deer as a facilitator of exotic species invasion in mature forests of virginia

William McSheaConservation Ecology Center, Smithsonian Conservation Biology Institute, Front Royal, VA

Deer as a facilitator of exotic species invasion in mature forests of Virginia

Appalachian Mountains/Ridge and Valley

• World-class temperate forest both for diversity and extent

• Large blocks of public forest

• Primarily deciduous forest

• Mature 2° forest (>100 yrs)

• Close to urban centers

SNP & M,GW&J NFs

Invasive plant and animal species – you know the story

White-tailed deer – you know the story

Stressors on eastern deciduous forests

• Forest loss, fragmentation, and parcelization• Loss of critical species (American chestnut, wolves, cougars)• Climate change (temperature, storm severity and frequency)

Browsing by deer alters forest succession pathways and sustainability

When Should We Worry About “Overabundant” Species

• Cause significant economic loss • Lower diversity across the landscape• Unsustainable demographics of rare species• Alters pathways for productivity or succession

A species that, other than as a result of an introduction, historically occurred, or currently occurs, in a particular ecosystem.

Invasive Species

Exotic SpeciesAny species, including its seeds, eggs, spores or other biological material capable of propagating that species, that is not native to a particular ecosystem.

An exotic species whose introduction does, or is likely to, cause economic, human health or environmental harm.

Native Species

In US – 20,000 native plant sp.; 4,000 alien plants; 400 invasive plantsLets agree to worry about invasive (native or exotic) first & exotic later

Abundant ground cover – little of it native species

Hidden native species are denied light but also invisible to deer??

The problem – low recruitment and diversity of native plants

• White-tailed deer prefer browsing native vegetation compared to exotic species, so native woody species not only compete with invasive species for light, but must survive preferential herbivory by deer.

The Solutions• Remove invasives. But - when invasive plants are removed - do plant populations increase or does herbivory increase? • Reduce deer numbers. But - when deer are removed - do natives increase or do invasive plants respond more quickly?

(-) (-)(-)

Can you increase recruitmentof woody seedling or species richness of herbaceous plants by fixing only one problem???Are you promising something you can not deliver when you try only one??

(+)Direct consumption

Competition for light and nutrients

Predictions:1) Reduced invasiveswill increase herbivory2) Reduced herbivorywill increase invasives

Project Details• Co-Investigators – Norm Bourg, Chad Stewart• C & O Canal and SCBI – 330 Plots• 4 x 4 m plot with 4 1x1 m subplots within each• ID and count all native herbaceous and woody plants

< 30cm high, within each sub plot. All saplings ( > 30 cm) within each plot. For grasses and ferns estimate % cover.

• Identify all exotic species, count and estimate % cover.

• Calculated species richness, species diversity (Shannon index, H’), and number of individuals per plot in 2005 (pre) and 2007 and 2009

Do Nothing

179 plots NN

45 plots NF

53 plots PN

49 plots PF

Measured all plots in summer 2005 andselect management category

Pulled Invasives twice each year: may and august

Timeline

• 325 plots established 2005 (+ 8 in 2006)• Invasives pulled twice each year & fences maintained• Plots resurveyed in 2007 and 2009

0

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Control Pulled Fenced Pull/Fence

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Treatments

After 4 years: Woody Species (> 30 cm)

native (all)

native (trees)

invasives (all)

Fencing (not pulling) increased native species (total or just trees)

Invasive species highest when deer excluded

Neither treatment achieved needed stocking rate after 4 years

Recommendedstocking rate forseedlings

2009 Sapling Data

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Control Fenced Only Pulled Only Pulled and Fenced

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lot

Treatment

Red MapleNo treatments increased numbers

2005 2009

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Control Fenced Only Pulled Only Pulled and Fenced

Treatment

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Quercus SpeciesOnly combined treatment increased numbers

*

2005 2009

Oak Saplings in 2009Treatment N of Plots N of Saplings Density

Control 183 4 0.02/plotPulled 55 0 0.00/plotFenced 50 6 0.12/plotPull/Fence 48 16 0.33/plot

Forestry matters: decline of oaks in eastern forests JWM 2007, McShea et al. based on FIA data

• Decline in proportion of intermediate-sized oaks in eastern forests: 32% in 1989 down to 21% in 2000

• stem density ofmaples doubled from660 (+ 201) in 19891303 (+246) in 2000

• no significant changein mast production yet

• maples are immune todeer and invasives and(lack of fire), whileoaks are not

SIGEO OriginsSmithsonian Tropical Research Institute (STRI)

In 1980, Steve Hubbell and Robin Foster established a 50-hectare (124-acre) forest plot in Panama. The survey methodology was unprecedented in scale and scope.

Every tree greater than or equal to 1cm diameter was identified, measured, tagged, and mapped. All trees are re-censured every 5 years.

Barro Colorado Island

Setting the Stage for this StudyStudy took place on SIGEO Plot at SCBI

A “base layer” of tree communityis overlaid with multiple explanatory and ancillary layers to understand forestcommunity structure

26 ha plot is created in 2008With 20 x 20 m grid cellswith each post marked with row and columnDue to be resurveyed in 2013

SCBI - SIGEO Plot Location

Tree Measurements- all stems > 1cm dbh measured, mapped, tagged & identified to species

Layers subsequently added: DEM, Downed woody debri; Soil nutrients and carbon; Seedlings; Small mammals; Invasive worms; Invasive exotic plants

Initial census of 25.6 ha completed:

- 40,180 stems (29,914 living); 26 Families, 38 genera

- 62 species of free-standing trees & shrubs, 54 of which are native;

Composition:

- Mature secondary Eastern mixed deciduous forest – dominant species:

Canopy: Understory:- Liriodendron tulipifera Asimina triloba- Carya glabra Carpinus canadensis- Carya tomentosa Cornus florida- Quercus alba Cercis canadensis- Q. rubra Amelanchier arborea- Nyssa sylvatica- Fraxinus americana Shrubs: Lindera benzoin (>17,000 stems)

- Succeeding to oak-hickory forest (8 spp. oaks, 4 spp. hickory + 2 Juglans spp.)

SCBI - SIGEO Forest Dynamics Plot

26 ha site includes 4 ha deer exclosure erected in 1990

8 ft fence with 4 ft of American wire fencing(set with larger mesh on bottom)and 6 strands of high tensile wire (4 ft)8 ft 2”x2” spacers with 6 slots (10o angle) hold high tensile fence in place

Added Twist to Plot

Idea for study came mainly from distribution of Microstegium in plotrelative to deer exclosure

4 ha deer exclosure erected in 1990 prior to invasion of most exotic species

Protocol for invasive species study• All 20 x 20 m grid cells examined in 2010• All invasive species recorded as present/absent• All individual clusters counted except Microstegium (5 abundance classes based on %)• Distributions matched with data layers

1) Are invasive plant distributions shaped by distribution of deer2) Underlying mechanisms for impact

Common invasive plants in forest

Wineberry Rubus phoenicolasius

Japanese barberry Berberis thunbergii

Common invasive plants in forest

Japanese stiltgrass Microstegium vimineum

Multiflora rose Rosa multiflora

Experimental DesignFour step process

1) Use canonical correlations to determine tree communities within each grid cell and select section of grid (reference plot) that “matched” deer exclosure

2) Insure that the range of all parameters within deer exclosure and reference plot are encompassed by the remaining squares (Phosphorus issue)

3) Permutation trees (conditional inference tree; party in R) were constructed using all squares outside of deer exclosure and reference plot (p < 0.99 to prune tree)

4) Repeated process using significant variables identified in first tree - using gird cells from deer exclosure and reference plot while adding “Fence” as variable

Colors reflect community types, created using a cluster analysis basedon basal area of the 10 dominant tree species in each unit (80%).

SIGEO study area (20 x 20 m2 quadrats)

Deer Exclosure (4 ha)erected 1990

Reference Plot (4 ha) matching communities

Norm Bourg

Jenny McGarvey and Xiaoli Shen

　 Min Max Mean SD

SPECIES ABUNDANCE

ROMU individual number (clumps) 0 33 1.01 2.96

BETH individual number (clumps) 0 138 6.44 11.79

RUPH individual number (clumps) 0 125 6.33 12.96

%MIVI cover in category 0 = absent; 1 = 1-25%; 2 = 26-50%; 3 = 51-75%; 4 = 76-100% 0 4 1.76 1.2

Comfrey individual number 0 116 10.99 18.83

PREDICTOR VARIABLES

No. of deer fecal pellet groups 0 52 1.93 4.99

Distance from quadrat center to forest edge (m) 21 523 282.89 115.5

Transformed aspect† -1 1 -0.008 0.72

Slope (degrees) 1.9 21.4 10.34 3.81

Topographic convergence index‡ 0 280.8 34.45 30.35

Woody species richness 3 20 10.16 2.85

No. of woody stem 7 582 60.37 71.22

pH 3.96 6.11 5.09 0.47

ECEC (estimated cation exchange capacity, cmol/kg) 6.98 41.26 18.13 5.65

Nitrogen (mg/kg) 2.13 9.09 4.3 1.16

Phosphorus (mg/kg) 10.31 35.05 20.1 5.82

% canopy openness (hemispherical photo) 3.69 20.89 9.07 2.16

Wild Comfrey Cynoglossum virginianum

11.39 ± 1.17 38.49 ± 4.48 4.15 ± 0.79

21.79 ± 2.20 6.21 ± 1.71 0.01 ± 0.01

Test Case – known deer-dispersed plant

Multiflora rose Rosa multiflora

More abundant in exclosureHigher nitrogen levels increase abundance

1.00 + 0.15

0.22 + 0.051.55 + 0.430.29 + 0.10

4.58 + 1.01

Japanese barberry Berberis thunbergii

Soil cations important (organic content)Twice as abundant outside of exclosure

2.17 + 0.39

1.80 + 0.333.26 + 0.25

5.32 + 0.80 11.16 + 1.0

Wineberry Rubus phoenicolasius

High nitrogen and low shrub density increases abundance- but in cells with low nitrogen more abundant with deer

3.25 + 0.51

7.11 + 1.290.22 + 0.092.25 + 0.36

20.9 + 2.68 9.82 + 1.28

Japanese stiltgrass Microstegium vimineum

Highest abundances found with low density of shrubs and high pH or high Levels of P and understory openness

Absence found almost only within deer exclosure

0 = absent; 1 = 0-25%; 2 = 26-50%; 3 = 51-75%; 4 = 76-100%.

1.58+0.14 2.20+0.12 0.49+0.06

1.76+0.13 2.60+0.1 1.39+0.09 1.67+0.16 2.33+0.12

Hard Seed Crops Critical for Wildlife Populations in Eastern Forests

• In order to produce seed we need mature trees• In order to produce mature trees we need:

– Lower densities of deer– Control of invasive plants– Periodic light reaching forest floor – disturbance

• Fire, harvest, hurricanes, insect outbreaks

These are not problems with seedling establishment but problems with the transition from seedling to sapling

A “Perfect” World• Overall Lower Density of Deer• Removal of invasive, exotic species• Create Spatial and Temporal Heterogeneity• Increase the Movement of Ungulates with

Respect to Resources (Predators)• Increased Avenues for Flow of Productivity and

thereby Increased Biodiversity

What does this all mean?• Deer do play a role in establishment of some exotic species in this forest• For one species (multiflora rose) the distribution of the plant was not influenced by deer – bird-dispersed seeds• Mechanism may not be the same for each exotic species• Mechanisms for facilitation are not clear from this study

• Deer actively spread seeds through feces or on coat – possible but unlikely• Deer activity changes soil characteristics and nutrient levels – possible (N, ECEC, P lower)• Absence of deer reduces leaf litter disturbance - possible• Native plants established before arrival of most invasives (1990 fence) - possible

Virginia Working Landscapes

Bill McShea1, Maria Van Dyke1, and James Barnes2

1. Smithsonian Conservation Biology Institute, Front Royal, VA2. Piedmont Environmental Council, Warrenton, VA

Mission: To encourage the sustainable use of Virginialandscapes for biodiversity

www.vaworkinglandscapes.org

Landowners

Develop a Conservation Network:

Grassland Research:Measuring biodiversity on grasslands in the Northern Shenandoah Valley and Piedmont Regions

1. Richness and Abundance of:• Birds• Pollinators• Native Plants

2. Effects of Management programs on these three biota

3. Conversion of introduced (CSG) to native grasslands (WSG)

Wildlife WSG

Not hayed Not grazed

New and/orManaged WSG

Hayed or ≤ 2 yrs old

Managed CSG

Hayed or grazed

Wildlife CSG

Not hayed Not grazed

All Fields = 20 Acres

BirdsSurvey Protocol TechniquesField Identification Training

Native Bees and ButterfliesSurveys Protocol TechniquesEcology and Life HistoryIdentification Training

PlantsSurvey Protocol TechniquesField and Microscope Identification

Citizen Science

Gypsy moths

DEERDEERMast Production

Oak Trees

Under-story seedlings

Pathogens

Humans

Sunlight

Ticks

Non-mast seeds

Fruit -eating birds

Ground-nesting birds

mesopredators

Soil MoistureSoil Nutrients

Rodents

Oak seedlings

Mast-eating birds

Pathogens

Fire