Subarctic forest Subarctic forest advance Subarctic forest Subarctic forest advance – advance advance – – – empirical empirical- empirical- mpirical - based results vs based results based results based results based results based results vs based results based results vs vs modeled predictions vs vs. modeled predictions vs. vs modeled predictions modeled predictions modeled predictions modeled predictions modeled predictions modeled predictions Ann A A Ann A A nnik nnik ika H ika H ik H i ik H i Hofgaar H f Hofgaar H f f gaard gaard d d d NINA NINA NINA NINA LANL, 28 LANL, 28 January 2015 LANL, 28 LANL, 28 January 2015 January 2015 January 2015
Today Today • Background • C • Common expect tations ti • Definitions • Change and consequences • Results from northern Europe Results from northern Europe • Circumpolar pattern
Why Why focus focus on the on the subarctic forest? subarctic forest? Models predict: Models predict: • Rapid advance of trees and shrubs in response to global warming i p t l b l i • Loss of 40-50% of current tundra Effect Effects of Effects of Effect s of changed forest distribution: s of changed forest distribution: 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 q Basic questions 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 Canada Central Canada Central Russia Central Russia PPS Arctic PPS Arctic PPS Arctic PPS Arctic Eastern Canada Eastern Canada 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 Northern Northern Norway Norway • Many students, assistants, locals • 31 Institutes • 10 C 10 Countries t i • 35 presentations at the IPY 2010 Conference • 27 presentations at the IPY 2012 Conference
Forest-tundra ecotone (FTE) Tree species Tree species Tree species lin Tree species line line lin Predictions of rate and spatial and spatial configuration change are challenging Krummho Krummholz Krummho Krummholz lz lz lin line line lin e FTE FTE Episodic and chronic drivers with Tree Tree Tree Tree lin line lin line e shifting freq enc shifting frequency and intensity Forest lin Forest line Forest lin Forest line e 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 land use, Soil organic matter ecological & climatic influences Nutrient stock Nutrient stock are cross-cutting at regional Unoccupied space and local and local scales Periglacial processes
Drivers of Drivers of FTE FTE Hofgaard et al. 2012 modified after Holtmeier and Broll, 2005
Impact of snow cover change Impact of snow cover change Deeper, more insulative snow Bigger Bi er Hi h Higher stiffer winter soil trees/shrubs temperature Enhanced Increased tree/shrub active layer growth depth Increased winter decomposition and nutrient mineralization
Tree/shrub height growth in Tree/shrub height growth in th t th t d the tundra environment the tundra environment d i i t t ng height No grazing warming No grazing ambient temp Sapli Grazing ambient temp 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 other 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
Current FTE movemen Current FTE movement t Enhanced climatic conditions vance Recover from disturbance event Recover from disturbance event Adv Release from chronic disturbance Constant climate Constant frequency of disturbance events Stationary Constant chronic disturbance Worsened climatic conditions Retreat 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 ca. 2000 Sturm et al. 2001
Decreased forest cover Abisko, Decreased forest cover Abisko N Sweden , N Sweden Van Bogaert et al. 2011
Age structure evidence Age structure evidence Treeline trees (black bars) and tree saplings beyond treeline (open bars) Aune, Hofgaard & Söderström, 2011
Khibiny Khibiny Mountains, FTE change Mountains, FTE change 1958 - 2008 1958 2008 1958 - 2008 1958 2008 25 ‐ 30 m altitudinal advance in 50 years 25 30 l i di l d i 50 Mathisen et al. 2014
Sites vs region - example Sites vs region - example Sites vs. region example Sites vs. region example Treeline Main drivers at Main drivers at individual sites individual sites individual sites individual sites 9-10 vutsrponmlkihgfedcaVUTSPNGF Regional 1 P recipitation & temperature temperature 2-3-6 Grazing increase: 2 o C 4-5 Temperature Forest line since early 20th i l 20 h 7 C limatic/unclear century 8 Arctic harsh climate Winter precipitation Site-based analyses: Site-based analyses: Regional latitudinal advance: Regional latitudinal advance: • Age structure Birch treeline 340 m/year • Spatial configuration Pine treeline 10 m/year • Average rate is of the order of g • Recruitment pattern Birch forest line 156 m/year 100 m/year • Remote sensing Pine forest line 71 m/year • Loss of tundra could be • Considerable spatial and temporal estimated as being ca. 2% at the Advance, Stationary Advance Stationary, Retreat Retreat variation variation end of current century d f t t • Data source related uncertainty of Advance rate of ca. 0.6 m/yr Hofgaard et al. 2013 around 10 m/year calculated for 1958-2008
Circumpolar Circumpolar pattern? pattern? Models predict: Models predict: • Rapid advance of trees and shrubs i in response to global warming p t l b l i • 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. • Regional variation R i l i i • Empirical-based results do not confirm model predictions
General conclusions G G General l l conclusions l l i i • 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 ll d tundra l l oss of 40-50% - a f 40 50% serious overestimate i • Multi-site analyses are needed to refine regional and circumpolar forest advance scenarios • Further synthesis activities will prove helpful
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