Allometry and evaluation of in situ optical LAI ... - Tree Physiology

More documents

Recommendations

Info

726 JONCKHEERE, MUYS AND COPPIN Table 4. Regression models used to establish allometric equations in the experimental Scots pine stand in Pijnven Forest, Hechtel-Eksel, Belgium. Regression Equation Logarithmic lny = a + bln x (10) Gaussian −b( x−c) y = ae 2 (11) Log-normal 2 ( −(ln( b−x) −c) / 4) y = a/( b − x) e (12) Rayleigh x b c y = a −e − ( / ) ( 1 ) (13) Power y ax b = (14) Polynomial 2 y = ax + bx+ c (15) Linear y = ax+ b (16) 7, 23, 38, 53 and 68°. A built-in optical filter blocked incoming radiation with wavelengths above 490 nm, to ensure maximum contrast between leaf and sky. A reference measurement was taken in an open area directly before and after the point measurements to estimate above-canopy irradiance. A 90° view lens cap blocked the investigator from the field of view. Leaf area index was automatically calculated by the built-in C2000 software (Li-Cor 1992), based on inversion of the Poisson light extinction model for comparing transmittances. The extinction model is based on four assumptions: (1) foliage is an optical black body that absorbs all the light it receives; (2) light-blocking plant elements are randomly distributed in the canopy; (3) plant elements have the same projection as simple geometrical convex shapes; and (4) plant elements are small compared to the area covered by each ring. Additional information on the operational theory of the instrument has been given by Welles and Norman (1991) and Jonckheere et al. (2004). Measurements with TRAC The TRAC instrument measures the gap fraction at the solar zenith angle. Specifically, TRAC measures the photosynthetic photon flux (PPF) through a canopy (Chen and Cihlar 1995). The instrument is carried by a person walking at a rate of about 0.3 m s –1 , and accounts for both canopy gap fraction and canopy gap size distribution. The measured gap fraction is used to estimate effective LAI (LAI e), and the gap size distribution is used to estimate the element-clumping index (Ωe). The value of Ωe, which quantifies the effects of nonrandom spatial distribution of foliage, was obtained by comparing the measured gap size distribution with a theoretical gap size distribution associated with a canopy having randomly distributed foliage elements (Chen and Cihlar 1995). The value of Ωe depends on the leaf inclination angle (G-function), which in turn varies with the view angle. However, for a view angle of 57.5°, G ≅ 0.5 and can be considered independent of leaf inclination. Therefore, by keeping the solar radiation angle constant at 57.5°, Ωe and LAI e values for the stand can be obtained with TRAC alone (Leblanc et al. 2002). Measuring LAI by hemispherical photography To estimate LAI by digital hemispherical photography, images of the canopy are acquired from below through a hemispherical (fisheye) lens with a 180° field of view. Leaf area index is computed from TREE PHYSIOLOGY VOLUME 25, 2005 the hemispherical photographs from gap fraction estimates in different zenithal and azimuthal ranges. We used a Kodak DCS 660 digital camera (Eastman Kodak, New York, NY) with an 8-mm fisheye lens (8 mm, f/4, Sigma, Tokyo, Japan). The camera and lens were placed in a self-leveling mount oriented by corner pins on a tripod. The top of the lens was 1.3 m above ground and the camera was oriented such that the magnetic north was always located at the top of the photographs. Photographs were taken during overcast conditions to minimize glare from direct sunlight. For every grid point, six photographs were taken with an aperture of f/8 at five shutter speeds (1/60, 1/125, 1/250, 1/500 and 1/1000 s) and at the highest possible resolution, i.e., 3040 × 2008 pixels. The photograph resulting in the best contrast between sky and canopy was selected visually from these bracketed exposures. A total of 30 photographs provided input for analysis by the Gap Light Analyzer (GLA) software which computes gap fraction data and LAI (Frazer et al. 1999, Frazer et al. 2001). First, a threshold value was interactively selected for each photograph to distinguish between visible sky and foliage, thereby converting the RGB color images to black and white. The border can be difficult to estimate because of the penumbral effect; therefore, all photographs were analyzed by the same experienced operator to reduce variation (Beaudet and Messier 2002). To compare LAI estimates from the hemispherical photography technique with the LAI-2000 results, gap fractions from the photographs were similarly separated in five zenith rings from 0–75°. The five zenith rings were divided into sky sectors by 10° azimuth angles (36 azimuth sectors). In a second step, LAI e was calculated for each photograph from the gap fraction data obtained by inversion of the Poisson model using 4 and 5 rings, representing 0–60° and 0–75°, respectively (Stenberg et al. 1994). The software takes terrain corrections, based on local aspect inputs and lens corrections, into account. As input to the GLA program, data for the terrain corrections were gathered in the field and data for the lens correction were supplied by Sigma. We then converted LAI e to corrected LAI by correcting for clumping and nonleafy material. Foliage clumping Estimates of LAI acquired by optical techniques are based on light interception by both foliage and stems, and assume a random foliage distribution (Chen and Cihlar 1995). The resulting index is therefore considered to be the effective leaf area index (LAI e) (Chen et al. 1991). Consequently, when converting LAI e to LAI it is necessary to correct both for light obstruction from non-leafy canopy material and for the effects of foliage clumping. Foliage clumping occurs mostly at the shoot level in conifers (Stenberg et al. 1994). Foliage clumping also occurs at the branch and crown levels for most forest types (Chen and Black 1992), as a result of branching patterns and irregular tree distribution (Chen et al. 1997). We applied several corrections to investigate the influence of clumping at the shoot, branch and tree levels on the conversion of LAI e measured optically to corrected LAI. Shoot-level clumping We corrected for clumping at the shoot level based on the method of Gower and Norman (1991), which Downloaded from http://treephys.oxfordjournals.org/ by guest on December 6, 2012
determines shoot clumping as the ratio of the projected area of the needles within the shoot to the vertically projected shoot area. This technique was further developed by Fassnacht et al. (1994) and corrected by Chen (1996) to include a greater number of shoot projections for the calculation of needle-to-shootarea ratio, γe. The shoot area is interpreted as the imaginary surface area of a sphere enveloping the leaf-clump and is calculated as a weighted mean ratio of the projected shoot silhouette area (Ap) for different projection angles. Chen (1996) found good agreement between results of shoot analysis for three projection (camera incidence) angles and shoot analysis for 21 and 39 projection angles, respectively, and thus recommended the use of the method by which each shoot from a calibration sample is imaged and analyzed from three camera incidence angles to estimate the correction parameter (Chen 1996): with: LAIc = LAIeγ e (1) γ e = An/ As (2) A s (0 ° , 0 ° )cos (15 ° ) (45 ° ,0 ° )cos (45 ° ) (90 ° , 0 ° )cos(75 ° ) A p + A p + A p = 2 cos(15 ° ) + cos(45 ° ) + cos(75 ° ) where LAI c is LAI corrected for shoot-level clumping, LAI e is effective LAI as estimated by the optical devices, γe is the ratio of needle-to-shoot area, An is half the total needle area for all the needles of a shoot, As is half the total shoot imaginary surface area, and Ap (θ, φ) is the projected area for given angles of θ and φ, where θ and φ are the azimuth and zenith projection angles in relation to the main axis of the shoot, respectively. Ten intact shoot samples were taken from each model tree and different model branches to obtain the within-shoot clumping factor. Half the total needle area in the shoot (An) was measured as the projected area of needles multiplied by a correction factor dependent on the hemi-cylindrical shape of the needles. Projected area for the needles of the shoots was determined with a Li-Cor planimeter, and the correction factor of 1+π/2 (Bond-Lamberty et al. 2003) was applied. The imaginary shoot area was obtained from the projected shoot area (Ap) at three viewing angles (0, 45 and 90°) with a Kodak DCS 660 digital camera. The corresponding images were digitally analyzed according to the methodology developed by Chen (1996) to assess the shoot silhouette area. Mean γe (= 2.001) was calculated as the arithmetic mean of the shoot samples. Stand-level clumping To correct for clumping within a stand at all scales greater than the shoot, including within-crown clumping, Ωe was obtained from TRAC (Leblanc 2002) as: Ω e ⎛ ( Fm − Fmr) ⎞ lnF = ⎜1 + ⎟ ⎝ 1− F ⎠ lnF m ALLOMETRY AND EVALUATION OF OPTICAL LAI DETERMINATION 727 where Fm is the measured total canopy gap fraction and Fmr is the gap fraction of an imaginary canopy with the same LAI as the clumped canopy, but where the foliage elements are considered spatially random. We measured Fm with TRAC along transects in the stand on October 7, 2002, (see Figure 1 for m mr (3) (4) sampling strategy) as the transmittance of direct or diffused radiation at the zenith angle of 57.5°. We derived Fmr from the measured gap size distribution by a gap removal approach. All transects were analyzed with the TRACWin software provided with the instrument. The calculated value for Ωe was 0.836. We calculated LAIB as (Chen and Cihlar 1995): LAI = LAI / Ω (5) B e e where LAI B is LAI corrected for branch- and tree-level clumping and LAI e is the effective LAI as estimated by the optical devices. Plant area index To correct for clumping at the within-shoot and above-shoot levels and account for the contribution of non-photosynthetic components of the canopy in the optical measurements, we calculated the LAI associated specifically with foliage (LAIF) as did Chen et al. (1997): LAI F ( 1 − α) LAI γ = Ω e e e where LAI F is LAI corrected for within-shoot and above-shoot level clumping and α is the woody-to-total area ratio. The value of α was derived from destructive sampling (Chen et al. 1997): α = W/LAItotal, where W is the woody area index and LAItotal is the total LAI from woody and green foliar material combined. Our calculation of α was based on in situ measurements of foliage and woody area per tree, derived from destructive measurements of the branches and measured total tree height, bole length, crown dimensions and crown length, and plotted against DBH. We applied these relationships to determine the mean α of the whole stand and obtained a value of 0.18, which is consistent with the analysis of the digital hemispherical images, where the amount of woody material was estimated by means of image classification, assuming the stems and branches seen on the photographs were simple cone shapes (Barclay et al. 2000). Results TREE PHYSIOLOGY ONLINE at http://heronpublishing.com Allometric relationships Allometric relationships at the branch and tree levels Regression equations at the branch and tree levels were derived from measurements of the six experimental trees (Table 5). The relationships in Table 5 are the best fits of the fitted regressions and were in all cases significant at P < 0.001. They were applied in this study to calculate canopy cover and PLA at the stand level (Table 5). Allometric relationships between tree height and bole length and between tree height and tree diameter were established by means of the Rayleigh equation (Table 6). At the branch level, a good relationship was found between branch cross-sectional area and needle leaf area (as well as needle dry mass) (r 2 ≥ 0.80) (Figure 2). In agreement with other studies (e.g., Mencuccini and Grace 1995), significant regression relationships were found at the tree level between basal area and DBH as independent variables, and needle area, (6) Downloaded from http://treephys.oxfordjournals.org/ by guest on December 6, 2012
Page 1 and 2: Tree Physiology 25, 723-732 © 2005
Page 3: ALLOMETRY AND EVALUATION OF OPTICAL
Page 7 and 8: et al. 1997), was calculated from P
Page 9 and 10: the LAI values estimated from direc

Allometry and evaluation of in situ optical LAI ... - Tree Physiology

Create successful ePaper yourself

Delete template?

Save as template?