Evaluating LAI estimation using time-lapse game camera images

Background/Question/Methods

Measures of Leaf area index (LAI) serve as an important indicator of forest canopy structure, which in turn provides an indicator of forest productivity and ecosystem exchange.  Changes in leaf area index are responsive to seasonally variable environmental drivers and are therefore dependent on interannual shifts in seasonal cycles.  Our goal was to assess the use of time-lapse photography using commercial off-the-shelf game cameras to evaluate canopy closure as a proxy for leaf area index in a mixed conifer forest in northeast Washington State.  Cameras were placed at study areas such that they captured an image looking straight up at the canopy.  LAI was estimated concurrently using an LAI 2200 tall canopy analyzer.   The ratio of canopy pixels to sky pixels was assessed by thresholding the blue band data from the camera.  This was then compared to LAI as measured with the LAI 2200.  We assessed the viability of this method in general, as well as factors influencing the accuracy of the method, including sun angle and variation in LAI.

Results/Conclusions

Preliminary results indicate that the cameras do provide a reasonable method for estimating LAI in an open canopy ponderosa pine forest (R² = 0.86).  This indicates that there is promise in this approach, however it is likely that refinements in the timing of canopy closure assessment and camera to LAI relationships can be improved.  Further results will indicate the degree to which camera-based estimates of LAI vary with sun angle and LAI.