Phenology of a young Oak tree

--- Katy-Jane Angwin, Thoa K. Pham, Peter C. Solin

Fall in Wyoming is highly variable, often with extreme temperature fluctuations and snowstorms. One may wonder how any plant is able to survive in such variable daily conditions. Oak trees (Quercus Spp.) can, therefore, be presumed to be particularly resilient in order to survive in Wyoming, to withstand temperatures in a state where not so long ago most people struggled to survive the winters.

Figure 1. Oak tree located at
N 41º 18.821’ and W 105º 34.837’.
This picture was taken on October 5, 2013.

Our experiment tracked an oak tree undergoing phenological changes from fall to winter. From September 19 to October 23, 2013, every other day we randomly collected two leaves from the oak tree located in Prexy’s pasture at the University of Wyoming (Figure 1).

Using a spectrophotometer, we measured the leaf reflectance across 11 visible and near-infrared wavelengths of light and calculated the Normalized Difference Vegetation Index (NDVI) to assess change over time. NDVI is an index that measures the difference in the absorption and reflection of light across red and near-infrared wavelengths. Healthy leaf absorbs most of blue and red light and reflects green and the near-infrared (NIR) light. Unhealthy leaf reflects more visible light and less NIR. This difference provides important information about plant physiological conditions. In general, normal end-of-season senescence result in NDVI decrease.

Because plants respond to cumulative temperature changes, not just daily events, we incorporated the minimum temperatures for each day in our research and gathered daily temperature from the National Climatic Data Center. During this experiment, there were three winter snowstorms where temperature dipped to below -6º C (20º F).

Figure 2. NDVI values for oak leaves collected across a 5 week time period from Prexy’s Pasture in Laramie, Wyoming, in response to winter storm events. This figure also shows minimum temperature data for Laramie, Wyoming, for the same 5 week time period. Regression equations and trendlines are presented.

We hypothesized that the relative NDVI values of the oak tree (Figure 2) would decrease as the temperature decreased.  Historical data has shown that NDVI values peak in the summer and trough in the winter. Our results showed distinct decreases in NDVI values, influenced by an increase in the red reflectance and a decrease in NIR values, coinciding with decreasing temperatures. Overall, our data shows the red reflectance values increase while the NIR regions of the spectrum decrease, which indicates the oak tree is entering the end-of-season state and is consistent with the decrease in NDVI. Trend lines also showed that the NDVI values of oak leaves would continue to decrease as colder winter weather moved into the area, which confirms our hypothesis.