Fall Changes in a Wyoming Maple Tree

-- Elijiah J. Attebury, John R. Copeland, Mary L. Harris, Elissa M. Paranto

What is the timing of senescence in 2013 for a maple tree on the University of Wyoming campus?  Can the change in spectral reflectance signature from one maple tree be characterized by a decline in the NDVI over time as leaves from the tree senescence?  Our study attempted to answer these questions.

Figure 1: Progression of senescence of a maple tree located near the Wyoming Union on the
University of Wyoming campus, from (a) September 24 (Julian Date 267),  (b) October 3 (JD 278),
(c) October 8 (JD 281), & (d) October 15( JD 288), 2013.

Senescence is the process of biological aging of deciduous leaves through time at the cellular level resulting in leaf deterioration (Figure 1). Whereas, NDVI is a vegetation index calculated using intensity of reflectance of visible red light and a band of infrared reflectance which is usually high in vigorous vegetation: NDVI= ((near infrared reflectance- red reflectance) / (near infrared reflectance + red reflectance)) x 100.

We sampled several leaves on each sampling date and tore them into pieces. We then mixed the pieces together and measured their reflectance with an Alta 2 hand-held spectrometer at 11 wavelengths ranging from 470 to 940 nm. Through sampling the reflectance of this one tree over a period of 43 days, we found that the NDVI rose at first to a maximum, and then proceeded to decline, until the leaves were brown and falling from the tree (Figure 2).

Figure 2: Graph of NDVI vs Time for a maple tree on the University of Wyoming campus 2013.
The trend line shows that NDVI declined with time as the fall season progressed.

The decline of NDVI is consistent with the progression of the senescence of a deciduous tree.  More detailed research, in another year, with daily measurements of reflectance and ambient weather conditions, might help define both natural, uninterrupted senescence and also the interrupting effects of early fall frosts on the phenological procession of this maple tree.