Acorns and Habitat: Oaks Support a Diversity of Forest Wildlife
1. Acorns & Habitat: Oaks Support
a Diversity of Forest Wildlife
Mike Fargione
Field Research and Outdoor
Programs Manager,
Cary Institute of Ecosystem
Studies
Fargione
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Oaks
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2. Oaks: Foundation Trees
• Directly and indirectly
impact Eastern forests.
• Their loss would impact
energy and nutrient
flow, hydrology, food
webs and biodiversity.
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3. Oaks as Habitat and Food
• Trunks, branches and leaves
provide nest platforms,
cavities, camouflage and
roosts.
• Buds and leaves are food for
insects, mammals and birds.
• Fruit (acorns) are important
food for birds and mammals
during the dormant season.
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4. Oak Groups:
“Reds” “Whites”
• Northern Red Oak
• Black Oak
• Scarlet Oak
• Acorns grow for 2
seasons; overwinter
before germination.
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• White Oak
• Chestnut Oak
• Swamp Oak
• Acorns grow in 1
season; germinate 1st
autumn.
5. Mast and Masting
• Mast are fruit of plants that
produce seeds in boom and
bust cycles.
• Mast Years are years with
abundant (boom) seed crop.
• Individual trees are
“masting” if they produce a
large quantity of seeds that
year.
• Soft (fleshy pulp) versus
hard (hard seed coat) mast.
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So#
Mast
Hard
Mast
6. Hard Mast
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• Hard outer shell that protects contents and
maintains moisture.
• Seeds can be stored for extended periods.
• Includes many dominant trees in E. North
American forests.
Steven
J.
Baskauf
h;p://bioimages.vanderbilt.edu/"
7. Oak Masting Behavior
• Boom or bust – many years
of small crops interrupted
by occasional booms in
acorn production.
• Synchrony among related
species in a region.
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8. Masting As An Evolved Strategy
• Seed production to maximize
plant reproduction, not to
feed wildlife.
• Small mammal populations
rise and fall in response to
acorn availability.
• Acorn fluctuations reduce
seed predators after poor
years and overwhelm them in
years of plenty, allowing
some acorns to escape and
germinate.
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9. “Acorns are the most important
wildlife food in deciduous forests of
North America, the ecological
equivalent of manna from heaven.”
McShea and Healey, 2002. Oak Forest Ecosystems – Ecology and Management for
Wildlife
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10. Wildlife Winter Survival Strategies
• Migrate.
• Switch to other readily-
available foods.
• Store sufficient fat to hibernate
or persist when food is scarce.
• Forage for and/or collect and
store persistent, high-energy
food for lean times.
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11. Acorn Nutritional Value -
Highly-digestible, high-energy, low
protein in a convenient, storable packet!
“Reds” “Whites”
Fat 14-23% 3-9%
Cell-
Solubles 50-70% 50-70%
Protein 5-8% 5-8%
Tannins 6-10% <2%
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12. Acorns as Wildlife Food -
Abundant acorns means:
• Consume large quantities in
short time, reduce foraging
time and building fat
reserves.
• Reduced foraging time
lowers their energy costs and
shortens exposure to
predators.
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13. How Many Does it Take?
• 12 acorns satisfies daily
energy requirements of a
ruffed grouse.
• 26-37 acorns satisfies
daily energy requirements
of a wild turkey at 32F. 6
extra acorns for every
additional 18F
temperature drop.
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14. Acorn Advantage?
Some studies have shown:
• Fall mortality of wild turkeys may increase when
heavy hunting pressure is combined with mast
failure.
• Mean body mass of fawn and yearling deer is
positively correlated to acorn abundance.
• Yearling deer reproductive rates (ovulation and #
fetuses) and male antler characteristics ( size and
# points) are positively correlated with acorn
abundance the previous fall.
• Black bear reproductive success is higher in areas
with excellent versus poor acorn production.
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15. Oak - Wildlife Species Profiles
• White-footed Mice
• Grey Squirrels
• White-tailed Deer
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16. White-footed Mouse
(Peromyscus leucopus)
• Mostly nocturnal, woodland, spending
time both on ground and in trees;
• Food seasonal. Primarily seeds
(particularly acorns and black cherry
pits), but also eat some vegetable
and animal matter. Animal matter
includes moth pupae and bird eggs.
• Store acorns individually in cavities/
tunnels and beneath debris.
Consume all the acorns they store.
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17. Mice – Oak Interactions
Abundant acorns increase mouse survival and
reproduction. Mouse numbers impact other wildlife.
More mice means:
• More gypsy moth pupae are eaten
and less oak foliage is damaged.
• More black-legged ticks and greater
chance they will be infected with
Lyme Disease the following summer.
In turn, higher-than-usual risk of
Lyme disease to humans during
second summer.
• Increase rates of predation on
ground-nesting bird eggs.
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Photo:
R.
Moore
18. Grey Squirrels
(Sciurus carolinensis)
• Arboreal; nests in tree cavities or
leaf nests among branches.
• Food primarily nuts, seeds, buds,
fungi and bird eggs.
• Acorns comprise a large portion
of their seasonal diet.
• Grey squirrels do not recover all
the acorns they hide; as such
they are important in dispersing
and planting acorns.
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19. Acorn Feeding Strategies of Grey
Squirrels – Critical to Oak Forests
• Eat now or store for later? –
critical to regeneration.
• Disperse individual nuts under
leaf litter or soil; = favorable
microclimate and protection
from other predators.
• Critical role in successfully
dispersing and aiding
germination of acorns.
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20. Acorn Storage Strategies of Grey
Squirrels
• Acorns loose nutritional
value upon germination
• Cache spring-germinating
acorns (“reds”) and
consume fall-germinating
(“whites”).
• Immediately eat insect-
damaged “reds”.
• Notching’ (embryo excision)
of fall-germinating acorns to
permit prolonged storage.
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21. White-tailed Deer
(Odocoileus virginianus)
• Browsers exhibiting highly-
selective food preferences
based on availability.
• Voluntarily restrict activity
and food intake during
winter, changing behavior
and seeking protected areas;
goal to conserve energy not
maximize food availability.
• Obligatory period of fall
‘lipogenesis’ or fat storage.
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22. Deer-Oak Interactions
• Body condition is critical to
over-winter survival.
• Highly-nutritious, fat-rich
acorns are important,
preferred deer food during
Fall-Spring.
• Acorn availability alters deer
habitat use.
• A large % of undamaged,
fallen acorns are consumed
by deer (1 kg/deer/day).
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23. Seedling Use by Deer
• Deer need to consume 6+
pounds of dormant buds per
day to maintain their body
weight.
• Deer loose significant
amounts of weight on a
browse-dominated diet (up
to 25-30% and survive).
• Oak seedlings are a
preferred winter browse.
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24. Seedling Use by
Deer at Cary
1994 - 2016
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0
500
1000
1500
2000
2500
3000
3500
#Budsavailable
Mean Available Buds Per Year - 1994-2016
0
2
4
6
8
10
12
14
16
%BudsBrowsed
Mean % Browsing Per Year - 1994-2016
25. “I now suspect that just as a deer
herd lives in mortal fear of its
wolves, so does a mountain live in
mortal fear of its deer.”
Aldo Leopold
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26. Overabundant Deer:
Effects on Forests
Leads to:
• Over browsing and reduced deer food.
• Loss of seed crop and existing plant
seedlings needed for regeneration.
• Reduced plant numbers and species.
• Loss of desired timber species and
increases in undesirable plants/
invasive species.
• Loss of forest structure in seedling/
sapling zone.
• Reduction in abundance of forest-
dwelling animals.
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27. Deer Impacts on Oak Forests
Deer exclosure study, buck-only
hunting site, Dutchess County, NY
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Unfenced
Fenced
28. Deer Impacts on Oak Forests
• Over browsing by deer threatens
the survival and perpetuation of
oak-dominated forests.
• Loss of oaks and acorns would
be devastating for a host of
animal species and an economic
loss of timber resources.
• Management for oaks may
require significant reductions
in deer populations.
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29. Interactions in Eastern Oak Forests*
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Oak trees and their acorn
production cycles are the basis for
complex food webs that directly and
indirectly impact many wildlife
species.
30. Interactions in Eastern Oak Forests*
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*
Adapted
from
Os<eld,
Jones
and
Wolff,
1996
31. Oak – Wildlife Interactions
in Eastern Forests
Maintaining oak dominated forests and
the acorns they produce is a worthy goal
for public and private lands.
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32. Further Reading
Ellison, et. al. 2005. Loss of foundation species: consequences for
the structure and dynamics of forested ecosystems. Frontiers in
Ecology and the Environment, 3(9):479-486.
Johnson, P.S., Shifley, S.R. and R. Rogers. 2011. The Ecology and
Silviculture of Oaks, 2nd Ed. U.S. Department of Agriculture, Forest
Service, Northern Research Station: 674 pp.
McShea, W.J. and W.M. Healy, Editors. 2002. Oak Forest Ecosystems,
Ecology and Management for Wildlife. The John Hopkins University
Press. 423 pp.
Ostfeld, R.S. 2011. Lyme Disease, The Ecology of a Complex System.
Oxford University Press. NY, NY. 216 pp.
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