Management of logging slash –
ecology & risk
Brenda R Baillie
Presented to the Te Mana O Te Wai Tairāwhiti freshwater symposium, Tolaga
Bay New Zealand , 4th May 2017
Outline
Woody debris in planted forest streams
To provide information to
assist with the
management of woody
debris in streams
Aim
Ecology of woody debris in streams
Risks & management of logging slash in streams
Loss or reduction of the indigenous wood component from
our river systems
(a) prior to human settlement; (b) prior to European settlement; (c) current day (McGlone, et al.,
2004; McGlone, 1989; Pairman, 2014). Figure compiled by M. Heaphy, Scion, New Zealand.
Forest cover in NZ
bank undercutting, tree mortality, wind, snow
floods, landslides/slips, debris flows
Wood input into streams
Functions of wood in stream ecosystems
pool
plunge
pool
sediment storage
aerates the water
bank undercutting/
channel widening
pool formation
bank protection
debris
dam
retention of organic
matter– instream
processing
sediment
loss
flow deflection
Provides habitat and food for
aquatic insects, koura,
native fish, trout and birds
Drawing by
Murray
Simpson
Ecological role of
wood
Photo by B. Baillie
Wood - roost habitat for whio (Blue duck)
0
2
4
6
8
10
12
14
16
unde
rcut
ban
k
log
jam
sing
le lo
g
rock
cav
e
root
plate
log&
unde
rcut
ban
k
log
jam
&ro
ck
Num
ber
of
roosts
Photo by B. Baillie
Wood – habitat for fish & aquatic invertebrates
Woody debris pools provided
habitat for most of the larger
long-fin eels & banded kokopu
Debris dams supported higher
densities & diversity of aquatic
invertebrates c.f. substrate
Distribution of pre-harvest and harvest woody debris
volumes by diameter class
0
5
10
15
20
25
30
35
40
45
50
1-4 5-9 10-14 15-19 20-24 25-29 30-34 35-39 40-44 45-49 50+
Diameter Classes (cm)
Wo
od
y d
eb
ris v
olu
me (
m3)
Pre-harvest
Harvest
Small
Wood
Large Wood
Woody debris in planted forests
a a
ab
b
0
100
200
300
400
Method 1 stream clean
Method 2 ground-base
Method 3 haul back from stream
Method 4 haul across stream
Ave
rag
e h
arv
est w
oo
dy d
eb
ris v
olu
me
s m
³ /ha
Volumes with different letters are significantly different (P< 0.05)
Effects of harvest methods & practices on woody
debris in streams
Environmental RiskDebris dams, channel blockage Downstream off-site movement
Damage to downstream environments &infrastructure Clean-up costs
Gisborne Herald May 3, 2017
Environmental risk – movement of logging slash
Pre- harvest Post-harvest 2 years after harvest
Woody debris is most mobile in the first 2 years following harvest. After that,
riparian vegetation and sediment assist in retaining wood within the stream
system (16 sites) (unpublished data)
Log Tag trial-100 pieces (mean stream width 3.6m)
Most movement within the first 6 months,
Pieces < 5m in length most unstable and moved the
furthest distance downstream
Declined thereafter as pieces had moved into more stable positions
R
I
S
KLow
High
1 2 3 4 5
Years since harvest
•Relocated into more stable position
•Locked in by sediment & vegetation
Wood Movement
What makes a piece of wood more stable?
More stable – less risk
suspended
partly
suspended
partly buried
length
longer than
stream width
rootwad
Management of logging slash(small streams, moderate floods)
Site specific, stream size & characteristics, limits to what you manage
upslope practices important
harvest method
riparian buffers
retirement of very steep areas
Minimise input of logging slash into streams
leave it – Safety first!!
downstream accessible sites to remove
natural accumulation points
debris traps - machine accessible sites
trees effective trapping logging slash
Steep inaccessible streamsPhoto courtesy of Ernslaw
Photo courtesy of Ernslaw
Accessible high-risk streams
• stream-cleaned to reduce the
risk to the stream environment,
downstream communities,
infrastructures such as culverts
& bridges
• safety issues
• $$$
Effects of logging slash management on streams
Mainly above the
channel
•shade
•temperature control
•maintain water flow
•lower risk of
movement
downstream
•large stable pieces,
important habitat
Slash mainly in
water
•impedes flow
• √
• √
• √
• √
•sedimentation
accumulation
•risk movement
•NB: degree & duration of effects, highly variable, stream specific
Stream-clean
•increase in light
•increase in stream
temperature
•decrease in oxygen
•increase in algae
•loss of sensitive
aquatic insects &
fish
•no risk
Pre-harvest Post-harvest Post-flood
Sources of
stable wood
Windthrow, unmerchantable pieces ($$$ to remove)
Meet stability criteria
Provide long-term source of wood
Contribute to stream ecosystem function & habitat biodiversity
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