51 Gentili et al Ecol Complex 2015

(1)

Review

Potential

warm-stage

microrefugia

for

alpine

plants:

Feedback

between

geomorphological

and

biological

processes

R. Gentili

a,

*,

C. Baroni

b

,

M. Caccianiga

c

,

S. Armiraglio

d

,

A. Ghiani

a

,

S. Citterio

a

a_Dipartimento_di_Scienze_{dell’Ambiente}_e_del_Territorio,_{Universita` degli}_Studi_di_Milano_Bicocca,_Piazza_della_Scienza_1,_I-20126_Milano,_Italy

b_Dipartimento_di_Scienze_della_Terra,_{Universita` di}_Pisa_and_CNR,_Istituto_di_Geoscienze_e_Georisorse,_Via_S._Maria_53,_I-56126_Pisa,_Italy

c

DipartimentodiBioscienzeUniversita` degliStudidiMilano,ViaCeloria26,20133Milano,Italy

d

MuseoCivicodiScienzeNaturalidiBrescia,ViaOzanam4,25128Brescia,Italy

Contents

1. Introduction... 88

2. Literaturescreening... 89

3. Landformsworkingaswarm-stagemicrorefugia... 90

3.1. Mountainsummits(andrelictsurfaces)... 90

3.1.1. Landform-vegetationunit... 90

3.1.2. Climaticcontrol... 90

3.1.3. Microclimatefeatures... 90

3.1.4. Microrefugiumfunction ... 90 ARTICLE INFO

Articlehistory:

Received21July2014

Receivedinrevisedform11October2014 Accepted13November2014

Availableonline

Keywords:

Resilience Mesorefugia Periglacialrefugia

Evolutionarygeomorphology Marginalpopulation Microclimate

ABSTRACT

Duringinterglacialstages,microrefugiaaresitesthatsupportlocallyfavorableclimateswithinlargerareas with unfavorable warmer climates. Despite recent theoretical representations of microrefugia, an appropriateecologicalcharacterizationisstilllacking,mostlyforwarmperiods.Acrossmountain/alpine areas,cold-adaptedplantspeciescouldadoptdifferentstrategiestomanagetheeffectsofclimatewarming: (A)migrationtowardhigherelevationsandsummits;(B)insituresilienceofcommunitiesandspecies populationswithinmicrorefugia;andC)adaptationandevolutionbygeneticdifferentiation.Thisreview aimstodistinguishandcharacterizefromanecologicalperspectiveglacial,nival,periglacialandcomposite landformsanddepositsthatmayfunctionaspotentialmicrorefugiaduringinterglacialwarmperiods.

We conducteda literaturescreeningrelated to thegeomorphological processesand landforms associatedwithvegetationandplantcommunitiesinalpine/mountainenvironmentsofEurope.They includeglacialdepositsrockglaciers,debris-coveredglaciers,compositeconesandchannels.InAlpine regions,geomorphologicnichesthatconstantlymaintaincold-airpoolingandtemperatureinversions arethemaincandidatesformicrorefugia.Withinsuchmicrorefugia,microhabitatdiversitymodulates theresponsesofplantstodisturbancescausedbygeomorphologicprocessesandsupportstheiraptitude forsurvivingunderextremeconditionsonunstablesurfacesinisolatedpatches.Currently,European marginalmountainchainsmaybeconsideredasexamplesofmacrorefugiawhererelictboreo-alpine speciespersistwithinpeculiargeomorphologicalnichesthatactasmicrorefugia.

Thisreviewcontributestoidentifyingpotentialwarm-stagemicrorefugiaareasacrossalpineand mountainregionsanddeterminingcertainlandformsthatplayormayplaysuchroleunder global-changescenarios.Theoccurrenceofwarm-stagemicrorefugiawithintheselocationsmaybeofgreat importanceforthemodelingoffuturedistributionsofspeciesandassessingtheriskofextinctionfor alpinespecies.Microrefugia mayhaveimportantimplicationsinmicro-evolutionaryprocessesthat occuracrossalternatingclimaticphases.

ß₂₀₁₄_Elsevier_B.V._All_rights_reserved.

* Correspondingauthor.Tel.:+390264482700;fax:+390264482996.

E-mailaddresses:rodolfo.gentili@unimib.it(R.Gentili),baroni@dst.unipi.it(C.Baroni),marco.caccianiga@unimi.it(M.Caccianiga),botanica@comune.brescia.it

(S.Armiraglio),alessandra.ghiani@unimib.it(A.Ghiani),sandra.citterio@unimib.it(S.Citterio).

lists

available

at

ScienceDirect

Ecological

Complexity

j o

u r

n

a l

h o

m e p

a g

e :

w w w

. e l s e v i e r

. c

o m

/ l o

c a t e / e c

o

c o

m

(2)

3.2. Debris-coveredglaciers... 90

3.2.3. Microclimatefeature... 90

3.2.4. Microrefugiumfunction ... 90

3.3. Moraineridgesanddeglaciatedforelands... 92

3.4. Nivationniches/snowpatches... 93

3.4.4. Microrefugium... 93

3.5. Rockglaciers... 93

3.6. Alpinecompositedebriscones(debrisslopes/scree) ... 94

3.7. Alpinecorridors(compositechannels/avalanchechannelsandtracks)... 94

3.8. Icecaves... 94

3.9. Otherlandforms... 95

3.10. Primarytopographicfactors:elevation,aspectandslope... 95

4. Thefeedbackbetweengeomorphologicalandbiologicalprocesses ... 95

5. Survivalstrategiesforhigh-altitudespecies ... 96

6. MicrorefugiainEuropeanmarginalmountainchains... 96

7. Conclusion... 97

Acknowledgements... 97

References... 97

1. Introduction

Global

temperatures

have

increased

over

the

last

century

(IPCC,

2007),

and

in

recent

decades,

the

climate

has

become

more

extreme

in

many

regions

of

the

world,

with

further

changes

expected

in

the

nature,

extent

and

incidence

of

different

weather

events,

such

as

exceptional

summer

heat

waves

in

certain

temperate

regions

or

in

mountain

areas

(Abeli

et

al.,

2012a;

Mastrandrea

and

Tavoni,

2013).

An

urgent

challenge

in

conserva-tion

biogeography

is

to

determine

how

species

will

respond

to

such

climate

changes.

To

avoid

extinction,

species

may

respond

to

climate

change

by

migrating

to

new

areas

and

adapting

through

phenotypic

plasticity

and

adaptive

evolution

to

the

new

environ-mental

conditions

(Ghalambor

et

al.,

2007).

In

addition

to

such

mechanisms,

there

has

been

an

increasing

acknowledgment

of

the

role

of

refugium

areas

in

facilitating

the

long-term

survival

of

species

and

populations

throughout

several

climatic

oscillations,

which

contributes

to

the

preservation

of

biological

diversity

from

extinction.

Historically,

the

notion

of

ecologic

refugium/refugia

was

referred

to

as

areas

of

survival

for

species

during

quaternary

glacial

phases

with

unfavorable

climate

conditions.

For

instance,

Battandier

(1894)

introduced

the

impor-tance

of

Mediterranean

glacial

refugia

in

North

Africa.

Subsequently,

the

‘‘refuge

hypothesis’’

was

postulated

for

birds

of

the

Amazon

forest

as

a

theory

of

ice-age

speciation

events

based

on

climate

change

(Haffer,

1969).

Favarger

and

Robert

(1966)

considered

that

the

Maritime

Alps,

South-Eastern

Alps

and

Jura

Mountains

were

important

migration

areas

(e.g.,

refugia)

in

which

alpine

taxa

concentrated

during

quaternary

glaciations.

Holder

et

al.

(1999)

assumed

a

‘‘glacial

refugium

hypothesis’’

for

the

Northern

Hemi-sphere

and

proposed

that

during

the

Pleistocene,

glacier

expansion

favored

intraspeciﬁc

diversity

by

isolating

populations

in

ice-free

refugia.

Recently,

the

terms

refugia/refugium

were

similarly

applied

to

Neotropical

forest

regions

and

areas

where

species

survive

during

warm-stage

interglacial

periods

(Birks

and

Willis,

2008;

Rull,

2009).

Stewart

et

al.

(2010)

deﬁned

a

refugium

as

‘‘a

region

or

regions

that

a

species

inhabits

during

the

period

of

a

glacial/

interglacial

cycle

that

represents

the

species’

maximum

contraction

in

geographical

range.’’

Rull

et

al.

(1988)

introduced

the

term

‘‘microrefugium,’’

which

was

theoretically

centered

on

a

biogeographic

perspective

(Rull,

2009)

as

a

‘‘small

area

with

local

favorable

environmental

features,

in

which

small

populations

can

survive

outside

their

main

distribution

area

(the

macrorefugium),

protected

from

the

unfavorable

regional

environmental

conditions.’’

From

its

name,

macrorefugium

can

be

(3)

main

(continuous)

range

,’’

and

it

can

be

further

distinguished

from

various

terms

(cryptic

refugia

and

microrefugia)

and

synonyms

(interglacial

refugia,

periglacial

refugia

and

northern

refugias)

(

Holderegger

and

Thiel-Egenter,

2009;

Rull,

2010;

Dahlberg,

2013

).

In

addition,

Olson

et

al.

(2012)

deﬁned

mesorefugia

as

‘‘

large

areas

that

contain

nested

clusters

of

microrefugia

with

similar

species

assemblages

that

have

functioned

as

a

refugium

over

millennia

.’’

However,

the

occurrence

of

micro-

and

macrorefugia

for

some

regions

and

climatic

periods

(e.g.

northern

Europe)

is

still

debated

(

Tzedakis

et

al.,

2013;

Rull,

2014

).

It

has

been

widely

accepted

that

during

the

Pleistocene,

numerous

arctic

and

alpine

species

migrated

in

deglaciated

areas

or

tolerated

glacial

conditions

within

nunatak

refugia

(

Schneeweiss

and

Schoenswetter,

2011

).

Despite

the

debate

regarding

ecological

refugia

for

animal

and

plant

species

during

glacial

periods

having

reached

a

climax

across

literature

works,

warm-stage

refugia

have

still

been

poorly

considered

(

Stewart

et

al.,

2010

).

During

interglacial

warm

periods,

forest

and

cold-adapted

species

endured

through

an

upward

migration

to

mountain

refugia

(

Bush,

2002

)

or

persisted

in

other

refugial

areas

(

Bhagwat

and

Willis,

2008

)

or

within

microrefugia

(

Rull,

2009

).

In

interglacial

stages,

microrefugia

are

sites

supporting

locally

favorable

cold/

fresh

climates

interspersed

in

larger

areas

with

unfavorable

warm

climate,

and

they

allow

populations

of

species

to

persist

outside

of

their

main

distributions

(

Stewart

et

al.,

2010;

Dobrowski,

2011

).

The

terminology

‘‘microrefugia’’

as

deﬁned

by

Rull

(2009)

will

be

used

throughout

the

manuscript.

Gottfried

et

al.

(2012)

observed

a

signiﬁcantly

higher

abun-dance

of

thermophilic

species

in

the

mountain

summits

of

Europe

over

the

last

decade.

In

addition,

recent

projections

of

high

mountain

species

habitat

shifts

that

result

from

temperature

increases

indicate

that

there

will

be

a

decline

of

cold

habitats

and

ﬂora

by

the

end

of

twenty-ﬁrst

century

(

Thuiller

et

al.,

2005;

Pauli

et

al.,

2012;

Dullinger

et

al.,

2012

).

However,

future

extinction

rates

could

be

overestimated

if

microrefugia

are

not

considered

in

models

(

Mosblech

et

al.,

2011

).

In

mountain

areas,

climate

is

a

key

limiting

factor

for

plant

life,

and

it

is

directly

to

topography,

which

determines

speciﬁc

microhabitat

conditions

(

Harris,

2008;

Gentili

et

al.,

2013

).

For

instance,

different

slopes

and

aspects

modulate

the

effects

of

temperature

and

water

balance

for

plant

species

(topo-climate).

According

to

Abbott

(2008)

and

Crawford

(2008)

topographic

heterogeneity

may

favor

the

survival

of

several

arctic

and

alpine

species

that

are

able

to

occupy

a

range

of

habitats,

which

may

vary

in

temperature,

soil-moisture

content

and

wind

exposure.

For

these

reasons,

topographic

locations

that

constantly

maintain

cold-air

pooling

and

temperature

inversions

can

be

predicted

as

the

main

candidates

for

warm-stage

microrefugia.

From

this

perspective,

Mosblech

et

al.

(2011)

indicated

that

local

topo-graphic

shelter

effects

can

increase

the

probability

of

a

species

surviving

in

a

microrefugium.

Birks

and

Willis

(2008)

speciﬁed

certain

microrefugia

(that

they

called

‘‘cryptic

refugia’’)

with

micro-environmentally

favorable

conditions,

such

as

north-facing

slopes,

steep

cliffs,

sea-cliffs

and

cool

ravines.

During

the

current

phase

of

climate

warming,

they

noted

that

(micro-)refugia

within

alpine

areas

can

be

niches

where

alpine

species

might

be

able

to

grow

below

the

altitudinal

forest

limit

(e.g.,

warm-stage

microrefugia).

According

to

Grabherr

et

al.

(1995)

,

more

than

one-third

of

the

alpine/nival

ﬂora

is

restricted

to

azonal

habitats,

especially

rocks,

debris

slopes

and

snowbeds

that

represent

nonstandard

habitats

created

by

disturbances.

Such

landscape

locations

may

have

climatic

environmental

patterns

that

are

constantly

dissimilar

from

regional

patterns

(

Dobrowski,

2011

).

Despite

such

recent

characterizations

and

theoretical

repre-sentations

of

microrefugia,

an

appropriate

representation

of

their

biogeographical

roles

and

ecological

characterizations

is

still

lacking

(

Rull,

2009;

Mee

and

Moore,

2014

),

especially

for

mountain

regions

where

future

climatic

scenarios

are

predicted

to

produce

signiﬁcant

effects

(

Thuiller

et

al.,

2005;

Thuiller,

2007

)

and

for

which

microrefugia

could

play

an

important

role

for

the

survival

of

alpine

plants.

Even

if

mountain

regions

appear

to

be

especially

well-suited

to

providing

potential

microrefugia,

their

number,

characteristics,

location

and

spatial

extent

remain

cryptic,

both

for

paleoecological

analyses

(

Rull,

2009;

Holderegger

and

Thiel-Egenter,

2009

)

and

current

analyses

(

Keppel

et

al.,

2012

).

According

to

the

recent

works

of

Dobrowski

(2011)

and

Ashcroft

et

al.

(2012)

,

the

most

suitable

level

to

which

microrefugia

can

be

identiﬁed

is

based

on

microclimatic

and

microtopographic

characterizations

of

landscapes

and

by

using

species

distribution

models

(

Keppel

et

al.,

2012;

Keppel

and

Wardell-Johnson,

2012;

Patsiou

et

al.,

2014

).

However,

most

of

the

predictive

models

for

the

potential

distribution

of

species

in

alpine

environments

and

all

of

the

refugial

hypotheses

rarely

consider

the

role

of

geomorpho-logical

processes

acting

in

mountain

areas

as

potential

forces

capable

of

creating

local

microhabitat

conditions

for

hosting

species

during

adverse

climatic

conditions

and

geomorphological

processes.

For

this

reason,

in

this

review,

going

over

such

a

mere

technical

approach,

we

present

a

literature

screening

on

the

mountain

landforms

or

geomorphological

features

that

have

been

predicted

or

inferred

to

function

as

current

or

future

(micro-)refugia.

The

speciﬁc

goals

of

this

work

are

to

a)

characterize

geomorphological

processes

and

landforms

of

alpine

life

zones

in

European

mountains

and

depict

their

function

in

creating

ecological

heterogeneity

and

acting

as

potential

microrefugia;

and

b)

evaluate

the

role

of

such

landforms

in

certain

marginal,

low

mountain

chains

of

Europe

that

host

relict

alpine

ﬂora

and,

even

now,

function

as

microrefugia.

2. Literature

screening

We

focused

on

literature

data

to

the

geomorphological

processes

and

landforms

associated

with

vegetation

and

plant

communities

in

alpine/mountain

environments.

Even

if

mountains

with

an

alpine

zone

occur

at

all

latitudes

from

the

wet

tropics

to

polar

regions

(

Grabherr

et

al.,

2010

),

in

this

paper,

we

refer

to

the

alpine

life

zone

of

Europe

in

which

both

vegetation

patterns

and

geomorphologic

processes

have

been

deeply

studied.

In

particular,

we

considered

the

mountain

chains

of

Europe

that

host

alpine

habitats

and

ﬂora

and

are

included

in

the

‘‘alpine

ﬂoristic

system’’

(

Aeschimann

et

al.,

2004)

,

which

comprises

the

Alps

in

sensu

stricto

and

alpine

marginal

chains.

Alpine

marginal

chains

include

the

Pyrenees,

Central

Massif,

Jura,

Vosgi,

Black

Forest

Mountains,

Apennines

(northern

and

central),

Carpathians,

Dinaric

Alps,

Corsica

and

Balcan

mountains.

In

this

study,

some

northern

or

eastern

Europe

mountain

chains

were

considered

as

well

as

north

American.

We

reviewed

the

literature

from

the

Web

of

Knowledge

database

(Thomson

Reuters)

through

a

search

of

the

online

database

using

several

combinations

of

the

following

queries:

landforms/geomorphological

processes*,

vegetation

or

plant

com-munity*,

plants*,

ﬂora*,

refugia*,

and

Alps/Alpine*.

Distinct

searches

were

performed

for

the

main

geomorphologic

processes

of

alpine

environments

in

relation

to

vegetation*/plant

communi-ty*,

which

included

alpine

corridor*,

debris

ﬂow*,

debris-covered

glacier*,

glacial

deposit*,

mountain

summit/peak*,

nivation

niche*,

rock/debris

fall*

rock

glacier*,

snow

avalanche*,

and

composite/

debris/talus

cone*.

Certain

gray

literature

data

(i.e.,

journal

that

is

not

cataloged

in

the

Web

of

Knowledge

and

Scopus

databases

or

is

not

shown

after

a

search)

were

extrapolated

from

reference

lists

of

papers

collected

(4)

often

included

articles

to

marginal

local

mountain

chains

(

Table

1 ).

We

organized

the

collected

literature

data

based

on

the

main

mountain

landforms

that

can

be

found

along

slopes

according

to

the

main

mountain/alpine

landforms

and

deposits

above

or

(sometime)

across

treeline

(

Fig.

1 ):

(a)

mountain

summits

above

trimlines

(including

relict

surfaces);

(b)

glacial

landforms

and

deposits,

such

as

glacial

forelands,

moraines,

and

debris-covered

glaciers;

(c)

nival

landforms

and

deposits,

such

as

nivation

niches/

snow

patches;

(d)

periglacial

landforms

and

deposits,

such

as

rock

glaciers;

(e)

composite

landforms

(polygenic),

such

as

alpine

composite

cones,

alpine

corridors

(composite

channels/avalanche

channels)

and

tracksu`;

(f)

other

landforms

(ice

caves,

roches

moutonne´es,

etc.).

The

main

topographic

features

were

also

considered,

such

as

elevation,

slope

and

aspect.

We

then

characterized

each

landform

according

to

its

vegeta-tion

features

(landform-vegetation

units;

see

Baroni

et

al.,

2007

),

climatic

controls,

microclimate

features

of

active

landforms

and

microrefugium

functions.

We

did

not

consider

wetlands

and

peat

bogs

that

have

already

been

considered

as

refugia

and

where

only

highly

specialized

plant

species

and

communities

occur.

3. Landforms

working

as

warm-stage

microrefugia

3.1. Mountain

summits

(and

relict

surfaces)

3.1.1. Landform-vegetation

unit

Mountain

summits

consist

of

highest

elevations

of

mountain

chains

above

the

trimlines

and

sharp

ridges

(

Are´te´s

).

They

include

the

top

levels

of

mountains

and

relict

surfaces

that

were

not

affected

by

Pleistocene

glacial

action

and

include

peaks,

crests

and

crowns

affected

by

frost

action

mainly

under

periglacial

conditions

(

Fig.

1 a).

In

alpine

areas,

generally

deglaciated

terrains

along

sharp

ridges

and

on

horns

extend

downward

to

the

highest

portions

of

accumulation

basins

as

glacial

basins

reduce

their

size

(

Baroni

and

Orombelli,

1996

).

The

acceleration

of

glacier

contractions

(

Haeberli

et

al.,

1999

)

has

been

increasingly

furnishing

new

and

wider

portions

of

the

highest

accumulation

basins,

which

are

suitable

for

refugia

during

warm

periods

(e.g.,

the

Careser

Glacier,

Carturan

et

al.,

2013

).

Erosional

processes,

(both

periglacial,

glacial

or

gravity-based)

of

mountain

summits

are

mainly

controlled

by

the

type,

structure

and

mass

strength

of

the

rock

and

geometry

of

the

slope

(

Cruden

and

Hu,

1999

).

On

summits,

ﬂoristic

and

vegetation

patterns

are

a

function

of

the

elevation

level,

latitudinal

range

and

habitat

availability

(

Kazakis

et

al.,

2007

).

In

temperate

mountains

(e.g.,

European

Alps),

the

limit

of

higher

plant

life

lies

at

approximately

3000–4000

m,

where

only

specialized

plants

(cushion)

or

those

with

nival

aptitudes

are

currently

able

to

survive

under

harsh

conditions;

these

plants

include

Leucanthe-mopsis

alpina

and

Ranunculus

galcialis

(

Ko¨rner,

2003;

Grabherr

et

al.,

2010

).

3.1.2. Climatic

control

Weathering

erosional

processes

of

mountain

summits

(peri-glacial

or

because

of

gravity)

are

mainly

controlled

by

relationships

among

the

climate,

type,

structure

and

mass

strength

of

the

rock,

geometry

of

the

slope,

and

biota

(

Cruden

and

Hu,

1999

).

3.1.3. Microclimate

features

In

mountain

summits,

thermal

conditions

have

periglacial

characteristics

and

may

vary

according

to

aspect.

Summits

may

beneﬁt

from

favorable

climatic

conditions

during

the

summer

season

because

of

direct

solar

radiation

(heating)

and

reﬂection

of

rock

outcrops

by

the

local

availability

of

melt

water

from

snow.

Such

processes

can

induce

differentiated

environments

with

marked

seasonality,

even

if

rigid

temperatures

in

winter

are

mitigated

by

thermal

inversion

(

Berry,

1992

).

3.1.4. Microrefugium

function

In

mountain

areas,

climate

warming

is

projected

to

shift

species’

ranges

to

higher

elevations,

even

if

the

alpine

plants

react

individualistically

to

climate

change

(

Grabherr

et

al.,

2010

).

Currently,

the

local

diversity

of

the

majority

of

boreal

and

temperate

mountain

peaks

is

increasing

because

of

rising

temperatures

(

Klanderud

and

Birks,

2003;

Pauli

et

al.,

2007

).

Even

if

some

species

have

been

found

as

relicts

on

summits

of

marginal

alpine

chains

(

Table

1 ),

the

role

of

mountain

peaks

as

microrefugia

is

undeﬁned

and

debated,

especially

for

the

middle-long

period

if

the

increasing

trend

continues

(

Pauli

et

al.,

2012

).

According

to

Dobrowski

(2011)

,

mountain

peaks

are

expected

to

play

a

limited

role

as

microrefugia

because

they

show

the

strongest

similarity

to

the

free-air

environment

and

exhibit

the

smallest

diurnal

temperature

variance

with

respect

to

bottom

valleys

and

valley

slopes.

However,

Randin

et

al.

(2009)

projected

species

distribution

models

and

indicated

that

local

scale

models

can

predict

persistent

alpine

species

habitats

toward

higher

elevations,

where

the

aspect

and

local

topographic

conditions

may

locally

preserve

favorable

micro-habitats.

3.2. Debris-covered

glaciers

3.2.1. Landform-vegetation

unit

Debris-covered

glaciers

(or

black

glaciers)

are

glaciers

whose

ablation

area

is

mainly

covered

by

a

continuous

layer

of

supraglacial

debris

(

Fig.

1 b).

Vegetation

cover

is

mainly

discontin-uous

and

patchy

with

a

species

assemblage

that

may

be

similar

to

that

of

alpine

and

subalpine

glacier

forelands,

with

the

latter

enriching

high-altitude

species

such

as

Androsace

alpina

(

Caccia-niga

et

al.,

2011

).

3.2.2. Climatic

control

As

a

general

rule,

debris-covered

glaciers

follow

the

course

of

glacier

retreats

and

advances

under

a

regional/continental

climatic

regime.

Under

the

present

climatic

conditions,

debris-covered

glaciers

exhibit

a

far

less

negative

mass

balance,

which

may

even

be

positive

in

certain

circumstance,

than

white

glaciers

(

Fickert

et

al.,

2007;

Diolaiuti

and

Smiraglia,

2010

),

so

they

may

show

a

long

persistence

within

warm

climatic

periods.

3.2.3. Microclimate

feature

Variations

in

debris

thickness

inﬂuence

the

debris

surface

temperature

because

of

debris

thermal

conductivity

and

low

albedo.

The

temperature

pattern

of

supraglacial

debris

is

characterized

by

a

high

daily

excursion

with

radiative

heating

during

the

daytime

and

cooling

by

sensible

heat

transfer

during

the

night

(

Brock

et

al.,

2010

).

3.2.4. Microrefugium

function

Debris-covered

glaciers

can

provide

habitat

with

favorable

microclimatic

conditions

for

numerous

plant

species

wherever

the

glacier

surface

is

sufﬁciently

stable

(

Table

1 ).

High-altitude

taxa

have

been

found

below

their

altitudinal

limits

on

such

glaciers

because

of

the

cooler

subsurface

soil

temperatures

(

Fickert

et

al.,

2007

).

Moreover,

shrubs

and

trees

are

able

to

germinate

and

grow

across

their

surface

once

they

become

stable

and

thick

enough

(

Pelﬁni

et

al.,

2012

).

Debris-covered

glaciers

may

act

as

a

dispersal

agent

for

alpine

species;

this

role

could

have

important

implica-tions

during

glacial

periods

and

particularly

during

warm

periods

(5)

Table 1

Landformsfunctioningorpredictedtofunctionas(micro-)refugiaduringwarmperiods.

Landforms Microrefugia Function Species Area Reference

Mountainsummits Alpinespeciesfoundinmarginal/ deglaciated/lowelevationchain

Current function

Leucanthemopsisalpina, Senecioincanus

N-Apennines Abelietal.(2012a,b);

Alessandrinietal.(2003)

Speciespredictedtopersistafter aprojectedwarmingeffectsof +5K

Predicted function

Androsacealpina TyroleanAlps Grabherretal.(2010)

Alpinespeciesfoundinmarginal/ deglaciated/lowelevationchain

Currentfunction Androsacevandelli,Potentilla nivalis

Pyrenees Go´mezetal.(2003)

Debris-coveredglaciers Alpinespeciesfoundbelowthe treeline

Deducted/predicted function

Artemisiaglacialis,Carex curvula,Ranunculusglacialis, Saxifragabryoides,etc.

WesternAlps Caccianigaetal.(2011)

Moraineridgesand deglaciatedforelands

Currentfunction Juncusjacquinii N-Apennines Gentilietal.(2006)

Currentfunction Violacomollia Prealps CredaroandPirola(1977)

HighAlpine-nivalspeciesfound inmarginal/almostdeglaciated/ lowelevationchain

Currentfunction Androsacealpina,Ranunculus glacialis,Saxifraga

Climaticnichesforalpinespecies Inferred/predicted function

‘‘Snowbedplantsand bryophites’’(speciesnot declared)

SwedishScandes Kullman(2010)

Currentfunction Salixherbacea,Silenesuecica N-Apennines Abelietal.(2012a,b);

Alessandrinietal.(2003)

Current

function

Cerastiumcerastioides N-Apennines Alessandrinietal.(2003)

Currentfunction Poaminor,Salixretusa, Soldanellaalpina,Veronica alpina,etc.

DinaricAlps SurinaandSurina(2010)

Currentfunction Salixherbacea DinaricAlps Redzic(2011)

Currentfunction Andreaeanivalis Tatramountain FudariandKucˇera(2002)

Rockglaciers Sameplantsgrowinginglacier foreﬁeldsorinmountain summits

Inferred/predicted function

Androsacealpina,Poalaxa, Oxyriadigyna,Saxifraga bryoides,etc.

CentralSwissAlps Burgaetal.(2004)

Cold-demandingandsnowbed

CentralItalianAlps Gobbietal.(2014)

Currentfunction Empetrumhermaphroditum, Juncustrifidus,Luzula alpino-pilosa

N-Apennines TomaselliandAgostini (1990)

Alpinecomposite debriscones

Alpinespeciesfoundbelowthe treeline

Inferred/predicted function

Leucanthemopsisalpina, Oxyriadigyna,Poalaxa

CentralItalianAlps Baronietal.(2007)

Climaticnichesforalpinespecies Inferred/predicted function

Cerastiumuniflorum, Cryptogrammacrispa, Soldanellaalpina,etc.,

FrenchAlps Bodin(2010)

Currentfunction Doronicumgrandiflorum, Oxyiriadigyna

Corsicanmountains Gamisans(2003)

Currentfunction Ranunculusglacialis Carpathianmassif Ronikier(2010)

Currentfunction Poaalpina,Sileneacaulis, Linariaalpina,etc.

C-Apennines DiPietroetal.(2008)

Currentfunction Carexfoetida N-Apennines Tomaselli(1991)

Currentfunction Polygonumviviparum,Salix retusa,S.serpyllifolia, Soldanellaalpina

DinaricAlps Redzˇic´ etal.(2011)

Currentfunction Cryptogrammacrispa,Luzula alpino-pilosa,Oxyriadigyna, etc.

Pyrenees Go´mezetal.(2004)

Currentfunction Andreaearupestris, Cryptogrammacrispa, Gymnomitrionspp., Polytrichumalpinum,etc.

NorthernBhoemia Ru˚zˇicˇkaetal.(2012)

Alpinecorridors Alpinespeciesfoundbelowthe treeline

CentralItalianAlps Gentilietal.(2010)

Alpinespeciesfoundbelowthe treeline

Currentfunction Campanulapulla,Arabis alpina,Linariaalpina

AustrianAlps Komposchetal.(2013)

Icecaves Alpinespeciesfoundbelowthe treeline

Currentfunction Rhytidiadelphustriquetrus, Seligeriapusilla,etc.

(6)

3.3. Moraine

ridges

and

deglaciated

forelands

3.3.1. Landform-vegetation

unit

Moraine

consists

of

rock

and

soil

material

picked

up

and

transported

by

glaciers

and

then

deposited.

Deglaciated

forelands

(or

foreﬁelds)

are

valley-ﬂoor

deposits

that

are

exposed

to

erosion

after

glacial

retreat

(

Fig.

1 a).

They

are

subjected

to

paraglacial

modiﬁcation

by

mass

movements,

frost

sorting,

wind

and

water

transport,

sediment

storage

(sandur),

and

they

are

also

subjected

to

vegetation

colonization;

in

addition,

glacioﬂuvial

processes

(7)

contribute

to

sediment

reworking

(

Eichel

et

al.,

2013

).

Areas

with

thin

debris

deposits

may

uncover

bedrock

areas

with

roches

muoutonne´es

that

produce

peculiar

vegetation

mosaics

(

Parolo

et

al.,

2005

).

On

moraines,

depending

on

the

surface

age

and

stabilization,

vegetation

may

consist

of

different

plant

communities

in

a

continuum

of

successional

phases

from

pioneer

communities

growing

on

active/recent

moraines

(e.g.,

Saxifraga

spp.

and

O. digyna

),

to

alpine

grasslands

(often

dominated

by

Poa

alpina

)

on

stabilized

deposits

and

to

subalpine

discontinuous

shrubland

(

Salix

spp.)

and

woodlands

(

Picea

excelsa

and

Larix

decidua

)

(

Birks,

1980;

Matthews,

1992;

Caccianiga

et

al.,

2001;

Caccianiga

and

Andreis,

2004

).

3.3.2. Climatic

control

Moraines

and

deglaciated

forelands

follow

the

course

of

glacial

retreats

and

advances

under

a

regional/global

climatic

regime.

3.3.3. Microclimate

features

Distance

from

the

glacier

along

with

local

differences

in

topographic

features

(e.g.,

height

of

the

ridge

crest)

above

the

glacier

foreland

primarily

affect

the

microclimate

of

these

landforms

and

deposits

(

Matthews,

1992

).

Secondary

factors

may

include

the

sedimentary

and

textural

characteristics

of

the

glacial

deposits

and

water

availability,

such

as

from

glacial

melting.

3.3.4. Microrefugium

function

Until

recently,

glacial

deposits

and

foreland

and

moraine

ridges

have

been

reported

as

performing

the

role

of

glacial

refugia

and/or

microrefugia

for

plant

and

animal

species

during

cold

periods

(

Lutz

et

al.,

2000;

Bhagwat

and

Willis,

2008;

Kaltenrieder

et

al.,

2009

).

In

particular,

several

paleoenviron-mental

studies

based

on

pollen

data

reported

that

at

the

southern

boundary

of

the

Alps,

morainic

amphitheaters

harbored

several

alpine

species

(

Ravazzi

et

al.,

2004;

Kaltenrieder

et

al.,

2009

;

Table

1 ).

Moraine

ridges

and

glacial

forelands

are

expected

to

function

as

warm-stage

microrefugia

favoring

snowbed

sites

and

communities

with

a

persistence

of

alpine

species

(

Kullman,

2010

)

because

of

the

frontal

recession

of

glaciers.

In

addition,

old/ancient

glacial

deposits

increase

habitat

heteroge-neity

(microreliefs

and

slope

discontinuities)

and

micro-climatic

conditions

at

lower

elevations

of

mountainous

areas

or

in

peripheral

mountain

chains

(

Rossi

et

al.,

2014

),

and

they

favor

the

persistence

of

alpine

species

within

niches.

Ancient

moraines

may

perform

a

microrefugium

function

for

the

conservation

of

genetic

diversity

at

the

population

level.

Mimura

et

al.

(2013)

found

that

older

moraine

populations

of

the

alpine

species

Salix

arctica

showed

higher

within-population

genetic

variation

compared

with

the

younger

moraine

populations,

which

is

generally

attributed

to

differences

in

the

establishment

age

associated

with

plant

densities

among

moraines.

3.4. Nivation

niches/snow

patches

3.4.1. Landform-vegetation

unit

Nivation

niches

are

ground

depressions

and

valleys

in

which

snow

persists

longer

than

in

the

surrounding

area

because

of

precipitation,

topography

and

geomorphological

processes

such

as

snow

avalanche

deposition

(

Christiansen,

1998

;

Fig.

1 c).

Nivation

niches

are

generally

dominated

by

herbaceous

vascular

plants

(e.g.,

Alchemilla

spp.,

Luzula

alpino-pilosa

,

etc.)

and

mixed

to

low-shrub

species

such

as

Salix

herbacea

(

Ferrari

and

Rossi,

1995;

Rossi

et

al.,

2006

).

In

alpine

and

mountain

area

terrain,

the

distribution

and

duration

of

snow

and

vegetation

are

closely

linked.

Snow

accumulation

and

snowmelt

erosion

within

niches

produces

complex

habitats

where

different

types

of

vegetation

can

grow

in

relation

to

the

snow

duration

and

micro-morphologic

and

topographic

characteristics,

such

as

grain

size,

slope

and

aspect

(

Palacios

et

al.,

2003

).

3.4.2. Climatic

control

The

patterns

of

snow

distribution

in

alpine

terrain

are

a

consequence

of

interactions

between

climatic

variables,

such

as

radiation,

precipitation

and

wind,

and

topographic

variables,

primarily

slope

and

aspect

(

Keller

et

al.,

2005

).

3.4.3. Microclimate

features

Snow

cover

affects

the

air

and

soil

temperatures

(until

the

occurrence

of

snow

melt)

and

various

abiotic

conditions,

such

as

soil

moisture

and

nutrients.

With

a

complete

disappearance

of

the

snow

cover,

a

greater

amount

of

solar

energy

is

absorbed

by

the

soil

surface,

which

heats

the

ground

and

air

(

Wipf

and

Rixen,

2010

).

3.4.4. Microrefugium

In

low

mountains

or

in

peripheral

mountain

chains,

nivation

niches

and

long-lasting

snow

cover

have

been

reported

as

the

principal

factors

favoring

the

persistence

of

a

scattered

alpine

belt

at

the

highest

peaks

and/or

survival

of

high

mountain

species

within

small

microsites

(

Stanisci

et

al.,

2005;

Grabherr

et

al.,

2010;

Abeli

et

al.,

2012a

;

Table

1 ).

3.5. Rock

glaciers

3.5.1. Landform-vegetation

unit

Rock

glaciers

are

tongue-shaped

bodies

made

of

coarse

debris

that

may

develop

after

a

glacier

retreat

and

under

permafrost

conditions

(

Fig.

1 d).

Active

and

inactive

rock

glaciers

consist

of

a

debris

layer

covering

ice-supersaturated

debris

or

pure

ice

(

Berthling,

2011

).

The

former

is

capable

of

moving

downslope

because

of

gravity

and

frost

action,

whereas

the

latter

generally

does

not

show

any

movement.

Depending

on

the

degree

of

activity

and

movement,

the

presence

of

permafrost

and

quality

of

substrate

rock

glaciers

host

different

types

of

vegetation,

from

pioneer

to

nival

vegetation

or

late

successional

communities

(

Cannone

and

Gerdol,

2003;

Burga

et

al.,

2004

).

3.5.2. Climatic

control

In

certain

regions,

the

spatial

distribution

of

rock

glaciers

reﬂects

the

climatic

conditions,

particularly

the

mean

annual

air

temperature

and

total

annual

precipitation

(

Baroni

et

al.,

2004

).

Active

rock

glaciers

can

be

used

as

indicators

of

mountain

permafrost

that

is

linked

to

annual

isotherms

of

approximately

2 ₈

C

(

Barsch,

1996

).

3.5.3. Microclimate

feature

In

active

rock

glaciers,

the

presence

of

permafrost

could

result

in

low

temperature

values,

even

during

the

summer

season.

Microclimates

may

also

greatly

vary

depending

on

topographic

heterogeneity.

3.5.4. Microrefugium

function

Rock

glaciers,

both

active

and

relict

(with

or

without

perma-frost,

respectively),

may

function

as

habitat

for

numerous

alpine

plant

species

whose

distribution

is

linked

to

grain

size

and

substrate

stability

(

Cannone

and

Gerdol,

2003;

Burga

et

al.,

2004;

Rieg

et

al.,

2012;

Gobbi

et

al.,

2014

),

thus

increasing

the

habitat

heterogeneity

at

a

local

scale

and

enhancing

species’

survival

(

Table

1 ).

Active

rock

glaciers

may

play

a

further

role

because

of