Level and Pattern of Overstory Retention Interact to Shape Long-term Responses of Forest Understories to Timber Harvest
Charles B. Halpern1, Juraj Halaj2, Shelley A. Evans1, Martin Dovciak3
Introduction
Results
Q1. Species composition
p
A
%
0
4
100%:
40%A:
15%A:
40%D:
15%D:
48-65
61-77
59-87
*Abam = Abies amabilis, Abco = A. concolor, Abgr = A. grandis, Tshe =
Tsuga heterophylla
40%D
40%D
40% Dispersed
(40%D)
15% Dispersed
(15%D)
Q1. Species composition. Compositional change (movement in NMS
space) was reduced by greater retention and by the presence of dispersed
trees in harvested areas. Retained forest patches showed small changes
in composition; however, in some blocks, changes were greater in the
more exposed patches of 15%A-p (vs. 40%A-p).
Control
Five 1-ha residual patches (A-p) separated by cleared areas (A-c)
Two 1-ha residual patches (A-p) separated by cleared areas (A-c)
Dispersed trees (dominants or co-dominants); basal area = 40%A
Dispersed trees (dominants or co-dominants); basal area = 15%A
Change in % cover
Early-seral herbs
Forest herbs
Late-seral herbs
Cleared (A-c)
Dispersed (D)
10
5
Level: P = 0.001
Pattern: P < 0.001
Pattern: P = 0.007
0
Level: P < 0.001
Pattern: P < 0.001
L x P: P = 0.002
-5
-10
Cleared (A-c)
Dispersed (D)
Q2. Seral group responses to level and
pattern of retention
Early-seral shrubs
20
A-c D
40%
Forest shrubs
15
10
A-c D
15%
Level: P < 0.001
Pattern: P = 0.006
5
A-c D
40%
A-c
Level: P < 0.001
Pattern: P < 0.001
L x P: P = 0.004
A-c D
40%
D
A-c D
15%
A-c D
40%
Systematic sampling grid: 40 m × 40 m spacing
Numbers of plots: 32-37 permanent sample plots per treatment
Bryophytes and herbs: 24 quadrats (0.1 m2) per plot
Shrubs: four, 6-m line intercepts per plot
Sampling dates: pre-treatment and 10-11 yr post-treatment
Q3. Moderating effects of dispersed trees. Compared to cleared areas
(A-c), dispersed retention (D) moderated effects of harvest. For earlyseral herbs and shrubs, effects of dispersed trees were stronger at higher
levels of retention (significant level × pattern interaction).
Q4. Aggregates as refugia
Change in % cover
15
10
5
0
Bryophytes
Early-seral herbs
Forest herbs
Level: P < 0.001
Pattern: P = 0.001
Level: ns
Pattern: ns
Level: P < 0.001
Pattern: P = 0.018
L x P: P = 0.039
-5
-10
Late-seral herbs
5
Control
Aggregated (A)
Dispersed (D)
Level: P = 0.019
Pattern: P = 0.002
20
Early seral
Forest generalist
A-p
-5
-10
Level: ns
Level: ns
Early-seral shrubs
Forest shrubs
15
10
A D
15%
100% A D
40%
Level: P < 0.001
Pattern: P < 0.001
5
0
-5
100% A D
40%
A D
15%
A
100%
Level: P < 0.001
Patern: P = 0.001
100% A D
40%
A D
15%
A D
15%
D
40% 15%
Q2. Seral group responses to level and pattern of retention. For
most plant groups, changes in cover were reduced at higher levels of
retention (40% < 15%) and in dispersed (vs. aggregated) treatments.
However, bryophytes showed large and comparable declines in all
treatments. (Note: changes in the controls are shown for comparison.)
Bryophytes
Late-seral herbs
-15
-10
100%
Bryophytes
0
-15
40-m spacing
Late seral
30
20
10
100%
A-p A-c D
40%
A-p A-c D
15%
Q5. Extirpations of late-seral herbs. Extirpations of late-seral
herbs were 2-4× more common in the cleared areas of aggregated
treatments (A-c) and under low levels of dispersed retention (15%D)
than in retained forest patches (A-p) or controls (100%).
Conclusions and
Management Implications
Ours is one of the few experiments to explore the individual and joint
effects of level and spatial pattern of overstory retention in shaping
vegetation responses to regeneration harvests. In these forests, both
factors have strong and potentially interacting effects.
● Level of retention. Increasing levels of retention can temper the
effects of disturbance and environmental stress, reducing colonization
of early-seral species and loss of late-seral herbs. However, even
relatively high levels of retention may not be sufficient to mediate loss
of bryophytes that are highly sensitive to overstory removal.
20
● Q1. Compositional trajectories in ordination (NMS) space
● Q2-4. Cover of forest-floor bryophytes and early-seral,
forest-generalist, and late-seral species
56-m radius
● Q5. Extirpations of late-seral herbs, expressed as the % of species
lost from all plots within a treatment unit or post-harvest environment
(1-ha patches or cleared areas within aggregated treatments)
D
40% 15%
40
40% 15%
A-c D
15%
Dispersed or Control
Change in % cover
●
●
●
●
●
Aggregated
A-c D
15%
0
-5
-10
Field Methods and Analyses
Sampling design
100%
50
0
-15
p
A
%
0
4
15%D
15
A-c
60
%
0
0
1
40%D
Bryophytes
20
40%A-c
p
A
%
5
1
Tshe 70-80 759-1781
Abgr 140-170 182-335
Abam 110-140 512-1005
c
A
%
5
1
40-53
40-66
9-33
40%A-c
p
A
%
0
4
Washington
Butte
975-1280
Little White Salmon 825-975
Paradise Hills
850-1035
D
%
5
1
%
0
0
1
36-52
72-106
p
A
%
5
1
40%D
p
A
%
0
4
Tshe 110-130 310-500
Abco
165
258-475
c
A
%
5
1
D
%
5
1
%
0
0
1
D
%
5
1
4-7
34-62
Paradise Hills, Washington
c
A
%
5
1
c
A
%
5
1
945-1310
1460-1710
D
%
5
1
p
A
%
5
1
Oregon
Watson Falls
Dog Prairie
%
0
0
1
Little White Salmon, Washington
40%A-c
15%D
Block
D
40% 15%
15%A
100% A D
40%
Tree
Basal
Slope Forest Stand density
area
(%) zone* age (yr) (no./ha) (m2/ha)
100%
40%A-c
Species extirpations (% of species)
p
A
%
5
1
40%D
Butte, Washington
40%A
40%A
Elevation
(m)
%
0
0
1
Aggregated:
cleared area (A-c)
Response variables
40%D
p
A
%
0
4
Aggregated:
1-ha patch (A-p)
100%
Q2. Seral group responses to level and pattern of retention.
How do level and pattern of retention affect the abundance
of bryophytes and key seral groups?
Q5. Extirpations of late-seral herbs. Which post-harvest
environments mediate loss of late-seral herb species?
A-p
p
A
%
5
1
100%
(Control)
Questions
Q4. Aggregates as refugia. Do retained forest patches serve as
refugia for disturbance-sensitive or shade-dependent species?
A-c
A
40%D
Q1. Species composition. How do level and pattern of retention
affect compositional changes within and among treatments?
Q3. Moderating effects of dispersed trees. Do the presence of
dispersed trees moderate responses relative to cleared areas?
Dog Prairie, Oregon
40%A-c
Experimental treatments
Study Areas
Q5. Extirpations of late-seral herbs
A-p
Watson Falls, Oregon
We present results from a long-term experiment in variable-retention
harvests in mature, coniferous forests of the Pacific Northwest, the
DEMO Study. It tests regional standards for live-tree retention on
federal forests within the range of the Northern Spotted Owl. These
include a minimum of 15% live-tree retention in each harvest unit,
primarily in the form of large (0.2-1 ha) aggregates. We assess the
effects of retention level (15% vs. 40% of original basal area), pattern
(aggregated vs. dispersed), and their interaction on compositional
changes and on the abundance of bryophytes and vascular understory
species with differing seral roles and sensitivities to disturbance and
environmental change.
Locations and pre-treatment characteristics
of the five experimental blocks
Q3. Moderating effects of dispersed trees
D
%
5
1
For most plant groups, changes in cover were tempered by higher
levels of retention and by the presence of dispersed trees. Although
retained patches (1 ha) in aggregated treatments were stable, changes in
adjacent cleared areas were greater than in dispersed treatments.
Bryophytes showed large declines regardless of treatment, suggesting
that threshold levels of disturbance and environmental stress were
exceeded. Late-seral herbs were sensitive to level and pattern of
retention: species’ losses were most frequent in the cleared areas of
aggregated treatments and at low levels of dispersed retention. In
contrast, these were the environments where early-seral species were
most abundant. Retained patches in aggregated treatments provided
refugia for bryophytes and late-seral herbs, but may be susceptible to
edge effects. The ability to achieve multiple ecological objectives with
variable retention may require spatial partitioning of post-treatment
habitats to include areas of dispersed retention, larger-sized aggregates,
and clearings.
Experimental Design
Designing variable-retention systems to achieve specific ecological
objectives requires an understanding of how forest organisms respond
to the amount (level) and spatial pattern of trees retained through
timber harvest. Greater retention should favor closed-canopy species
and inhibit disturbance-dependent, early-seral species. Although
dispersed retention can reduce microclimatic extremes relative to
cleared areas (clearcuts), it may be ineffective for species that are
sensitive to disturbance or changes in light, temperature, or humidity.
Maintaining these species through harvest may require undisturbed
forest patches (or aggregates).
c
A
%
5
1
Variable retention has replaced clearcut logging as the principal
method of regeneration harvest in many regions of the world. A
primary motivation for retaining live trees through harvest is to
enhance the ecological values of managed forests, including the
maintenance and recovery of late-seral species. However, the ability to
achieve these goals may hinge on the level (amount) or spatial pattern
of retention. Here we present decade-scale results from a large-scale
experiment in the Pacific Northwest in which we assess understory
responses to varying levels (15 vs. 40%) and patterns (aggregated vs.
dispersed) of overstory retention. We quantify changes in abundance
of bryophytes and early- to late-seral vascular plants — groups for
which we expected differing responses to disturbance and
environmental changes associated with canopy removal.
Change in % cover
Abstract
University of Washington, Seattle, WA; 2Allevia Health, Inc., Corvallis, OR; 3State University of New York, Syracuse, NY
Change in % cover
1
100% 40%
100% 40%
A-p
15%
A-p
100% 40%
A-p
15%
15%
A-p
Q4. Aggregates as refugia. Changes in cover of bryophytes and
late-seral herbs did not differ among residual forest patches and
controls. However, declines tended to be greater with increasing
exposure of the patches (100% < 40%A-p < 15%A-p).
Examples of 1-ha aggregates: 15%A (left), 40%A (right)
● Pattern of retention. Effects on the understory were reduced in
dispersed relative to aggregated treatments, particularly at higher
levels retention where shading was sufficient to limit the
establishment of early-seral herbs and shrubs. Higher levels of
dispersed retention also greatly reduced loss of late-seral herbs.
● Forest aggregates as refugia. Although 1-ha aggregates serve
as refugia, they may be susceptible to edge effects (increased light,
temperature, and wind damage), particularly at lower levels of
retention, i.e., greater exposure. Smaller-sized aggregates, which are
permitted under current retention standards, are more susceptible to
these effects and are less likely to serve as refugia.
● Minimum retention standards in the Pacific Northwest.
Regardless of pattern, our long-term observations suggest that 15%
retention — the minimum standard for federal matrix lands in the
Pacific Northwest — is not sufficient to retain the abundance or
diversity of species characteristic of older forests in this region.
● Multiple objectives for management. The ability to achieve
multiple objectives within the context of variable-retention harvests,
i.e., timber production, provision of early-seral habitat, and
maintenance of late-seral species, will require the spatial partitioning
of habitats to include areas of dispersed retention, clearings, and large
undisturbed aggregates.
Analyses
Experimental treatments at Butte, Washington. The
15%A treatment is out of the image to the west. The 75%A
treatment (three circular 1-ha gaps) was not included in this
study. Treatment dimensions are 320 m x 400 m.
● Q1. NMS ordination through time of plots representing different treatments or within-treatment environments
● Q2-Q4. One-way ANOVA and a priori contrasts testing effects of retention level, pattern, and their interaction
● Q5. Qualitative comparison of species’ extirpation rates among treatments or post-harvest environments
Acknowledgement. This a product of the Demonstration of Ecosystem
Management Options (DEMO) Study, a joint effort of the USDA Forest
Service Region 6 and Pacific Northwest Research Station. Research partners
include the University of Washington, Oregon State University, University of
Oregon, Gifford Pinchot and Umpqua National Forests, and the Washington
State Department of Natural Resources (http://www.fs.fed.us/pnw/rmp/demo).
Funding was provided by the USDA Forest Service, PNW Research Station.