Transcript 1. Motivation - Forest Landscape Ecology Lab

Boreal forest land-use change
and its effects on carbon storage
Erica A. Howard
Michelle Steen-Adams
FEM 875, Forest Landscape Change, Fall 2002
Climate change & the carbon cycle
*(Fluxes in
billion tons
C per year)
Increased CO2 in the atmosphere is causing temperatures to rise
1. Motivation
Why should we care about
boreal forests?
Provide fresh water
Support wildlife/biodiversity
Provide timber, petro,
and mineral resources
Allow for recreation
Support native peoples
Regulate climate
Store carbon
Boreal forests & C cycling
Boreal forests now store
several hundred Pg of C
Boreal forests could store
much more C in the
future…
…or could release some
of that C to the
atmosphere
1. Motivation
Boreal forests in a
global perspective
Definitions:
Forests between the
summer & winter positions
of the Arctic front (Landsberg &
Gower 1997)
Forests between ~50-70N
Forests with a characteristic
community composition
Boreal-temperate boundary
at the -16ºC (avg monthly)
isotherm (R&W 1998)
Boreal-”taiga-tundra”
(boreal woodland?)
boundary -- 1330 growing
degree-days (R&W 1998)
The circumpolar range of the boreal forest.
About two-thirds of the area is in Eurasia.
The sector in eastern Canada lies farthest
from the North Pole.
Boreal forests in a
global perspective
Area:
12-14 x 108 ha; ~25% of
global forest cover. (L&G
1997; IPCC LULUC&F)
~20% is wetlands; much of
this is peatlands (Lafleur et
al. 1997)
The circumpolar range of the boreal forest.
About two-thirds of the area is in Eurasia.
The sector in eastern Canada lies farthest
from the North Pole.
Boreal forests in a
global perspective
Carbon content:
~88-143 Pg C in
aboveground vegetation
(Schlesinger 1997; IPCC)
~203-471 Pg C in soil
organic matter and surface
litter. (Schlesinger 1997; IPCC)
FROM: http://www.fao.org/docrep/003/y0900e/y0900e06.htm#P20_4486
“Source: Dixon et al., 1994; Schlesinger, 1997.
Boreal forests account for more
carbon than any other terrestrial
ecosystem (26 percent of total
terrestrial carbon stocks), while
tropical and temperate forests account
for 20 and 7 percent, respectively
(Dixon et al., 1994).”
Regional boreal
forests
Russia
Scandinavia
China
Canada
Alaska
22
65%
22%
44
2%
n/a
n/a
34%
1%
26
…These have very different histories.
Estimates of carbon balance at
varied spatial and temporal scales
Regional estimates (for the present, or very recent
years):
IPCC LULUC&F 2000: inverse modeling results
(Rayner et al 2000; Bousquet et al. 1999) show sinks in Siberia and
North America (but not necessarily boreal N.A.)
Myneni et al. 2001: remote sensing & forest
inventory results show (for 1981-1999) increase in
biomass woody C in Eurasian boreal, but losses in
Canadian boreal forests except for fragments with
gains in N. Saskatchewan & Alberta
Estimates of carbon balance at
varied spatial and temporal scales
Stand-specific estimates:
NEE uptake up to ~2.7 Mg C/ha/yr (eddy flux)
• Very few recently disturbed stands
NEE emissions of ~ -2.5 Mg C/ha/yr in one
managed stand (drained?)
Need more data in disturbed systems -- there are
some chronosequence and paired-site studies
Estimates change through time:
Kurz & Apps find that Canada’s boreal forests
switched from net sink to net source in the 70s or
80s as a result of changing fire frequency
interacting with stand succession
Comparison of estimates of biomass
and NPP of boreal forests
(Jiang et al. 2002)
Country
Biomass
(Mg C ha-1)
Alaska, USA
43-181
Canada
26-214
Russia, Europe 23-166
Russia, Siberia 56-237
China
56-318
NPP (kg C ha-1/yr)
250-1660
1170-3800
1270-3140
310-6750
1810-7800
C balance depends
on disturbance
Historical boreal C
balance: intimately tied
to fire and pests
Future C balance: will
also be influenced by
logging
“Land use, land-use change and
forestry activities, also known as
‘carbon sinks’, can provide a
relatively cost-effective way of
combating climate change, either by
increasing the removal of
greenhouse gases from the
atmosphere (e.g. by planting trees or
managing forests), or by reducing
emissions (e.g. by curbing
deforestation). There are pitfalls,
however…”
UN Framework Convention on Climate Change
website: http://unfccc.int/issues/lulucf.html
1. Motivation
Biophysical drivers of boreal forest
landscape structure: disturbances
Fire
Stand-replacing fire  very common in Canada;
intervals 30-200 yrs (L&G 1997)
Low-intensity fire  very common in Siberia
(Schulze et al. 1999)
Larocque et al. 2000:
Fires may be frequent enough to keep forest stands
in early successional species (e.g., jack pine)
…or fires may occur less frequently, allowing
directional succession
Age-class dynamics
and/or succession
dominate landscape
pattern in most
unmanaged boreal
landscapes
Biophysical drivers of boreal forest
landscape structure: disturbances
Other non-anthropogenic disturbances:
Windthrow
Insect/pest outbreak  very common; dominant in Eastern Canada;
can be low intensity or mortality inducing
Age-class dynamics
and/or succession
dominate landscape
pattern in most
unmanaged boreal
landscapes
Biophysical drivers of boreal forest
landscape structure:
abiotic/biotic template
Landform/drainage/soil texture:
Soil moisture, temperature, nutrient availability
Growing season length
Deciduous/evergreen habit
Moss or lichen types
Microbial biomass and species composition
Seed source
Land Uses of Boreal Forests:
Pre-Industrial Era
Note: People and forests migrated into Canada concurrently since the
last Ice Age - there is no real “pre-indigenous” boreal forest here
Agriculture and grazing, esp. in Scandinavia
(Ostlund et al. 2002)
Selective Logging, fuelwood extraction (e.g. late
19th c. Norway (Storaunet et al. 2000), also Canada (Weir and
Johnson 1998)
Small-scale resource extraction: tar (late 18thearly 19th c. boreal Sweden and Finland; specific
wood production—e.g. ax-handles; peat
Hunting/gathering (Ostlund et al. 2002)
Medicine trees; Magical/ spiritual uses
Culturally-modified trees/
Forest land uses in boreal Sweden
(Ostlund et al. 2002)
Boreal Forest Structure
characteristic of
Pre-Industrial era
Dominant ecological processes/ drivers
Disturbance and succession
“Old-Growth” conditions
Age of oldest trees
Landscape structure: forest
matrix and interspersed patches
Multiple age class stands But no “equilibrium” age class distribution?
Effects of pre-industrial era
land use on boreal forest landscapes
In Scandinavia, impacts of human
activities limited to local scales;
biophysical processes, especially fire,
drove landscape pattern.
Mixedwood boreal forest, Canada
(1883-1994): white spruce  aspen and
jack pine dominated forests.
Land Uses of Boreal Forests:
Industrial Era
Mining/ Drilling
Recreation/ Road building
Settlement/ Agriculture
Forest Harvesting
(Intensive) Forest management
Thinning
Herbicide application
Logging
Fire Suppression
Change in processes driving
landscape pattern:
Fire  Industrial logging
(Axelsson and Ostlund 2001, Jiang et al. 2002)
Effects of forest management
on forest structure
Changes in species composition:
Decrease in the deciduous component of
Scandinavian boreal forests
Management in Canada sometimes favors
deciduous trees (aspen)
Conversion of “old-age” forests to young
forests
Changes in Patch structure: Fragmentation
Uneven-aged  even-aged stands **
Potential to reduce site productivity
Carbon Dynamics: Effects of
landscape structure
•Succession and age class structure
•recently disturbed stands are a net source
•(decomposition continues, NPP is suppressed)
•recovering stands have highest NPP
•older stands produce more litter (leaf and woody)
•regeneration may be a problem following harvest or very
high intensity fire
•Species distribution (conifer vs. broadleaf; understory type)
•deciduous foliage decomposes faster than evergreen
•deciduous broadleaf - more productive than conifers during
summer, but shorter growing season
•foliage decomposes faster than mosses, lichens, wood
Effects of Forest Harvesting on
Boreal Forest Carbon Dynamics
China (Jiang et al. 2002--modelling study):
Harvest/ disturbance levels influenced
biomass, litter, and soil carbon stocks
Rotation length influenced C stocks
Sensitivity of soil carbon stocks relative to
biomass and litter carbon stocks at short
and long time-scales
Carbon Dynamics: Effects of
landscape structure
•Disturbance affects amplitude of the annual C
balance:
•Many forms of disturbance enhance both growing
season productivity and winter respiration (in the short
term) (Zimov et al. 1999)
-species shifts (in both overstory and understory) from
evergreen to deciduous habit may be one cause
-Since NPP and respiration are only partially
correlated and may not vary linearly with the
effects of disturbance, this may be a clue that big
change in the C balance may be in store for the
future
Key points
“The boreal forest is a patchwork of forest
age-classes and plant assemblages resulting
form the interaction among fire histories,
insect outbreaks, and topography.” (Larocque et al.
2000)
Net ecosystem exchange (NEE) depends
strongly on the vegetation composition
Net biome productivity (NBP) (or modified
NEP) may be less than one 1/1000th of
instantaneous NPP (Schulze et al. 1999). So
disturbance is KEY.
Ways that human activities may
influence carbon stocks and mean
annual carbon sequestration
Nitrogen deposition
Climate change (temperature and
precipitation)
Increased CO2 concentration
Intensive forest management
Effects of climate change on
boreal forests
In upland Scandinavia: Increased
temperature and/ or CO2 did not significantly
influence tree growth; Carbon sequestration
may be less than expected. Rasmussen et al. (2002)
Effects of increased temperature and nitrogen
deposition: enhanced forest productivity and
timber yield (model based on Finnish Scots
pine stands) (Pussinun et al. 2002)
Increased temperature  lower carbon
stocks (Pussinun et al. 2002)
Limits to our Knowledge /
Important Future Research
Directions
Effects of alteration of forest landscape
structure due to fire suppression
Regeneration
success: Do
managed forests
recover to the same
level of productivity
as unmanaged
forests?
• How much do small
landscape elements
(like beaver ponds)
matter?
Present and on-going
research questions
What factors limit ecosystem recovery?/
How long do human alterations persist
on the landscape?
What does “sustainable forestry” mean?:
forestry for single species versus whole
communities (Hanski 2002).
How long does it take to achieve oldgrowth conditions in boreal forests?