Leaf angle distribution
Encyclopedia
The Leaf Angle Distribution (or LAD) of a plant canopy refers to the mathematical description of the angular orientation
of the leaves
in the vegetation
. Specifically, if each leaf is conceptually represented by a small flat plate, its orientation can be described with the zenith
and the azimuth angles of the surface normal
to that plate. If the leaf has a complex structure and is not flat, it may be necessary to approximate the actual leaf by a set of small plates, in which case there may be a number of leaf normals and associated angles. The LAD describes the statistical distribution of these angles.
follow an erectophile LAD while those at the bottom of the canopy are more planophile. This may be interpreted as a strategy by that plant species to maximize exposure to light, an important constraint to growth and development. Yet other species (notably sunflower
) are capable of reorienting their leaves throughout the day to optimize exposure to the Sun: this is known as heliotropism
.
, transmittance
and absorption
of solar light in the vegetation layer, and thus also on its growth and development. LAD can also serve as a quantitative index to monitor the state of the plants, as wilting
usually results in more erectophile LADs. Models
of radiation
transfer need to take this distribution into account to predict, for instance, the albedo
or the productivity of the canopy.
or air turbulence
. Nevertheless, when these environmental conditions are suitable or can be controlled, it is possible to acquire data on leaf orientation.
This may be done, for instance, with a Spatial Coordinate Apparatus, which is an articulated mechanical device capable or recording the position in three-dimensional space of three separate points forming a small triangle. The orientation of the triangle is computed from these coordinates.
Yet another approach is to use a laser scanner
: this instrument can record the angular and distance coordinates of the intersection of a laser beam with objects within its range. The LAD of canopy leaves can be derived from such measurements.
Extensive data sets of LAD have been recorded, especially during intensive field campaigns, such as the First ISLSCP Field Experiment (FIFE) or the SAFARI 2000 field campaign. See the external links below for getting access to these data.
Orientation (geometry)
In geometry the orientation, angular position, or attitude of an object such as a line, plane or rigid body is part of the description of how it is placed in the space it is in....
of the leaves
Leaf
A leaf is an organ of a vascular plant, as defined in botanical terms, and in particular in plant morphology. Foliage is a mass noun that refers to leaves as a feature of plants....
in the vegetation
Vegetation
Vegetation is a general term for the plant life of a region; it refers to the ground cover provided by plants. It is a general term, without specific reference to particular taxa, life forms, structure, spatial extent, or any other specific botanical or geographic characteristics. It is broader...
. Specifically, if each leaf is conceptually represented by a small flat plate, its orientation can be described with the zenith
Zenith
The zenith is an imaginary point directly "above" a particular location, on the imaginary celestial sphere. "Above" means in the vertical direction opposite to the apparent gravitational force at that location. The opposite direction, i.e...
and the azimuth angles of the surface normal
Surface normal
A surface normal, or simply normal, to a flat surface is a vector that is perpendicular to that surface. A normal to a non-flat surface at a point P on the surface is a vector perpendicular to the tangent plane to that surface at P. The word "normal" is also used as an adjective: a line normal to a...
to that plate. If the leaf has a complex structure and is not flat, it may be necessary to approximate the actual leaf by a set of small plates, in which case there may be a number of leaf normals and associated angles. The LAD describes the statistical distribution of these angles.
Examples of Leaf Angle Distributions
Different plant canopies exhibit different LADs: For instance, grasses and willows have their leaves largely hanging vertically (such plants are said to have an erectophile LAD), while oaks tend to maintain their leaves more or less horizontally (these species are known as having a planophile LAD). In some tree species, leaves near the top of the canopyCanopy (forest)
In biology, the canopy is the aboveground portion of a plant community or crop, formed by plant crowns.For forests, canopy also refers to the upper layer or habitat zone, formed by mature tree crowns and including other biological organisms .Sometimes the term canopy is used to refer to the extent...
follow an erectophile LAD while those at the bottom of the canopy are more planophile. This may be interpreted as a strategy by that plant species to maximize exposure to light, an important constraint to growth and development. Yet other species (notably sunflower
Sunflower
Sunflower is an annual plant native to the Americas. It possesses a large inflorescence . The sunflower got its name from its huge, fiery blooms, whose shape and image is often used to depict the sun. The sunflower has a rough, hairy stem, broad, coarsely toothed, rough leaves and circular heads...
) are capable of reorienting their leaves throughout the day to optimize exposure to the Sun: this is known as heliotropism
Heliotropism
Heliotropism is the diurnal motion of plant parts in response to the direction of the sun.It is found in some members of family Malvacea e.g Malva or Lavetara...
.
Importance of LAD
The LAD of a plant canopy has a significant impact on the reflectanceReflection (physics)
Reflection is the change in direction of a wavefront at an interface between two differentmedia so that the wavefront returns into the medium from which it originated. Common examples include the reflection of light, sound and water waves...
, transmittance
Transmittance
In optics and spectroscopy, transmittance is the fraction of incident light at a specified wavelength that passes through a sample. A related term is absorptance, or absorption factor, which is the fraction of radiation absorbed by a sample at a specified wavelength...
and absorption
Absorption (electromagnetic radiation)
In physics, absorption of electromagnetic radiation is the way by which the energy of a photon is taken up by matter, typically the electrons of an atom. Thus, the electromagnetic energy is transformed to other forms of energy for example, to heat. The absorption of light during wave propagation is...
of solar light in the vegetation layer, and thus also on its growth and development. LAD can also serve as a quantitative index to monitor the state of the plants, as wilting
Wilting
Wilting refers to the loss of rigidity of non-woody parts of plants. This occurs when the turgor pressure in non-lignified plant cells falls towards zero, as a result of diminished water in the cells...
usually results in more erectophile LADs. Models
Scientific modelling
Scientific modelling is the process of generating abstract, conceptual, graphical and/or mathematical models. Science offers a growing collection of methods, techniques and theory about all kinds of specialized scientific modelling...
of radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
transfer need to take this distribution into account to predict, for instance, the albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
or the productivity of the canopy.
Measuring LAD
Accurately measuring the statistical properties of leaf angle distributions is not a trivial matter, especially for small leaves. Clinometers can be used but may be rather bulky or inconvenient. Most leaves are quite light and tend to move with the slightest breezeWind
Wind is the flow of gases on a large scale. On Earth, wind consists of the bulk movement of air. In outer space, solar wind is the movement of gases or charged particles from the sun through space, while planetary wind is the outgassing of light chemical elements from a planet's atmosphere into space...
or air turbulence
Turbulence
In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic and stochastic property changes. This includes low momentum diffusion, high momentum convection, and rapid variation of pressure and velocity in space and time...
. Nevertheless, when these environmental conditions are suitable or can be controlled, it is possible to acquire data on leaf orientation.
This may be done, for instance, with a Spatial Coordinate Apparatus, which is an articulated mechanical device capable or recording the position in three-dimensional space of three separate points forming a small triangle. The orientation of the triangle is computed from these coordinates.
Yet another approach is to use a laser scanner
3D scanner
A 3D scanner is a device that analyzes a real-world object or environment to collect data on its shape and possibly its appearance . The collected data can then be used to construct digital, three dimensional models....
: this instrument can record the angular and distance coordinates of the intersection of a laser beam with objects within its range. The LAD of canopy leaves can be derived from such measurements.
Extensive data sets of LAD have been recorded, especially during intensive field campaigns, such as the First ISLSCP Field Experiment (FIFE) or the SAFARI 2000 field campaign. See the external links below for getting access to these data.
LAD functions
In general, LAD are modelled with one- or two-parameters functions including ellipsoidal, rotated-ellipsoidal and Beta functions. Comparison between different LAD functions with in-situ measurements show that, two-parameter functions (especially Beta function) may perform better than one-parameter functions.External links
- http://www.daac.ornl.gov/FIFE/Datasets/Vegetation/Leaf_Angle_Data.html
- http://www.daac.ornl.gov/S2K/guides/kt_canopy_structure.html