Comparison of Autumn NDVI Values for Quaking Aspen (Populus tremuloides) Leaves in Southeastern, Wyoming

--- Matthew Doonan, Colleen Friday, Rowdy Nelson, Kelsey Welter

As one of the most common deciduous trees in the Rocky Mountain West, the quaking aspen is an important member of many ecosystem communities in the state of Wyoming (Populus). The patterns of pigment concentration levels in the leaves of aspens change the rate and efficiency of photosynthesis throughout the year (Castro). Thus, these pigment concentrations impact ramet (individual trees of the aspen clone community) health and survival rates.

Aspen growing in sun (left column) and shade (right column)
in September 10, and November 5, 2015.

This study looked at two quaking aspen ramets of the same group in the University of Wyoming Cheney Plaza garden in Laramie, Wyoming throughout the entire autumn season (September through November) 2015. One tree received full sunlight for the majority of every day, while the other tree was shaded by a significantly larger pine to the west, and received only a few hours worth of sunlight each day.

NDVI values with a linear trend line for the aspen trees growing in sun and shade
in the autumn of 2015 in Laramie, Wyoming

The study tracked the spectral reflectance of individual leaves from each tree that were representative of the entire tree on a weekly basis, and used the spectral reflectance to calculate the NDVI values for the leaves collected. Our results showed that while the sun and shade tree leaves did not have significantly different NDVI values at the beginning of autumn, by November there was a noteworthy difference between the NDVI values, with the shade tree having lower values.

Changes in spectral reflectance of dry bean (Phaseolus vulgaris) leaves in response to atrazine

--- Daniel M. Adamson

Drift and carryover of herbicides present challenges to land managers because of damage to sensitive crops. For many chemical groups, herbicide injury to plants may not be visible for several days or weeks after application. Remote sensing offers the potential to detect herbicide injury prior to visually apparent symptoms. This study tested the ability of a hand-held reflectance spectrometer to detect changes in leaf reflectance of dry bean (Phaseoulus vulgaris) plants in response to atrazine.

Dry bean (Phaseolus vulgaris) plants 21 hours (left) and 213 hours (right) after atrazine application.
After 213 hours, symptoms were visible as interveinal chlorosis (right-middle) and necrosis (right-right)

Atrazine was applied at 100% and 25% rates on V1 dry beans in a greenhouse. Results show that significant changes to the spectral reflectance curve occurred on treated plants within 21 hours of application. Indices such as NDVI and GNDVI were poor indicators of plant health because of the reduction in reflectance at 560nm and 645 nm.

Orange Normalized Difference Vegetation Index (ONDVI) of
dry bean (Phaseoulus vulgaris) leaves in response to two rates of atrazine application. 

An alternative index using reflectance at 600 nm (Orange NDVI) was able to capture relatively small changes in reflectance of treated plants in this region.

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.