Response of Forest Insects and their Natural Enemies to Experimental Ice Storms in a Northeastern Forest Final Report to the Edna Bailey Sussman Foundation Wendy Leuenberger State University of New York College of Environmental Science and Forestry Introduction Concomitant with warming temperatures, climate change will increase the frequency and intensity of extreme weather events. Ice storms are one such event that can have major environmental, societal, and economic consequences as they can encompass large areas and affect both natural ecosystems and human infrastructure and commerce. A novel experiment at Hubbard Brook Experimental Forest (HBEF) in New Hampshire simulated ice storms of varying severity this winter by spraying water into the canopy under freezing conditions to develop an empirical understanding of their effects on forests. I interned with HBEF investigating the effects of these experimental ice storms on avian and caterpillar communities. I also developed and disseminated a lesson plan inspired by my research.
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· Web viewI reared gypsy moth caterpillars (Lymantria dispar dispar) in the lab to determine whether treatment affected caterpillar growth rates. Caterpillars were fed sugar maple
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Response of Forest Insects and their Natural Enemies to Experimental Ice Storms in a Northeastern Forest
Final Report to the Edna Bailey Sussman FoundationWendy Leuenberger
State University of New York College of Environmental Science and Forestry
Introduction
Concomitant with warming temperatures, climate change will increase the frequency and
intensity of extreme weather events. Ice storms are one such event that can have major
environmental, societal, and economic consequences as they can encompass large areas and
affect both natural ecosystems and human infrastructure and commerce. A novel experiment at
Hubbard Brook Experimental Forest (HBEF) in New Hampshire simulated ice storms of varying
severity this winter by spraying water into the canopy under freezing conditions to develop an
empirical understanding of their effects on forests. I interned with HBEF investigating the effects
of these experimental ice storms on avian and caterpillar communities. I also developed and
disseminated a lesson plan inspired by my research.
Methods
Ten plots were chosen for ice storm manipulation at Hubbard Brook Experimental Forest in New
Hampshire, USA. Plots encompassed mixed hardwood stands co-dominated by American beech
(Fagus grandifolia), sugar maple (Acer saccharum), and yellow birch (Betula alleghaniensis)
with an age of approximately 100 years. Fixed area plots (20 m × 30 m; approximately
basketball court-sized) were assigned to one of five treatments, with two plots in each treatment
(Fig. 1). Icing in winter 2016 occurred between January 18th and February 11th. Pilot methods for
application of ice storms were described in Rustad and Campbell (2012) and updated methods
will be published by these authors and additional investigators.
Figure 1. Experimental process of simulating ice storms: application (a), ice accretion (b), branches breaking (c), and downed branches (d) (Rustad and Campbell 2012).
Figure 2. The author measuring ice accretion in a high treatment plot (¾ inch/19 mm ice accretion). Photo courtesy of the Hubbard Brook Research Foundation.
Figure 1. Terrain (LiDAR) map of ice storm experiment plots with colors representing the target amount of experimentally-applied ice accretion. Hubbard Brook is in the northern part of the figure and the main road is south of the plots. Plots 1 and 8 will be iced in both 2016 and 2017 while all other icings were applied only in 2016. The plots were located within Hubbard Brook Experimental Forest in New Hampshire.
In addition to these ten ice storm plots, I sampled six additional untreated plots (n = 16
plots total) to address the role of edge effects. Untreated plots were located 100 m away (n = 3)
and 250 m away (n = 3) from the nearest ice storm plot (Fig. 2). Plots were located in areas with
comparable vegetation species and structure to the manipulation plots. For point counts, I also
sampled an additional location between manipulation plots 1 and 2 (plot ISEa), for a total of 17
point count locations.
Fig 2. Location of plots for the ice storm experiment at Hubbard Brook Experimental Forest in New Hampshire. Ice storm treatments are located within ISE1-ISE10. Additional control plots include ISEa and plots A100-C250. Circles represent the 15 m point count radii. I visited each point count location six times in 2015 and 2016.
.I assessed avian communities using point counts, where I recorded birds during six 10-
minute intervals per year in the ice storm plots. I restrained my analyses to only include foliage
gleaning birds such as warblers and vireos. These species search leaves and branches for food,
such as caterpillars.
I used plasticine caterpillars to estimate relative predation rates and identify predators of
Lepidopteran larvae. Plasticine is a type of non-drying clay commonly used for stop-motion
photography. In comparative studies, these simple models suffer levels of attack similar to real
larvae, and thus provide a useful comparison of predation intensity, causation, and impact across
treatments. I created and deployed 50 light green geometrid (inchworm) models in each plot for
six days. Caterpillars were glued to leaf petioles or next to leaves in life-like typical day time
inchworm poses (Fig. 3, Howe et al. 2009). Caterpillars were checked every other day to look for
evidence of predation and removed if predated (Bereczki et al. 2014). Remaining caterpillars
were removed after six days (Bereczki et al. 2014). All caterpillars were examined under 1.75×
magnification for evidence of predation that could have been missed in the field (Howe et al.
2009). ‘Wounds’ on retrieved caterpillars were ascribed to birds, invertebrate predators, or small
mammals based on characteristic damage (Low et al. 2014).
Figure 3. (a) Plasticine caterpillar (left) next to a notodontid caterpillar (right) at Hubbard Brook Experimental Forest, NH, in June 2015. (b) Caterpillar model with clear evidence of bird predation.
I reared gypsy moth caterpillars (Lymantria dispar dispar) in the lab to determine
whether treatment affected caterpillar growth rates. Caterpillars were fed sugar maple or beech
leaves for three days. Caterpillars and leaves were weighed before and after the experiment to
determine relative growth and consumption rates.
I expected that caterpillar predation and avian use of treated areas would increase due to
additional structural heterogeneity resulting from downed limbs and other ice storm-related
damage. I predicted that caterpillars would grow better on leaves from trees in the higher
treatment plots as there is more light reaching the leaves, resulting in more nutritious leaves.
Results
I recorded observations of 125 birds from 13 species during point counts. Of these, 99
observations from eight species were foliage gleaners with more observations in 2016 (n = 76)
than in 2015 (n = 23). I recorded between two and 13 observations of foliage gleaners at each
plot over the course of two years. The most common species (> 10 observations) were red-eyed
vireos (Vireo olivaceus; n = 42), black-throated blue warblers (Setophaga caerulescens; n = 16),
and black-capped chickadees (Poecile atricapillus; n = 10). The index of abundance for birds is
greater in the high treatment plot than in plots without treatment. No other differences among
treatments were found.
Fig 4. Detection-corrected index of avian abundance with 95% confidence intervals for foliage gleaning birds in response to experimental ice storm treatments. Letters indicate significant differences among treatments (α = 0.05). Icing was applied to two 20 m × 30 m plots per treatment during January and February, 2016 at Hubbard Brook Experimental Forest in New Hampshire.
In 2015, 202 caterpillars were predated by birds (n = 63), invertebrates (n = 123), and
small mammals (n = 23). In 2016, 177 caterpillars were predated by birds (n = 59), invertebrates
(n = 99) and small mammals (n = 23). I recovered 792 out of 800 caterpillars per year, for a
recovery rate of 99%. The main model explaining caterpillar predation by birds is that predation
rates were lower at the additional control plots than the ice storm experiment plots. This result
indicates that predation rates are linked more to the density of caterpillars than to the effects of
ice storm treatments, as birds learn that plasticine caterpillars aren’t food and stop trying to eat
them over time. Small mammal and invertebrate predation was primarily driven by whether or
not the caterpillars came unglued and fell to the ground where they were more available to these
predators.
Caterpillar growth rates increased with ice storm treatment for both sugar maple and
birch. Growth rates were higher on sugar maple than on beech, as sugar maple leaves are more
nutritious than beech leaves.
Fig 5. Relative growth rates of gypsy moth caterpillars on American beech and sugar maple leaves from ice storm treatment plots. Treatments include control (0 mm), low (6 mm), medium (13 mm) and high (19 mm) of ice accretion. Icing was applied to two 20 m × 30 m plots per treatment during January and February, 2016 at Hubbard Brook Experimental Forest in New Hampshire.
bab
a
c
b
aa
c
Implications
Birds are spending more time in the higher treatment plots. While there is no evidence of
changes in predation rates by birds, caterpillars grow faster on leaves from the higher treatment
plots. Therefore, my conjecture is that birds are attracted to the canopy gaps caused by the
experimental ice storms to search for more or higher quality food.
Lesson plan
I worked with Jackie Wilson from the Hubbard Brook Research Foundation to improve the
lesson plan I piloted last year. She aligned the lesson plan with Next Generation Science
Standards for middle and high school students, which ties the lesson into public school
curriculum to make it easier for teachers to implement the plan. The lesson plan is now publicly