Phenology of a young Aspen tree in Simpson Plaza

--- Jack M Brown, James R. Schaffarzick, Stuart D. Thrash,  Zachariah M. Tuthill

Quaking aspen (Populous tremuloides) are known for their leaf color changing events that take place in the mid to late fall from a deep green to yellow/orange, then falling off of the tree for the winter, which involves changing phenological processes.

These phenological events are quantified by measuring the amount of light reflected by the leaves in visible and infrared regions, and using that data to calculate Normalized Difference Vegetation Index (NDVI).  NDVI is a measure of the vigor of a plant. This study monitored the fall phenology of a young aspen tree in Simpson Plaza on the campus of the University of Wyoming (Figure 1).

Figure 1. Young aspen tree our group
monitored in Simpson Plaza on the
campus of the University of Wyoming

Observations were taken a total of 27 times from September 19 to October 26, 2013. Each observation included:  a) Collecting 3-4 leaves from the bottom branch of the tree, b) Cutting leaves up (excluding stems), c) Homogenizing leaves (mixing them up), and d) Measuring percent reflectance values with a handheld spectrometer.

Reflectance values as well as NDVI values changed as the fall wore on, but 3 early season winter storm events drastically changed the leaf color, which affected the phenological events. NDVI values initially rose (leaves were still green), then declined when temperatures dropped below freezing at night (leaves were changing color/dying), or after winter storms were present (Figure 2).

Figure 2. NDVI Values of young aspen tree measured from September 19 to October 26, 2013.
Trend line fitted through these points accounted for 82% of the variation in the dataset.

After monitoring an aspen tree on campus for over a month, we concluded that early season winter storm events can have a dramatic effect on leaf phenological events that normally take place during the pre-winter season. If a winter storm rolls in early, it can change the timing of these events, as well as damage and kill aspen leaves.