Ex 4: (Phase 3.) The Effects of Shade Among Strata in an Understory Protection Strip System
This phase of crop plan design deals with activating and evaluating the effects of adjacent stand shade.
1. Select the "UP-no shade" Crop Plan developed in Phase 1. Activate the Crop Plan event (bring the yellow highlight to it) by right-clicking, then click on Select current CP from the drop-down menu.
1. Use Excel Edit-Copy to make a copy of the UP-no shade Crop Plan
2. Move the cursor to a cell in the first column below the UP-no shade Crop Plan. Leave a few blank rows for appearance.
3. Use Excel Edit-Paste to paste a copy of the UP-no shade Crop Plan
2. Select the Crop Plan event at the top of the newly copied CP by right- or double-clicking.
1. Edit the event using the MGM Edit event () button. Call it simply UP since we will now add adjacent stand shade calculations.
3. Activate the Options event in the UP Crop Plan.
1. Choose the Adjacent Shade tab and check the “Use shade…” box. This will activate the spatial stand-to-stand shade calculation, which imposes a species-specific reduction on growth. As the label warns, this annual shade calculation will considerably slow down MGM.
2. Ensure the latitude, longitude, growing season and diffuse fraction are appropriate for your region. The seasonal shade calculation is relatively insensitive to small changes in these parameters. Differences less than 2 degrees of latitude, or less than a week of growing season will have little impact on projections. At this time longitude is not used, and the diffuse fraction of total radiation is quite stable at about 0.5 across the western boreal.
4. Activate the third Visualize event.
5. On the MGM Toolbar, click on to execute only the current Crop Plan (UP) up to and including the current event.
1. Note the status bar at the bottom of Excel, which will provide an indication of the processing MGM is doing. Before the first Grow event is executed, you will see the solar track being calculated for each day in the growing season at the latitude specified in the Options event. Once the sun has been tracked, the “seasonal sky” generated is saved and used for the entire projection. This sky view is brightest in the south near the maximum altitude the sun reaches at the current latitude. These sky brightness values are used in conjunction with the position of the stands and the density of the tree species within them to determine seasonal-average shade at the top of each stand stratum.
2. Once the sun is tracked, the projection year (stand age) is shown in the status bar. Note the projections are considerably slower – an 80-year projection takes 15 minutes on a typical 2007-vintage computer. The sun track is fixed once, but tree sizes and densities change during projection, so the shade needs to be calculated at the beginning of every year.
3. When MGM stops after the third Visualize event, take a look at the WindProtection, WestRetention, and MC stand worksheets. Near the top of these worksheets, in cell E3, the “Toplight” value is the seasonal average light transmission percentage at the top of that stand’s canopy. This value is 0% if adjacent stands cut out all the incoming light, 100% if adjacent stands do not shade this one, or an intermediate value if there is partial shade). The tallest strips (WindProtection) have 100% light, while the shortest (MC) have the lowest light (~70% in this example) due to their considerably taller neighbouring strata. The landings and retention zones have quite high light (>90%) since the former are relatively large and the latter are only shaded by thin strips of relatively open aspen trees. The shade determination is done on a random selection of ten spatial locations at the top of each stand stratum, so there is some variation (<3%) in Toplight values from run to run. Each stand’s Toplight value is used to reduce the growth rates of the trees. At 100% light, the trees attain the full growth they would get in a normal, unshaded stand. The growth reduction is non-linear, with little effect until <80% light, then progressively stronger reductions for lower light levels.
6. On the MGM Toolbar, click on () to execute all Crop Plans on this worksheet.
1. The “no shade” projection takes a few minutes, and the "UP" projection with shading turned on will take 15 minutes to complete.
7. When complete, note the area-weighted deciduous volume is half that of the previous run, where we did not consider adjacent stand effects. Shade cast by the spruce retention and wind protection strips suppresses the aspen growth.