Climate Change, Arctic Plant Communities
and Nutrient Feedbacks
Rebecca Hale
Biogeochemistry of Northern Ecosystems
20 April 2005
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 progress…thinking about effects on
nitrogen cycling
Climate change in the Arctic!
•
Temperature
• 2-3ºC increase in Arctic since 1960’s/70’s
• 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)
Changes in N Cycling
•
•
N deposition
indirect effects due to warming and CO2
• increased N mineralization
• altered C:N 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
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
Arctic Plants
tussock tundra
shrub tundra
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
Changes in Plant Community
[the abridged version]
•
decreased biodiversity,
loss of lichens and
mosses
• Chapin et al. (1995): 3050% 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
Shrubs!
•
increase in evergreen and deciduous shrub abundance
and biomass with warming and fertilization
• Betula nana increased from 25% to >90% 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
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
Betula nana
dwarf birch
Arctic willow
Viccinium uliginosum
Alpine bilberry
Alnus crispa
Green alder
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
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
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?
•
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