12.4.1 Canopy architecture

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Plant canopies are structurally diverse due to unique spatial patterns that different species adopt for intercepting light and an even diversity of plant species which occupies a natural community. For example, there is considerable penetration of sunlight through the canopy of a dry eucalypt forest. Conversely in dense rainforest or in a radiata pine plantation, only sunflecks reach the ground. A considered glance from ground to tree top reveals why these dissimilarities occur (Figure 12.24).


Figure 12.24 A fish-eye view of tropical forest in Cameroon, West Africa, showing species diversity and canopy layering. Experiments were in progress at this site on microclimate response to forest management. Trees had been clear felled mechanically, clear felled manually or selectively cleared. Growth rate of newly planted saplings was measured in these plots and compared to growth in undisturbed plots (shown here). Hemispherical photographs were used to calculate change in canopy cover and solar radiation load on different plots. Hemispherical photographs can also be used to calculate potential contribution from sunflecks by plotting the sun's path across a photograph

(Photograph courtesy D. Eamus)

The first reason is simply based on canopy density or the quantity of leaf area per unit canopy volume. This index is substantially less for a dry eucalypt forest than that for a rainforest or pine forest. The second reason relates to the display of the foliage. Adult leaves of eucalypts are typically pendulous, allowing much of the incident light and energy to pass uninterrupted through the canopy and reach the ground. Conversely, in a diverse rainforest many species display their leaves at shallower angles to the horizontal thereby absorbing a larger proportion of incident radiation and preventing much of the incident light and energy being transmitted to the ground.

In a complex system like a rainforest the canopy is arranged in horizontal layers, the distribution of leaf area with height being associated with the development in space and time of the diversity of species. However, even in monocultures it is convenient to consider the canopy as being horizontally uniform and the level of radiation constant in any layer. Even here, attenuation of light through the canopy is complicated by changing availability, quality and direction of incident light and it is necessary to make some simplifying assumptions when calculating the proportion that is intercepted.

The Beer–Lambert Law, which describes absorption of light by plant pigments in solution, provides a simple approach which has been applied widely to a range of canopies. This function demonstrates that the absorption of light will be more or less exponential with increasing intercepting area down through the canopy. Absorption of sunlight by photosynthesis occurs within a well-defined spectral band (400–700nm) and matches a peak in energy distribution across the wavelength spectrum of sunlight transmitted to the earth’s surface through our atmospheric window (Figure 12.1).

Sunlight in this waveband can be represented as either a quantum flux or a radiant energy flux. Quantum flux, or more explicitly, photosynthetic photon flux density (PPFD), is simplified here to ‘photon irradiance’ (Q) and has units of µmol quanta m–2 s–1 (‘µmol quanta’ rather than ‘µmol photons’ because quantum energy derived from photons drives photosynthesis). For the sake of making a clear distinction, radiant energy flux is simplified to ‘irradiance’. In the present example, irradiance coincides with photosynthetically active radiation (PAR) and is expressed as joules (J) per square metre per unit time. Depending on the application, time can span seconds, days or years, and is then coupled with either joules, megajoules (MJ) or gigajoules (GJ).

On clear days, PAR represents about half of the total shortwave (solar) radiation or radiant energy flux I (expressed as J m–2 s–1) incident on a canopy, and is totally responsible for photosynthesis. If changes in the spectral distribution of energy as it passes through the canopy are ignored, I and PPFD can be used interchangeably in the analysis below. In practice PPFD is attenuated more rapidly than I (that is, there is a proportionally larger change in PAR than total solar radiation (I) in moving from top to bottom) because leaves are relatively transparent to the near-infrared part of the solar beam.

Application of the Beer–Lambert Law shows that at any level of cumulative area F within the canopy, the rate of change of photon irradiance, Q, within the canopy is given by:

dQ/dF = -kQF (12.5)

where k is the extinction or foliar absorption coefficient, a dimensionless parameter. k measures the fraction of incident photons absorbed by a unit of leaf area or conversely the fraction of leaf area projected onto the horizontal from the direction of the incident beam. For many species, foliage in the vertical plane is distributed approximately symmetrically about the midpoint of the canopy and most absorption of light will occur in the middle of the canopy. After integration, QF at any level F is given by:

QF = Q0ekF (12.6)

where Q0 is the PAR incident at the top of the canopy. At the base of the canopy F is equal to the leaf area index (LAI), a dimensionless number which expresses total projected leaf area of the canopy as a ratio of the ground area over which it is displayed. Thus the level of interceptance is an exponential function of the product kF. If a value of 0.5 is assigned to k, then 95% light interception occurs at kF = 3 which is equivalent to an LAI of 6 m2 leaf area m–2 ground area. Maximum values of LAI vary with species, site, stress and season (Section 6.4).

In practice k is not a constant value for any canopy and varies with solar elevation, the ratio of direct to diffuse beam irradiance and any changes in canopy structure or leaf inclination and orientation which occur seasonally or in response to the movement of leaves (e.g. heliotropism). For the majority of canopies, k varies from 0.3 to 1.3. Canopies with erectophile leaves (e.g. grasses) and high leaf angles to the horizontal or with a clumped distribution have a lower k and intercept less light per unit of foliage compared to canopies with planophile leaves (e.g. clovers) and low leaf angles or a regular distribution. The cumulative leaf area required to intercept 95% of the radiation incident at the top of the canopy will be greater for canopies dominated by erectophile leaves or having a clumped distribution. In many species and plant communities leaf inclination may change from erectophile at the top of the canopy to planophile at the bottom. This allows more even distribution and interception of light and reduces the proportion of leaves which is exposed at the top of a canopy to levels of light which are saturating for photosynthesis and, conversely, reduces the proportion of leaves at the bottom of a canopy which is exposed to levels below the light-compensation point for photosynthesis. For a canopy with leaves distributed randomly with respect to orientation and inclination, k is approximately 0.5 (Monteith and Unsworth 1990) and this value is commonly assigned to k in the literature.