Canopy architectural properties estimation using digital hemispherical photography (DHP)

1. Canopy architectural properties estimation using digital hemispherical photography (DHP) Rende (Cs), IT, October, 4th to 7th , 2011 Chianucci Francesco* , Cutini Andrea (francesco.chianucci@entecra.it) • Materials and methods • The study involved 10 permanent plots of deciduous coppice stands in conversion into high forest under different silvicultural treatments established in previous studies (Cutini et al. 1998, Cutini et al. 2010). • In summer 2011, data were collected from 15 grid points inside each plots. • DHP were taken at late noon using Nikon Coolpix 4500 with the FC- lens converter. Camera was set up following Pekin and MacFarlane(2009). • Images were analysed using GLA- Gap Light Analyzer (Fraser et al. 1999) and WinSCANOPY (Regent Instruments, Ste-Foy, Quebec) software. • The output variables were LAI, canopy openness, clumping and canopy transmitted light. • LAI estimates were made with the PCA – LAI2000 (Licor – NE, USA) at the same time and location. • It represents a widely used instrument due to its ability to provide immediate LAI estimates, even if it is restricted by general tendency towards underestimating LAI (Cutini et al. 1998). • Canopy transmitted light was measured by means of Accupar ceptometer (Decagon, WA–USA) in midday at the same grid points. • The outputs variables from DHP were compared with the other two methods by statistical means across the full sample and significance of correlation coefficient (Pearson’s r). • Goal • The aim of this research is: • Compare light estimates and canopy structural properties from digital hemispherical photography (DHP) with indirect methods. • In addition, we test the possibility of using DHP as a tool for evaluating the effect of different silvicultural treatments on canopy properties. • Background • Studies on properties of canopy are important in ecological research. • Nevertheless, is very difficult and time consuming to measure directly canopy architectural properties. • On the other hand, indirect methods using technical instrumentations such as PCA-LAI 2000 or Ceptometers are very expensive, thus reducing the use of these methods on a wide-scale. • Hemispherical photography is the cheapest alternative to the other indirect methods. • It provides indirect estimation of forest light environment and structural parameters such as canopy openness, LAI and canopy light transmittance. FIGURE 2 – Averaged LAI estimates (± SE) from PCA and Digital Hemispherical Photography – NC: no corrections (5 rings). C: corrections for clumping • Results and conclusions • The studied stands presented differences owing to silvicultural treatments. • Averaged PCA-LAI2000 estimates range from 2.54 to 4.28 m2m-2 in thinned stands while was 2.89 to 4.99 m2m-2 in unthinned stand using 5 rings values. Thinning was effective in reducing LAI (Figure 1). • However, differences between the two data sets were not high (roughly -20% in thinned stands), even though significant using GLA software outputs (p<0.05). • Average LAI values from photography were higher than those from PCA using Winscanopy Software (+7-21%), in particular when corrections for clumping (C) were applied (Figure 2). On the contrary, LAI estimates from DHP are similar or smaller than those from PCA using GLA software. Latter software results more appropriate for light microclimate assessment. • Average transmitted light (%) values were 4.49% using Ceptometer while were 6.56% using DHP. • Overall, correlations between photography and the other two methods are satisfactory (Table 1). • The main drawbacks related to DHP are the perceived sensitivity to exposure and gamma correction (Figure 3 – data not published). • With the release of higher quality camera such as Digital single lens reflex (DSLR) and other different photography techniques (Pekin and Macfarlane 2009) these constraints could be reduced, thus improving the accuracy of photograph method. • It may result in a lower underestimation comparing with LAI 2000 (Cutini et al. 1998), but comparisons with direct reference measurements is strongly needed to demonstrate accuracy of the photograph method. • Also, latter technique needs to be tested in forest stand with medium-high LAI values (>6m2m-2). FIGURE 1 – Averaged LAI estimates (± SE) from PCA and Digital Hemispherical Photography for thinned and unthinned stands TABLE 1 – Correlation coefficient (Pearsons’ r) and significance of DHP values versus PCA and Accupar estimates. NC: no corrections; LX: corrections for clumping. ***= p<0.001; **= p<0.01; *= p<0.05 Acknowledgements The study has been performed for the XXV PHD degree in Forest Ecology. Field work by Tessa Giannini, Giovanni Vagnoli, Giovanni Venturi is greatly acknowledged. FIGURE 3 – Variations of LAI estimates with exposure. 0: exposure metered in clearing References Cutini A., Matteucci G., Scarascia Mugnozza G. 1998 – Estimation of leaf area index with the Li-Cor LAI 2000 in deciduos forests. For. Ecol. Man. 105: 55-65 Cutini A., Chianucci F., Giannini T. 2010 – Effetti del trattamento selvicolturale su caratteristiche della copertura, produzione di lettiera e di seme in cedui di faggio in conversione. Ann. CRA – SEL 36: 109-124. Fraser GW., Canham CD., Lertzman KP 1999 – Gap Light Analyzer (GLA): Imaging software to extract canopy structure and gap light transmission indices from true-color fisheye photographs, user’s manual and program documentation. Simon Fraser Univ., Barnaby, BC. Pekin B., Macfarlane C. 2009 – Measurement of Crown Cover and Leaf Area Index using Digital Cover Photography and Its application to Remote Sensing. Remote sensing 1: 1298 – 1320