Climate Change, Arctic Plant Communities

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Climate Change, Arctic Plant Communities and
Nutrient Feedbacks
Rebecca Hale Biogeochemistry of Northern
Ecosystems 20 April 2005
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A glimpse of things to come

• Global warming (!) has a disproportionate effect
  on Arctic ecosystems
• Effects of climate change on tundra ecosystems?
• Change in plant community composition
• Changes in plant physiology, structure, function
• Effects of plant change?
• A work in progressthinking about effects on
  nitrogen cycling

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Climate change in the Arctic!

• Temperature
• 2-3ºC increase in Arctic since 1960s/70s
• Direct effect on plants
• Indirect effects
• permafrost thawing, increased active soil depth
• extended growing season
• soil warming and changes in soil processes (i.e.
  decomposition, N-mineralization)

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Changes in N Cycling

• N deposition
• indirect effects due to warming and CO2
• increased N mineralization
• altered CN ratios and associated changes in
  decomposition rates
• indirect effects due to plant community and
  physiology changes
• changes in litter decomposition
• changes due to altered water regime

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Welcome to the Tundra

• plant growth is 1 nutrient limited
• most biomass in below ground plant parts (stems,
  rhizomes, storage tissue)
• controls over plant community composition change
  across environmental gradient
• lower tundra - biotic and abiotic controls
  (higher biomass more competition)
• higher tundra - become gradually more abiotic,
  very little competition

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Arctic Plants

• tussock tundra

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shrub tundra
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Methods

• remote sensing
• different scales spatially and temporally tell
  us different things
• historic aerial photographs small spatial,
  large temporal scale
• satellites lots of them, fall all over the
  spatial/temporal scale spectrum
• paleoclimatic records
• global warming in Holocene what was the change
  in plant community composition then?
• Modeling
• experimental manipulations
• control changes over the past years in plant
  community compositional changes and physiological
  changes have often been in agreement with
  experimental and modeling results

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Changes in Plant Communitythe abridged version

• decreased biodiversity, loss of lichens and
  mosses
• Chapin et al. (1995) 30-50 decline in species
  richness in tussock after 9 yr warming exp. (loss
  of forbs and mosses)
• Jonasson et al. (1991) loss of diversity in
  lichens and mosses with 3 yr fertilization

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Shrubs!

• increase in evergreen and deciduous shrub
  abundance and biomass with warming and
  fertilization
• Betula nana increased from 25 to gt90 tussock
  tundra biomass after 15 years fertilization
• Paleoclimatic methods corroborate these results
• early Holocene warming shrub invasions in
  Alaska arctic
• increased shrub growth and development
• short term versus long term results
• Hartley et al (1999) soil warming increases shoot
  production

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Shrubs Part II

• increased shrub growth and development
• short term versus long term results
• Hartley et al (1999) soil warming increases
  shoot production after 3 years, but had no effect
  after 5 years

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Betula nana dwarf birch
Arctic willow
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Viccinium uliginosum Alpine bilberry
Alnus crispa Green alder
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Why?!?!?

• loss of lichen/moss abundance
• competition with vascular plants
• light
• nitrogen
• negative response to N fertilization
• response to acid rain, increased T, decreased
  snow cover, increased CO2, altered fire regimes
  are other possibilities for lichen loss

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Plant Composition

• warming
• increased growing season (up to 7 days earlier)
  is correlated with increased photosynthetically
  active period
• modeling results versus field results
• increased N mineralization/soil fertility
• fertilization experiments have caused changes in
  plant community composition are these
  experiments realistic?
• increased N min has been demonstrated in field
  control plots, but increases in experimental
  warming plots have been small and variable

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Still to come so what?

• what are the effects of these compositional
  changes on ecosystem functions?
• Nutrient partitioning
• differentiation in timing, depth and chemical
  form of N uptake in arctic tundra
• dominant species use the most available forms of
  N
• how will this be affected by increased abundance
  of dominant and loss of rarer species?

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• effects of plant communities on soil processes
  and fertility
• lichens/mosses large role in maintaining soil
  moisture
• increased transpiration with increased shrub
  abundance
• changes in litter quality with change in dominant
  species

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