fossil fuel emissions concentrated?
A: In the Northern Hemisphere.
Q: What do these inversion studies suggest about the tropics?
A: That the tropics are likely a net source, not a major sink.
Q: Have the conclusions about the northern land sink been confirmed by later studies?
A: Yes, multiple times — though the exact processes and locations remain uncertain
Q: What remains poorly understood despite the observed northern sink?
A: The underlying ecological and physiological processes driving it.
What is a major limiting factor for plant growth in high northern latitude ecosystems like boreal forests?
Nitrogen availability (N-limitation).
Why is nitrogen often unavailable to plants in cold ecosystems?
Because low temperatures and wet soils slow microbial decomposition of soil organic matter
How does nitrogen become available to plants
Through mineralization — microbial breakdown of organic matter into nitrate (NO₃⁻) and ammonium (NH₄⁺).
What effect can warming have on nitrogen availability in cold soils?
It can increase decomposition, leading to more N mineralization and greater plant uptake.
What is the competing effect of warming on soils?
Increased microbial respiration, releasing more CO₂ and CH₄ into the atmosphere
explain the competing factors of warming at high latitudes on the carbon sink
soil respiration ↑ > microbial CO2 and CH4 release ↑ > Atmos GHGs ↑ > soil carbon loss ↑ > carbon source ↑
however
N mineralisation ↑ > plant N uptake ↑ > plant growth ↑ > biomass carbon sink ↑
Q: What forest type was used in Melillo et al. (2011)?
A: A northern temperate mixed forest (oak, maple, pine) at Harvard Forest.
Q: What effect did warming have on nitrogen cycling in (Melillo et al 2011) experiment?
Nitrogen mineralization increased significantly.
Did plant biomass increase with warming (Mellilo et al 2011)
Yes — vegetation carbon increased due to enhanced N availability.
What happened to soil respiration during the first 6 years of Melillo et al 2011
: It increased more than biomass growth, leading to a net carbon loss.
What was the net carbon balance by year 7 of the Mellilo et al 2011 experiment?
Roughly neutral — large plant growth compensated for earlier soil carbon loss.
Outline Melilla et al 2011 timespan
Year 0–1:
* Warming starts
* Soil microbes accelerate → N mineralization ↑
* Soil respiration ↑ rapidly (carbon loss)
Years 1–6:
* Vegetation grows faster, but…
* Respiration still exceeds growth
* Net result: carbon loss to atmosphere
Year 7:
* Plant growth catches up
* Net carbon balance ≈ 0 (loss offset by gain)
Beyond? (not measured):
* Continued warming could tip balance again:
– Soil C stores continue depleting?
– Nutrient limitation returns?
– Mortality rises?
What does FACE stand for in climate/ecosystem research
Free-Air CO₂ Enrichment
What is the purpose of FACE experiments
To study the effects of elevated CO₂ on plants and ecosystems under natural field conditions
Why are FACE experiments more realistic than growth chambers or greenhouses
Because they allow plants to grow in open-air conditions with natural light, temperature, soil, and competition
What are the main limitations of FACE experiments
They are extremely expensive, logistically complex, and have been run at only a few sites worldwide
Where are most existing FACE sites located?
In temperate forests, grasslands, and croplands — mostly in developed countries
What is the significance of the AmazonFACE project?
It is the first FACE experiment in a natural tropical rainforest, aiming to study CO₂ effects in the most productive and carbon-dense biome on Earth.
Why is AmazonFACE especially important for climate science?
Because tropical forests are a critical part of the global carbon cycle, yet their response to rising CO₂ is poorly understood.
What questions is AmazonFACE trying to answer
Will elevated CO₂ increase tropical forest productivity? Or will it be limited by nutrients (e.g. phosphorus), warming, or mortality
What photosynthetic metric is commonly used in FACE studies to assess CO₂ response?
Asat – Light-saturated CO₂ uptake rate
Why do C₄ plants respond less to elevated CO₂?
Because they already concentrate CO₂ internally, so additional CO₂ in the air offers limited benefit
What does DMP stand for in plant growth studies?
Dry Matter Production — the total mass of plant tissue (excluding water content)
How does DMP respond to elevated CO₂ compared to Asat?
The response of DMP is much smaller and more variable than the increase in photosynthesis (Asat).
What plant structural traits were measured in （Ainsworth et al 2004) FACE synthesis?
Plant height, stem diameter, leaf number, LAI, SLA, DMP, and crop yield.
Which traits showed modest or inconsistent responses to elevated CO₂? (Ainsworth et al 2004)
Traits like LAI (leaf area index), height, leaf number, and DMP.
What is SLA (specific leaf area), and how did it respond to elevated CO₂? (Ainsworth et al 2004)
SLA = leaf area per unit leaf mass; response varied by species, often showing reduced SLA (thicker leaves).
What trend is observed in long-term tree growth under elevated CO₂ in FACE studies? CITE
An initial increase in growth (NPP), followed by a decline toward ambient levels. (Norby et al 2010)
What does the NPP growth pattern in (Norby et al 2010) suggest about carbon limitation
That plants are not strongly carbon-limited over the long term in natural settings.
How high was the peak growth stimulation in the early years of elevated CO₂? (Norby et al 2010)
Up to 33% increase in NPP.
summarise Norby et al 2010
Short-term ≠ Long-term.
Plants may initially grow faster with more CO₂, but over time, nutrients, ecological feedbacks, and acclimation reduce the effect.
Most natural ecosystems are not carbon-limited over the long term.
Q: Why does nitrogen boost the long-term growth response to CO₂?
Because biomass production needs nitrogen (and other nutrients), not just carbon
What does (Norby et al 2010) suggest about ecosystems under elevated CO₂ without added nutrients?
That growth responses will be limited, even if photosynthesis increases.
What are the plant-independent parts of the water cycle
Evaporation, precipitation, condensation, sublimation, infiltration, percolation, runoff, and groundwater flow
What are the plant-dependent parts of the water cycle?
Plant uptake, transpiration, water storage in biomass, and alteration of runoff
How do plants return water to the atmosphere
Through transpiration – the loss of water vapor from stomata in leaves
How do plants affect water retention in ecosystems
By storing water in their tissues, slowing surface runoff, and enhancing infiltration through root systems
How does water availability affect vegetation?
It limits plant growth, controls vegetation type, and determines ecosystem productivity
What happens to the water plants absorb but don’t transpire?
It may be used in photosynthesis, growth, or stored in tissues temporarily.
What is the term for the combined water loss from evaporation and transpirationEvapotranspiration
Evapotranspiration
How do different plant types affect the water cycle
Deep-rooted plants can access groundwater, trees increase transpiration, and dense vegetation can reduce runoff
What is Sustainable Development Goal 6?
Clean water and sanitation, including the availability and sustainable management of water
How are plants connected to SDG 6?
Plants both depend on and influence water availability and quality — especially through transpiration, retention, and ecosystem structure.
Why are ecosystems mentioned in SDG 6 targets
Because ecosystems like wetlands and forests are crucial to water cycling, storage, and quality, and are sensitive to water availability.
Why are peatlands singled out under SDG 6?
Because they are critical for both water regulation and carbon storage, and are highly water-dependent.
What defines a peatland?
An ecosystem where organic matter accumulates in waterlogged conditions, forming peat
What are the four main types of peatlands (after Keddy 2002)?
Swamp – Trees rooted in hydric soils, not peat (e.g., mangroves)
Marsh – Herbaceous plants in hydric soils (e.g., reed beds)
Bog – Mosses, sedges, trees in deep peat, little water movement
Fen – Sedges and grasses rooted in peat, with high water flow
How does water availability affect peatland carbon sequestration
Peatlands store more carbon when water levels are high; drying reduces storage and increases CO₂ emissions
What happens when peatlands dry out?
They lose their water-retention function, release stored carbon, and degrade ecologically.
