Ecological and Aesthetic
Effects of VariableRetention Harvests in the
Northwestern United
States: Initial Results
from the DEMO Study
Keith B. Aubry and Charles E. Peterson
U.S. Forest Service, PNW Res. Station
Green-tree Retention - NWFP
‹ 15% of each harvest unit
z 70% intact patches (aggregates) 0.2-1.0 ha
z 30% dispersed trees / small clumps
‹ Largest, oldest decadent trees
and snags.
‹ Retained indefinitely.
‹ Ecological value largely untested –
professional judgments.
The DEMO Study
‹ Demonstration of Ecosystem
Management Options (DEMO).
‹ First operational-scale, replicated
experiment to study the effects of
variable-retention harvests in mature
Douglas-fir forests in the
northwestern United States.
‹ Broad array of response variables.
The DEMO Study
‹ Research questions:
z How does the level of green-tree
retention affect ecological attributes,
microclimatic conditions, and public
perceptions of visual quality?
z At a given level of retention, do
effects vary with the pattern of
retained trees?
z How do responses vary over time?
DEMO Study Areas
‹ Experimental
treatments were
implemented at 6
locations (blocks)
in W. Washington
and Oregon.
DEMO Experimental Design
‹ Treatment units: 13 ha
‹ 6 treatments – strongly contrasting
levels and patterns of green-tree retention.
‹ Aggregates & gaps:
z 1-ha circles
‹ Dispersed retention:
z Dominants and
Co-dominants
‹ Randomized complete block design.
DEMO
‹ Stands 65-170
years old.
‹ Upland forests
- riparian areas
& roads avoided.
‹ Harvest
methods and
silvicultural
prescriptions
varied among
blocks, but not
within blocks.
Unharvested
Control
DEMO Response Variables
‹ Vascular plants, mosses and liverworts.
‹ Truffles and mushrooms.
‹ Canopy- and litter-dwelling arthropods.
‹ Wildlife
z Terrestrial salamanders
z Small mammals, squirrels, bats
z Breeding birds
‹ Microclimatic conditions.
‹ Public perceptions of visual quality.
DEMO – Systematic Sampling Grid
Buffer:
40 m
56 m
40m
Aggregate:
1 ha
Sampling Response Variables
‹ Sampling conducted
before and after harvest.
‹ Pre-treatment
sampling: 1994 to 1996
‹ Harvests: 1997 to 1998
‹ Initial post-treatment
sampling: 1998 to 2004
‹ Next round: 2008
Forest-floor Bryophytes
‹ Declines in species richness of mosses
‹ Pattern had
little effect.
Change in Richness
and liverworts were large at both 40% and
15% retention.
8
Aggregated
Dispersed
4
0
a
ab
-4
-8
bc cd
P < 0.0001
100
75
40
cd d
15
Level of retention (%)
Late-seral Herbs
‹ At the treatment level,
pattern of retention had
little effect on species losses.
z Higher losses
Extirpations
in clearcut areas
offset lower losses
in aggregates.
z Losses were
intermediate in
dispersed-retention
treatments.
6
5
Harvested
areas
4
3
2
Aggregates
1
0
D
100%
A
40%
D
A
15%
Post-harvest environment
Litter-dwelling Arthropods
‹ Carabid beetles and interior-forest spiders
declined significantly in all harvest
treatments (15% & 40% retention).
‹ Neither level nor pattern of
retention affected the magnitude
of treatment responses.
‹ 5-7 years after harvest, none of
the DEMO treatments provided
suitable habitat for the forestdependent arthropods we studied.
Litter-dwelling Arthropods
‹ Interior-forest species in both groups
responded to habitat edges in the aggregates.
‹ Carabid beetles and interior-forest spiders
declined significantly with
proximity to forest edge.
‹ Habitat was better in
aggregates than in
clearcut areas, but
poorer than in
undisturbed forest.
Small Mammals
‹ Among interior-forest species:
only the fungivorous western
red-backed vole responded
negatively to level of retention.
‹ WRBV was significantly reduced or extirpated
in all harvested areas – persisted only in
aggregates or undisturbed forest.
However, as with interior-forest arthropods,
aggregates provided lower-quality habitat than
undisturbed forest – edge effects were evident.
z
Summer Microclimate
‹ Maximum air temperatures in 15% and 40%
dispersed treatments were no different than in
harvested areas of aggregated treatments (0%).
35
a
a
ab
o
Air temperature ( C)
‹ Mean air temperatures
in 15% dispersed
treatment same as
harvested areas (0%).
30
b
25
Maximum
20
Mean
a
15
0
ab
15
bc
40
c
100
Level of retention (%)
‹ Microclimatic conditions in core areas of 1-ha
aggregates were similar to undisturbed forest.
Public Perceptions
‹ Passionate opposition
to clearcutting in the
Pacific Northwest.
‹ Public perceptions
strongly influenced by
pattern of retention.
z Visual quality rated higher in dispersed treatments,
but only at levels of retention >15%.
‹ For aggregated-retention treatments,
increasing the level of retention results in only
small increases in perceived visual quality.
Major Short-term Findings
‹ Retention levels >15% will likely be needed
to effectively retain sensitive plants and
animals, ameliorate harsh microclimatic
conditions, and gain public acceptance.
‹ At 15% and 40% retention, treatment-level
responses for most taxa were unaffected by
the pattern of green trees retained.
z Ecological and microclimatic benefits of intact
forest in aggregates were offset by detrimental
effects of intervening clearcut areas.
Major Short-term Findings
‹ Aggregates provided "lifeboats" for many
species that are sensitive to timber harvest.
‹ Aggregates at least 1 ha in size appear to
be necessary for retaining many interiorforest species, at least in the short-term.
‹ Retaining both large aggregates (>1 ha)
and dispersed trees at levels >15% may be
the most effective strategy for maintaining
biological diversity after timber harvest in
mature Douglas-fir forests of the PNW.
Thank you!