Lecture 3a: Effects of rising CO2 - terrestrial Flashcards
TERRESTRIAL : topics covered
*Direct and indirect effects
*Example cases from forest, grassland and savanna
* C3 versus C4 paradigms in long-term change
Free Air Carbon Dioxide Enrichment (FACE)
experiment in Switzerland
We know that rising CO2 is having an impact on plants from the results of experiments like the one above: inn the woodlands of Switzerland Free Air Carbon Dioxide Enrichment (FACE)
A central tower with instruments on it is in the centre and around it more towers all with tubes all the way down. They are integrated with a computer control system that knows which way the wind is blowing: sonic anemometer to measure wind speed and direction. These tubes double concentrate the CO2 reaching the surrounding trees by fumigation from the towers. It is however very expensive.
The carbon pumped in is generated by biomass burning.
^ This adds an outdoor option to improve growth which would previously only have been possible in growth chambers
also see: CO2 fertilization experiments: DUKE FOREST FREE AIR CO2 ENRICHMENT
Effects of our current atmosphere on C3 plants
Our current atmosphere is 21% O2 and 0.04% CO2 thus when plants open their stomata to take in CO2 they also take in a lot of O2 making the process inefficient.
Rubisco’s catalytic inefficiency means that plants invest hugely in Rubisco
RUBISO fixes 10^15 g C per year, about 1/7th of all atmospheric CO2
RUBISO is slow, catalyzing 3 – 10 reactions per second (compare to carbonic anhydrase, at 500,000 rxns / sec.) To make up for Rubisco’s catalytic deficiencies, plants make LOTS of it.
In C3 plants, 50% of soluble leaf protein can be Rubisco. Algae and C4 plants (adapted to hot dry conditions) have carbon-concentrating mechanisms so can get by with much less Rubisco protein.
Some plants have additional CO2-concentrating steps: C4 mechanism
Because Rubisco has an oxygenase as well as a carboxylase activity, some plants concentrate CO2 to promote the carboxylation reaction using PEPC
Rubisco carboxylation uses CO2, a reaction that is competitively inhibited by O2
PEPC uses HCO3- (bicarbonate), so competition with O2 is not a problem
What happens in the mesophyll and bundle sheath cells in C4:
- Atmospheric CO2 enters mesophyll cells and is converted to bicarbonate.
PEPC carboxylates PEP to produce OAA, a four-carbon compound. - OAA (or a derivative) is transported to a bundle sheath cell and decarboxylated, releasing CO2 at Rubisco, which initiates the Calvin-Benson cycle. A three-carbon compound returns to the mesophyll cell.
It was thought that all C4 plants had Kranz anatomy
In plants with Kranz anatomy, bundle sheath cells form a ring around the vascular tissue, and mesophyll cells form a ring around them
We are now aware that ~1% known C4 plants with dimorphic chloroplasts – a division of function within the same cells
C4 photosynthesis is only advantageous in dry hot sunny regions
C3 plants have a slight advantage at cool
temperature (the additional carboxylation steps of C4 require energy)
Because photorespiration increases with temperature, C4 plants have an advantage higher temperatures
Because carbon-fixation in C4 plants is not
carbon-limited, they are able to take advantage
of high light intensities.
using the C4 PEPC CO2 concentrating process it is possible for plants in hot dry conditions to open their stomata for just one hour at dawn to collect enough CO2 for the rest of the day and avoid they evaporation that would otherwise cause their dessication
As C4 is not carbon limited they can take advantage of high light availability. However in low light levels and temperate environment C4 is limited due to the extra energy required to carry out the PEPC step
Increasing available CO2 improves growth in C3 plants – this is used in glass houses to grow tomatoes
^initial impact though in long-term (over years) plants can acclimatise to lower level to reduce water loss
However in C4 plants even doubling CO2 will not benefit them as they already function at high effic.
Key points for understanding/revision of C3 and C4:
1.C3 and C4 plants respond very differently to increased concentrations of atmospheric CO2,
at least in the short-term. This arises from key differences in anatomy and physiology that we
are still uncovering at the present time.
- Understanding the differences between paradigms and the influence of time has led to key
advances in prediction of the outcomes of the impacts of increased atmospheric CO2 on mixed
plant communities (see the following two topic sessions). - For example, the C3 vs C4 paradigm responses to elevated atmospheric CO2 are being challenged
through longer-term experiments (up to 20 years, compared to 3-5 years). These findings are
very exciting in terms of understanding long-term outcomes of enhanced atmospheric CO2.
Plant response is not linear - long-term experiments over decades are necessary to comprehend the long-term impact of global warming effects
Forests
see diagram in notes from:
G B Bonan Science 2008;320:1444-1449
Hydrology: the movement of water – water is drawn up through the tree by water evaporation from the leaves, by impacting the old growth in an area it impacts all aspects of the environment (vegetation dynamics)
Fate of carbon, within forest and beyond:
see diagram in notes
CO2 isn’t the only requirement of plants
We know that NPK all contribute hugely also and this is why we use fertilisers
^ Because of this we cannot look at CO2 alone in experiments relating to it
also see: Ecology letters 2002, roots and funghi accelerate carbon and nitrogen cycling in forests exposed to elevated CO2
^Less root-derived C was sequestered in all ingrowth cores in the elevated CO2 plots.
The magnitude of this difference depended on soil nitrogen availability.
C leaks from roots into surrounding soil feeding fungi. Roots need to be connected with fungi for efficient carbon fixation
A big diff between carbon above and below ground
Grasslands
Prairie Heating & C02 Enrichment (PHACE) Experiment, Cheyenne, WY Experiment, Morgan et al., unpublished (2006-2010):
*Northern mixed-grass prairie
*Thirty 3-meter diameter rings
*2 [C02] (ambient & 600 ppm)
*2 temps (ambient & +1.5/3.0 C)
Preliminary Results:
*C02 enhanced productivity
*Warm temperatures increased C4 C02 production response g -40m
experiment with rings of soil integrated with additional CO2 and heat lamps. To simulate climate change and rising greenhouse gases
CO2 enhanced productivity
Warming temp. Increased C4 plant productivity
Key paper
Unexpected reversal of C3 versus C4
grass response to elevated C02 during
a 20-year field experiment
Peter B. Sarah Hobbie,3 Tali D. Lee,’ Melissa A. pastore3
Reich et al., Science 360, 317-320 (2018) 20 April 2018
Unexpected reversal of C3 versus C4
grass response to elevated C02 during
a 20-year field experiment
Peter B. Sarah Hobbie, Tali D. Lee,’ Melissa A. pastore, Reich et al., Science 360, 317-320 (2018) 20 April 2018
a long-term (20-year) FACE experiment in Minnesota, USA
During approximately the first 12 years (1998–2009), results were as expected: C3 plots averaged a 20% increase in total biomass at eCO2 relative to ambient CO2 in contrast to C4 plots that averaged a 1% increase.
During the subsequent 8 years (2010–2017), the pattern reversed: C3 plots averaged 2% less and C4 plots 24% more biomass in eCO2 than in ambient CO2.
Proof that you must not assume long-term outcomes from short-term effects
Summary of the effects of rising CO2 on grassland:
*Over the first 12 years of a 20-year free-air CO2 enrichment experiment with 88
C3 or C4 grassland plots biomass was markedly enhanced at eCO2 relative to
ambient CO2 in C3 but not C4 plots, as expected.
*During the subsequent 8 years, the pattern reversed. Soil net nitrogen
mineralization rates, an index of soil nitrogen supply, exhibited a similar shift
*CO2 first enhanced but later depressed rates in C3 plots, with the opposite true
in C4 plots, partially explaining the reversal of the eCO2 biomass response.
*These findings challenge the current C3-C4 eCO2 paradigm and show that even
the best-supported short-term drivers of plant response to global change might
not predict long-term results.
Long-term resistance to simulated climate change in an infertile grassland:
Acid grassland, Buxton, Derbyshire, UK
^low nutrient grassland
Shelters lower the amount of water reaching the covered patches simulating drought predicted in future due to global warming
In experiment soil heating cables were used to warm the soil see photos in notes
Less water = less growth
Summary of experiment above: In unproductive, grazed grassland at Buxton in northern England (U.K.), one of the longest running experimental manipulations of temperature and rainfall reveals vegetation highly resistant to climate shifts maintained over 13 yr.
Resistance to simulated climate change was in the form of:
(i)constancy in the relative abundance of growth forms and maintained dominance by long-living, slow-growing grasses, sedges, and small forbs;
(ii)immediate but minor shifts in the abundance of several species that have
remained stable over the course of the experiment;
(iii) no change in productivity in response to climate treatments with the exception
of reduction from chronic summer drought; and
(iv) only minor species losses in response to drought and winter heating.
In upland cold environments such as tundra plants are resistant to change
Key points for understanding/revision of terrestrial systems:
1.In terrestrial systems…..the overall long-term impacts of higher atmospheric CO2 concentrations
are often different to the short-term fertilisation responses and which are clearly linked, through time to nutrient availability e.g. through altered mineralisation rates.
2.The C3 vs C4 paradigm responses to elevated atmospheric CO2 are being challenged through longer-term experiments (up to 20 years, compared to 3-5 years). These findings are very exciting in terms of understanding long-term outcomes of enhanced atmospheric carbon dioxide.
3.In ecosystems such as cool temperate upland grasslands, nutrient scarcity and prevailing low
temperatures (eg. Southern Pennines of the UK- Buxton) serve to limit any potential biomass enhancement and/or species composition shifts over the longer-term.
Savanna
*Mainly grasses and some widely spaced trees – support wildlife and livestock, and provide many other ecosystem services, such as nutrient cycling, water balance, and climate regulation.
*Key to the healthy functioning of savannas is a continuous grass layer, which many animal species depend on for habitat and food.
*Savanna grasses also carry regular fires that maintain the open, well-lit environment. Otherwise, it becomes dominated by shrubs and trees, a phenomenon that we referred to as “woody encroachment”.
*The establishment, growth and survival of savanna trees are affected by browsing animals, fire, and the availability of water and nutrients. Changes in any of these factors can lead to an imbalance between trees and grasses and ultimately to woody encroachment. Increase in tree/shrub density
*African savannas are estimated to have experienced increases in woody cover of around 2.5% per decade, with these shifts being most substantial in systems dominated by grazing herbivores.
*When savannas become dense bushlands, they become less suitable for wildlife and domestic grazers and lose many of the plant and animal species that are adapted to open, sunny conditions.
*Because the grasses are shaded out, fires can no longer spread. It then becomes impractical and expensive to reduce tree cover and restore the grasses. This is an issue that many livestock and crop farmers are struggling with.
- The CO2 “fertilisation” effect can greatly increase plant growth. It has been put forward as part of the reason for woody encroachment in savannas. The high concentration of CO2 in the atmosphere also increases stored carbon reserves in savanna trees, which allows them to recover faster after disturbance. Direct impact of rising CO2
*Grassland scientists and livestock farmers have long observed that heavy grazing can lead to more woody encroachment. Ecologists have explained this by two mechanisms: grazing leading to less competition from grasses, allowing trees to grow faster; and fewer fires to stunt trees, because the grass is kept short.
*While elevated CO2 makes woody encroachment more likely, managing grazing to ensure a healthy grass layer can help to reduce this. Regular burning and reintroducing browsing herbivores may also help control the threat that trees can pose to the savanna.
Savanna revision points
1.Species with habitats susceptible to considerable alterations will probably experience a severe local decline in the next few decades.
2.Loss of suitable habitats and decreased food availability, which has been forecasted for most large carnivores, might also induce these species to shift their home ranges in search of alternative food sources.
3.These may include areas where they are more likely to experience more conflict with humans.
4.Large carnivores require long-term conservation, management strategies, and more research to develop a deeper understanding of climate change impacts and establish pre-emptive measures ensuring population viability in the coming decades.
Note: increasing level of blindness observed in leopards in recent years – usually in areas where plants protect themselves with long spines – so it’s likely related to eye damage incurred during hunting. Vision damage results in decreased hunting efficiency and lower survival rate
