Aspen leaf reflectance through 2013 fall Season

--- Jake M. Hogan, Amanda R. O'Donnell, Matthew B. Manore, Patrick T. Snead

Figure 1: The aspen tree our group studied is the

foremost tree.  This photograph was taken on

October 11, 2013. The leaves on this tree

remained relatively unchanged in color from

the leaves in this photograph.

As the fall season progress, leaf color of aspen (Populus tremuloides) and other deciduous trees begin to change. This is due to the changes in the pigments in the leaves.

The objective of this project was to track the change of aspen leaf color growing in Prexy’s Pasture at the University of Wyoming (Figure 1) from September 19 (Julian date: 262) to Oct. 24 (Julian date: 297), 2013.

When the study began the leaves were green because they reflecting high amounts of green light in the visible part of the spectrum, meaning that their chlorophyll was still active. Using a digital spectrometer, we measured leaf spectral reflectance in the visible (470-700 nm) and infrared (735-940 nm) regions of the spectrum.

The percent reflectance values in red (645 nm) and infrared (735-810 nm) measured each day was used to calculate the normalized difference vegetation index values of the leaves, which indicate changes in leaf’s red and infrared reflectance (Figure 2).

The leaf reflectance remained relatively the same for much of the study; however, as the study drew to a close the reflectance of green light began to decline slightly and the non-visible wavelengths were reflected more which demonstrated that changes were occurring within the tree (Figure 2).

Figure 2:  NDVI for aspen trees leafs in front of the Agriculture building at the
University of Wyoming in Laramie, Wyoming; data were collected from
September 19, 2013 through October 24, 2013.  NDVI values were
calculated using the near infrared and red reflectance values in the
following formula (NIR – Red) / (NIR + Red).

The colder day and night temperatures were beginning to affect the leaves resulting in higher infrared wavelength reflectance, especially on Oct. 16, 2013 (day 289), even though the leaf color remained relatively unchanged. The study showed the hardiness of the tree that this group studied and the water and nutrient availability that reduced stress on the tree. The tree remained relatively unchanged for most of the study period, through three winter storms and freezing night temperatures.