Climate Change, Arctic Plant Communities and Nutrient Feedbacks

1. Climate Change, Arctic Plant Communities and Nutrient Feedbacks Rebecca Hale Biogeochemistry of Northern Ecosystems 20 April 2005

2. 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

3. 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)

4. 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

5. 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

6. Arctic Plants tussock tundra

7. shrub tundra

8. 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

9. Changes in Plant Community[the 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

12. 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

13. 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

14. Betula nana dwarf birch Arctic willow

15. Viccinium uliginosum Alpine bilberry Alnus crispa Green alder

16. 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

17. 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

18. 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?

19. 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