based results vs based results based results based results based - - PowerPoint PPT Presentation

Subarctic forest Subarctic forest advance advance Subarctic forest Subarctic forest advance – advance – – – empirical mpirical empirical- empirical-

• based results

based results based results based results vs vs vs vs modeled predictions modeled predictions modeled predictions modeled predictions based results based results based results based results vs vs. vs

• vs. modeled predictions

modeled predictions modeled predictions modeled predictions

A i A ik H H f d A i A ik H H f f d Ann nnik ika H Hofgaar gaard Ann nnik ika H Hofgaar gaard d NINA NINA NINA NINA LANL, 28 LANL, 28 January 2015 January 2015 LANL, 28 LANL, 28 January 2015 January 2015

Today Today

• Background
• C

ti

• Common expect

tations

• Definitions
• Change and consequences
• Results from northern Europe

Results from northern Europe

• Circumpolar pattern

Why Why focus focus on the

• n

subarctic forest? the subarctic forest?

Models predict: Models predict:

• Rapid advance of trees and shrubs

i p t l b l i in response to global warming

• Loss of 40-50% of current tundra

Effect Effects of s of changed forest distribution: changed forest distribution: Effect Effects of s of changed forest distribution: changed forest distribution:

• Climatic (albedo, GHG)
• Ecological
• Socio-economical

Rates of Rates of predicted advance: predicted advance:

• Average 2000 m/y
• Largest values imply 6000 m/y

Largest values imply 6000 m/y

Common expectations Common expectations

• Climate is considered one of the most

important factors controlling forest- tundra ecotone dynami ics

• As

As temperatures increase temperatures increase, the forest-

• the forest

tundra ecotone is expected to shift upwards and northwards upwards and northwards

• The response is expected to be shown

The response is expected to be shown by swift tree and shrub advance

Basic questions q Basic q questions

• Are trees invading the Arctic?

Are trees invading the Arctic?

• Can the question be answered in a

simplistic way? Where why how??? simplistic way? Where, why, how???

• Can site-based responses be translated

into region wide general pattern? into region-wide general pattern?

• Invasion to what rate?
• Wh

What response and rate regul lati ing f f actors are dominating and at what scales?

• Implications of forest advance?

Central Central Canada Canada Central Russia Central Russia Eastern Canada Eastern Canada

PPS Arctic PPS Arctic PPS Arctic PPS Arctic

Stud Studie ies includ s include: :

• seed production •seed quality
• regeneration •growth responses
• age structure •spatial pattern •soil
• animal interference •land use

animal interference land use

• socio-economy •mapping
• experiments •remote sensing
• >60 Graduate students
• climate data •historical archives
• 8 Postdocs

8 Postdocs

• 32 PIs
• Many students, assistants, locals
• 31 Institutes
• 10 C

t i 10 Countries

• 35 presentations at the IPY 2010 Conference
• 27 presentations at the IPY 2012 Conference

Northern Northern Norway Norway

Forest-tundra ecotone (FTE)

Tree species Tree species Tree species lin Tree species line lin line Predictions of rate and spatial Krummho Krummholz lz Krummho Krummholz lz lin line lin line e

FTE

and spatial configuration change are challenging Tree Tree Tree Tree lin line lin line e

FTE

Episodic and chronic drivers with shifting freq enc Forest lin Forest line Forest lin Forest line e shifting frequency and intensity Large set of abiotic and biotic impact factors (e.g. temperature, snow, wind, fire, herbivory, land use); with variable influence through time and space

Characteristics of FTE Characteristics of FTE

Forest Tundra

Temperature gradient

Vegetation cover

Geological, topographic, land use

Soil organic matter Nutrient stock

land use, ecological & climatic influences

Nutrient stock Unoccupied space

are cross-cutting at regional and local

Periglacial processes

and local scales

Drivers Drivers of

• f FTE

FTE

Hofgaard et al. 2012 modified after Holtmeier and Broll, 2005

Impact of snow cover change Impact of snow cover change

Hi h Deeper, more insulative snow Bi er Higher winter soil temperature Bigger stiffer trees/shrubs Increased Enhanced active layer depth Increased winter tree/shrub growth decomposition and nutrient mineralization

Tree/shrub height Tree/shrub height growth in growth in th t d i t th t d i t the tundra environment the tundra environment

No grazing warming No grazing ambient temp

ng height

Grazing ambient temp

Sapli Experimental period 1999‐2008. Error bars represent 1 SE.

The experiment show grazing controlled response to environmental change, with climate (warming) as a secondary force. This herbivore‐driven concealing of expected climate‐driven tree/shrub expansion emphasizes the necessity to consider changes in grazing regimes and

• ther disturbances along with climate change.

Hofgaard et al. 2010

Role of disturbance through time Role of disturbance through time

Holocene climate trend d Disturbance events Chronic disturbance Stand density

Hofgaard 1997

t Current FTE movement Current FTE movemen

vance Adv

Constant frequency of disturbance events

Stationary Retreat

Enhanced climatic conditions Recover from disturbance event Recover from disturbance event Release from chronic disturbance Constant climate Constant chronic disturbance Worsened climatic conditions Frequent disturbance events Intensified chronic disturbance

Site results examples Site results -

• examples

Retreating: etreating: etreating: etreating: No recruitment; seeds are produced, but seedlings are lacking; trends sensitive to tree death Stationary: Stationary: Seedlings are common, but low or no recruitment to the Advancing: Advancing: tree layer; trends Seedlings and young trees are sensitive to tree death common; trends not sensitive to tree death

Increased shrub cover Brooks Range, Alaska Increased shrub cover Brooks Range, Alaska

• ca. 1950

Sturm et al. 2001

• ca. 2000

Decreased forest cover Abisko, , N Sweden Decreased forest cover Abisko N Sweden

Van Bogaert et al. 2011

Age structure Age structure evidence evidence

Treeline trees (black bars) and tree saplings beyond treeline (open bars)

Aune, Hofgaard & Söderström, 2011

Khibiny 1958 2008 1958 - 2008 Khibiny Mountains, FTE change Mountains, FTE change 1958 2008 1958 - 2008

25 30 l i di l d i 50 25‐30 m altitudinal advance in 50 years Mathisen et al. 2014

Main drivers at Main drivers at 1 P temperature 2-3-6 Grazing 4-5 7 C 8 9-10 vutsrponmlkihgfedcaVUTSPNGF

• Birch treeline 340 m/year
• Pine treeline 10 m/year
• Birch forest line 156 m/year
• Pine forest line 71 m/year
• Considerable spatial and temporal

variation variation

• Data source related uncertainty of

around 10 m/year

Sites vs region - example Sites vs. region example Sites vs region - example Sites vs. region example

Site-based analyses: Site-based analyses: Age structure Spatial configuration Recruitment pattern Remote sensing Advance Advance, Stationary Stationary, Retreat Retreat Advance rate of ca. 0.6 m/yr calculated for 1958-2008 individual sites individual sites

Treeline

individual sites individual sites recipitation & Temperature

Forest line

Regional temperature increase: 2oC i l 20 h limatic/unclear Arctic harsh climate Winter precipitation since early 20th century Regional latitudinal advance: Regional latitudinal advance:

• Average rate is of the order of

g 100 m/year

• Loss of tundra could be

estimated as being ca. 2% at the d f t t end of current century

Hofgaard et al. 2013

Circumpolar Circumpolar pattern? pattern?

Models predict: Models predict:

• Rapid advance of trees and shrubs

i p t l b l i in response to global warming

• Loss of 40-50% of current tundra

within current century Rates of Rates of predicted advance: predicted advance:

• Average 2000 m/y
• Largest values imply 6000 m/y

Result Results to s to come: come:

• Rees, Hofgaard, Cairns, Timoney

et al. in prep. R i l i i

• Regional variation
• Empirical-based results do not

confirm model predictions

l i General conclusions G l l i G General l conclusions

• Yes, trees and shrubs are moving north, but ........
• Where – local to regional perspective
• Wh

Why – causal b l back kground d

• Mismatch between predictions and observations
• Mismatch between results based on exp

periments vs. natural (both rate and species-specific responses)

• Rate of advance – not km/year but meters/decade?
• M d

Modelled d tundra l l oss of 40-50% - a serious overestimate

• ll

f 40 50% i

• Multi-site analyses are needed to refine regional and

circumpolar forest advance scenarios

• Further synthesis activities will prove helpful

Closing comments Closing comments

• Herbivores can dominate the dynamics of the zone at

regi ion- and d speci ies-specifi ific l l evel ls b by modif difyi ing e.g. recruitment, survival and growth of trees and shrubs

• Disturbance-driven modification of expected climate-

driven tree and shrub expansion emphasises the need to consider changes in grazing regimes and other perturbations (fire, insects etc.) along with climate change

• Between-site and between-region variation has to be

considered

• If not - misleading interpretations regarding rates of

climate-driven encroachment will prevail

Thanks! Thanks! Thanks! Thanks!

Financial thanks to: Financial thanks to: The Research Council of Norway Norwegian Institute for Nature Research The Government of Canada U.S. National Science Foundation Scott Polar Research Institute Russian Academy of Science, Moscow State University International Arctic Science Committee