Changes in sensecense rates of cottonwood trees under different light conditions

--- Amanda Pennino, Wyatt Good and Tyler Therkildson

In this study, spectral reflectance of leaves from two cottonwood trees (Populus deltoides) grown in full sun and shade conditions were used to measure the rate of senescence, or leaf drop, between trees. Light reflectance was measured using an ALTA II Spectrometer on 3 three leaves for each tree twice a week between September 15 and October 18. Light reflectance was then converted to average reflectance values and then used in a normalized vegetation index (NDVI) to assess for leaf greenness.

Changes in the NDVI values of cottonwood tree (orange - exposed to sun, blue - shaded) leaves

It was found that the tree growing in full sun portrayed lower NDVI values overall compared to the shade tree, insinuating that it’s chlorophyll content breakdown and senescence rate was quicker than the tree in shade. Additional studies should take place over a longer period and to an improved number of trees to further validate these results and to explore similar trends in other deciduous trees.

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.

Populus tremuloides vigor in comparison to daily high and low temperatures

-- Jennifer Bayne, Clint Beiermann, Alicia Brown, Spencer Northup, Preston Talbert

The weather that accompanies the fall season in Wyoming tends to be highly variable; extreme temperature fluctuations and snowstorms are not out of the ordinary beginning in September. The vegetation and plant life in the area begin the process of dormancy once the temperature begins to drop. Presence of temperature fluctuation between September and November cause an uncertainty as to when the leaves will begin to change and eventually fall; whether it is early in fall or later.

We selected an aspen tee approximately 30 yards south of the UW Agriculture building (figure above). Leaf samples were collected twice a week at 1:10 pm starting on September 11th and ending on November 6th, 2014.

We measured the spectral reflectance of the leaves and calculation the normalized difference vegetation index (NDVI) and recorded the minimum and maximum temperatures for that day. Our results showed a direct correlation between daily low temperatures and NDVI values.

Changes in Phenological Characteristics of Populus tremuloides in Response to Early Frost

-- Albert Adjesiwor, Michael Flaherty, Ben Butkus, Ariana Potetz

Quaking aspen (Populus tremuloides Michx.) is an important forest tree species in North America. The changing global climate is expected to result in inclement weather conditions such as severe frost. Wyoming is characterized by high summer temperatures and extremely low winter temperatures. A study was conducted to evaluate the response of a young quaking aspen tree to early fall frost. 

Leaf NDVI changes in young aspen from early to late fall 2014

It was observed that frost and cold temperatures (Fig 1) resulted in early leaf senescence and abscission which affected how leaves interact with electromagnetic radiation (EMR). Leaf senescence due to frost resulted in decreased chlorophyll content of leaves and reduced photosynthetic activity. 

This resulted in reduced plant vigor (Fig 2) as measured by the normalized difference vegetation index (NDVI) shown in Figure 1. Remote sensing will be a good tool for assessing how young aspen trees respond to frost damage.

Monitoring the Phenology of Quaking Aspen at UW

-- Parmeshwor Aryal, Gregory F. Galli, and Jacob A. Rohrer

Plant response to seasonal change varies based on type of plant species and the growth stage. The Normalized Difference Vegetation Index (NDVI) has been predominantly used to determine the effects of environmental conditions on vegetation performance and their phenological changes over the growing season. A study was conducted to understand the phenology of quaking aspen (Populus tremuloides Michx.) in response to the seasonal environmental changes.

Spectral reflectance values of adaxial (top) and abaxial (bottom) foliar surface of a young quaking aspen tree were obtained and NDVI values were calculated for both surfaces from early fall (September 11, 2014) to late fall (October 21, 2014).

Phenological changes in a quaking aspen from September 11 to October 21, 2014

As the fall season progressed into winter, the NDVI values of both surface of leaf decreased over the time which was congruent with the visual changes in leaf color. The NDVI values of adaxial surface was found to be higher than that of abaxial surface. That suggested that the NDVI is effective indicator for phenological changes during growing season and for determining different reflectance between two sides of the leaves.

Use of remote sensing tool to study the senescene of Gambel oak leaves

-- E Kelienpaste, VR Joshi, A. Renneisen, S. Swoboda-Colberg, D. Smith

The interaction of electromagnetic radiation with leaf pigments in plants has become an essential diagnostic variable for examining phenology. This study is an attempt to understand the phenological event of senescence of the Gambel oak during the fall season through the application of remote sensing tools. A portable spectrometer was used to record reflectance values of the leaves. NDVI values were calculated by dividing the difference of infrared and red reflection values by the sum of infrared and red reflection values. NDVI values were regressed over time to develop the best line of fit representing the relationship between these two variables. The second order polynomial regression showed the best fit model. Initially lower NDVI values increased slightly with improved weather conditions for first few days and then dropped sharply with decreasing air temperatures. The results of this study show that declining air temperatures are an important factor in senescence. It shows NDVI values of Gambel oak leaves decrease as the fall season progresses.

Phenology and senesence observations of two balsam poplars in different locations with varying amounts of sunlight on UW campus

-- Balke Balzan, Shelby Baumgartner, Taylor Miller, Lee Noel, Luke Nieslanik

Many trees have adapted to tolerate cold, harsh conditions. Balsam poplar (Populus balsamifera) trees are no exception and have also developed mechanisms for surviving unfavorable circumstances, such as losing leaves in the fall season. Three random leaves were selected from two Balsam poplar trees five to seven times a week. One tree was located in a high sun exposure area and the other tree was located in a shaded area. A spectrometer was used to measure leaf reflectance values.

Normalized Difference Vegetation Index (NDVI) was calculated to observe a trend in each tree over time. Our group hypothesized that the tree with more sun exposure would keep its leaves longer into the fall season. However, results did not support this hypothesis. Both trees had very similar NDVI values throughout the course of the study. According to this experiment sun exposure is not a noticeable limiting factor in Balsam poplar senescence.

Monitoring phenology of White oak and Eastern cottonwood trees using remote sensing

-- Tealand Stende; Jesse Shenefelt, Amanda Lee, Paul Ratigan, Abdelaziz Nilahyane

As the fall season progresses and the temperature decreases, leaf color of deciduous trees begins to change as a result of changes in the pigments of the leaves. Using the ALTA II reflectance spectrometer, the reflectance values were measured and the Normalized Difference Vegetation Index (NDVI) was calculated for the Eastern Cottonwood (Populus deltoides) and the White oak (Quercus alba). Tree leaves wee collected at the University of Wyoming Campus in order to monitor and understand on a local scale how these trees respond to climate changes during the fall season, and then draw a general picture comparing both trees in terms of phenological events.

The results revealed that the decline in NDVI is consistent with the progression of the senescence of both trees, but different trends regarding change in leaf color and the leaf-out timing were apparent. The NDVI values for the oak tree were associated with the quadratic model with an R**2 = 74%, while the NDVI values for the cottonwood tree lowed a linear trend (R**2 = 73%).