Light Extinction in Terrestrial Ecosystems

Light extinction in terrestrial ecosystems is a matter of great interest, as availability of and competition for light can strongly influence plant composition, structure, and growth, and consequently shape ecosystem characteristics.

In a forest, for example, incoming visible light is mostly attenuated by the upper layer formed by leaves and branches belonging to tall trees (canopy). The underlying, shaded understory and undergrowth vegetation is impacted, and so are the functions it holds, like supporting biodiversity, providing habitat to several animals, and nutrient cycling.

Study of light interception and absorption by vegetation is also connected with activities like farming and silviculture, since optimal plant yield is related to the ability of light to penetrate crop (e.g., in narrow rows) and make it grow fast and efficiently. A quantitative knowledge of light extinction can be important in order to understand and model crop productivity or linked phenomena like water balance.

Light can penetrate through canopies (this term defines also the upper layer of crop) and reach shaded lower plants and the ground by direct illumination (e.g., through gaps in foliage and canopy), transmission through leaves (with an intensity lower than the incoming one), and reflection by leaves or ground of incoming direct and diffuse radiation. Also, some light is absorbed (e.g., by leaves) or reflected away (Figure 2).

Given the many and different terrestrial ecosystems influenced by light extinction and their big variability even on short time (seasons) and space scales, it is difficult to develop generally applicable models. One of the commonest models used to describe light transmission through a canopy is a modified Lambert-Beer law, obtained assuming a homogeneous (with respect to attenuation) canopy with small, randomly dispersed leaves:

where /in is the incoming PPDF (on the top of the canopy, where canopy depth z = 0), /z is available PPDF at canopy depth z and LAI(z) is the cumulated 'leaf area index' from the top of the canopy to depth z. The leaf area index is the ratio of the total one-sided green leaf surface to the surface of the ground underneath the canopy (or the projected needle area per unit of ground surface). Values are dimen-sionless and range from 0.2-2.5 (tundra) up to 10 (tropical forest) with higher values for conifers. LAI field measures can be achieved by destructive sampling or by litter traps, allometric relationships (e.g., linking LAI to tree diameter or height) and light interception methods (e.g., calculating LAI from the inversion of Lambert-Beer relationship or other models, based on measurements of light transmission through canopy). The extinction coefficient k is species or canopy related and can span from 0.3 to 2.0. Plants with vertically oriented leaves usually have lower values (e.g., for cereals, k < 0.5).

Even if the above relationship linking canopy density and structure to light transmission has showed to be relatively robust with respect to violations of its underlying, unrealistic assumptions, many more complex models describing light attenuation in forest canopies or in single plants can be found in literature to account for the fact that, for example, many plants do not have a homogeneous structure with respect to radiation (e.g., leaves are not randomly spatially distributed or they are of a finite size).

These models can be based on aggregate parameters related to canopy structure, like LAI, leaf orientation, and average leaf area density (m_1, total leaf upper surface per unit of volume), can explicitly account for factors like sun position, foliage clumping and distribution, ground and

leaf optical properties or diffuse radiation, or can be based on more elaborate approaches, like 3D vegetation models. Also, importance of direct evaluation of light extinction by field measurements (e.g., at several canopy depths) is increasing. Data can be collected using several methods, like hemispherical photography, and are also used for obtaining information about canopy structure and for existing model calibration and input data collection.

See also: Allometric Principles; Autotrophs; Forest Models; Leaf Area Index; Plant Growth Models; Radiation Balance and Solar Radiation Spectrum; Tropical Rainforest.

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