Figure 12.25 A fish-eye view of tropical forest in Cameroon, West Africa, showing species diversity and canopy layering. (Photograph courtesy D. Eamus)
12.3 - Forest production and light
Tree canopies cast shadows, especially on clear days, indicating absorption of radiant energy. Averaged over space and time, the photosynthetically active component of that energy is efficiently employed and drives growth. Acquisition of energy and carbon by plants is thus determined by total leaf area, leaf surface distribution within the canopy and photosynthetic capacity of individual leaves. Productivity will ultimately depend on distribution of photosynthetic performance throughout the canopy as a whole, which in turn, is determined by the photosynthetic capacity of individual leaves and the distribution of sunlight.
Plant canopies are structurally diverse because of unique spatial patterns that different species adopt for intercepting light and the 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 12.25).
Experiments were in progress at this site in Cameroon on microclimate responses 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. Hemispherical photographs were used to calculate change in canopy cover and solar radiation load on different plots (shown here). Hemispherical photographs can also be used to calculate potential contribution from sunflecks by plotting the sun's path across a photograph.
The first reason for such dissimilarities is 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 to 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.
12.3.1 - Canopy architecture and light interception
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 of light 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 decline exponentially with increasing intercepting area down through the canopy. Absorption of sunlight by photosynthesis occurs within a well-defined spectral band (400–700 nm) and matches a peak in energy distribution across the wavelength spectrum of sunlight transmitted to the earth’s surface through our atmospheric window (as shown in Figure 12.1 at the start of the chapter).
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, PAR), 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 of the canopy) 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:
\[ \mathrm{d} Q / \mathrm{d} F = -kQ_F \tag{12.2} \]
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, \(Q_F\) at any level \(F\) is given by:
\[ Q_F = Q_0 e^{–kF} \tag{12.3} \]
where \(Q_0\) 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.
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 with a higher \(k\) (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.
12.3.2 - Canopy productivity
Photosynthesis is driven by the fraction of radiation intercepted by the canopy and gross photosynthetic production (\(A_g\)), a measure of the total amount of CO2 fixed in photosynthesis, can be expressed as:
\[ A_g = A_0 [1 - exp(-kS_{a}W_{l})] \tag{12.4} \]
\(A_0\) is the gross photosynthetic production at full light interception and \(S_{a}W_{l}\) expresses LAI as the product of specific leaf area (\(S_{a}\), the ratio of leaf area:leaf dry mass) and dry mass of leaf organic matter (\(W_{l}\), often approximated as leaf dry mass).
Efficiency of light conversion to biomass (\( \varepsilon \))
As implied by Equation 12.4, there is a proportional relationship between production of dry mass and interception of radiation (see Fig. 12.22), while LAI is a major determinant of photosynthetic production. The slope of this relationship is a measure of the conversion efficiency (\( \varepsilon \)) of light (photon irradiance if based on PAR) or irradiance (\(I\) if based on shortwave radiation) to dry mass. \( \varepsilon \) has units of g MJ–1 and values based on photon irradiance are approximately twice those based on \(I\). \( \epsilon \) can be considered to be the canopy-scale equivalent of \( \varphi \), the quantum yield of individual leaves. Values of \( \varepsilon \) based on absorbed radiation are net of any light or radiation that is reflected upward from the direction in which the incident value is measured.
This proportional relationship was first clearly defined in the above terms for the seasonal growth of temperate agricultural and horticultural crops in Britain (Monteith 1977, Fig. 12.22). It has since been shown to hold for a range of vegetation types and environments. Proportionality occurs because photosynthesis by most leaves in a canopy tends to be light limited. Consequently any increase in light intercepted or absorbed results in an increase in dry mass production. As crops grow from establishment or plant communities develop from a state of initial colonisation to maturity, LAI increases, and a greater leaf surface results in greater levels of light interception and rates of growth.
12.3-Ch-Fig-12.26.png
Figure 12.26 Above-ground dry mass production and intercepted radiation for eucalypts in plantations in Tasmania and Victoria show a linear relationship. Closed symbols refer to different species or provenances of the same species at Tasmanian sites. Open symbols refer to Eucalyptus globulus growing in Victoria. (Original data C.L. Beadle and G. Inions)
As predicted by theory, linear relationships between above-ground dry mass production and intercepted radiation have been observed for eucalypts in plantation forests in southeast Australia: \( \varepsilon \) was around 0.45 g MJ–1 (based on \(I\), Figure 12.26). Conversion efficiency was independent of species and provenance within species, and for one species, Eucalyptus globulus, was independent of site. Differences in growth rate between species, in this case during the early phase of forest growth, can be entirely a function of more rapid development of LAI in one species compared to another.
Comparative analyses of \( \varepsilon \) for different vegetation types are commonly based on above-ground dry mass (\(W_{a}\)) because of lack of information on below-ground biomass (\(W_{b}\)). \( \varepsilon \) based on \(W_{a}\) will be less than that based on \( W_{a} + W_{b} \) by the ratio \(W_{a} : (W_{a} + W_{b}) \). Partitioning of dry mass to roots may be substantially higher on a resource-poor compared to a resource-rich site. Consequently, a comparison of \( \varepsilon \) between sites based on \(W_{a}\) rather than \( W_{a} + W_{b} \) would lead to a relative underestimate of the efficiency of conversion of light to dry mass on poorer sites. Similarly, stress in response to soil water deficit, high vapour pressure deficit (leading to stomatal closure) or extremes of temperature will reduce \( \varepsilon \) and may also change the partitioning of dry mass. Reductions in \( \varepsilon \) occur in response to stomatal closure or to stresses of sufficient severity to reduce the quantum yield of photosynthesis. In effect, plants reduce photosynthesis by redirecting absorbed energy away from photochemistry and into photoprotective pathways which disperse absorbed energy as heat. \( \varepsilon \) thus embodies the photosynthetic history of a crop over a given interval and integrates the effects of all environmental variables on photosynthetic utilisation of absorbed radiation.
In summary, forest canopies consist of multiple layers. A logarithmic gradient of sunlight availability exists from upper to lower layers, and leaf properties adjust accordingly.