Acorns and Habitat: Oaks Support a Diversity of Forest Wildlife

Acorns & Habitat: Oaks Support a Diversity of Forest Wildlife

Mike Fargione Field Research and Outdoor

Programs Manager, Cary Institute of Ecosystem

Studies

Fargione -‐ Oaks & Wildlife

Oaks: Foundation Trees

•  Directly and indirectly impact Eastern forests.

•  Their loss would impact energy and nutrient flow, hydrology, food webs and biodiversity.


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.


Oak Groups:

“Reds” “Whites”

•  Northern Red Oak

•  Black Oak

•  Scarlet Oak

•  Acorns grow for 2 seasons; overwinter before germination.


•  White Oak

•  Chestnut Oak

•  Swamp Oak

•  Acorns grow in 1 season; germinate 1st autumn.

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.


So# Mast Hard Mast

Hard Mast


•  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/"

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.


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.


“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


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.


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% Fargione -‐ Oaks & Wildlife

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.


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.


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.


Oak - Wildlife Species Profiles

•  White-footed Mice

•  Grey Squirrels

•  White-tailed Deer


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.


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.


Photo: R. Moore

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.


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.


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.


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.


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).


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.


Seedling Use by Deer at Cary 1994 - 2016


0

500

1000

1500

2000

2500

3000

3500

# B

uds

avai

labl

e

Mean Available Buds Per Year - 1994-2016

0 2 4 6 8

10 12 14 16

% B

uds

Bro

wse

d

Mean % Browsing Per Year - 1994-2016

“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


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.


Deer Impacts on Oak Forests

Deer exclosure study, buck-only hunting site, Dutchess County, NY


Unfenced Fenced

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.


Interactions in Eastern Oak Forests*


Oak trees and their acorn production cycles are the basis for complex food webs that directly and indirectly impact many wildlife species.

Interactions in Eastern Oak Forests*

Fargione -‐ Oaks & Wildlife * Adapted from Os<eld, Jones and Wolff, 1996

Oak – Wildlife Interactions in Eastern Forests

Maintaining oak dominated forests and the acorns they produce is a worthy goal for public and private lands.


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.
