Rapid assessment of forest canopy and light regime using smartphone hemispherical photography

Abstract Hemispherical photography (HP), implemented with cameras equipped with “fisheye” lenses, is a widely used method for describing forest canopies and light regimes. A promising technological advance is the availability of low‐cost fisheye lenses for smartphone cameras. However, smartphone camera sensors cannot record a full hemisphere. We investigate whether smartphone HP is a cheaper and faster but still adequate operational alternative to traditional cameras for describing forest canopies and light regimes. We collected hemispherical pictures with both smartphone and traditional cameras in 223 forest sample points, across different overstory species and canopy densities. The smartphone image acquisition followed a faster and simpler protocol than that for the traditional camera. We automatically thresholded all images. We processed the traditional camera images for Canopy Openness (CO) and Site Factor estimation. For smartphone images, we took two pictures with different orientations per point and used two processing protocols: (i) we estimated and averaged total canopy gap from the two single pictures, and (ii) merging the two pictures together, we formed images closer to full hemispheres and estimated from them CO and Site Factors. We compared the same parameters obtained from different cameras and estimated generalized linear mixed models (GLMMs) between them. Total canopy gap estimated from the first processing protocol for smartphone pictures was on average significantly higher than CO estimated from traditional camera images, although with a consistent bias. Canopy Openness and Site Factors estimated from merged smartphone pictures of the second processing protocol were on average significantly higher than those from traditional cameras images, although with relatively little absolute differences and scatter. Smartphone HP is an acceptable alternative to HP using traditional cameras, providing similar results with a faster and cheaper methodology. Smartphone outputs can be directly used as they are for ecological studies, or converted with specific models for a better comparison to traditional cameras.

[1]  A. Cutini,et al.  Digital hemispherical photography for estimating forest canopy properties: current controversies and opportunities , 2012 .

[2]  D. J. Fitter,et al.  STAND STRUCTURE AND LIGHT PENETRATION , 1980 .

[3]  L. Tichý.  Field test of canopy cover estimation by hemispherical photographs taken with a smartphone , 2016 .

[4]  A. Gonsamo,et al.  Measuring fractional forest canopy element cover and openness - definitions and methodologies revisited , 2013 .

[5]  Richard A. Fournier,et al.  Modelling light obstruction in three conifer forests using hemispherical photography and fine tree architecture , 1996 .

[6]  Instantaneous and potential radiation effect on underplanted European beech below Norway spruce canopy , 2012, European Journal of Forest Research.

[7]  Margaret C. Anderson.  Studies of the Woodland Light Climate: I. The Photographic Computation of Light Conditions , 1964 .

[8]  M. Symonds,et al.  A brief guide to model selection, multimodel inference and model averaging in behavioural ecology using Akaike’s information criterion , 2010, Behavioral Ecology and Sociobiology.

[9]  C. Messier,et al.  Effects of light and intraspecific competition on growth and crown morphology of two size classes of understory balsam fir saplings , 2001 .

[10]  T. W. Ridler,et al.  Picture thresholding using an iterative selection method. , 1978 .

[11]  T. Mitchell Aide,et al.  Soundscape analysis and acoustic monitoring document impacts of natural gas exploration on biodiversity in a tropical forest , 2017 .

[12]  Margaret C. Anderson.  Studies of the woodland light climate. II. Seasonal variation in the light climate , 1964 .

[13]  Alemu Gonsamo,et al.  Sampling gap fraction and size for estimating leaf area and clumping indices from hemispherical photographs , 2010 .

[14]  R Core Team,et al.  R: A language and environment for statistical computing. , 2014 .

[15]  Johannes E. Schindelin,et al.  Fiji: an open-source platform for biological-image analysis , 2012, Nature Methods.

[16]  R. Hill.  A lens for whole sky photographs , 2007 .

[17]  H. Maas,et al.  Validation of geometric models for fisheye lenses , 2009 .

[18]  Jianchu Xu,et al.  On the exposure of hemispherical photographs in forests , 2013 .

[19]  Michael Nobis,et al.  Automatic thresholding for hemispherical canopy-photographs based on edge detection , 2005 .

[20]  M. Rautiainen,et al.  Estimation of forest canopy cover: A comparison of field measurement techniques , 2006 .

[21]  Margaret C. Anderson.  Stand Structure and Light Penetration. II. A Theoretical Analysis , 1966 .

[22]  P. Rich.  Characterizing plant canopies with hemispherical photographs. , 1990 .

[23]  B. Courbaud,et al.  Forest radiative transfer models: which approach for which application? , 2014 .

[24]  Alemu Gonsamo,et al.  Application note: CIMES: A package of programs for determining canopy geometry and solar radiation regimes through hemispherical photographs , 2011 .

[25]  S. Pacala,et al.  Forest models defined by field measurements : Estimation, error analysis and dynamics , 1996 .

[26]  C. Canham,et al.  Sapling growth in response to light and nitrogen availability in a southern New England forest , 2000 .

[27]  Craig Macfarlane,et al.  Estimating forest leaf area using cover and fullframe fisheye photography: Thinking inside the circle , 2007 .

[28]  Gaston M. Diaz,et al.  Enhanced Gap Fraction Extraction From Hemispherical Photography , 2015, IEEE Geoscience and Remote Sensing Letters.

[29]  A. Gonsamo,et al.  Measuring fractional forest canopy element cover and openness - definitions and methodologies revisited , 2013 .

[30]  D. Coombe,et al.  Hemisperical and Woodland Canopy Photography and the Light Climate , 1959 .

[31]  P. Reich,et al.  Evaluation of several measures of canopy openness as predictors of photosynthetic photon flux density in deeply shaded conifer-dominated forest understory , 1999 .

[32]  S. Hale,et al.  Relationships between canopy transmittance and stand parameters in Sitka spruce and Scots pine stands in Britain , 2009 .

[33]  Andrea Cutini,et al.  Estimation of canopy properties in deciduous forests with digital hemispherical and cover photography , 2013 .

[34]  Lionel Hernández,et al.  Comparison of four different programs for the analysis of hemispherical photographs using parameters of canopy structure and solar radiation transmittance , 2011 .

[35]  D. Chrimes,et al.  Overstorey density influence on the height of Picea abies regeneration in northern Sweden , 2005 .

[36]  Margaret C. Anderson.  LIGHT RELATIONS OF TERRESTRIAL PLANT COMMUNITIES AND THEIR MEASUREMENT , 1964 .

[37]  Christian Messier,et al.  Use of a spatially explicit individual-tree model (SORTIE/BC) to explore the implications of patchiness in structurally complex forests , 2003 .