Disturbance, Microclimate, and Vegetation
Responses to Variable-Retention Harvests
Disturbance
Don McKenzie
Microclimate
Troy Heithecker (MS)
Bryophytes
Cara Nelson (PhD)
Martin Dovciak
Shelley Evans
Jim Saracco
Vascular plants
Cara Nelson
Martin Dovciak
Shelley Evans
Working model, questions, and variables of interest
2. Microclimatic stresses
(light, temperature, soil moisture)
3. Bryophytes
4. Herbs
• early-seral
• late-seral
1. Logging disturbance
(bare soil, slash)
Disp.
40%
15%
How much to retain?
Does pattern matter?
100%
Agg.
Treatments
Cut areas
Aggregates • dispersed vs.
aggregated
• refugia
• edge effects
1. Disturbance and post-harvest ground conditions
Do they vary with level or pattern of retention?
40%
15%
Dispersed
Aggregated
Disturbed soil (year 1)
ns
12
ns
***
WF
CF
Cover (%)
10
Yarding method
ground-based,
slash piled
8
A
40%
6
DP
BU
15%
D
4
PH
2
LWS
0
Pattern
Level
Block
Source of variation
helicopter
Slash cover (year 1)
**
85
*
Forest structure
**
LWS
Cover (%)
80
PH
A
15%
75
70
DP
BU
WF
40%
D
65
60
large opengrown trees,
dense vine maple
CF
Pattern
Level
Block
Source of variation
second-growth
1. Disturbance and post-harvest ground conditions
Do they vary with level or pattern of retention?
Yes and no.
• soil disturbance was minimal, but
• slash cover was greater at lower retention and in
aggregated treatments
Greater variation was found among locations due to
• different yarding systems (ground-based vs. helicopter)
• different forest structures (tree size, shrub cover)
Results have implications for plants that are sensitive to
physical disturbance or burial.
2a. Microclimate vs. retention level
Are current minimum standards for retention sufficient
to moderate understory microclimate?
0%
“clearcut”
15%D
40%D
100%
= microclimate station (light, temperature, soil moisture)
Transmitted light (moles m-2 day-1)
Transmitted light (year 6-7)
0%
40
30
a
b
15%
20
c
c
10
40%
p < 0.001
0
0% 15%
40%
100%
Level of retention
100%
Selection of warm, sunny days
Temperature (oC)
25
20
15
Warm, sunny
10
Cool, cloudy
5
0
00:00 04:00 08:00 12:00 16:00 20:00
Time of day
Mean air temperature (year 6-7)
28
no difference
o
Temperature ( C)
a
ab
24
bc
c
20
16
p = 0.001
0% 15%
40%
Level of retention
100%
Max. air temperature (year 6-7)
36
no difference
a
32
ab
o
Temperature ( C)
a
b
28
24
20
p = 0.006
0% 15%
40%
Level of retention
100%
2a. Microclimate vs. retention level
Are current minimum standards for retention sufficient
to moderate understory microclimate?
Not for temperature.
• mean air temp in 15%D = “clearcut”
• max. air temp in 15 - 40%D = “clearcut”
• mean and max. soil temp in 15 - 40%D = “clearcut”
Conclusion: species sensitive to changes in temperature
(or humidity) may not benefit from dispersed retention,
even at retention levels as high 40%.
2b. Microclimate: Forest aggregates
How does microclimate vary with distance from edge?
How does microclimate compare to uncut forest?
15%A
100%
= microclimate station
1-ha
aggregate
50
Butte (dense forest)
Little White Salmon (open forest)
-2
Transmitted light (moles m day
-1
Transmitted light (year 6-7)
40
30
S-W
20
N-E
10
100%
100%
0
-60
-40 -20
Aggregate
0
20 40 60
Harvest area
Distance from edge (m)
-60
-40 -20
Aggregate
0
20 40 60
Harvest area
Distance from edge (m)
Mean air temperature (year 6-7)
Little White Salmon (open forest)
28
o
Temperature ( C)
Butte (dense forest)
26
S-W
24
N-E
100%
100%
22
-60
-40 -20
Aggregate
0
20 40 60
Harvest area
Distance from edge (m)
-60
-40 -20
Aggregate
0
20 40 60
Harvest area
Distance from edge (m)
Butte
Butte
LWS
Trees/ha
Canopy
cover
1,780
336
81%
55%
Little White Salmon (LWS)
2b. Microclimate: Forest aggregates
How does microclimate vary with distance from edge?
• light declined steeply, especially along N/E edges
• temperature declined less steeply
• edge-related gradients are shaped by forest structure
(canopy openness, stem density)
How does microclimate compare to uncut forest?
• 1-ha sized aggregates can support core areas with
microclimate similar to undisturbed forest
• smaller aggregates may be compromised by edge
effects
3a. Bryophytes: Treatment effects
Do initial responses vary with level or pattern of retention?
• mosses and liverworts –
diverse groups
• many associated with CWD
and other substrates
• sensitive to disturbance and
environmental stress
Hylocomium splendens
• re-establishment can be slow
Scapania bolanderi
Change in % cover
Bryophytes: change in total cover (year 1)
5
Treatment means
Cut areas only
0
0
-5
-20
-5
a
-10
-15
5
ab ab
b b
no difference
15% 40%
-10
Agg.
Disp.
-15
no difference
100%
15% 40%
Level of retention
-20
Change in no. of species per plot
Bryophytes: change in species richness (year 1)
4
Treatment means
Cut areas only
2
2
0
0
a
-2
-4
-6
4
c
bc
bc
b
15% 40%
-2
Agg.
Disp.
Agg.< Disp.
b ab b a
100%
15% 40%
Level of retention (%)
-4
-6
3a. Bryophytes: Treatment effects
Do initial responses vary with level or pattern of retention?
Mostly no.
• declines in bryophytes cover were large at both
moderate and low levels of retention
• pattern had little effect on the magnitude of decline
• tendency for greater loss of richness in aggregated
treatments than in dispersed treatments
Conclusion: greater retention is needed to support the
diversity and abundance of bryophytes found in older
forests.
3b. Bryophytes: Role of substrates
Does substrate affect responses to retention?
15%
40%
Tree base
NE
CWD
SW
Forest
floor
100%
Mosses: Cover (year 8)
Tree base
CWD
NE
Forest floor
SW
Cover (%)
50
40
NE
SW
30
20
10
0
15% 40%
100%
15% 40%
100%
Level of retention
15% 40%
100%
3b. Bryophytes: Role of substrates
Does substrate affect responses to retention?
Yes.
• CWD is an important substrate for bryophytes
• however, its suitability as a substrate is greatly reduced
at lower levels of retention (less so for forest floor)
Conclusion: negative effects of harvest can be mediated
through changes in substrate quality.
3c. Bryophytes: Persistence within aggregates
Do 1-ha aggregates serve as refugia?
Are bryophytes sensitive to forest edge?
1-ha aggregate
40%A
Bryophytes: change in cover vs. proximity to edge (year 1)
8
Mosses
4
no decline
Change in cover (%)
0
-4
-8
0.4
ns
Liverworts
0.2
decline
0.0
-0.2
-0.4
-0.6
r = -0.50
p = 0.008
-50 -40 -30 -20 -10
0 (forest edge)
Distance from edge (m)
3c. Bryophytes: Persistence within aggregates
Do 1-ha aggregates serve as refugia?
Are bryophytes sensitive to forest edge?
Yes, aggregates serve as refugia in the short term, but
some taxa are sensitive to edge.
• mosses showed no response to edge
• liverworts declined, but only within 10 m of the edge
Conclusion: 1-ha aggregates are large enough to retain
the diversity of bryophytes found in older forests.
Smaller aggregates may be compromised by edge
effects.
4a. Herbs: Treatment effects
Do responses vary with level or pattern of retention?
Early-seral herbs. Respond
positively to disturbance and
increases in resources.
Late-seral herbs. Sensitive to
disturbance and environmental
stress.
Cirsium vulgare
Pyrola picta
Early-seral herbs: changes in richness (year 6-7)
Change in no. of species / plot
Treatment means
Within treatments
7
6
5
4
3
2
1
0
level of retention
is important
Agg.
Disp.
100%
15% 40%
Level of retention
Agg. cut
Disp.
Agg. uncut
100%
15% 40%
Level of retention
Late-seral herbs: changes in richness (year 6-7)
Change in no. of species / plot
Treatment means
Within treatments
Agg. uncut
0
Disp.
Disp.
-1
Agg.
-2
Agg. cut
-3
-4
15% 40%
100%
Level of retention
15% 40%
100%
Level of retention
4a. Herbs: Treatment effects
Do responses vary with level or pattern of retention?
Yes for level. Generally no for pattern.
• herbs showed strong responses to level of retention, but
more subtle responses to pattern
• in aggregated treatments, small changes within forest
patches were balanced by large changes in adjacent
harvest areas
4b. Herbs: Persistence within aggregates
Do 1-ha aggregates serve as refugia for forest herbs?
Are herbs sensitive to forest edge?
1-ha forest aggregate
40%A
Herbs: change in richness with proximity to edge (year 2)
Change in richness
(no. species / band)
3
2
1
Early-seral herbs
0
-1
Forest herbs
-2
ns
-3
-4
Forest aggregate
-5
-50
-40
-30
-20
-10
Harvest area
0
10
Distance from edge (m)
20
30
Edge-related declines of herb species (year 1-2)
Is there is a correlation between decline in cover and
proximity to forest edge?
• 23 herbs tested
• number of significant correlations:
year 1: 3 species (13%)
year 2: 8 species (35%)
• correlation coefficients (declines) all increased in year 2
4b. Herbs: Persistence within aggregates
Do 1-ha aggregates serve as refugia for forest herbs?
Are herbs sensitive to forest edge?
Aggregates serve as refugia in the short term.
• no forest species were lost
• few early-seral species invaded
However, some species are sensitive to edge.
• 35% of herbs showed edge-related declines
• declines were more frequent and stronger over time
Conclusion: 1-ha sized aggregates can serve as refugia,
but temporal trends suggest the need for continued
study.
General Conclusions
How much to retain?
• Microclimatic studies suggest that current minimum
standards for retention (15%) are not sufficient to
moderate air and soil temperatures
• For most plant groups, level of retention is important
• Conservation of more sensitive groups – bryophytes
and late-seral herbs – may require greater levels of
retention
General Conclusions
Does pattern matter?
• At the treatment-scale, there were surprisingly few
effects of pattern
• In aggregated treatments, benefits of undisturbed
patches are balanced by larger changes in adjacent
cleared areas
• Pattern may become more important in the future if
aggregates serve as dispersal sources for disturbancesensitive species
General Conclusions
Do forest aggregates function as intended?
• In the short term, 1-ha aggregates serve as refugia
• Light and temperature are elevated at the edge, but
core areas can resemble undisturbed forest
• Herbs and bryophytes are less sensitive to edge than
would be suggested by microclimatic gradients
• Further study is needed to determine whether
aggregates will serve as dispersal sources for adjacent
harvested area
Published Information
~ DISTURBANCE & GROUND CONDITIONS ~
Halpern, C.B., D. McKenzie. 2001. Disturbance and post-harvest ground conditions in a structural
retention experiment. For. Ecol. Manage. 154:215-225.
~ MICROCLIMATE ~
Heithecker, T.D., C.B. Halpern. 2007. Edge-related gradients in microclimate in forest aggregates
following structural retention harvests in western Washington. For. Ecol. Manage. 248:163-173.
Heithecker, T.D., C.B. Halpern. 2006. Variation in microclimate associated with dispersedretention harvests in coniferous forests of western Washington. For. Ecol. Manage. 226:60-71.
~ FOREST-FLOOR BRYOPHYTES ~
Dovčiak, M., and others. 2006. Persistence of ground-layer bryophytes in a structural-retention
experiment: initial effects of level and pattern of retention. Can. J. For. Res. 36:3039-3052.
Nelson, C.R., C.B. Halpern. 2005. Short-term effects of timber harvest and forest edges on
ground-layer mosses and liverworts. Can. J. Bot. 83:610-620.
~ UNDERSTORY VASCULAR PLANTS ~
Halpern, C.B., D. McKenzie, S.A. Evans, D.A. Maguire. 2005. Early responses of forest
understories to varying levels and patterns of green-tree retention. Ecol. Appl. 15:175-195.
Nelson, C.R., C.B. Halpern. 2005. Edge-related responses of understory plants to aggregated
retention harvest in the Pacific Northwest. Ecol. Appl. 15:196-209.
Nelson, C.R., C.B. Halpern, J.A. Antos. 2007. Variation in responses of late-seral herbs to
disturbance and environmental stress. Ecology 88:2880-2890.