Comparing autumn senescence from trends in NDVI for two aspen trees in Laramie, WY

--- Erika Alderete, Barbara Jean Bender, Kirk Scheffler, Jennie Schoedel

Aspen trees (Populous tremuloides) are one of the many varieties of deciduous tree that experience visible pigmentation change from green chlorophyll dominant to yellow/red carotene/anthocyanin dominant throughout the fall season.  Although many aspen phenology studies have been done to investigate why and how annual senescence occurs, there is a lack of research detailing how location attributes of different yet nearby trees affect the progression of aspen senescence.

Aspen #1 (clump), Aspen #2 (solo) trees selected for this study

Previous studies range from tracking the timing of pigment changes to identifying other natural stressors contributing to leaf death.  In our experiment we explored the effect of proximity to other aspen trees for autumn senescence and leaf loss.  We chose two aspen trees to collect data from to determine this effect, one in a stand of five trees and another completely isolated from any other aspen trees.

Changes in the NDVI values of the clumped (brown) and solo (blue) aspen trees

Our results indicate that although the onset of dormancy was earlier for the secluded tree, there were in fact no statistically significant differences in senescence timing or rate.

Fertilizer can extend the growing season of tomatoes grown in high tunnel

-- Jenna Meeks

Do you ever feel there just aren’t enough hours in the day? How about enough days in the growing season for your home-grown tomatoes? The use of fertilizer may extend the growing season for tomato plants grown in a high-tunnel, a greenhouse like structure heated by solar radiation (Figure 1).

High-tunnel (Photo: Jenna Meeks)

One way to measure the benefits of fertilizers on plant growth is by analyzing leaf reflectance data. Reflectance is determined using a specific instrument which can see more than the visible rays detected by human eyes. It also encompasses the invisible region and this is where cellular differences in plant leaves can be seen as variations in reflectance.

The normalized differential vegetation index (NDVI), calculated using two wavelengths, red in the visible region and infrared in the invisible region, is used as a proxy for plant vigor. Therefore, NDVI is a tool to determine if fertilized tomato plants are healthier than unfertilized plants as the normal growing season comes to an end.

I studied the effect of four fertilizer treatments: a) Control (no fertilizer); b) 200 pounds of nitrogen as NPK fertilizer; c) a substitution of 75% of the N requirement with compost and; and d) a substitution of 25% of the N requirement with compost. Reflectance values were recorded from each plant four times with an Alta II Reflectance Spectrometer and the data were analyzed in Microsoft Excel.

The control plot displayed a lower NDVI than the fertilizer treatments, suggesting fertilized plants maintain their health for a longer period than unfertilized plants (Figure 2). The treatments with compost showed higher vigor than the control plot for half of the sampling dates, yet the 200lb N treatment was consistently higher than the control. These analyses are helpful to growers as they develop their fertilizer plan and crop rotations in their high-tunnels.