Within the next 50–100 years, the warming climate will have major effects on boreal and northern hardwood forests situated near the prairie–forest border of central North America. This biome boundary shifted to the northeast during past episodes of global warming, and is expected to do so again. The climate of the future will likely lead to higher mortality among mature trees, due to the greater frequency of droughts, fires, forest-leveling windstorms, and outbreaks of native and exotic insect pests and diseases. In addition, increasing populations of native deer and European earthworm invasions will inhibit the establishment of tree seedlings. The expected net impact of these factors will be a “savannification” of the forest, due to loss of adult trees at a rate faster than that at which they can be replaced. This will cause a greater magnitude and more rapid northeastward shift of the prairie–forest border, as compared with a shift solely attributable to the direct effects of temperature change.

1. Dave Hansen
Univ. of MN
Climate change, invasive
species and forests
Lee E. Frelich
Director, The University of Minnesota Center
for Hardwood Ecology
Vice President, Eastern Native Tree Society
Chair, Board of Directors, Great River
Greening
John Knuerr

2. Svante August Arrhenius
(Nobel prize, Chemistry,1903)
Developed the hothouse theory for
CO2 in 1896, and in 1905 predicted that
raising CO2 content of the atmosphere
would cause an increase in mean global
temperature similar in magnitude to
modern predictions

3. Evidence that CO2 increase and climate change are caused by humans
1. Keeling curve corresponds to population and known emissions of CO2
2. Suess effect
3. Temperature profile of the atmosphere—warmer lower atmosphere and
cooler stratosphere
4. Existing ‘Greenhouse effect’ of 56 degree F before human enhancement
of heat trapping gases in the atmosphere
5. Models with human greenhouse gas enhancement match observed
temperatures over the last 100 years, models with only natural
forcings on climate do not

5. Climate change occurs in the context of large year to
year variability—summer temperatures for 2009

6. …and spatial variability
Summer 2009—Example of regional temperature anomalies
versus global mean in a warming climate. A cold summer in
central and eastern North America, although 80% of the
world had above average temperatures.

8. Tree ring analyses showing climate over the several hundred years
Comparison of the original Mann ‘hockey stick’ reconstruction of temperature
(red) with implementation of valid criticisms of the PC methodology (green).
From Wahl and Amman 2006.
http://www.cgd.ucar.edu/ccr/ammann/millennium/CODES_MBH.html

9. Tree ring analyses from 14 sites
Osborne and Briffa 2006, instrumental record for northern hemisphere (red)
and for records closest to proxy sites (green).

10. Oerlemans, 2005 temperature reconstruction of global mean temperature
based on 169 glaciers.

11. Temperature evidence from boreholes
Borehole temperature data from
Huang, Pollock and Shen, Nature, 2000

12. Modeling of temperature
showing how natural and
human influences work
together.
From McCarthy, 2009, Science

13. Natural versus human CO2 forcing on the climate

15. British Meteorological Office, December 2009

16. Spruce White pine
Migration maps for tree response to past climate change
in 1000s years before present (M.B. Davis 1983)

17. Current and simulated future
range of black spruce, from
Lenihan and Neilson 1995.
300 mile shift is equal to
distance moved in ~ 2000
years in paleorecord

18. Forest cover of central North America (green)
DeFries, R., M. Hansen, J.R.G. Townshend, A.C. Janetos, and T.R. Loveland
(2000), 1 Kilometer Tree Cover Continuous Fields, 1.0, Department of Geography,
University of Maryland, College Park, Maryland, 1992-1993.
It is possible that the pbf will move 500 km to the north
and east, deforesting an area 2X the size of California

19. Native species have migrated in the past without going extinct
what’s different now?
Rate of climate change—many species cannot migrate fast enough
Fragmented environment—makes species to movement more difficult
Habitat loss—less habitat now and can support fewer species
Invasive species—can reproduce and move fast, they have opportunity
to displace native species during times of rapid change
Exotic diseases and pests—can spread faster in a warmer climate
Deer grazing—will increase in the north, can extirpate native plants
TNC
David Augustine

20. Minneapolis Star
tribune
Before and after the 1999 blowdown
in the Boundary Waters, northern MN

21. Wind plus fire = major forest transformation
Nick Fisichelli and Roy Rich, Cavity Lake Burn, Seagull Lake, July 2007.
Photo: Dave Hansen, University of MN

22. Several forces are at work that may push MN forests
towards savanna:
If soils become drier for any reason sites can support
fewer trees:
Warmer soils
Higher evaporation to precipitation ratio
European earthworms stripping the insulating duff layer

23. Photo: Dave Hansen
More drought = trees under stress and forest dieback

24. Native insects play a major role in forest change
Benign native insects can have outbreaks in a warmer climate.
For example, mountain pine beetle in British Columbia—a
native insect that caused massive tree mortality over 30 million
acres of lodgepole pine forest, and could threaten jack pine
in MN

25. Sylvania Wilderness maple and hemlock forest in 1988

26. Sylvania in 2006

27. Global warming or
Global worming?
Earthworms are ecosystem
engineers that can alter the
structure of soil, and change
the H2O, N and P cycles, C
dynamics and seedbed
characteristics on a
regional scale

28. Photos: Dave Hansen, University of MN
Soil profile, no earthworms (left), with earthworms (right).
The loss of the duff layer will affect summer soil temperature

29. Earthworm invasion in maple forest causes a decrease
in tree ring width and loss of seedling density and herb species
on the forest floor
1.6
1.4
Ring width index
1.2
1
Worm free
0.8
0.6 worm invaded
0.4
0.2
0
-5 0 5 10 15 20 25 30
Invasion timing (years)

30. Recent literature shows that Earthworms also facilitate
germination and establishment of European buckthorn
and garlic mustard by changing the seed bed
Photos: The Nature Conservancy
Kathleen Knight

31. Exotic earthworms + deer =
major transformation of the
forest. Bare soil and lower
nutrient status should favor
hemlock over maple, but
deer favor maple.
Both dominant species are
disfavored, combined with
warmer climate and sandy
soils, probably means
Sylvania will become oak
savanna in the future

32. Summary of impacts on Lake States
tree species
Global warming and rate of migration:
All species
Sudden oak death: red oak, pin oak
Deer: white cedar, yellow birch,
white pine, oaks
Balsam woolly adelgid: balsam fir
Emerald ash borer: green ash,
black ash, white ash
Paul Jost Asian long-horned beetle: red maple,
sugar maple, aspen
Hemlock woolly adelgid: hemlock
Mountain pine beetle: jack pine
Native insects: eastern larch, others
Earthworms: sugar maple and others

33. Overall scheme for change at the prairie-forest border
proposed by Frelich and Reich

34. Lee Frelich
We can expect a messy
transition
History shows that
development of a new
ecosystem takes 100s
to 1000 years to develop
Old
Dave Hansen
Transitional
New

35. Questions?
Layne Kennedy
Lee Frelich and clones at work during Ham Lake Fire, Seagull Lake, May 6, 2007