Reports

Objectively determining the beginning and ending of green up at Tonzi:

Upward facing cameras (z-cams) have been installed in three locations at Tonzi since late 2009.  Each of these cameras sees branches from several trees and allows us to calculate gap fraction.  The cameras take four images per day a 30min intervals before sunset.  The images that pass the gap fraction calculations with acceptable quality are averaged to produce a daily gap fraction (Raw) and further smoothed with a five day running mean (Smooth).  By plotting the time series of gap fraction, the beginning and ending of the period where the trees put on leaves (green up) can be determined.  However an objective (algorithmic) method is needed to do this consistently. 

The method developed is to fit a line to the stable (smoothed) no-leaf period from day 1 to day 60 (Top) and fit another line to the stable (smoothed) full-leaf period day 120 to day 180 (Bottom).  Then during the period from day 61 day 119 Raw points that are more than 0.025 gap fraction below the projection of the Top line and more than 0.025 gap fraction above the projection of the Bottom line are considered the 'Mid' points and used to fit a third line called 'Green Up'.  Where Green Up intersects Top and Bottom determine the beginning and ending green up date respectively.  Figures 1-3 show some examples from each of the cameras.

Figure 1. Z-cam 1 year 2020 green up date determination has green up starting on day 77 and ending on day 101

 

Figure 2. Z-cam 2 year 2010 green up date determination has green up starting on day 76 and ending on day 101.  Here there is a wobble in the green up period but the resulting dates are reasonable.

 

Figure 3. Z-cam 3 year 2012 green up date determination has green up starting on day 87 and ending on day 116.  Here, the gap fraction pulls away from the Top line more slowly than other years but the line fits still result in reasonable dates.

 

Figure 4 summarizes all the dates for each year and each camera.  The beginning and ending dates can shift by more than 20 days depending on the year.  And the time it takes to go from no leaves to full leaves change by nearly 15 days.  And for most year all three cameras report very similar dates.

Figure 4. Beginning and ending dates of green up for the Tonzi oak trees based on gap fraction calculated from upward looking cameras (z-cams).  Beginning dates from the three cameras in blue and ending dates from each camera in orange.