Using Unmanned Aircraft Systems (UAS) to Quantify Mistletoe in Urban Environments
David L. Kulhavy1, Christopher M. Schalk1,*, Reid A. Viegut1, Daniel R. Unger1, Schaeffer W. Shockley1, and I-Kuai Hung1
1Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University, Nacogdoches, TX 75962, USA. *Corresponding author.
Urban Naturalist, No. 20 (2019)
Abstract
Phoradendron leucarpum (American Mistletoe) is a hemiparasitic plant that infects deciduous trees across the United States. We examined the feasibility of using an unmanned aircraft system (UAS) to detect and quantify American Mistletoe in an urban environment compared to ground-count surveys. On average, regardless of tree height, we detected more American Mistletoe plants using the UAS compared to the ground-count surveys; our estimates of American Mistletoe load nearly doubled when we used the UAS. In the ground-count surveys, our ability to accurately count the number of American Mistletoe plants decreased with increasing tree height. These results demonstrate that UAS can help researchers and managers to accurately predict the parasite load of trees to produce a more accurate hazard rating as well as help quantify the resource availability for wildlife in urban environments.
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D.L. Kulhavy, C.M. Schalk, R.A. Viegut, D.R. Unger, S.W. Schockley, and I-K. Hung
22001199 URBAN NATURALIST No. 20N:1o–. 1200
Using Unmanned Aircraft Systems (UAS) to Quantify
Mistletoe in Urban Environments
David L. Kulhavy1, Christopher M. Schalk1,*, Reid A. Viegut1, Daniel R. Unger1,
Schaeffer W. Shockley1, and I-Kuai Hung1
Abstract - Phoradendron leucarpum (American Mistletoe) is a hemiparasitic plant that
infects deciduous trees across the United States. We examined the feasibility of using an
unmanned aircraft system (UAS) to detect and quantify American Mistletoe in an urban
environment compared to ground-count surveys. On average, regardless of tree height, we
detected more American Mistletoe plants using the UAS compared to the ground-count surveys;
our estimates of American Mistletoe load nearly doubled when we used the UAS. In
the ground-count surveys, our ability to accurately count the number of American Mistletoe
plants decreased with increasing tree height. These results demonstrate that UAS can help
researchers and managers to accurately predict the parasite load of trees to produce a more
accurate hazard rating as well as help quantify the resource availability for wildlife in urban
environments.
Introduction
Mistletoes (order Santalales) are hemiparasitic plants that infect deciduous
trees and rely on the sap from the xylem of their host trees for water and essential
elements (Gairola et al. 2013, Sangüesa-Barreda et al. 2012). A mistletoe plant
produces photosynthetic products and can alter the carbon balance in the host tree,
leading to drought stress and water deficits (Gairola et al. 201 3, Sangüesa-Barreda
et al. 2012). Tree infection results from a 6-step process that includes seed deposition,
germination with growth of a radicle attached to a twig, plant attachment,
initial infection with the haustoria (Year 1), shoot and sinkers for plant establishment
(Year 2), and plant growth (Year 3) (Coder 2016). Fruit matures in the winter
and is consumed by birds that carry the seeds to other trees via defecation (Coder
2016, Whittaker 1984). Infections are greatest in open forest stands with tall, opencrowned
trees, and infections are usually concentrated at the top outer branches
of the host trees (Coder 2016). On a tree, mistletoes are generally distributed in
clumps in the upper and outer parts of the crown due to initial dispersal patterns by
birds (Overton 1996, Sangüesa-Barreda et al. 2012, Sayad et al. 2017). Mistletoe
plants may be aggregated in individual trees; infestations increase in trees with
established mistletoe plants that disperse seeds within the same canopy (Ward and
Paton 2007). If abundant enough, mistletoe infection can affect branch integrity
due to girdling or breakage, and even cause mortality of the host tree (Mathiasen et
al. 2008).
1Arthur Temple College of Forestry and Agriculture, Stephen F. Austin State University,
Nacogdoches, TX 75962, USA. *Corresponding author - schalkc@sfasu.edu.
Manuscript Editor: Leonie K. Fischer
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Traditionally, surveys of mistletoes have been conducted using ground counts to
quantify the number of plants (Coder 2016, Hawksworth 1977, Smith 1969). The
use of small unmanned aircraft systems (UAS) is increasing in natural-resource
management. Components on a UAS include a GPS receiver for global navigation
of the aircraft position and airspeed, inertial navigation to measure aircraft
altitude, an internal barometer to measure altitude above the takeoff location for
each flight, controls for both the camera and the UAS, and a flight-data recorder for
UAS position and altitude values for each image (Hugenholtz et al. 2012, Whitehead
and Hugenholtz 2014, Whitehead et al. 2014). These components allow UAS
to measure a number of natural-resource parameters. For example, Kulhavy et al.
(2016) compared data collected by a UAS with those collected by conventional
ground surveys of urban forest-tree condition rating using the Council of Tree and
Landscape Appraisers (CTLA) method with no difference in measurements of 6
variables—trunk condition, growth, crown structure, insects and diseases, crown
development, and life expectancy. Dwyer and Tincher (2018) used a UAS to view
nest contents of Pandion haliaetus (L.) (Osprey), which would normally be impossible
to assess from below.
Phoradendron leucarpum (Rafinesque) Reveal and M.C. Johnston (American
Mistletoe, hereafter, Mistletoe) is a native mistletoe that infects deciduous trees
across the US, including those in urban environments. It is important to quantify
mistletoe accurately in urban areas, as an increased infection load on a tree can
cause limb breakage, drought stress, and mortality of part of the crown of the host
tree. For example, Coder (2016) proposed a hazard-rating system for Mistletoe
in the southeastern US based on the number of plants in the tree, but application
of that hazard rating is dependent on being able to accurately know the number
of plants present on a host tree. By using a small UAS, we wanted to quantify
the number of plants seen by the UAS and compare this number to ground-based
counts. We sought to examine the feasibility of employing a UAS to detect and
count Mistletoes in an urban environment because the aircraft can be maneuvered
for both nadir (straight down) and oblique (not straight down) imagery that can
be recorded for analysis in a laboratory setting. We predicted that the UAS would
provide a more accurate estimate of the number of Mistletoe in a tree because it
would be able to document plants present on the crown of the host tree that would
otherwise be obscured when using ground-count surveys.
Field Site Description
We conducted the study in the city of Nacogdoches, Nacogdoches County, TX,
USA. Nacogdoches (31°36'12.6504''N, 94°39'19.7532''W) has a population of
33,900 (as of 2017) in an area of 69.9 km2 (Kendig Keast Collaborative 2017). The
area is characterized by a subtropical humid climate with hot summers and mild
winters. Average annual precipitation is 1213 mm, and average annual temperature
is 18.8 °C (Chang et al. 1996). The trees (n = 100) used in the study are located
Figure 1 (following page). Location of 100 host hardwood trees of Phoradendron leucarpum
(American Mistletoe) in the city of Nacogdoches, TX, USA.
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D.L. Kulhavy, C.M. Schalk, R.A. Viegut, D.R. Unger, S.W. Schockley, and I-K. Hung
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Figure 1. [Caption is on preceding page.]
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in urban green spaces including the campus of Stephen F. Austin State University,
Banita Creek Park, Festival Park, Pecan Park, Pioneer Park, and in downtown Nacogdoches
(Fig. 1). In Nacogdoches, there are 22 parks totaling 158.4 ha. For this
study, we surveyed hardwoods in community parks that were evenly distributed
across the city (Kendig Keast Collaborative 2017).
Methods
We surveyed 100 hardwood trees from 6 January 2018 to 14 March 2018, allowing
for maximum visibility of Mistletoe plants during leaf-off of the host trees.
For each tree, we recorded species, trunk diameter (cm) at 1.4 m in height, and total
height. Ground surveys consisted of 1 observer slowly walking around each host
tree and counting the number of Mistletoe plants observed. Following the ground
count, we flew a UAS DJI Phantom 4 Pro (Dà-Jiāng Innovations Science and Technology
Co., Ltd., Shenzen, China) around each tree using the 20-MP camera to
record both nadir and oblique imagery in both video and still images (Fig. 2). We
took the imagery from the Phantom 4 to the GIS Laboratory in the Arthur Temple
College of Forestry and Agriculture for analysis and to count Mistletoe plants. We
also determined the average time and costs associated with each survey method.
To avoid missing or double-counting plants, we counted the crown and oblique
images of the tree from the north in a clockwise direction. We also uploaded representative
aerial images that are freely available from an iNaturalist.org project
entitled “Mistletoes of Nacogdoches” (https://www.inaturalist.org/projects/mistletoes-
of-nacogdoches). These records provided locality data for each tree, as well
as a visual record of both the Mistletoe and the host tree. We used linear regression
to assess the relationship between ground-based and UAS-based surveys, on both
Mistletoe count and infestation rate. In order to determine if detectability of Mistletoe
plants was affected by tree height, we separated the observations into 2 groups
based on the total height of each host tree. The break value was the average height
(16.7 m) of the 100 host trees surveyed, resulting in the “tall trees” group of 51 and
the “short trees” group of 49. We then conducted the same regression analysis on
each of the 2 groups.
Results
We surveyed a total of 100 hardwood trees for Mistletoe (Table 1). They ranged
in diameter from 25 cm to 122 cm and in height from 10.6 m to 33.5 m. Ground
counts of Mistletoe plants per tree ranged from 1 to 81 plants; counts from the
UAS ranged from 1 to 158 plants. The UAS method required a larger initial investment
compared to the ground-count method (Table 2). However, both methods
were comparable regarding the time spent to assess the Mistletoe load in each tree
(Table 2). On average, regardless of tree height, when we used the UAS, we detected
more Mistletoe plants than we identified during ground co unts.
We detected an average ± SD of 10.2 ± 14.6 plants per tree in the ground surveys
and 17.1 ± 27.6 plants per tree from the UAS surveys (nearly a 60% increase
in the number of Mistletoe plants detected). For all trees (n = 100), the number of
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Mistletoe plants observed in UAS surveys per tree can be predicted from ground
counts using equation 1 (R2 = 0.7379; P < 0.001; Fig. 3A).
UAS count = 1.623(ground count) + 0.5333 Equation 1
Figure 2. An example of
(A) a nadir, and (B) oblique
UAS image of Phoradendron
leucarpum (American
Mistletoe) in a hardwood
tree.
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For tall trees, we observed an average of 12.6 ± 15.6 Mistletoe plants in the
ground surveys and 22.0 ± 30.0 plants from the UAS (nearly a 57% increase in
the number of Mistletoe plants detected). For tall trees (n = 51), the number of
Mistletoe plants observed in UAS surveys per tree can be predicted from ground
counts using equation 2 (R2 = 0.6484; P < 0.001; Fig. 3B).
UAS count = 1.5217(ground count) + 2.7551 Equation 2
For short trees (n = 49), we observed 7.7 ± 12.9 Mistletoe plants per tree in the
ground surveys and 12.0 ± 24.2 plants from the UAS (~64% increase in the number
of Mistletoe plants detected). For short trees, the number of Mistletoe plants per
tree observed in UAS surveys can be predicted from ground counts using equation
3 (R2 = 0.8694; P < 0.001; Fig. 3C).
UAS count = 1.7522(ground count) – 1.4402 Equation 3
Discussion
We found that UAS provided a more accurate count of the parasite load of
Mistletoe on their hardwood hosts in eastern Texas compared to ground surveys.
In general, ground surveys underestimated the number of Mistletoe plants found in
the crowns of these trees; our estimates of Mistletoe load nearly doubled when we
used the UAS. Though our best-fit lines were likely influenced somewhat by influential
outliers of trees with large mistletoe counts, we nevertheless suggest when
only ground surveys can be conducted (e.g., when a UAS is unavailable or airspace
Table 2. Average time and cost per assessment method assuming a $20.00-per–hour pay scale for
hourly wages. N/A = not applicable.
Method completion time (min) Method cost ($)
Item UAS Ground survey UAS Ground survey
Initial equipment purchase N/A N/A 1500.00 100.00
Field assessment per tree 3 8 1.00 2.66
Lab assessment per tree 5 N/A 1.66 N/A
Totals per tree assessed 8 8 2.66 2.66
Table 1. List of 100 trees surveyed for Phoradendron leucarpum (American Mistletoe).
Scientific name Common name Count
Quercus nigra (L.) Water Oak 56
Quercus falcata Michaux Southern Red Oak 15
Celtis laevigata Willdenow Sugarberry 8
Fraxinus pennsylvanica Marshall Green Ash 7
Betula nigra (L.) River Birch 5
Ulmus americana (L.) American Elm 3
Carya illinoinensis (Wangenh.) K. Koch Pecan 3
Ulmus alata Michaux Winged Elm 2
Carya tomentosa Sargent Mockernut Hickory 1
Total 100
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restrictions prevent the use of a UAS), our models can help provide a more accurate
estimate of the number of Mistletoe plants present. In particular, this method may
be particularly useful for taller trees where accurate ground-based assessments of
Mistletoe load are more difficult to obtain compared to those on shorter trees. While
the UAS method was more accurate compared to ground surveys, the UAS was
more expensive overall due to the initial cost of a UAS purchase, but the time spent
to assess Mistletoe load was equivalent between both methods.
In the US, UAS are flown under 2 Federal Aviation Administration (FAA) regulations:
FAA 336, the Special Rule for Model Aircraft that can be used for training
Figure 3. Comparison
of Phoradendron
leucarpum (American
Mistletoe) counts
observed from the
ground to those observed
from a UAS
for: (A) all trees surveyed,
(B) trees that
were greater than 16.7
m in height, and (C)
trees that were ≥16.7
m in height.
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pilots; and FAA 107 is the UAS license after completion of an exam at a certified
FAA center. UAS flight rules include maintaining the UAS in the pilot’s line of
sight, flying only during daylight hours, flying less than 122 m above the ground,
using a UAS that weighs less than 25 kg, and not flying over people without their
knowledge (FAA 2016a, 2016b, 2018). For this UAS method to be utilized, it is
important that personnel adhere to both FAA and local regulations. For example,
for this project, pilots flew under FAA 107 and obtained permission from the entities
that controlled the area.
Mistletoes are an important resource for wildlife because they provide food in
the form of nectar and fruit, as well as habitat for nest sites for insects and birds
(Watson 2001, Whittaker 1984). Mistletoe is dispersed primarily by birds; thus,
more-accurate counts of plants may provide evidence of increased resource use
of Mistletoe by birds in urban environments that previously estimated (Whittaker
1984). When these plants are established, accurate counts may provide insights as
to their use by host-restricted insect herbivores (Whittaker 1984). Trees already
infected are more likely to be continually infected from seeds in the crown (Coder
2016). Taller trees have more Mistletoe plants (Carlo and Aukema 2005, Coder
2016), which underscores the value of these taller trees for birds as a reliable resource
for both food and nesting sites. For our study, the taller-tree model (Fig. 3B)
highlighted the value of using UAS for counting Mistletoes.
With our method, the applicability of using a UAS to survey for Mistletoe in
other environments depends on the ability of the UAS to capture images of the
tree from different aspects, which was feasible at our study site as the trees often
occurred in urban greenspaces, were spaced far apart, and were assessed during
winter when tree leaves did not obscure Mistletoe plants. The utility of this survey
method is most applicable in similar situations. The utility of using a UAS to capture
oblique images of tree crowns in large urban forests may be more challenging,
depending on factors such as tree density, spacing, and timing. However, if oblique
images cannot be obtained, there is still utility in being able to capture nadir images
of tree crowns in these forests (Fig. 2A), as the tops of trees are otherwise difficult
to examine and incorporate into rating systems (Smith 1969).
Tree symptoms appear with increasing Mistletoe infections including branch
dieback, branch girdling, breakage, and even death of the host tree (Mathiasen
et al. 2008). If the parasite load is high enough, Mistletoe infection sites are a
platform for fungal and insect infection that can further degrade the branches
(Mathiasen et al. 2008). Further, trees under drought stress are more prone to
branch breakage with Mistletoe infections (Coder 2016). Coder’s (2016) rating
system, based on the number of Mistletoe plants in a host tree, could be used to
make management decisions and prioritize treatment efforts. Underestimates of
counts and parasite load may delay treatment for Mistletoe, including potential
tree removal, highlighting the importance for UAS to aid in the evaluation of
Mistletoe load on trees in urban environments.
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Acknowledgments
We thank USDA NIFA McIntire-Stennis and Stephen F. Austin State University for
funding. We also thank E. Lozano for creating Figure 1, C.P. Randle for informative discussions
on mistletoe taxonomy and systematics, and L. Fischer and 2 anonymous reviewers
for their constructive comments on the manuscript.
Literature Cited
Carlo, T.A., and J.E. Aukema. 2005. Female-directed dispersal and facilitation between a
tropical mistletoe and a dioecious host. Ecology 86:3245–3251.
Chang, M., L.D. Clendenen, and H.C. Reeves. 1996. Characteristics of a humid climate.
Center for Applied Studies in Forestry, Stephen F. Austin State University, Nacogdoches,
TX, USA. 211 pp.
Coder, K.D. 2016. American Mistletoe: Tree infection, damage, and assessment manual.
Warnell Outreach, School of Forestry and Natural Resources, University of Georgia
Publication No. 36, Athens, GA, USA. 38 pp.
Dwyer, J.F., and M.C. Tincher. 2018. Use of small unmanned aircraft systems (UAS) to
assess and quantify risk of entanglement in synthetic baling twine in Pandion haliaetus
(Osprey) Nests. Urban Naturalist 17:1–6.
Federal Aviation Administration (FAA). 2016a. Model-aircraft operating standards, US Department
of Transportation, Advisory Circular No.91-57A. Available online at https://
www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-57A_Ch_1.pdf. Accessed
2 November 2018.
FAA. 2016b. Summary of small unmanned aircraft rule (Part 107). FAA News, , Washington,
DC. Available online at https://www.faa.gov/uas/media/Part_107_Summary.pdf.
Accessed 2 November 2018.
FAA. 2018. Unmanned aircraft systems. Available online at https://www.faa.gov/uas/getting_
started/. Accessed 2 November 2018.
Gairola, S., A. Bhatt, Y. Govender, H. Baijnath, S. Proches, and S. Ramdhani. 2013. Incidence
and intensity of tree infestation by the mistletoe Erianthemum dregei (Eckl.
& Zeyh.) V. Tieghem in Durban, South Africa. Urban Forestry and Urban Greening
12:315–322.
Hawksworth, F.G. 1977. The 6-class Dwarf Mistletoe rating system. USDA Forest Service
General Technical Report RM-48. Rocky Mountain Forest and Range Experiment Station,
Fort Collins, CO, USA. 7 pp.
Hugenholtz, C.H., B.J. Moorman, K. Riddell, and K. Whitehead. 2012. Small unmanned
aircraft systems for remote sensing and earth-science research. Eos, Transactions of the
American Geophysical Union 93:236–237.
Kendig Keast Collaborative. 2017. City of Nacogdoches master plan for parks, recreation,
and trails. Available online at https://www.ci.nacogdoches.tx.us/1267/Parks-Recreation-
and-Trails-Master-Plan. Accessed 2 November 2018.
Kulhavy, D.L., D.R. Unger, I. Hung, and Y. Zhang. 2016. Comparison of AR dronequadricopter
video and the visual CTLA method for urban-tree hazard rating. Journal
of Forestry 114:517–523.
Mathiasen, R.L., D.L. Nickrent, D.C. Shaw, and D.M. Watson. 2008. Mistletoes: Pathology,
systematics, ecology, and management. Plant Disease 92:988–1006.
Overton, J.M. 1996. Spatial autocorrelation and dispersal in mistletoes: Field and simulaUrban
Naturalist
D.L. Kulhavy, C.M. Schalk, R.A. Viegut, D.R. Unger, S.W. Schockley, and I-K. Hung
2019 No. 20
10
tion results. Vegetatio 125:83–98.
Sangüesa-Barreda, G., J.C. Linares, and J.J. Camarero. 2012. Mistletoe effects on Scots
Pine decline following drought events: Insights from within-tree spatial patterns,
growth, and carbohydrates. Tree Physiology 32:585–598.
Sayad, E., E. Boshkar, and S. Gholami. 2017. Different role of host and habitat features in
determining spatial distribution of mistletoe infection. Forest Ecology and Management
384:323–330.
Smith, R.B. 1969. Assessing Dwarf Mistletoe on Western Hemlock. Forest Science
15:277–285.
Ward, M.J., and D.C. Paton. 2007. Predicting mistletoe seed-shadow and patterns of seed
rain from movements of the Mistletoebird, Dicaeum hirundinaceum. Austral Ecology
32:113–121.
Watson, D.M. 2001. Mistletoe: A keystone resource in forests and woodlands worldwide.
Annual Review of Ecology and Systematics 32:219–249.
Whitehead, K., and C.H. Hugenholtz. 2014. Remote sensing of the environment with small
unmanned aircraft systems (UASs), part 1: A review of progress and challenges. Journal
of Unmanned Vehicle Systems 2:69–85.
Whitehead, K., C.H. Hugenholtz, S. Myshak, O. Brown, A. LeClair, A. Tamminga, T.E.
Barchyn, B. Moorman, and B. Eaton. 2014. Remote sensing of the environment with
small unmanned aircraft systems (UASs), part 2: Scientific and commercial applications.
Journal of Unmanned Vehicle Systems 2:86–102.
Whittaker, P.L. 1984. The insect fauna of mistletoe (Phoradendron tomentosum, Loranthaceae)
in southern Texas. Southwestern Naturalist 29:435–444.