Air–fuel ratio

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio

Air–fuel ratio (AFR) is the mass ratio of air to a solid, liquid, or gaseous fuel present in a combustion process. The combustion may take place in a controlled manner such as in an internal combustion engine or industrial furnace, or may result in an explosion (e.g., a dust explosion, gas or vapor explosion or in a thermobaric weapon).

The air–fuel ratio determines whether a mixture is combustible at all, how much energy is being released, and how much unwanted pollutants are produced in the reaction. Typically a range of fuel to air ratios exists, outside of which ignition will not occur. These are known as the lower and upper explosive limits.

In an internal combustion engine or industrial furnace, the air–fuel ratio is an important measure for anti-pollution and performance-tuning reasons. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the stoichiometric mixture, often abbreviated to stoich. Ratios lower than stoichiometric (where the fuel is in excess) are considered "rich". Rich mixtures are less efficient, but may produce more power and burn cooler. Ratios higher than stoichiometric (where the air is in excess) are considered "lean". Lean mixtures are more efficient but may cause higher temperatures, which can lead to the formation of nitrogen oxides. Some engines are designed with features to allow lean-burn. For precise air–fuel ratio calculations, the oxygen content of combustion air should be specified because of different air density due to different altitude or intake air temperature, possible dilution by ambient water vapor, or enrichment by oxygen additions.

Internal combustion engines

In theory, a stoichiometric mixture has just enough air to completely burn the available fuel. In practice, this is never quite achieved, due primarily to the very short time available in an internal combustion engine for each combustion cycle.

Most of the combustion process is completed in approximately 2 milliseconds at an engine speed of 6,000 revolutions per minute. (100 revolutions per second; 10 milliseconds per revolution of the crankshaft. For a four-stroke engine would mean 5 milliseconds for each piston stroke, and 20 milliseconds to complete one four stroke, 720 degree cycle (the Otto cycle). This is the time that elapses from the spark plug firing until 90% of the fuel–air mix is combusted, typically some 80 degrees of crankshaft rotation later. Catalytic converters are designed to work best when the exhaust gases passing through them are the result of nearly perfect combustion.

A perfectly stoichiometric mixture burns very hot and can damage engine components if the engine is placed under high load at this fuel–air mixture. Due to the high temperatures at this mixture, the detonation of the fuel-air mix while approaching or shortly after maximum cylinder pressure is possible under high load (referred to as knocking or pinging), specifically a "pre-detonation" event in the context of a spark-ignition engine model. Such detonation can cause serious engine damage as the uncontrolled burning of the fuel-air mix can create very high pressures in the cylinder. As a consequence, stoichiometric mixtures are only used under light to low-moderate load conditions. For acceleration and high-load conditions, a richer mixture (lower air–fuel ratio) is used to produce cooler combustion products (thereby utilizing evaporative cooling), and so avoid overheating of the cylinder head, and thus prevent detonation.

Engine management systems

The stoichiometric mixture for a gasoline engine is the ideal ratio of air to fuel that burns all fuel with no excess air. For gasoline fuel, the stoichiometric air–fuel mixture is about 14.7:1 i.e. for every one gram of fuel, 14.7 grams of air are required. For pure octane fuel, the oxidation reaction is:

25 O₂ + 2 C₈H₁₈ → 16 CO₂ + 18 H₂O + energy

Any mixture greater than 14.7:1 is considered a lean mixture; any less than 14.7:1 is a rich mixture – given perfect (ideal) "test" fuel (gasoline consisting of solely n-heptane and iso-octane). In reality, most fuels consist of a combination of heptane, octane, a handful of other alkanes, plus additives including detergents, and possibly oxygenators such as MTBE (methyl tert-butyl ether) or ethanol/methanol. These compounds all alter the stoichiometric ratio, with most of the additives pushing the ratio downward (oxygenators bring extra oxygen to the combustion event in liquid form that is released at the time of combustions; for MTBE-laden fuel, a stoichiometric ratio can be as low as 14.1:1). Vehicles that use an oxygen sensor or other feedback loops to control fuel to air ratio (lambda control), compensate automatically for this change in the fuel's stoichiometric rate by measuring the exhaust gas composition and controlling fuel volume. Vehicles without such controls (such as most motorcycles until recently, and cars predating the mid-1980s) may have difficulties running certain fuel blends (especially winter fuels used in some areas) and may require different carburetor jets (or otherwise have the fueling ratios altered) to compensate. Vehicles that use oxygen sensors can monitor the air–fuel ratio with an air–fuel ratio meter.

Other types of engines

In the typical air to natural gas combustion burner, a double-cross limit strategy is employed to ensure ratio control. (This method was used in World War II). The strategy involves adding the opposite flow feedback into the limiting control of the respective gas (air or fuel). This assures ratio control within an acceptable margin.

Other terms used

There are other terms commonly used when discussing the mixture of air and fuel in internal combustion engines.

Mixture

Mixture is the predominant word that appears in training texts, operation manuals, and maintenance manuals in the aviation world.

Air–fuel ratio is the ratio between the mass of air and the mass of fuel in the fuel–air mix at any given moment. The mass is the mass of all constituents that compose the fuel and air, whether combustible or not. For example, a calculation of the mass of natural gas—which often contains carbon dioxide (CO
₂), nitrogen (N
₂), and various alkanes—includes the mass of the carbon dioxide, nitrogen and all alkanes in determining the value of m_fuel.

For pure octane the stoichiometric mixture is approximately 15.1:1, or λ of 1.00 exactly.

In naturally aspirated engines powered by octane, maximum power is frequently reached at AFRs ranging from 12.5 to 13.3:1 or λ of 0.850 to 0.901.

The air-fuel ratio of 12:1 is considered as the maximum output ratio, whereas the air-fuel ratio of 16:1 is considered as the maximum fuel economy ratio.

Fuel–air ratio (FAR)

Fuel–air ratio is commonly used in the gas turbine industry as well as in government studies of internal combustion engine, and refers to the ratio of fuel to the air.

${\displaystyle \mathrm{F}\mathrm{A}\mathrm{R}=\frac{1}{\mathrm{A}\mathrm{F}\mathrm{R}}}$

Air–fuel equivalence ratio (λ)

Air–fuel equivalence ratio, λ (lambda), is the ratio of actual AFR to stoichiometry for a given mixture. λ = 1.0 is at stoichiometry, rich mixtures λ < 1.0, and lean mixtures λ > 1.0.

There is a direct relationship between λ and AFR. To calculate AFR from a given λ, multiply the measured λ by the stoichiometric AFR for that fuel. Alternatively, to recover λ from an AFR, divide AFR by the stoichiometric AFR for that fuel. This last equation is often used as the definition of λ:

${\displaystyle \lambda =\frac{\mathrm{A}\mathrm{F}\mathrm{R}}{{\mathrm{A}\mathrm{F}\mathrm{R}}_{\text{stoich}}}}$

Because the composition of common fuels varies seasonally, and because many modern vehicles can handle different fuels when tuning, it makes more sense to talk about λ values rather than AFR.

Most practical AFR devices actually measure the amount of residual oxygen (for lean mixes) or unburnt hydrocarbons (for rich mixtures) in the exhaust gas.

Fuel–air equivalence ratio (ϕ)

The fuel–air equivalence ratio, ϕ (phi), of a system is defined as the ratio of the fuel-to-oxidizer ratio to the stoichiometric fuel-to-oxidizer ratio. Mathematically,

${\displaystyle \varphi =\frac{\text{fuel-to-oxidizer ratio}}{(\text{fuel-to-oxidizer ratio}{)}_{\text{st}}}=\frac{m_{\text{fuel}}/m_{\text{ox}}}{{\left(m_{\text{fuel}}/m_{\text{ox}}\right)}_{\text{st}}}=\frac{n_{\text{fuel}}/n_{\text{ox}}}{{\left(n_{\text{fuel}}/n_{\text{ox}}\right)}_{\text{st}}}}$

where m represents the mass, n represents a number of moles, subscript st stands for stoichiometric conditions.

The advantage of using equivalence ratio over fuel–oxidizer ratio is that it takes into account (and is therefore independent of) both mass and molar values for the fuel and the oxidizer. Consider, for example, a mixture of one mole of ethane (C
₂H
₆) and one mole of oxygen (O
₂). The fuel–oxidizer ratio of this mixture based on the mass of fuel and air is

${\displaystyle \frac{m_{{\text{C}}_2^{}{\text{H}}_6^{}}}{m_{{\text{O}}_2^{}}}=\frac{1\times (2\times 12+6\times 1)}{1\times (2\times 16)}=\frac{30}{32}=0.9375}$

and the fuel-oxidizer ratio of this mixture based on the number of moles of fuel and air is

${\displaystyle \frac{n_{{\text{C}}_2^{}{\text{H}}_6^{}}}{n_{{\text{O}}_2^{}}}=\frac{1}{1}=1}$

Clearly the two values are not equal. To compare it with the equivalence ratio, we need to determine the fuel–oxidizer ratio of ethane and oxygen mixture. For this we need to consider the stoichiometric reaction of ethane and oxygen,

C₂H₆ + 7⁄2 O₂ → 2 CO₂ + 3 H₂O

This gives

${\displaystyle (\text{fuel-to-oxidizer ratio based on mass}{)}_{\text{st}}={\left(\frac{m_{{\text{C}}_2^{}{\text{H}}_6^{}}}{m_{{\text{O}}_2^{}}}\right)}_{\text{st}}=\frac{1\times (2\times 12+6\times 1)}{3.5\times (2\times 16)}=\frac{30}{112}=0.268}$

${\displaystyle (\text{fuel-to-oxidizer ratio based on number of moles}{)}_{\text{st}}={\left(\frac{n_{{\text{C}}_2^{}{\text{H}}_6^{}}}{n_{{\text{O}}_2^{}}}\right)}_{\text{st}}=\frac{1}{3.5}=0.286}$

Thus we can determine the equivalence ratio of the given mixture as

${\displaystyle \varphi =\frac{m_{{\text{C}}_2^{}{\text{H}}_6^{}}/m_{{\text{O}}_2^{}}}{{\left(m_{{\text{C}}_2^{}{\text{H}}_6^{}}/m_{{\text{O}}_2^{}}\right)}_{\text{st}}}=\frac{0.938}{0.268}=3.5}$

or, equivalently, as

${\displaystyle \varphi =\frac{n_{{\text{C}}_2^{}{\text{H}}_6^{}}/n_{{\text{O}}_2^{}}}{{\left(n_{{\text{C}}_2^{}{\text{H}}_6^{}}/n_{{\text{O}}_2^{}}\right)}_{\text{st}}}=\frac{1}{0.286}=3.5}$

Another advantage of using the equivalence ratio is that ratios greater than one always mean there is more fuel in the fuel–oxidizer mixture than required for complete combustion (stoichiometric reaction), irrespective of the fuel and oxidizer being used—while ratios less than one represent a deficiency of fuel or equivalently excess oxidizer in the mixture. This is not the case if one uses fuel–oxidizer ratio, which takes different values for different mixtures.

The fuel–air equivalence ratio is related to the air–fuel equivalence ratio (defined previously) as follows:

${\displaystyle \varphi =\frac{1}{\lambda }}$

Mixture fraction

Further information: Mixture fraction

The relative amounts of oxygen enrichment and fuel dilution can be quantified by the mixture fraction, Z, defined as

${\displaystyle Z=\left[\frac{s{Y}_{\mathrm{F}}-Y_{\mathrm{O}}+Y_{\mathrm{O},0}}{s{Y}_{\mathrm{F},0}+Y_{\mathrm{O},0}}\right]}$,

where

${\displaystyle s={\mathrm{A}\mathrm{F}\mathrm{R}}_{\mathrm{s}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{h}}=\frac{W_{\mathrm{O}}\times v_{\mathrm{O}}}{W_{\mathrm{F}}\times v_{\mathrm{F}}}}$,

Y_F,0 and Y_O,0 represent the fuel and oxidizer mass fractions at the inlet, W_F and W_O are the species molecular weights, and v_F and v_O are the fuel and oxygen stoichiometric coefficients, respectively. The stoichiometric mixture fraction is

${\displaystyle Z_{\mathrm{s}\mathrm{t}}=\left[\frac{1}{1+\frac{Y_{\mathrm{F},0}\times W_{\mathrm{O}}\times v_{\mathrm{O}}}{Y_{\mathrm{O},0}\times W_{\mathrm{F}}\times v_{\mathrm{F}}}}\right]}$

The stoichiometric mixture fraction is related to λ (lambda) and ϕ (phi) by the equations

${\displaystyle Z_{\text{st}}=\frac{\lambda }{1+\lambda }=\frac{1}{1+\varphi }}$,

assuming

${\displaystyle \mathrm{A}\mathrm{F}\mathrm{R}=\frac{Y_{\mathrm{O},0}}{Y_{\mathrm{F},0}}}$

Percent excess combustion air

Ideal stoichiometry

In industrial fired heaters, power plant steam generators, and large gas-fired turbines, the more common terms are percent excess combustion air and percent stoichiometric air. For example, excess combustion air of 15 percent means that 15 percent more than the required stoichiometric air (or 115 percent of stoichiometric air) is being used.

A combustion control point can be defined by specifying the percent excess air (or oxygen) in the oxidant, or by specifying the percent oxygen in the combustion product. An air–fuel ratio meter may be used to measure the percent oxygen in the combustion gas, from which the percent excess oxygen can be calculated from stoichiometry and a mass balance for fuel combustion. For example, for propane (C
₃H
₈) combustion between stoichiometric and 30 percent excess air (AFR_mass between 15.58 and 20.3), the relationship between percent excess air and percent oxygen is:

${\displaystyle \begin{array}{rl}\mathrm{M}\mathrm{a}\mathrm{s}\mathrm{s}\mathrm{\%}{\mathrm{O}}_2\mathrm{i}\mathrm{n}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{a}\mathrm{n}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{s}& \approx -0.1433(\mathrm{\%}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}{\mathrm{O}}_2{)}^2+0.214(\mathrm{\%}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}{\mathrm{O}}_2)\\ \mathrm{V}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{m}\mathrm{e}\mathrm{\%}{\mathrm{O}}_2\mathrm{i}\mathrm{n}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{p}\mathrm{a}\mathrm{n}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{m}\mathrm{b}\mathrm{u}\mathrm{s}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{g}\mathrm{a}\mathrm{s}& \approx -0.1208(\mathrm{\%}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}{\mathrm{O}}_2{)}^2+0.186(\mathrm{\%}\mathrm{e}\mathrm{x}\mathrm{c}\mathrm{e}\mathrm{s}\mathrm{s}{\mathrm{O}}_2)\end{array}}$

Visible spectrum

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Visible_spectrum

White light is dispersed by a prism into the colors of the visible spectrum.

The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 380 to about 750 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 400–790 terahertz. These boundaries are not sharply defined and may vary per individual. Under optimal conditions these limits of human perception can extend to 310 nm (ultraviolet) and 1100 nm (near infrared). The optical spectrum is sometimes considered to be the same as the visible spectrum, but some authors define the term more broadly, to include the ultraviolet and infrared parts of the electromagnetic spectrum as well.

The spectrum does not contain all the colors that the human visual system can distinguish. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are also called pure colors or spectral colors.

Visible wavelengths pass largely unattenuated through the Earth's atmosphere via the "optical window" region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, and so the midday sky appears blue (apart from the area around the Sun which appears white because the light is not scattered as much). The optical window is also referred to as the "visible window" because it overlaps the human visible response spectrum. The near infrared (NIR) window lies just out of the human vision, as well as the medium wavelength infrared (MWIR) window, and the long-wavelength or far-infrared (LWIR or FIR) window, although other animals may perceive them.

Spectral colors

Color	Wavelength (nm)	Frequency (THz)	Photon energy (eV)
violet	380–450	670–790	2.75–3.26
blue	450–485	620–670	2.56–2.75
cyan	485–500	600–620	2.48–2.56
green	500–565	530–600	2.19–2.48
yellow	565–590	510–530	2.10–2.19
orange	590–625	480–510	1.98–2.10
red	625–750	400–480	1.65–1.98

 

Main article: Spectral color

Colors that can be produced by visible light of a narrow band of wavelengths (monochromatic light) are called pure spectral colors. The various color ranges indicated in the illustration are an approximation: The spectrum is continuous, with no clear boundaries between one color and the next.

History

Newton's color circle, from Opticks of 1704, showing the colors he associated with musical notes. The spectral colors from red to violet are divided by the notes of the musical scale, starting at D. The circle completes a full octave, from D to D. Newton's circle places red, at one end of the spectrum, next to violet, at the other. This reflects the fact that non-spectral purple colors are observed when red and violet light are mixed.

In the 13th century, Roger Bacon theorized that rainbows were produced by a similar process to the passage of light through glass or crystal.

In the 17th century, Isaac Newton discovered that prisms could disassemble and reassemble white light, and described the phenomenon in his book Opticks. He was the first to use the word spectrum (Latin for "appearance" or "apparition") in this sense in print in 1671 in describing his experiments in optics. Newton observed that, when a narrow beam of sunlight strikes the face of a glass prism at an angle, some is reflected and some of the beam passes into and through the glass, emerging as different-colored bands. Newton hypothesized light to be made up of "corpuscles" (particles) of different colors, with the different colors of light moving at different speeds in transparent matter, red light moving more quickly than violet in glass. The result is that red light is bent (refracted) less sharply than violet as it passes through the prism, creating a spectrum of colors.

Newton's observation of prismatic colors (David Brewster 1855)

Newton originally divided the spectrum into six named colors: red, orange, yellow, green, blue, and violet. He later added indigo as the seventh color since he believed that seven was a perfect number as derived from the ancient Greek sophists, of there being a connection between the colors, the musical notes, the known objects in the Solar System, and the days of the week. The human eye is relatively insensitive to indigo's frequencies, and some people who have otherwise-good vision cannot distinguish indigo from blue and violet. For this reason, some later commentators, including Isaac Asimov, have suggested that indigo should not be regarded as a color in its own right but merely as a shade of blue or violet. Evidence indicates that what Newton meant by "indigo" and "blue" does not correspond to the modern meanings of those color words. Comparing Newton's observation of prismatic colors with a color image of the visible light spectrum shows that "indigo" corresponds to what is today called blue, whereas his "blue" corresponds to cyan.

In the 18th century, Johann Wolfgang von Goethe wrote about optical spectra in his Theory of Colours. Goethe used the word spectrum (Spektrum) to designate a ghostly optical afterimage, as did Schopenhauer in On Vision and Colors. Goethe argued that the continuous spectrum was a compound phenomenon. Where Newton narrowed the beam of light to isolate the phenomenon, Goethe observed that a wider aperture produces not a spectrum but rather reddish-yellow and blue-cyan edges with white between them. The spectrum appears only when these edges are close enough to overlap.

In the early 19th century, the concept of the visible spectrum became more definite, as light outside the visible range was discovered and characterized by William Herschel (infrared) and Johann Wilhelm Ritter (ultraviolet), Thomas Young, Thomas Johann Seebeck, and others. Young was the first to measure the wavelengths of different colors of light, in 1802.

The connection between the visible spectrum and color vision was explored by Thomas Young and Hermann von Helmholtz in the early 19th century. Their theory of color vision correctly proposed that the eye uses three distinct receptors to perceive color.

Limits to visible range

See also: Color vision § Physiology of color perception

Photopic (black) and scotopic (green) luminous efficiency functions. The horizontal axis is wavelength in nm. See luminous efficiency function for more info.

The visible spectrum is limited to wavelengths that can both reach the retina and trigger visual phototransduction (excite a visual opsin). Insensitivity to UV light is generally limited by transmission through the lens. Insensitivity to IR light is limited by the spectral sensitivity functions of the visual opsins. The range is defined psychometrically by the luminous efficiency function, which accounts for all of these factors. In humans, there is a separate function for each of two visual systems, one for photopic vision, used in daylight, which is mediated by cone cells, and one for scotopic vision, used in dim light, which is mediated by rod cells. Each of these functions have different visible ranges. However, discussion on the visible range generally assumes photopic vision.

Atmospheric transmission

The visible range of most animals evolved to match the optical window, which is the range of light that can pass through the atmosphere. The ozone layer absorbs almost all UVA light (below 315 nm). However, this only affects cosmic light (e.g. sunlight), not terrestrial light (e.g. Bioluminescence).

Ocular transmission

Cumulative transmission spectra of light as it passes through the ocular media, namely after the cornea (blue), before the lens (red), after the lens (gray) and before the retina (orange). The solid lines are for a 4.5 year old eye. The dashed orange line is for a 53 year old eye, and dotted for a 75 year old eye, indicating the effect of lens yellowing.)

Before reaching the retina, light must first transmit through the cornea and lens. UVB light (< 315 nm) is filtered mostly by the cornea, and UVA light (315–400 nm) is filtered mostly by the lens. The lens also yellows with age, attenuating transmission most strongly at the blue part of the spectrum. This can cause xanthopsia as well as a slight truncation of the short-wave (blue) limit of the visible spectrum. Subjects with aphakia are missing a lens, so UVA light can reach the retina and excite the visual opsins; this expands the visible range and may also lead to cyanopsia.

Opsin absorption

Each opsin has a spectral sensitivity function, which defines how likely it is to absorb a photon of each wavelength. The luminous efficiency function is approximately the superposition of the contributing visual opsins. Variance in the position of the individual opsin spectral sensitivity functions therefore affects the luminous efficiency function and the visible range. For example, the long-wave (red) limit changes proportionally to the position of the L-opsin. The positions are defined by the peak wavelength (wavelength of highest sensitivity), so as the L-opsin peak wavelength blue shifts by 10 nm, the long-wave limit of the visible spectrum also shifts 10 nm. Large deviations of the L-opsin peak wavelength lead to a form of color blindness called protanomaly and a missing L-opsin (protanopia) shortens the visible spectrum by about 30 nm at the long-wave limit. Forms of color blindness affecting the M-opsin and S-opsin do not significantly affect the luminous efficiency function nor the limits of the visible spectrum.

Different definitions

Regardless of actual physical and biological variance, the definition of the limits is not standard and will change depending on the industry. For example, some industries may be concerned with practical limits, so would conservatively report 420–680 nm, while others may be concerned with psychometrics and achieving the broadest spectrum would liberally report 380–750, or even 380–800 nm. The luminous efficiency function in the NIR does not have a hard cutoff, but rather an exponential decay, such that the function's value (or vision sensitivity) at 1,050 nm is about 10⁹ times weaker than at 700 nm; much higher intensity is therefore required to perceive 1,050 nm light than 700 nm light.

Vision outside the visible spectrum

Under ideal laboratory conditions, subjects may perceive infrared light up to at least 1,064 nm. While 1,050 nm NIR light can evoke red, suggesting direct absorption by the L-opsin, there are also reports that pulsed NIR lasers can evoke green, which suggests two-photon absorption may be enabling extended NIR sensitivity.

Similarly, young subjects may perceive ultraviolet wavelengths down to about 310–313 nm, but detection of light below 380 nm may be due to fluorescence of the ocular media, rather than direct absorption of UV light by the opsins. As UVA light is absorbed by the ocular media (lens and cornea), it may fluoresce and be released at a lower energy (longer wavelength) that can then be absorbed by the opsins. For example, when the lens absorbs 350 nm light, the fluorescence emission spectrum is centered on 440 nm.

Non-visual light detection

In addition to the photopic and scotopic systems, humans have other systems for detecting light that do not contribute to the primary visual system. For example, melanopsin has an absorption range of 420–540 nm and regulates circadian rhythm and other reflexive processes. Since the melanopsin system does not form images, it is not strictly considered vision and does not contribute to the visible range.

In non-humans

See also: Evolution of color vision

The visible spectrum is defined as that visible to humans, but the variance between species is large. Not only can cone opsins be spectrally shifted to alter the visible range, but vertebrates with 4 cones (tetrachromatic) or 2 cones (dichromatic) relative to humans' 3 (trichromatic) will also tend to have a wider or narrower visible spectrum than humans, respectively.

Vertebrates tend to have 1-4 different opsin classes:

• longwave sensitive (LWS) with peak sensitivity between 500–570 nm,
• middlewave sensitive (MWS) with peak sensitivity between 480–520 nm,
• shortwave sensitive (SWS) with peak sensitivity between 415–470 nm, and
• violet/ultraviolet sensitive (VS/UVS) with peak sensitivity between 355–435 nm.

Testing the visual systems of animals behaviorally is difficult, so the visible range of animals is usually estimated by comparing the peak wavelengths of opsins with those of typical humans (S-opsin at 420 nm and L-opsin at 560 nm).

Mammals

Most mammals have retained only two opsin classes (LWS and VS), due likely to the nocturnal bottleneck. However, old world primates (including humans) have since evolved two versions in the LWS class to regain trichromacy. Unlike most mammals, rodents' UVS opsins have remained at shorter wavelengths. Along with their lack of UV filters in the lens, mice have a UVS opsin that can detect down to 340 nm. While allowing UV light to reach the retina can lead to retinal damage, the short lifespan of mice compared with other mammals may minimize this disadvantage relative to the advantage of UV vision. Dogs have two cone opsins at 429 nm and 555 nm, so see almost the entire visible spectrum of humans, despite being dichromatic. Horses have two cone opsins at 428 nm and 539 nm, yielding a slightly more truncated red vision.

Birds

See also: Bird_vision § Ultraviolet_sensitivity

Most other vertebrates (birds, lizards, fish, etc.) have retained their tetrachromacy, including UVS opsins that extend further into the ultraviolet than humans' VS opsin. The sensitivity of avian UVS opsins vary greatly, from 355–425 nm, and LWS opsins from 560–570 nm. This translates to some birds with a visible spectrum on par with humans, and other birds with greatly expanded sensitivity to UV light. The LWS opsin of birds is sometimes reported to have a peak wavelength above 600 nm, but this is an effective peak wavelength that incorporates the filter of avian oil droplets. The peak wavelength of the LWS opsin alone is the better predictor of the long-wave limit. A possible benefit of avian UV vision involves sex-dependent markings on their plumage that are visible only in the ultraviolet range.

Fish

See also: Vision_in_fish § Ultraviolet

Teleosts (bony fish) are generally tetrachromatic. The sensitivity of fish UVS opsins vary from 347-383 nm, and LWS opsins from 500-570 nm. However, some fish that use alternative chromophores can extend their LWS opsin sensitivity to 625 nm. The popular belief that the common goldfish is the only animal that can see both infrared and ultraviolet light is incorrect, because goldfish cannot see infrared light.

Invertebrates

The visual systems of invertebrates deviate greatly from vertebrates, so direct comparisons are difficult. However, UV sensitivity has been reported in most insect species. Bees and many other insects can detect ultraviolet light, which helps them find nectar in flowers. Plant species that depend on insect pollination may owe reproductive success to their appearance in ultraviolet light rather than how colorful they appear to humans. Bees' long-wave limit is at about 590 nm. Mantis shrimp exhibit up to 14 opsins, enabling a visible range of less than 300 nm to above 700 nm.

Thermal vision

Main article: Infrared sensing in snakes

Some snakes can "see" radiant heat at wavelengths between 5 and 30 μm to a degree of accuracy such that a blind rattlesnake can target vulnerable body parts of the prey at which it strikes, and other snakes with the organ may detect warm bodies from a meter away. It may also be used in thermoregulation and predator detection.

Spectroscopy

Earth's atmosphere partially or totally blocks some wavelengths of electromagnetic radiation, but in visible light it is mostly transparent

Main article: Spectroscopy

Spectroscopy is the study of objects based on the spectrum of color they emit, absorb or reflect. Visible-light spectroscopy is an important tool in astronomy (as is spectroscopy at other wavelengths), where scientists use it to analyze the properties of distant objects. Chemical elements and small molecules can be detected in astronomical objects by observing emission lines and absorption lines. For example, Helium was first detected by analysis of the spectrum of the Sun. The shift in frequency of spectral lines is used to measure the Doppler shift (redshift or blueshift) of distant objects to determine their velocities towards or away from the observer. Astronomical spectroscopy uses high-dispersion diffraction gratings to observe spectra at very high spectral resolutions.

Plant ecology

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Plant_ecology

A tropical plant community on Diego Garcia

Rangeland monitoring using Parker 3-step Method, Okanagan Washington 2002

Plant ecology is a subdiscipline of ecology that studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among plants and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, and competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands.

A global overview of the Earth's major vegetation types is provided by O.W. Archibold. He recognizes 11 major vegetation types: tropical forests, tropical savannas, arid regions (deserts), Mediterranean ecosystems, temperate forest ecosystems, temperate grasslands, coniferous forests, tundra (both polar and high mountain), terrestrial wetlands, freshwater ecosystems and coastal/marine systems. This breadth of topics shows the complexity of plant ecology, since it includes plants from floating single-celled algae up to large canopy forming trees.

One feature that defines plants is photosynthesis. Photosynthesis is the process of a chemical reactions to create glucose and oxygen, which is vital for plant life. One of the most important aspects of plant ecology is the role plants have played in creating the oxygenated atmosphere of earth, an event that occurred some 2 billion years ago. It can be dated by the deposition of banded iron formations, distinctive sedimentary rocks with large amounts of iron oxide. At the same time, plants began removing carbon dioxide from the atmosphere, thereby initiating the process of controlling Earth's climate. A long term trend of the Earth has been toward increasing oxygen and decreasing carbon dioxide, and many other events in the Earth's history, like the first movement of life onto land, are likely tied to this sequence of events.

One of the early classic books on plant ecology was written by J.E. Weaver and F.E. Clements. It talks broadly about plant communities, and particularly the importance of forces like competition and processes like succession. The term ecology itself was coined by German biologist Ernst Haeckel.

Plant ecology can also be divided by levels of organization including plant ecophysiology, plant population ecology, community ecology, ecosystem ecology, landscape ecology and biosphere ecology.

The study of plants and vegetation is complicated by their form. First, most plants are rooted in the soil, which makes it difficult to observe and measure nutrient uptake and species interactions. Second, plants often reproduce vegetatively, that is asexually, in a way that makes it difficult to distinguish individual plants. Indeed, the very concept of an individual is doubtful, since even a tree may be regarded as a large collection of linked meristems. Hence, plant ecology and animal ecology have different styles of approach to problems that involve processes like reproduction, dispersal and mutualism. Some plant ecologists have placed considerable emphasis upon trying to treat plant populations as if they were animal populations, focusing on population ecology. Many other ecologists believe that while it is useful to draw upon population ecology to solve certain scientific problems, plants demand that ecologists work with multiple perspectives, appropriate to the problem, the scale and the situation.

History

Main article: History of ecology

Alexander von Humboldt's work connecting plant distributions with environmental factors played an important role in the genesis of the discipline of plant ecology.

Plant ecology has its origin in the application of plant physiology to the questions raised by plant geographers. Carl Ludwig Willdenow was one of the first to note that similar climates produced similar types of vegetation, even when they were located in different parts of the world. Willdenow's student, Alexander von Humboldt, used physiognomy to describe vegetation types and observed that the distribution vegetation types was based on environmental factors. Later plant geographers who built upon Humboldt's work included Joakim Frederik Schouw, A.P. de Candolle, August Grisebach and Anton Kerner von Marilaun. Schouw's work, published in 1822, linked plant distributions to environmental factors (especially temperature) and established the practice of naming plant associations by adding the suffix -etum to the name of the dominant species. Working from herbarium collections, De Candolle searched for general rules of plant distribution and settled on using temperature as well. Grisebach's two-volume work, Die Vegetation der Erde nach Ihrer Klimatischen Anordnung, published in 1872, saw plant geography reach its "ultimate form" as a descriptive field.

Starting in the 1870s, Swiss botanist Simon Schwendener, together with his students and colleagues, established the link between plant morphology and physiological adaptations, laying the groundwork for the first ecology textbooks, Eugenius Warming's Plantesamfund (published in 1895) and Andreas Schimper's 1898 Pflanzengeographie auf Physiologischer Grundlage. Warming successfully incorporated plant morphology, physiology taxonomy and biogeography into plant geography to create the field of plant ecology. Although more morphological than physiological, Schimper's has been considered the beginning of plant physiological ecology. Plant ecology was initially built around static ideas of plant distribution; incorporating the concept of succession added an element to change through time to the field. Henry Chandler Cowles' studies of plant succession on the Lake Michigan sand dunes (published in 1899) and Frederic Clements' 1916 monograph on the subject established it as a key element of plant ecology.

Plant ecology developed within the wider discipline of ecology over the twentieth century. Inspired by Warming's Plantesamfund, Arthur Tansley set out to map British plant communities. In 1904 he teamed up with William Gardner Smith and others involved in vegetation mapping to establish the Central Committee for the Survey and Study of British Vegetation, later shortened to British Vegetation Committee. In 1913, the British Vegetation Committee organised the British Ecological Society (BES), the first professional society of ecologists. This was followed in 1917 by the establishment of the Ecological Society of America (ESA); plant ecologists formed the largest subgroup among the inaugural members of the ESA.

Cowles' students played an important role in the development of the field of plant ecology during the first half of the twentieth century, among them William S. Cooper, E. Lucy Braun and Edgar Transeau.

Distribution

Main articles: Phytogeography and Species distribution

World biomes are based upon the type of dominant plant.

Plant distributions is governed by a combination of historical factors, ecophysiology and biotic interactions. The set of species that can be present at a given site is limited by historical contingency. In order to show up, a species must either have evolved in an area or dispersed there (either naturally or through human agency), and must not have gone locally extinct. The set of species present locally is further limited to those that possess the physiological adaptations to survive the environmental conditions that exist. This group is further shaped through interactions with other species.

Plant communities are broadly distributed into biomes based on the form of the dominant plant species. For example, grasslands are dominated by grasses, while forests are dominated by trees. Biomes are determined by regional climates, mostly temperature and precipitation, and follow general latitudinal trends. Within biomes, there may be many ecological communities, which are impacted not only by climate and a variety of smaller-scale features, including soils, hydrology, and disturbance regime. Biomes also change with elevation, high elevations often resembling those found at higher latitudes.

Biological interactions

Further information: Biological interaction

Competition

Further information: Competition (biology)

Plants, like most life forms, require relatively few basic elements: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulphur; hence they are known as CHNOPS life forms. There are also lesser elements needed as well, frequently termed micronutrients, such as magnesium and sodium. When plants grow in close proximity, they may deplete supplies of these elements and have a negative impact upon neighbours. Competition for resources vary from complete symmetric (all individuals receive the same amount of resources, irrespective of their size) to perfectly size symmetric (all individuals exploit the same amount of resource per unit biomass) to absolutely size-asymmetric (the largest individuals exploit all the available resource). The degree of size asymmetry has major effects on the structure and diversity of ecological communities. In many cases (perhaps most) the negative effects upon neighbours arise from size asymmetric competition for light. In other cases, there may be competition below ground for water, nitrogen, or phosphorus. To detect and measure competition, experiments are necessary; these experiments require removing neighbours, and measuring responses in the remaining plants. Many such studies are required before useful generalizations can be drawn.

Overall, it appears that light is the most important resource for which plants compete, and the increase in plant height over evolutionary time likely reflects selection for taller plants to better intercept light. Many plant communities are therefore organized into hierarchies based upon the relative competitive abilities for light. In some systems, particularly infertile or arid systems, below ground competition may be more significant. Along natural gradients of soil fertility, it is likely that the ratio of above ground to below ground competition changes, with higher above ground competition in the more fertile soils. Plants that are relatively weak competitors may escape in time (by surviving as buried seeds) or in space (by dispersing to a new location away from strong competitors.)

In principle, it is possible to examine competition at the level of the limiting resources if a detailed knowledge of the physiological processes of the competing plants is available. However, in most terrestrial ecological studies, there is only little information on the uptake and dynamics of the resources that limit the growth of different plant species, and, instead, competition is inferred from observed negative effects of neighbouring plants without knowing precisely which resources the plants were competing for. In certain situations, plants may compete for a single growth-limiting resource, perhaps for light in agricultural systems with sufficient water and nutrients, or in dense stands of marsh vegetation, but in many natural ecosystems plants may be colimited by several resources, e.g. light, phosphorus and nitrogen at the same time.

Therefore, there are many details that remain to be uncovered, particularly the kinds of competition that arise in natural plant communities, the specific resource(s), the relative importance of different resources, and the role of other factors like stress or disturbance in regulating the importance of competition.

Mutualism

Further information: Mutualism (biology)

Mutualism is defined as an interaction "between two species or individuals that is beneficial to both". Probably the most widespread example in plants is the mutual beneficial relationship between plants and fungi, known as mycorrhizae. The plant is assisted with nutrient uptake, while the fungus receives carbohydrates. Some the earliest known fossil plants even have fossil mycorrhizae on their rhizomes.

The flowering plants are a group that have evolved by using two major mutualisms. First, flowers are pollinated by insects. This relationship seems to have its origins in beetles feeding on primitive flowers, eating pollen and also acting (unwittingly) as pollinators. Second, fruits are eaten by animals, and the animals then disperse the seeds. Thus, the flowering plants actually have three major types of mutualism, since most higher plants also have mycorrhizae.

Plants may also have beneficial effects upon one another, but this is less common. Examples might include "nurse plants" whose shade allows young cacti to establish. Most examples of mutualism, however, are largely beneficial to only one of the partners, and may not really be true mutualism. The term used for these more one-sided relationships, which are mostly beneficial to one participant, is facilitation. Facilitation among neighboring plants may act by reducing the negative impacts of a stressful environment. In general, facilitation is more likely to occur in physically stressful environments than in favorable environments, where competition may be the most important interaction among species.

Commensalism is similar to facilitation, in that one plant is mostly exploiting another. A familiar example is the epiphytes which grow on branches of tropical trees, or even mosses which grow on trees in deciduous forests.

It is important to keep track of the benefits received by each species to determine the appropriate term. Although people are often fascinated by unusual examples, it is important to remember that in plants, the main mutualisms are mycorrhizae, pollination, and seed dispersal.

Parasitism

Further information: Parasitic plant

Parasitism in biology refers to an interaction between different species, where the parasite (one species) benefits at the expense of the host (the other species). Parasites depend on another organism (their host) for survival in general, which usually includes both habitat and nutrient requirements at the very minimum. Parasitic plants attach themselves to host plants via a haustoria to the xylem and/or phloem. Many parasitic plants are generalists and are able to attack multiple hosts at the same time, greatly affecting community structures. Host species' growth, reproduction, and metabolism are affected by the parasite due to the nutrients, water, and carbon being taken by the parasite. They are also able to alter competitive interactions among hosts and indirectly affect competition in the community.

Commensalism

Commensalism refers to the biological interaction between two species in which one benefits while the other simply remains unaffected. The species that benefits is referred to as the commensal while the species that is unaffected is referred to as the host. For example, organisms that live attached to plants, known as epiphytes, are referred to as commensals. Algae that grow on the backs of turtles or sloths are considered as commensals, too. Their survival rate is higher when they are attached to their host, however they do not harm nor benefit the host. Nearly 10% of all vascular plant species around the world are epiphytes, and most of them are found in tropical forests. Therefore, they make up a large fraction of the total plant biodiversity in the world, being 10% of all species, and 25% of all vascular plant species in tropical countries. However, commensals have the capability to transform into parasites over time by which results in a decrease in success or an overall population decline.

Herbivory

Main articles: Herbivory and Plant defense against herbivory

Reindeer in front of herbivore exclosures. Excluding different herbivores (here reindeer, or reindeer and rodents) has different effects on the vegetation.

An important ecological function of plants is that they produce organic compounds for herbivores in the bottom of the food web. A large number of plant traits, from thorns to chemical defenses, can be related to the intensity of herbivory. Large herbivores can also have many effects on vegetation. These include removing selected species, creating gaps for regeneration of new individuals, recycling nutrients, and dispersing seeds. Certain ecosystem types, such as grasslands, may be dominated by the effects of large herbivores, although fire is also an equally important factor in this biome. In few cases, herbivores are capable of nearly removing all the vegetation at a site (for example, geese in the Hudson Bay Lowlands of Canada, and nutria in the marshes of Louisiana) but normally herbivores have a more selective impact, particularly when large predators control the abundance of herbivores. The usual method of studying the effects of herbivores is to build exclosures, where they cannot feed, and compare the plant communities in the exclosures to those outside over many years. Often such long term experiments show that herbivores have a significant effect upon the species that make up the plant community.

Other topics

Abundance

Main article: Abundance (ecology)

The ecological success of a plant species in a specific environment may be quantified by its abundance, and depending on the life form of the plant different measures of abundance may be relevant, e.g. density, biomass, or plant cover.

The change in the abundance of a plant species may be due to both abiotic factors, e.g. climate change, or biotic factors, e.g. herbivory or interspecific competition.

Colonisation and local extinction

Main article: Biogeography

See also: Colonisation (biology), Biological dispersal, Seed dispersal, Local extinction, and Soil seed bank

Whether a plant species is present at a local area depends on the processes of colonisation and local extinction. The probability of colonisation decreases with distance to neighboring habitats where the species is present and increases with plant abundance and fecundity in neighboring habitats and the dispersal distance of the species. The probability of local extinction decreases with abundance (both living plants and seeds in the soil seed bank).

Life forms

Main article: Plant life-form

See also: Raunkiær plant life-form

Reproduction

Main article: Plant reproduction

There are a few ways that reproduction occurs within plant life, and one way is through parthenogenesis. Parthenogenesis is defined as "a form of asexual reproduction in which genetically identical offspring (clones) are produced". Another form of reproduction is through cross-fertilization, which is defined as "fertilization in which the egg and sperm are produced by different individuals", and in plants this occurs in the ovule. Once an ovule is fertilized within the plant this becomes what is known as a seed. A seed normally contains the nutritive tissue also known as the endosperm and the embryo. A seedling is a young plant that has recently gone through germination. Another form of reproduction of a plant is self-fertilization; in which both the sperm and the egg are produced from the same individual- this plant is therefore a self-compatible titled plant.