Canopy structural and biophysical parameters

Forest canopy structure, and seasonal dynamics of foliage growth, encompasses far more than solely the size and shape of the canopy over time. The spatial arrangement of stems, branches, and leaves, dictates the nature of the exchange surface including how much light can penetrate through gaps in the architecture (Pretzsch, 1997). Due to the structurally complex nature of forest canopies a number of biophysical variables have been developed to quantify and describe particular canopy elements – facilitating comparisons of one canopy to another. Estimates of biomass (that of standing wood and the seasonally

dynamic foliage elements) are important inputs in carbon studies as well as a wide variety of other applications, however there are numerous other structural parameters that can also be quantitatively measured to give an insight into canopy structure, providing indicators of how open or dense the canopy is, or the spatial distribution of branch or leaf biomass. Structural and biophysical properties of vegetation, such as LAI, gap fraction, foliage profiles, clumping index, and leaf angle distributions are significant parameters related to ecosystem structure and function (Jonckheere et al., 2004; Danson et al., 2007; Jupp et al., 2009). These properties can be used to describe vegetation dynamics over time with repeated measurements.

Leaf area is highly correlated with vegetation productivity (Webb et al., 1983). LAI is a biological parameter that defines the area that interacts with solar radiation and carbon dioxide exchange with the atmosphere (Kozlowski et al., 1991; Clawges et al., 2007) and is therefore a primary factor controlling the optical properties and light transmission of the

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canopy (Kozlowski et al., 1991). This information is essential for modelling and simulation of ecological and hydrological processes. LAI was first defined by Watson (1947) as the total one-sided area of photosynthetic tissue per unit ground surface area, but the definition has more recently been updated to the maximum projected leaf area per unit ground surface area (Jonckheere et al., 2004), to take into consideration the irregular form of needles and leaves. LAI is a dimensionless quantity and therefore independent of scale, appropriate to a canopy that forms a continuous and homogeneous layer. The LAI of forest stands varies widely depending on species, age, and site condition. In temperate deciduous forests LAI is typically in the region of 3-6 and temperate evergreen coniferous forests may reach an LAI of 11 or 12 (Kozlowski et al., 1991). Seasonal LAI changes reflect phenological events such as leaf unfolding, expansion and senescence (Hertel et al., 2012). The vertical distribution and availability of light is one of the most important factors for production of biomass (Hertel et al., 2012), and is quantified by the Foliage Area Volume Density (FAVD), also called leaf area density, defined as the total one-sided leaf area per unit of layer volume (Wilson, 2011). FAVD is a vertical profile of LAI where the integral of the FAVD gives the LAI (Morsdorf et al., 2006).

LAI can be estimated indirectly via the gap fraction; the probability of a ray of light passing the canopy without encountering any plant elements (Danson et al., 2007). Gap fraction is a key structural parameter in its own right which offers a measure of how open or dense a canopy is; gap fraction equals zero where there is a complete gap, and equals one when there is plant element present. The pattern of light absorption through a canopy often matches the logarithmic expectation of Beer-Lambert Law and has therefore been adapted for vegetation canopies (Monsi & Saeki, 1953). Beer-Lambert Law relates the attenuation of light to the properties of material through which the beam is travelling; absorbance is related to the concentration of the solution, the length of the light path through the solution, and an extinction coefficient. For vegetation canopies, a layer of leaves tends to absorb a fraction of incident light (depending on structural and reflectance factors). Light that passes through the first layer of leaves may be partially absorbed by the second layer, and so on. If each layer of leaves absorbs the same proportion of incident light, the resultant curve of light absorption as a function of leaf area will have logarithmic form. Under the assumption that gap fraction (Pgap) is equivalent to transmittance (T), Beer-Lambert Law can be adapted as follows:

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(2.1)

where is the transmitted light intensity after travelling path length through the canopy, is the incident light intensity and is the extinction coefficient, which is the fraction of foliage area projected onto a perpendicular plane (‘G-function’; Ross, 1981), and is the leaf area index.

Equation 2.1 then is inverted to solve for LAI. is usually approximated to 0.5 to assume a spherical leaf distribution (Martens et al., 1993). However in practise the extinction coefficient has been found to vary with solar elevation, canopy structure, and ratio of direct to diffuse beam irradiance and therefore generally varies between 0.25 and 0.75 (Jarvis & Leverenz, 1983). The main assumptions of this approach are that the foliage elements are randomly dispersed, the canopy is homogeneous, and the canopy elements exhibit isotropic canopy radiation (equal transmittance in all directions) (Hopkinson & Chasmer, 2007). However, rather than being randomly located, canopy elements are generally clumped in natural forest stands. This non-random arrangement of foliage is particularly common among many conifer species and can be described by the Clumping Index (Ω) (Walter et al., 2003). Clustering of canopy elements increases the transmittance of light and leads to underestimation of LAI, therefore the term ‘effective LAI’ is usually used (Clawges et al., 2007). To produce the ‘true LAI’, the effective LAI derived from the gap fraction inversion is combined with the clumping index where a Ω value near to zero indicates a very high clumped canopy whereas a value of one means a random spatial distribution (Chen & Cihlar, 1995).

2.4.1 Direct methods of measuring vegetation canopy structure

Direct or semi-direct measurement techniques for measuring vegetation structure are those made directly on canopy elements, such as stratified clipping (Hosoi & Omasa, 2009) or point-quadrant sampling (Warren-Wilson, 1959), and have been widely used for

quantifying biophysical parameters in forests. One method of direct measurement is the felling and stripping of leaves of a single tree. After leaf collection, leaf area and dry mass is measured and the area divided by the measured mass to give an estimate of Specific Leaf Area, the projected area of fresh leaf per gram of dry leaf (SLA, cm2 g-1), a measure of leaf

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thickness (Clawges et al., 2007). The remainder of the leaves are dried and the total dry mass is multiplied by the average SLA to give a value of LAI (Breda, 2003). This method can also be used to obtain a vertical leaf area density profile of the foliage of a tree based on harvesting plant elements within defined height levels (Clawges et al., 2007). Light, and therefore SLA, scales with height in the canopy because sun-adapted foliage (compared with lower shady leaves) shows significantly higher photosynthetic activity (Marek et al., 1989).

These approaches are referred to as ‘destructive’ methods, as the plants being measured are damaged in some way as forest measurements are derived by active collection of plant material (Jonckheere et al., 2004). For this reason these methods clearly do not facilitate multi-temporal studies. The use of litterfall traps is a non-destructive direct method of calculating LAI of deciduous trees. This method collects leaves during leaf fall in traps of certain area distributed below the canopy and the area of collected leaves is measured and LAI determined by weight (Finotti et al., 2003). However, this method measures

cumulative LAI rather than being an accurate measure at a single time, making it unsuitable for multi-temporal studies.

For evergreen species, a destructive sampling or allometric equations approach needs to be adopted. Direct methods derive information of high accuracy and therefore are often used as reference measurements to validate indirect estimates, such as remote sensing

approaches (Baker et al., 2010). In fact, they are the only methods that measure ‘true’ LAI and associated variables as they directly separate the foliage. However, whilst direct methods are accurate for a specific location, they are inherently time consuming, labour intensive, and expensive, and therefore impractical for a wider level analysis (Lichti et al., 2002; Gibbs et al., 2007). Furthermore, the destructive nature of these approaches mean that repeated readings are impossible, to assess seasonal change, for instance (Houghton et al., 2009). With direct methods, it is not always practical, or even possible, to measure biomass by cutting down trees, especially in an urban setting or protected area.

2.4.2 Indirect methods of measuring vegetation structure

Indirect methods collect information from the canopy without coming into physical contact with its elements. Based on radiative transfer theory, indirect methods infer LAI from the measurements of light transmission through the canopy (Ross, 1981; Breda, 2003). This is the case for specially designed instruments such as the LAI-2000 Plant Canopy Analyser

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(Licor Inc., Nebraska) which measures diffuse radiation by means of a fisheye light sensor for five zenith angles (Jonckheere et al., 2004). This instrument has been used to estimate LAI in coniferous (Gower & Norman, 1991) and deciduous stands (Cutini et al., 1998). The Tracing Radiation and Architecture of Canopies (TRAC) (3rd Wave Engineering, Ontario, Canada) is a hand-held instrument which collects information on the gap size distribution of a canopy. This quantifies the effect of non-random spatial distribution of foliage (Ω), which can be used to convert effective LAI to true LAI (Jonckheere et al., 2004). Although the LAI-2000 and TRAC are both portable and non-destructive, they rely on a number of assumptions and in fact measure Plant Area Index (PAI) as they cannot distinguish between leaves and woody material. They are also unable to retain a permanent record of the canopy structure. Other indirect optical methods for characterising vegetation structure such as satellite remote sensing, hemispherical photography, and terrestrial laser scanning, are outlined next.