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

Debris dam in a native forest stream

Some wood eventually makes its way into lakes…

…..estuaries, beaches

& oceans

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

Pre-wood removal

Wood removal

Post-wood removal

What happens when you remove the wood

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

Environmental

Risk

Ecological

Benefit

Summary

Future – wood as a rehabilitation tool