Early

Vegetation

Development

on

an

Exposed

Reservoir

Implications

for

Dam

Removal

The

drawdown

of

Horsetooth

Reservoir

Colorado

for

dam

maintenance

provides

a

case

study

analog

of

vegetation

response

on

sediment

that

might

be

exposed

from

removal

of

a

tall

dam

Early

vegetation

recovery

on

the

exposed

reservoir

bottom

was

a

combination

of

-LRB-

-RRB-

vegetation

colonization

on

bare

moist

substrates

typical

of

riparian

zones

and

reservoir

sediment

of

shallow

dams

and

-LRB-

-RRB-

a

shift

in

moisture

status

from

mesic

to

the

xeric

conditions

associated

with

the

pre-impoundment

upland

position

of

most

of

the

drawdown

zone

Plant

communities

changed

rapidly

during

the

first

four

years

of

exposure

but

were

still

substantially

different

from

the

background

upland

plant

community

Predictions

from

the

recruitment

box

model

about

the

locations

of

Populus

deltoides

subsp

monilifera

-LRB-

plains

cottonwood

-RRB-

seedlings

relative

to

the

water

surface

were

qualitatively

confirmed

with

respect

to

optimum

locations

However

the

extreme

vertical

range

of

water

surface

elevations

produced

cottonwood

seed

regeneration

well

outside

the

predicted

limits

of

drawdown

rate

and

height

above

late

summer

stage

The

establishment

and

survival

of

cottonwood

at

high

elevations

and

the

differences

between

the

upland

plant

community

and

the

community

that

had

developed

after

four

years

of

exposure

suggest

that

vegetation

recovery

following

tall

dam

removal

will

follow

a

trajectory

very

different

from

a

simple

reversal

of

the

response

to

dam

construction

involving

not

only

long

time

scales

of

establishment

and

growth

of

upland

vegetation

but

also

possibly

decades

of

persistence

of

legacy

vegetation

established

during

the

reservoir

to

upland

transition

Introduction

Although

more

than

dams

have

been

removed

in

the

United

States

in

the

past

century

dam

removal

has

only

recently

received

significant

attention

from

the

scientific

community

-LRB-

Beyer

Hart

and

Poff

Graf

-RRB-

Reasons

for

dam

removal

include

unsafe

conditions

or

loss

of

function

associated

with

aging

or

sediment-filled

structures

and

more

recently

environmental

restoration

-LRB-

Hart

et

al.

Pohl

-RRB-

Dam

removal

decisions

involve

a

tradeoff

of

multiple

socioeconomic

and

ecological

costs

and

benefits

-LRB-

Stanley

and

Doyle

-RRB-

Potential

environmental

consequences

of

dam

removal

include

-LRB-

-RRB-

benefits

from

restoring

more

natural

flow

and

sediment

regimes

-LRB-

Poff

et

al.

Kondolf

-RRB-

and

-LRB-

-RRB-

removing

barriers

that

block

fish

passage

-LRB-

Lenhart

-RRB-

and

fragment

the

river

corridor

-LRB-

Nilsson

et

al.

-RRB-

Potential

negative

effects

include

the

impacts

of

releasing

stored

and

possibly

contaminated

sediment

and

enhancing

dispersal

of

undesirable

species

-LRB-

Bednarek

-RRB-

The

aspect

of

dam

removal

examined

here

is

the

ecological

fate

of

the

land

under

the

former

reservoir

pool

Trajectories

of

vegetation

response

on

lands

exposed

by

dam

removal

influence

higher-order

responses

such

as

human

and

wildlife

use

and

biogeochemical

processes

-LRB-

Shafroth

et

al.

-RRB-

In

some

cases

restoration

of

pre-dam

vegetation

may

be

a

management

goal

The

extent

to

which

this

can

be

accomplished

by

natural

colonization

and

subsequent

vegetation

change

may

significantly

affect

project

costs

In

some

cases

natural

colonization

may

be

the

default

management

action

because

of

budgetary

constraints

limited

mandates

or

lack

of

interest

in

restoration

Even

in

the

absence

of

a

specific

restoration

target

for

vegetation

on

the

exposed

surfaces

there

are

concerns

about

rapid

dominance

of

these

barren

areas

by

undesirable

weedy

non-native

species

and

the

need

to

provide

stabilizing

vegetation

to

minimize

erosion

Several

sources

of

information

support

projections

of

likely

future

vegetation

on

land

exposed

by

a

dam

removal

The

first

source

includes

studies

of

the

plant

communities

in

the

surrounding

upland

landscapes

in

the

riparian

zones

of

rivers

and

in

the

margins

of

lakes

and

reservoirs

The

description

of

these

communities

and

the

controls

on

their

composition

especially

vegetation

dynamics

on

disturbed

bare

ground

sites

provide

a

coarse

identification

of

possible

states

for

the

former

reservoir

pool

There

is

also

a

significant

literature

examining

vegetation

dynamics

within

periodically

exposed

lake

or

reservoir

shorelines

illustrating

the

importance

of

intra

and

inter-annual

water

level

fluctuation

-LRB-

Keddy

and

Rznicek

Hill

et

al.

-RRB-

and

positive

relations

between

species

richness

and

both

total

cover

and

substrate

fineness

-LRB-

Nilsson

and

Keddy

Nilsson

et

al.

-RRB-

Studies

of

floodplain

vegetation

colonization

and

dynamics

provide

information

on

likely

pioneer

species

and

subsequent

changes

associated

with

fluvial

processes

and

geomorphic

surfaces

within

river

bottomlands

in

many

regions

For

example

Friedman

et

al.

-LRB-

-RRB-

described

patterns

of

vegetation

change

on

the

floodplain

of

Plum

Creek

in

eastern

Colorado

where

species

richness

peaked

at

intermediate

ages

older

and

higher

surfaces

were

increasingly

dominated

by

rhizomatous

perennials

and

the

overall

species

list

was

non-native

In

the

western

United

States

much

of

the

focus

on

relations

between

streamflow

and

riparian

vegetation

has

centered

on

cottonwood

which

is

the

structurally

dominant

native

tree

Populus

deltoides

subsp

monilifera

-LRB-

plains

cottonwood

-RRB-

is

a

pioneer

species

with

a

relatively

narrow

regeneration

niche

The

requirements

for

a

bare

moist

surface

with

limited

drawdown

following

germination

have

been

represented

in

a

formal

recruitment

box

model

describing

the

floodplain

locations

and

patterns

of

water

stage

where

establishment

is

likely

for

P.

deltoides

subsp

monilifera

and

other

species

of

cottonwood

and

willow

with

similar

establishment

requirements

-LRB-

Mahoney

and

Rood

Rood

et

al.

-RRB-

This

model

is

a

clear

example

of

how

expectations

derived

from

the

study

of

riparian

plant

distributions

and

life

history

requirements

can

be

used

to

inform

an

assessment

of

recolonization

of

the

former

reservoir

pool

following

dam

removal

The

second

general

source

of

information

is

observations

from

actual

dam

removals

-LRB-

Bednarek

Stanley

and

Doyle

-RRB-

In

relatively

few

cases

have

environmental

effects

been

evaluated

following

dam

removal

and

these

studies

were

all

of

dams

less

than

m

tall

in

relatively

humid

settings

-LRB-

Hart

et

al.

-RRB-

Quantitative

analysis

of

vegetation

response

to

actual

dam

removal

is

rare

although

two

recent

studies

examine

vegetation

colonization

and

succession

within

the

former

reservoir

pools

of

small

dams

removed

in

Wisconsin

-LRB-

Lenhart

Orr

and

Stanley

-RRB-

A

final

source

of

information

is

case

studies

from

alterations

at

least

partially

analogous

to

dam

removal

including

breaching

of

beaver

and

debris

dams

accidental

human

dam

failures

and

dam

maintenance

activities

For

example

the

episodic

release

of

a

large

sediment

pulse

from

dam

maintenance

has

been

analyzed

as

a

surrogate

for

the

downstream

effects

of

the

type

of

sediment

pulse

that

might

be

produced

by

dam

removal

-LRB-

Wohl

and

Cenderilli

Zuellig

et

al.

-RRB-

In

this

study

we

examine

vegetation

colonization

and

early

dynamics

on

areas

exposed

when

a

reservoir

was

drained

for

four

years

to

facilitate

dam

repairs

We

describe

vegetation

pattern

in

terms

of

time

since

exposure

in

this

analog

to

dam

removal

in

order

to

supplement

the

sparse

empirical

database

available

to

scientists

resource

managers

and

policy

makers

involved

in

dam

removal

evaluations

Study

Site

Horsetooth

Reservoir

is

located

in

the

foothills

of

the

Rocky

Mountains

km

west

of

Fort

Collins

Colorado

in

the

transition

zone

between

two

physiographic

provinces

the

Colorado

Piedmont

subdivision

of

the

Great

Plains

to

the

east

and

the

southern

Rocky

Mountains

to

the

west

-LRB-

Fenneman

-RRB-

At

an

elevation

of

m

asl

the

study

site

is

in

the

rain

shadow

of

the

Rocky

Mountains

approximately

km

east

of

the

Continental

Divide

mean

annual

precipitation

ranges

from

cm

more

than

of

which

falls

between

April

and

September

Summer

in

the

study

area

is

typically

hot

with

a

mean

July

maximum

of

°C

Fall

is

cool

and

typically

dry

punctuated

occasionally

by

wet

and

sometimes

heavy

upslope

snowstorms

Winter

is

characteristically

dry

and

cool

to

cold

with

a

mean

minimum

January

temperature

of

°C

and

extreme

winter

minimum

temperatures

as

low

as

°C

-LRB-

Hansen

et

al.

-RRB-

The

transition

zone

between

these

physiographic

regions

described

by

Marr

-LRB-

-RRB-

as

the

Grassland-Lower

Montane

ecotone

is

characterized

by

a

rapid

change

in

elevation

and

a

shift

from

grassland

to

forest

In

this

transition

zone

localized

differences

in

soil

primarily

determine

the

dominant

vegetation

type

at

a

given

location

Grasslands

characteristically

dominate

deeper

finer

textured

soils

transitioning

to

shrublands

and

open

stands

of

ponderosa

pine

-LRB-

Pinus

ponderosa

-RRB-

on

shallow

rocky

soils

and

fractured

rock

outcrops

-LRB-

Marr

-RRB-

Existing

vegetation

on

the

steep

slopes

and

ridges

above

the

high

water

line

of

Horsetooth

Reservoir

matches

the

transition

from

shrublands

dominated

by

Cercocarpus

montanus

and

Rhus

trilobata

-LRB-

Rhus

aromatica

Great

Plains

Flora

Association

-RRB-

to

a

Pinus

ponderosa

community

type

described

by

Peet

-LRB-

-RRB-

for

rocky

slopes

below

m

This

open

xeric

forest

type

is

characterized

by

widely

scattered

Pinus

ponderosa

with

a

grass-dominated

understory

Despite

xeric

site

conditions

understory

species

diversity

and

cover

are

relatively

high

Cercocarpus

montanus

Rhus

trilobata

and

Yucca

glauca

dominate

the

shrub

layer

while

Stipa

comata

Bromus

tectorum

Helianthus

pumilus

Sporobolus

cryptandrus

Bouteloua

hirsuta

and

Verbascum

thapsus

are

important

in

the

herbaceous

layer

-LRB-

Peet

-RRB-

As

seen

in

portions

of

the

valley

not

inundated

by

Horsetooth

Reservoir

grasslands

dominate

toeslopes

above

the

valley

margins

as

well

as

the

valley

floor

where

finer-textured

soils

accumulate

to

greater

depths

These

grasslands

are

characteristically

dominated

by

Agropyron

smithii

Andropogon

scoparius

Bouteloua

curtipendula

B.

gracilis

Bromus

tectorum

and

Stipa

comata

-LRB-

Hansen

and

Dahl

-RRB-

Narrow

valley

floors

typically

support

scattered

stands

of

riparian

shrubs

and

trees

including

Salix

irrorata

Betula

occidentalis

Populus

deltoides

subsp

monilifera

and

Salix

amygdaloides

-LRB-

Marr

-RRB-

The

reservoir

is

situated

between

two

sharp

ridge

crests

or

hogbacks

formed

by

steeply

dipping

layers

of

shales

and

sandstones

The

reservoir

is

approximately

km

long

and

is

formed

by

four

large

earth-filled

dams

Horsetooth

Dam

closes

the

northern

end

of

the

valley

and

Soldier

Canyon

Dixon

Canyon

and

Spring

Canyon

Dams

close

breaches

in

the

eastern

hogback

ridge

created

by

pre-existing

cross-valley

drainages

The

structural

heights

of

the

dams

are

and

m

respectively

Construction

of

the

four

dams

creating

Horsetooth

Reservoir

occurred

between

and

With

a

total

off-channel

storage

capacity

of

approximately

Horsetooth

is

one

of

storage

reservoirs

built

as

part

of

the

Colorado-Big

Thompson

Project

which

stores

regulates

and

diverts

approximately

of

water

annually

from

the

Colorado

River

headwaters

on

the

west

slope

of

the

Continental

Divide

to

the

more

heavily

populated

east

slope

The

project

provides

water

for

irrigated

agriculture

municipal

and

industrial

use

hydroelectric

power

generation

and

water-based

recreation

Discovery

of

sinkholes

and

increased

seepage

from

the

reservoir

prompted

a

dam

modernization

project

and

reservoir

drawdown

that

began

in

the

fall

of

-LRB-

B.

Boaz

personal

communication

U.S.

Bureau

of

Reclamation

-RRB-

Methods

Field

Sampling

We

sampled

vegetation

along

transects

that

began

m

into

upland

vegetation

-LRB-

above

the

reservoir

's

high

water

mark

of

m

above

sea

level

-RRB-

and

extended

down

slope

-LRB-

perpendicular

to

the

shoreline

-RRB-

to

an

elevation

of

m

asl

Below

m

asl

the

reservoir

begins

to

separate

into

distinct

pools

with

different

water

surface

elevations

Transects

were

located

randomly

along

the

entire

length

of

the

shoreline

with

the

exception

of

the

following

excluded

areas

small

shallow

coves

on

the

west

side

of

the

reservoir

the

four

dams

and

m

on

either

side

of

each

dam

Along

each

transect

we

estimated

the

percent

cover

of

every

species

present

in

plots

in

mid-September

of

and

Plants

were

identified

to

species

when

possible

using

local

and

regional

floras

-LRB-

Great

Plains

Flora

Association

Weber

-RRB-

Plots

were

spaced

variably

depending

on

the

steepness

of

the

slope

so

that

they

were

evenly

distributed

along

the

elevational

gradient

On

steep

slopes

a

plot

was

sampled

every

meter

on

progressively

gentler

slopes

plots

were

sampled

at

intervals

of

or

m

in

order

to

achieve

as

close

as

practicable

to

plots

per

m

of

elevation

change

Transect

lengths

ranged

from

to

m.

Following

exclusion

of

plots

without

clear

hydrologic

history

as

described

below

the

numbers

of

plots

analyzed

per

transect

ranged

from

to

with

a

total

of

plots

each

year

The

proportion

of

surface

area

at

each

plot

that

was

occupied

by

cobble-sized

particles

or

larger

-LRB-

mm

diameter

-RRB-

was

estimated

in

the

field

A

single

elevation

of

each

plot

was

determined

using

a

total

station

surveying

instrument

registered

to

the

water

surface

elevation

and

tied

to

the

gage

measuring

long-term

reservoir

water

levels

Substrate

characteristics

and

topography

were

measured

in

General

observations

in

suggested

that

there

had

been

little

change

in

substrate

or

topography

between

years

Records

of

water

level

fluctuation

during

the

study

period

were

combined

with

plot

elevations

to

estimate

when

plots

were

exposed

Using

this

information

we

divided

the

transects

into

four

basic

zones

-LRB-

from

highest

elevation

to

lowest

-RRB-

UPL

an

upland

zone

above

the

level

of

reservoir

inundation

TOP

former

reservoir

bottom

exposed

during

the

growing

season

and

not

inundated

again

during

the

study

MID

former

reservoir

bottom

exposed

during

the

growing

season

and

not

inundated

again

during

the

study

and

BTM

former

reservoir

bottom

exposed

temporarily

during

the

and

growing

seasons

-LRB-

Figs.

and

-RRB-

Areas

excluded

from

analysis

consisted

of

-LRB-

-RRB-

portions

of

transects

below

m

asl

where

separate

pools

began

to

form

in

various

portions

of

the

reservoir

-LRB-

-RRB-

a

wave

action

zone

above

the

high

water

mark

of

the

reservoir

but

still

disturbed

and

-LRB-

-RRB-

a

narrow

band

between

the

MID

and

BTM

zones

that

was

exposed

in

and

but

was

not

re-exposed

in

The

wave

action

zone

and

the

zone

exposed

only

in

and

each

spanned

less

than

m

of

the

elevation

gradient

and

had

too

few

plots

for

meaningful

analysis

Fig.

water

surface

elevation

and

drawdown

zones

Vegetation

analysis

is

limited

to

four

distinct

elevation

zones

UPL

was

never

inundated

TOP

was

first

exposed

in

and

not

subsequently

MID

was

first

exposed

in

and

not

subsequently

BTM

was

first

exposed

in

and

was

subsequently

re-flooded

and

re-exposed

in

both

and

Reservoir

when

sampled

in

September

At

this

time

the

zone

was

in

the

third

year

of

exposure

in

the

second

year

and

in

the

first

year

Second-year

seedlings

of

Populus

deltoides

subsp

monilifera

are

evident

in

the

zone

and

several

mature

individuals

are

present

near

the

full

pool

elevation

of

the

reservoir

before

drawdown

Melilotus

spp.

strongly

dominates

the

zone

here

Data

Analysis

Plant

characteristics

and

communities

Plant

characteristics

are

summarized

by

elevational

zone

and

sampling

year

with

each

combination

denoted

by

the

zone

-LRB-

BTM

MID

TOP

and

UPL

-RRB-

subscripted

by

the

last

two

digits

of

the

year

of

sampling

-LRB-

and

-RRB-

The

resulting

eight

zone-year

combinations

are

grouped

into

classes

based

on

the

number

of

years

since

the

zone

was

last

exposed

or

drawn

down

-LRB-

years

and

Upland

-RRB-

There

was

no

preexisting

rooted

vegetation

in

any

of

these

zones

prior

to

their

first

exposure

and

no

apparent

survival

of

vegetation

in

the

BTM

zone

between

the

two

successive

years

it

was

drawn

down

Fractional

nativity

duration

and

wetland

index

were

based

on

the

aggregate

species

list

for

each

zone-year-transect

combination

Nativity

was

calculated

as

the

fraction

of

species

classified

as

native

and

duration

was

calculated

as

the

fraction

of

species

classified

as

perennial

based

on

McGregor

et

al.

-LRB-

-RRB-

and

USDA-NRCS

-LRB-

-RRB-

For

example

a

value

of

for

nativity

would

mean

of

the

species

were

classified

as

native

and

a

value

of

for

duration

would

mean

of

the

species

were

classified

as

purely

perennial

-LRB-

as

opposed

to

annual

biennial

or

with

a

mixed

duration

-RRB-

Wetland

indicators

-LRB-

Reed

USDA-NRCS

-RRB-

for

individual

plants

of

Obligate

-LRB-

OBL

-RRB-

Facultative

Wetland

-LRB-

FACW

-RRB-

Facultative

-LRB-

FAC

-RRB-

Facultative

Upland

-LRB-

FACU

-RRB-

and

Upland

-LRB-

UPL

-RRB-

were

assigned

numeric

values

of

to

respectively

-LRB-

Tiner

-RRB-

Thus

values

of

the

wetland

index

range

from

for

a

hydric

community

composed

entirely

of

obligate

wetland

species

to

for

a

xeric

community

composed

entirely

of

upland

species

Total

vegetative

cover

within

each

zone-year-transect

combination

was

adjusted

for

differences

in

substrate

by

analysis

of

covariance

using

rockiness

-LRB-

proportion

of

surface

occupied

by

cobble

or

larger

particles

-RRB-

as

a

covariate

This

analysis

-LRB-

Proc

Mixed

SAS

-RRB-

fit

separate

relationships

between

an

arcsine

square

root

transformation

of

cover

and

a

square

root

transformation

of

rockiness

for

each

zone-year

combination

We

report

total

vegetative

cover

means

and

confidence

intervals

for

each

zone-year

combination

adjusted

to

the

grand

mean

of

the

covariate

Direct

comparisons

of

observed

species

richness

across

zones

and

transects

were

not

possible

because

of

the

different

areas

and

numbers

of

plots

sampled

We

pooled

all

plots

sampled

in

each

zone-year

combination

without

regard

to

transect

in

order

to

develop

comparable

estimates

of

species

richness

We

used

two

procedures

A

first-order

jackknife

estimate

of

total

richness

is

based

on

the

number

of

observed

species

the

number

of

sampled

plots

and

the

number

of

species

observed

in

only

one

plot

-LRB-

McCune

and

Grace

-RRB-

We

also

used

a

bootstrap

procedure

to

estimate

the

total

number

of

species

as

the

asymptote

of

a

fitted

species-area

curve

For

each

zone-year

combination

we

drew

random

samples

of

each

number

of

plots

-LRB-

to

n

total

number

of

plots

-RRB-

averaged

the

total

number

of

species

for

each

number

of

plots

and

then

fit

a

curve

to

the

data

-LSB-

Michaelis-Menten

equation

following

Inouye

-LRB-

-RRB-

-RSB-

Differences

in

dominant

species

were

evaluated

by

calculating

species

cover

relative

to

the

total

cover

within

each

zone-year-transect

combination

Differences

in

overall

species

composition

between

zone-year

combinations

were

evaluated

using

relative

Sorensen

distance

-LRB-

McCune

and

Meford

McCune

and

Grace

-RRB-

expressing

a

percent

dissimilarity

of

the

distribution

of

cover

across

species

normalized

to

the

total

cover

within

a

zone-year

combination

This

index

was

based

on

cover

values

for

each

zone-year

obtained

by

first

averaging

all

plots

within

a

zone-year-transect

and

then

averaging

across

transects

within

each

zone-year

combination

Cottonwood

seedling

establishment

We

examined

a

priori

expectations

derived

from

Mahoney

and

Rood

's

-LRB-

-RRB-

recruitment

box

model

concerning

-LRB-

-RRB-

the

position

of

new

seedlings

relative

to

the

water

's

edge

during

the

period

of

seed

availability

and

-LRB-

-RRB-

rates

of

water

level

decline

that

could

be

survived

by

new

seedlings

For

these

analyses

we

pooled

plots

across

transects

and

years

We

then

calculated

average

cover

of

first-year

Populus

deltoides

subsp

monilifera

seedlings

for

each

year

in

elevational

zones

relative

to

the

elevations

of

the

water

surface

each

year

during

the

seed

release

and

germination

window

We

used

the

period

of

June

to

July

for

seed

release

and

germination

based

on

Segelquist

et

al.

-LRB-

-RRB-

and

our

antecdotal

observations

in

Fort

Collins

from

to

To

examine

the

effects

of

drawdown

rate

we

focused

on

plots

within

the

elevational

zone

corresponding

to

the

location

of

the

water

's

edge

during

the

germination

window

These

plots

were

both

predicted

and

observed

to

have

the

highest

probabilities

of

establishment

For

each

of

these

plot-year

combinations

we

estimated

the

drawdown

rate

as

the

difference

between

the

water

surface

elevation

on

the

last

day

the

plot

was

inundated

and

the

water

surface

elevation

days

later

In

some

cases

especially

in

when

drawdown

was

very

rapid

the

water

surface

elevation

days

later

was

below

the

limit

of

m

asl

at

which

pools

formed

in

the

reservoir

bottom

and

reliable

estimates

of

water

surface

at

individual

transects

were

not

possible

In

these

cases

we

used

m

asl

as

a

lower

bound

of

water

surface

elevation

thus

producing

a

conservative

-LRB-

low

-RRB-

estimate

of

the

rate

of

decline

experienced

by

seedlings

Average

cover

of

Populus

deltoides

subsp

monilifera

seedlings

was

calculated

for

classes

of

drawdown

rate

by

pooling

observations

of

current

year

germinants

in

both

and

Results

Plant

Characteristics

and

Communities

The

percentage

of

native

species

was

relatively

constant

over

time

with

median

values

ranging

from

for

exposed

for

one

year

to

for

exposed

for

four

years

-LRB-

Fig.

-RRB-

There

was

no

suggestion

of

a

trend

for

the

early

colonizing

communities

to

be

shifting

towards

the

higher

proportions

of

native

species

associated

with

the

upland

that

had

median

percentages

of

native

species

of

and

in

the

two

years

of

sampling

In

contrast

both

duration

and

wetland

index

showed

clear

trends

of

increasing

over

the

first

four

years

of

exposure

toward

values

observed

in

the

upland

zone

-LRB-

Fig.

-RRB-

Median

percentages

of

perennial

species

were

and

in

the

first

year

of

exposure

-LRB-

and

-RRB-

after

two

years

-LRB-

-RRB-

and

after

three

years

-LRB-

and

-RRB-

and

after

four

years

-LRB-

-RRB-

The

median

percentage

of

perennial

species

in

the

upland

zone

was

in

both

and

Median

values

for

wetland

index

in

the

first

year

of

exposure

were

and

-LRB-

and

-RRB-

which

are

in

the

range

of

wetland

vegetation

communities

-LRB-

Tiner

-RRB-

Wetland

index

increased

steadily

over

time

to

in

the

fourth

year

-LRB-

-RRB-

approaching

the

xeric

value

of

for

both

and

in

the

upland

zone

-LRB-

Fig.

-RRB-

Fig.

characteristics

by

drawdown

zone

and

time

exposed

The

unit

of

replication

is

a

transect

within

each

zone-year

combination

Wetland

index

fraction

native

and

fraction

perennial

are

calculated

for

each

plot

based

on

species

presence

and

then

averaged

across

plots

for

each

transect

within

a

zone-year

combination

Tops

and

bottoms

of

boxes

represent

and

percentiles

the

horizontal

line

is

the

median

whiskers

-LRB-

vertical

lines

-RRB-

include

the

minimum

of

times

the

inter-quartile

range

and

the

range

of

the

data

and

values

outside

the

whiskers

are

represented

by

asterisks

Patterns

in

both

species

richness

and

cover

were

complicated

by

differences

among

sampling

years

in

precipitation

and

among

zones

in

substrate

and

sampled

area

The

second

year

of

sampling

was

drier

than

which

contributed

to

lower

total

cover

in

the

upland

zone

and

lower

numbers

of

identifiable

species

We

recorded

a

total

of

distinct

taxa

In

some

cases

these

were

combinations

of

species

that

could

not

be

reliably

separated

in

the

field

at

the

time

of

sampling

Examples

among

the

more

dominant

species

include

conflation

of

Bromis

japonicus

and

B.

tectorum

and

Melilotus

spp.

which

included

Melilotus

alba

and

M.

officinalis

Differences

in

estimation

procedures

for

total

species

richness

did

not

appreciably

change

the

relative

patterns

-LRB-

Fig.

-RRB-

Based

on

the

first-order

jackknife

estimate

species

richness

was

and

species

in

the

first

year

following

exposure

-LRB-

and

-RRB-

compared

to

and

species

in

the

upland

zone

-LRB-

and

-RRB-

There

was

some

suggestion

of

an

intermediate

peak

in

richness

in

the

second

-LRB-

for

-RRB-

and

third

-LRB-

for

and

for

-RRB-

years

of

exposure

Fig.

richness

and

adjusted

total

cover

by

drawdown

zone

and

time

exposed

In

the

top

graph

asterisks

are

the

asymptotic

richness

from

Michaelis-Menten

species-area

curves

fit

to

bootstrapped

sets

of

plots

pooled

across

transects

-LRB-

Inouye

-RRB-

open

circles

are

first-order

jack-knife

estimates

of

richness

-LRB-

McCune

and

Grace

-RRB-

In

the

bottom

graph

solid

circles

are

estimates

of

mean

total

cover

adjusted

to

the

grand

mean

substrate

value

from

an

analysis

of

covariance

using

transect

within

a

zone-year

combination

as

the

unit

of

replication

The

vertical

lines

from

the

solid

circles

are

confidence

intervals

for

these

estimated

means

The

substrate

area

composed

of

large

particles

-LRB-

mm

diameter

-RRB-

was

a

significant

predictor

of

total

cover

-LRB-

P

-RRB-

with

more

rocky

substrate

associated

with

lower

total

cover

-LRB-

Fig.

-RRB-

Total

plant

cover

adjusted

with

analysis

of

covariance

to

the

mean

fraction

of

large

particles

was

in

the

first

year

of

exposure

-LRB-

and

-RRB-

There

was

some

suggestion

of

a

decline

in

cover

within

the

first

four

years

of

exposure

rather

than

a

trend

in

the

direction

of

the

generally

higher

-LRB-

-RRB-

upland

cover

values

Species

composition

was

most

similar

between

zone-year

combinations

within

a

given

time

of

exposure

-LRB-

Table

-RRB-

Species

composition

of

exposed

areas

became

progressively

more

similar

to

the

upland

composition

with

increasing

time

of

exposure

Composition

at

one

year

of

exposure

was

dissimilar

to

the

uplands

and

composition

at

four

years

of

exposure

was

dissimilar

to

the

uplands

There

was

however

substantial

turnover

of

species

within

the

first

four

years

with

generally

large

dissimilarities

between

the

different

years

-LRB-

e.g.

dissimilarity

between

one

and

four

years

exposed

-RRB-

Table

dissimilarity

matrix

Distance

measure

is

a

Sorensen

percent

dissimilarity

using

mean

cover

values

for

each

zone-year

combination

and

relativized

to

zone-year

unit

totals

-LRB-

McCune

and

Grace

-RRB-

Values

range

from

for

identical

species

cover

composition

to

for

no

similarity

of

species

cover

composition

Matrix

is

symmetrical

around

main

diagonal

Main

diagonal

entries

are

indicated

by

dashes

and

are

by

definintionYears

exposedZone

yearRelativized

Sorenson

distance

-LRB-

dissimilarity

-RRB-

Years

There

were

also

substantial

shifts

in

the

individual

dominant

species

related

to

time

exposed

-LRB-

Table

-RRB-

The

introduced

annual

Chenopodium

glaucum

with

a

FACW

wetland

indicator

value

strongly

dominated

the

first-year

communities

was

the

second

most

dominant

species

two

years

after

exposure

but

was

strongly

reduced

in

the

third

year

and

absent

from

fourth-year

and

upland

zones

Panicum

capillare

a

native

annual

was

the

second

most

dominant

species

in

the

first

year

of

exposure

increased

to

the

most

dominant

species

in

the

second

year

declined

in

the

third

and

fourth

years

and

was

absent

from

the

upland

zone

The

native

Rorippa

curvipes

is

classified

as

an

obligate

-LRB-

OBL

-RRB-

wetland

plant

and

was

important

in

the

first

year

of

exposure

declined

to

a

very

minor

presence

in

the

second

year

and

was

absent

in

the

third-year

fourth-year

and

upland

communities

Table

cover

of

selected

species

by

elevational

zone

and

time

exposed

Relative

cover

is

calculated

at

the

transect

level

for

each

zone-year

combination

and

then

averaged

across

transects

All

species

with

cover

ranks

among

the

top

four

in

any

zone-year

combination

are

included

Dashes

indicate

absenceSpeciesCharacteristicsRelative

Cover

-LRB-

-RRB-

by

Years

Exposed

and

Sampled

ZoneDurationNativityWetland

Ambrosia

tomentosaPNUPL

Bromus

inermisPEUPL

Bromus

japonicus

B.

tectorumAEUPL

Cercocarpus

montanusPNUPL

Chenopodium

Cirsium

Ericameria

nauseosaPNUPL

Lactuca

serriolaAEFAC

Melilotus

spp.

Panicum

Populus

deltoides

subsp

Rhus

aromaticaPNUPL

Rorippa

Salsola

collinaAEUPL

Suckleya

Verbascum

thapsusBEUPL

Verbena

A

annual

B

biennial

P

perennial

N

native

E

exotic

W

wetland

FACW

facultative

wetland

FAC

facultative

FACU

facultative

upland

UPL

upland

Some

of

the

dominant

species

in

the

second

through

fourth

years

of

exposure

were

substantially

less

important

or

absent

from

both

the

first

year

of

exposure

zone

and

the

upland

zone

These

included

Cirsium

arvense

Ericameria

nauseosa

Lactuca

serriola

Salsola

collina

Verbascum

thapsus

and

Verbena

bracteata

The

conflated

Bromus

japonicus

and

B.

tectorum

both

introduced

dominated

the

upland

zone

This

group

was

absent

in

the

first

year

of

exposure

and

gradually

increased

in

relative

cover

in

the

second

through

fourth

years

The

other

most

dominant

species

in

the

upland

Bromus

inermis

Cercocarpus

montanus

and

Rhus

aromatica

were

unimportant

or

absent

in

essentially

all

of

the

first

four

years

of

exposure

-LRB-

Table

-RRB-

Cottonwood

Seedling

Establishment

Plains

cottonwood

-LRB-

Populus

deltoides

subsp

monilifera

-RRB-

was

the

dominant

tree

species

colonizing

the

exposed

surfaces

-LRB-

Table

-RRB-

Mature

individuals

occurred

in

the

upland

near

the

margin

of

the

full-pool

reservoir

although

were

sampled

in

the

upland

plots

on

randomly

located

transects

Seedlings

of

Salix

amygdaloides

S.

exigua

Populus

angustifolia

P.

tremuloides

and

Tamarix

ramossima

were

present

but

rare

in

the

drawdown

zones

along

sampled

transects

First-year

cottonwood

-LRB-

Populus

deltoides

subsp

monilifera

-RRB-

seedlings

were

generally

distinguishable

from

previous

year

germinants

and

root

sprouts

by

their

cotyledons

and

absence

of

bud

scale

scars

when

sampled

in

mid-September

New

seedlings

were

strongly

concentrated

in

or

slightly

above

the

elevational

band

occupied

by

the

water

's

edge

during

the

period

of

seed

release

of

to

July

-LRB-

Fig.

-RRB-

However

smaller

numbers

of

new

seedlings

were

spread

over

a

considerable

range

of

elevations

from

m

above

the

zone

exposed

during

the

germination

window

to

more

than

m

below

the

optimum

zone

Drawdown

rate

had

a

strong

influence

on

the

cover

of

first-year

seedlings

as

sampled

near

the

end

of

the

growing

season

-LRB-

Fig.

-RRB-

For

seedlings

established

in

the

optimum

band

of

the

wetted

edge

during

the

germination

window

cover

decreased

substantially

at

drawdown

rates

greater

than

cm/day

However

the

range

of

drawdown

rates

survived

by

seedlings

was

wide

with

new

seedlings

present

with

measurable

cover

at

rates

of

greater

than

cm/day

The

zone

of

maximum

cover

of

current

year

germinants

was

to

m

above

the

water

level

on

September

in

and

m

above

in

using

the

conservative

m

asl

elevation

at

which

separate

pools

formed

in

the

reservoir

Actual

observed

water

levels

at

the

dam

on

September

were

m

asl

in

and

m

asl

in

Average

elevations

of

all

plots

containing

older

than

first-year

Populus

deltoides

subsp

monilifera

seedlings

in

were

m

above

the

maximum

water

level

in

that

year

and

m

above

the

conservative

m

asl

value

for

September

water

level

in

older

seedlings

were

m

above

the

maximum

and

m

above

the

conservative

September

level

Fig.

distribution

of

first-year

Populus

deltoides

subsp

monilifera

seedlings

in

relation

to

the

elevations

exposed

during

the

germination

window

of

June

to

July

Means

SE

are

based

on

plots

in

each

elevational

class

pooled

across

transects

and

sampling

yearsFig

of

first-year

Populus

deltoides

subsp

monilifera

seedlings

-LRB-

established

in

the

zone

of

the

wetted

edge

during

the

germination

window

-RRB-

in

relation

to

rate

of

drawdown

Means

SE

are

based

on

plots

in

each

drawdown

class

pooled

across

transects

and

sampling

years

Drawdown

is

calculated

for

the

period

following

the

date

a

plot

was

last

exposed

Discussion

Nilsson

et

al.

-LRB-

-RRB-

distinguished

land

as

pre-upland

or

pre-riparian

in

an

analysis

of

vegetation

response

to

inundation

from

dam

construction

This

distinction

is

similarly

useful

in

describing

the

response

of

vegetation

to

dam

removal

Large

fractions

of

the

land

under

the

reservoir

pools

of

the

shallow

dam

removal

sites

in

Wisconsin

examined

by

Orr

and

Stanley

-LRB-

-RRB-

and

Lenhart

-LRB-

-RRB-

were

pre-riparian

Contingent

on

changes

in

topography

from

sediment

accumulated

behind

the

dam

and

the

geomorphic

response

to

dam

removal

pre-riparian

areas

might

be

expected

to

support

marshland

or

riparian

vegetation

following

dam

removal

-LRB-

Shafroth

et

al.

-RRB-

In

contrast

we

sampled

slightly

more

than

meters

of

drawdown

at

Horsetooth

Reservoir

Essentially

all

of

the

area

exposed

was

pre-upland

If

this

had

been

a

permanent

dam

removal

the

exposed

area

would

not

be

expected

to

support

a

wetland

or

riparian

plant

community

in

the

long

term

The

patterns

of

vegetation

change

we

observed

are

a

combination

of

the

initial

colonization

of

a

riparian

bare

ground

disturbance

patch

and

the

vegetation

response

to

a

shift

from

mesic

to

xeric

site

conditions

The

mesic

to

xeric

transition

is

reflected

in

the

wetland

index

value

which

systematically

increased

from

a

hydric

or

wetland

community

in

the

first

year

of

exposure

to

upland

or

xeric

value

in

the

fourth

year

of

exposure

very

similar

to

the

upland

values

of

the

surrounding

landscape

vegetation

In

contrast

Lenhart

-LRB-

-RRB-

reported

wetland

index

values

at

sites

years

following

removal

of

shallow

dams

in

Wisconsin

that

were

still

indicative

of

hydric

or

wetland

communities

The

species

composition

at

the

Horsetooth

site

changed

dramatically

over

the

first

four

years

of

exposure

-LRB-

Tables

and

-RRB-

but

had

not

yet

approached

that

of

the

surrounding

upland

plots

The

initial

colonizing

community

of

a

bare

surface

might

be

expected

to

have

higher

fractions

of

short-lived

and

perhaps

non-native

species

than

later

vegetation

on

the

site

At

the

Horsetooth

site

the

fraction

of

perennials

steadily

increased

through

four

years

of

exposure

whereas

the

fraction

of

native

species

was

below

that

of

the

surrounding

uplands

and

did

not

change

appreciably

or

consistently

Substrate-adjusted

mean

values

for

total

cover

were

lower

than

the

upland

and

appeared

to

decline

somewhat

in

the

second

through

fourth

years

of

exposure

A

possible

explanation

for

these

patterns

in

cover

and

richness

is

that

the

mesic

conditions

in

the

first

year

of

exposure

produce

a

reasonably

high

plant

cover

composed

heavily

of

annual

species

This

is

followed

by

a

period

of

reorganization

replacing

annuals

with

deeper

rooted

perennials

Species

richness

peaks

during

this

transition

whereas

total

cover

tends

to

drop

in

the

drier

conditions

of

years

two

through

four

following

exposure

and

only

gradually

increases

to

the

ultimately

higher

cover

of

a

slowly

developing

perennial

community

present

on

the

dry

upland

sites

The

cottonwood

recruitment

box

model

of

Mahoney

and

Rood

-LRB-

-RRB-

expresses

the

regeneration

niche

of

these

disturbance-dependent

pioneer

riparian

tree

species

and

makes

predictions

about

where

recruitment

will

occur

along

rivers

in

central

and

western

North

America

in

relation

to

water

surface

elevations

-LRB-

Rood

et

al.

-RRB-

Cottonwood

seedling

establishment

requirements

are

well

known

from

multiple

laboratory

-LRB-

Fenner

et

al.

Mahoney

and

Rood

Segelquist

et

al.

Amlin

and

Rood

-RRB-

and

field

studies

-LRB-

Rood

et

al.

Auble

and

Scott

Shafroth

et

al.

Rood

and

Mahoney

Rood

et

al.

-RRB-

Seedling

recruitment

tends

to

occur

on

bare

ground

sites

that

are

moist

during

an

early

summer

period

of

seed

dispersal

and

germinability

and

that

remain

moist

enough

for

seedlings

to

survive

drought

stress

More

specifically

for

Populus

deltoides

subsp

monilifera

the

model

describes

suitable

establishment

sites

as

-LRB-

-RRB-

bare

ground

largely

created

by

fluvial

disturbance

-LRB-

-RRB-

wetted

by

water

surface

elevations

during

a

to

window

of

seed

release

dispersal

and

germinability

-LRB-

-RRB-

subject

to

water

table

declines

of

no

more

than

cm/day

and

-LRB-

-RRB-

generally

occurring

at

elevations

from

to

cm

above

the

water

surface

elevation

of

late

summer

base

flow

-LRB-

Mahoney

and

Rood

-RRB-

The

Horsetooth

drawdown

provided

bare

wetted

surfaces

over

a

much

greater

elevational

range

than

would

be

associated

with

riparian

floodplain

sites

or

removal

of

a

shallow

dam

Predictions

of

the

recruitment

box

model

were

met

in

two

respects

First

the

highest

cover

of

first-year

P.

deltoides

subsp

monilifera

occurred

in

the

elevation

zone

corresponding

to

the

water

's

edge

during

the

estimated

June

to

July

period

of

estimated

maximum

seed

release

and

germinability

for

the

area

-LRB-

Fig.

-RRB-

Second

cover

of

first-year

seedlings

dropped

sharply

at

locations

subject

to

drawdown

rates

cm/day

-LRB-

Fig.

-RRB-

However

there

was

appreciable

seedling

establishment

well

outside

the

bounds

predicted

by

the

recruitment

box

model

Wave

action

capillary

rise

and

plot

heterogeneity

in

elevation

are

likely

explanations

for

the

substantial

establishment

in

locations

m

above

the

water

's

edge

The

establishment

at

much

lower

elevations

as

far

as

m

below

the

water

's

edge

on

July

is

more

difficult

to

explain

It

appears

that

the

extreme

drawdown

rates

created

suitable

bare

moist

sites

far

below

the

elevation

of

the

optimum

but

close

enough

to

the

period

of

maximum

seed

availability

to

allow

some

of

the

tail

end

of

the

distribution

of

germinable

seed

to

establish

We

observed

maximum

cover

at

drawdown

rates

of

cm/day

compared

to

the

upper

limit

of

cm/day

used

in

the

recruitment

box

model

and

we

observed

some

seedlings

surviving

first-year

drawdown

rates

of

greater

than

cm/day

Some

survival

of

seedlings

at

drawdown

rates

of

and

cm/day

has

been

reported

in

laboratory

experiments

-LRB-

Mahoney

and

Rood

-RRB-

and

the

value

of

cm/day

is

probably

better

viewed

as

the

upper

end

of

the

most

suitable

range

of

rates

rather

than

as

an

absolute

limit

The

rapid

Horsetooth

drawdown

rates

produced

seedling

establishment

at

high

elevations

relative

to

late

summer

water

levels

The

elevation

zone

with

maximum

cover

of

first-year

seedlings

was

at

least

m

above

the

water

level

on

September

in

and

at

least

m

above

the

September

level

in

This

was

not

purely

a

first-year

phenomenon

as

the

average

elevations

of

older

than

current-year

seedlings

were

well

above

the

range

of

current

year

water

levels

more

than

m

above

the

maximum

level

of

the

current

year

and

more

than

m

above

the

level

on

September

of

the

current

year

Mahoney

and

Rood

-LRB-

-RRB-

discuss

several

factors

that

may

support

establishment

at

high

elevations

relative

to

the

water

's

edge

including

capillary

rise

lateral

subsurface

water

movement

and

advantageous

sequences

of

precipitation

events

Given

that

some

exposed

surfaces

at

Horsetooth

Reservoir

were

on

the

sides

of

sharp

ridges

with

very

limited

contributing

drainage

area

we

believe

the

most

likely

explanation

for

the

high

elevation

establishment

we

observed

is

local

variation

in

particle

size

and

subsurface

drainage

-LRB-

i.e.

fine

sediment

in

cracks

between

boulders

-RRB-

that

create

pockets

of

adequate

moisture

retained

from

inundation

and

channeled

from

rainwater-derived

percolation

Implications

for

Dam

Removal

Decisions

Given

the

multiple

dimensions

in

which

dams

differ

-LRB-

Poff

and

Hart

-RRB-

and

the

relative

paucity

of

long-term

dam

removal

data

sets

evaluations

and

decisions

about

dam

removal

in

the

near

future

will

be

based

largely

on

case

studies

and

site

assessments

rather

than

on

statistical

conclusions

from

a

sampled

population

of

previous

removals

A

first-order

approximation

of

future

vegetation

within

a

former

reservoir

pool

can

be

obtained

by

considering

topography

of

the

new

surface

in

relation

to

the

new

water

surfaces

substrate

characteristics

the

pre-dam

vegetation

and

its

character

as

pre-upland

or

pre-riparian

and

typical

wetland

riparian

and

upland

vegetation

communities

in

the

region

-LRB-

Shafroth

et

al.

-RRB-

For

shallow

dams

constituting

the

vast

majority

of

dams

removed

to

date

-LRB-

Hart

and

Poff

-RRB-

the

exposed

surfaces

will

be

at

elevations

close

to

the

surface

of

the

new

river

and

a

large

fraction

of

the

exposed

area

would

have

been

riparian

or

wetland

before

inundation

For

tall

dams

and

the

drawdown

case

we

report

the

vast

majority

of

the

exposed

land

is

pre-upland

not

significantly

influenced

by

a

river

before

dam

construction

substantially

above

the

water

surface

following

removal

or

exposure

and

likely

to

support

upland

vegetation

typical

of

the

region

There

are

two

primary

ways

such

a

first

approximation

could

be

misleading

The

first

is

that

the

post-dam

topography

and

substrate

characteristics

may

have

been

altered

substantially

by

the

period

of

inundation

This

is

especially

true

if

the

dam

has

accumulated

substantial

quantities

of

trapped

sediment

that

have

altered

topography

stored

nutrients

or

contaminants

or

altered

organic

content

or

particle

size

distributions

Our

results

showed

a

significant

effect

of

substrate

size

proportions

on

total

vegetation

cover

however

Horsetooth

Reservoir

was

not

accumulating

substantial

sediment

because

of

the

diversion

source

of

most

of

the

impounded

water

The

second

way

a

first

approximation

might

be

misleading

is

that

the

character

of

the

transition

between

extended

inundation

to

post-dam

site

conditions

will

influence

vegetation

response

over

some

time

scale

at

least

on

the

order

of

the

life

spans

of

the

initially

colonizing

plants

Erosion

of

accumulated

sediment

may

produce

substantial

changes

in

post-dam

topography

likely

resulting

in

a

topography

intermediate

between

the

pre-dam

and

immediately

exposed

conditions

This

was

not

a

major

factor

at

Horsetooth

Reservoir

because

of

the

relative

absence

of

accumulated

sediment

On

the

other

hand

extended

inundation

is

an

abnormal

source

of

disturbance

to

initiate

a

bare

ground

plant

succession

for

pre-upland

surfaces

such

as

those

exposed

by

the

Horsetooth

Reservoir

Thus

the

initially

colonizing

plant

community

was

considerably

more

mesic

-LRB-

lower

wetland

indicator

score

-RRB-

than

the

community

that

might

colonize

a

dry

bare-ground

surface

created

by

a

disturbance

such

as

fire

In

semi-arid

landscapes

establishment

of

pioneer

riparian

tree

species

such

as

P.

deltoides

subsp

monilifera

is

almost

always

associated

with

hydrologic

disturbance

and

bare

ground

sites

near

the

water

's

edge

although

mature

trees

can

often

persist

on

much

drier

sites

Thus

successful

establishment

of

riparian

cottonwood

during

the

drawdown

may

leave

a

transient

legacy

of

vegetation

on

the

exposed

reservoir

surfaces

that

would

be

atypical

of

the

sites

either

before

reservoir

construction

or

over

the

long

term

following

dam

removal

These

patterns

are

very

much

a

function

of

the

specifics

of

reservoir

drawdown

and

are

thus

potentially

amenable

to

management

as

part

of

a

dam

removal

For

example

cottonwood

would

be

unlikely

to

colonize

the

bulk

of

pre-upland

first-year

exposure

sites

if

the

drawdown

occurred

in

winter

and

the

area

was

dry

during

the

early

summer

period

of

seed

release

On

the

other

hand

the

summer

drawdowns

observed

at

Horsetooth

probably

increased

total

plant

cover

on

the

exposed

surfaces

which

might

be

desirable

from

the

perspective

of

erosion

mitigation

Weed

control

on

the

former

reservoir

pool

is

a

consideration

in

many

dam

removal

evaluations

Initial

colonization

of

the

reservoir

pools

at

Horsetooth

was

by

a

ruderal

community

-LRB-

Menges

and

Waller

Grime

-RRB-

with

high

fractions

of

short-lived

species

including

invasive

non-native

weeds

This

is

a

reasonable

expectation

of

a

riparian

bare

ground

disturbance

patch

and

generally

for

reservoir

pool

surfaces

that

might

be

exposed

by

dam

removal

where

non-native

species

are

common

in

the

regional

species

pool

-LRB-

e.g.

Orr

and

Stanley

Lenhart

-RRB-

Effective

weed

control

is

complicated

in

the

case

of

a

tall

dam

We

observed

a

dramatic

turnover

of

dominant

species

during

the

first

four

years

and

substantial

changes

in

wetland

character

of

the

vegetation

in

response

to

drier

site

conditions

This

suggests

that

-LRB-

-RRB-

species-focused

weed

control

will

have

to

be

maintained

over

multiple

years

with

shifting

targets

as

some

of

the

weeds

are

diminishing

naturally

and

being

replaced

by

other

species

and

-LRB-

-RRB-

planting

of

a

desired

or

transition

crop

to

preempt

space

and

resources

from

invading

weeds

will

have

to

consider

rapidly

changing

site

conditions

to

be

effective

in

both

the

initial

drawdown

and

immediately

subsequent

years

Stanley

and

Doyle

-LRB-

-RRB-

suggested

that

dam

removal

is

best

viewed

as

a

new

disturbance

with

a

consequent

set

of

positive

and

negative

effects

rather

than

a

simple

reversal

of

the

original

impacts

of

dam

construction

Sediment

transport

is

one

clear

example

of

hysteresis

where

the

pulse

release

of

accumulated

sediment

can

produce

a

post-removal

response

that

is

not

a

simple

mirror

image

or

reversal

of

the

response

to

dam

construction

Vegetation

of

the

former

reservoir

pool

is

another

such

asymmetrical

response

The

existing

plant

community

is

eliminated

quickly

-LRB-

weeks

to

months

-RRB-

following

inundation

of

the

reservoir

pool

from

dam

construction

Recovery

follows

a

different

path

on

a

different

time

scale

Lenhart

-LRB-

-RRB-

suggested

that

the

initial

establishment

of

stinging

nettle

-LRB-

Utrica

dioica

-RRB-

and

rice-cut

grass

-LRB-

Leersia

oryzoides

-RRB-

on

sediment

of

the

former

reservoir

pool

may

be

suppressing

the

development

of

woody

vegetation

on

shallow

dam

removal

sites

in

Wisconsin

We

saw

little

indication

of

this

type

of

effect

as

the

dominant

species

continued

to

shift

rapidly

in

the

first

four

years

of

the

Horsetooth

Reservoir

drawdown

However

the

changes

we

observed

in

the

first

four

years

suggest

that

recovery

of

the

vegetation

community

will

be

on

the

time

scale

of

years

to

decades

Vegetative

cover

composition

of

dominant

species

and

proportions

of

native

and

perennial

species

were

still

changing

after

four

years

and

were

substantially

different

from

the

upland

community

representing

the

likely

long-term

vegetation

Furthermore

the

establishment

and

survival

of

cottonwood

in

the

drawdown

zone

suggests

that

the

particular

nature

of

the

transition

from

decades

of

continuous

inundation

to

a

xeric

condition

may

leave

a

legacy

signature

in

the

vegetation

community

at

least

on

the

decades

to

century

time

scale

corresponding

to

the

lifespan

of

cottonwood

O O O O O O O O O O B I O O B O B I O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O B O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O B I O O O O B I O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O B O O B I O O O O O O O O O O O O O O O O O O B O B I O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 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colorado
cottonwood
dam removal
drawdown
horsetooth reservoir
recruitment box model
riparian