Simulations of below-canopy PAR 
at the Wind River Canopy Crane Facility

Here are some plots from the first run of the CrnSol model on the ellipsoid data from the WRCCF provided by Jiquan Chen. The CrnSol model is a derivative of the PJSol model developed by me at Los Alamos to study solar irradiance in pinyon-juniper woodland. It uses ray-casting to determine the transmission of solar irradiance through a canopy. Tree crowns are idealized as ellipsoids containing a "turbid medium" of leaves. The model can be run for any time period and irradiance at any specified point in 3d space is calculated. This version does not calculated diffuse input but will as soon as I merge the code into it.

For this trial run, I chose to simulate direct beam PAR below the canopy at the summer solstice. The mapped area is 400m x 200 m and Jiquan provided the ellipsoid data for 3610 trees. I chose to simulate a sub-plot of 50m x 50m with its origin (lower left) located at 250m East and 100m North.

The first plot below is direct PAR at ground level integrated over the simulated day of the summer solstice 21 June 1997. The percent cover in this plot (vertical crown projection) was calculated to be 76%.

Technical points for this run:

1. For this first try, the land surface was assumed to be level and flat. All the trees were assumed to have the same elevation at the base -- I ignored the elevation data provided and assigned every tree (and every simulated point) a "z" at the base of zero. The way to correct this is to use a high-resolution DEM for the crane site. I could have constructed one from the data for elevation at the base of the trees but this would have been biased. If there are more elevation data for the inter-tree spaces then this could be incorporated to produce a better DEM.
2. The leaf area density (LAD, m² leaf area / m³ ellipsoid volume) was assigned to be 0.25 to every ellipsoid for this simulation. I do not have any data to support this value but it would be nice to have some. This parameter controls how much light penetrates the canopy: transmission = exp(-LAD * PathLength). Later updates might add concentric ellipsoids with differing LAD's , and species-specific LAD values.

The red square in the "stem map" below shows the area that was used for the simulations here.

A gray-scale image of the simulated PAR at the ground surface -- high values are white, low values are black. Gaps are evident and are of various configurations. Note the sharp gradients of PAR at the edges of the gaps (white areas).

A map of below-canopy PAR is shown below. The color map produced by Excel for this is not very intuitive but the large, continuous bluish area is where the canopy has produced dense shade (compare to the image above). I chose five points at which to examine the daily course of PAR and have numbered them on the plot.

Point 1 is in the center of a large "dark" area. Point 2 is in the center of the largest gap seen above. Point 3 is on the western edge of that gap and point 4 is on the eastern edge of the gap. Finally, point 5 is between two gaps in a "darker" area. The daily course of PAR at each of the five points is shown below to illustrate the temporal heterogeneity of the light environment.

It is important to note that the absolute values of PAR (mol/m2) are not calibrated and probably not correct. However, the values can be compared relative to one another. Notice that penetration of direct beam only occurs significantly in the four hour period between 1000 and 1400. Point 5, between two gaps, has three periods of significant sunflecks. The line for Point 1, in the dark area is barely visible at about 0930.

Interestingly, the points that form an east-west transect across a gap show the sunfleck in the gap progresses from east to west from about 1130 to 1330. This is seen in the above plot by following the progression of black (W edge of gap), then blue (center of gap), then green (E edge of gap) lines.

To illustrate the spatial heterogeneity, a transect across the plot is shown below. Notice the large differences in PAR between gaps and non-gaps. A large gap and a small gap, of similar maximum PAR, are labeled to illustrate the size difference between them.

Last Updated on 6/25/97
By Scott N. Martens
Email: scott@vache.ucdavis.edu