Large woody debris inputs after harvesting in small stream buffers at Malcolm Knapp Research Forest

AbstractA series of small clearcuts were harvested in 1998 in the southeast corner of a 70 year old second growth stand in the Coastal Western Hemlock dry maritime subzone at Malcolm Knapp Research Forest. Each cutblock was bisected by a small stream, and 0m, 10m and 30m reserve zones were established adjacent to these streams. Each treatment was replicated 3 times and 3 unharvested streams were identified as controls. In the summers of 2005 and 2006 all logs greater than 7.5cm at mid-span diameter that were in decay classes 1 through 3 were measured and tagged. Post-harvest windthrow in the 10 and 30m buffers in the 2 years following harvesting produced a number of large spanning logs. Most conifer logs are still in decay class 1, while deciduous logs are mostly in decay classes 2 and 3. Most logs are still suspended above the stream. We will continue to track the fate of LWD in this system and will develop equations that relate years to in-stream recruitment to the species, spanning length and mid-span diameter.

Devesh Bahuguna, MSc. Candidate and Dr. Stephen J. Mitchell, Associate Professor Department of Forest Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4, deveshbahuguna@gmail.com

MethodsWe measured all LWD that fully or partially crossed the stream channel for a distance of 150m within each of the Treatment Units in the Malcolm Knapp Research Forest Riparian Buffers Study. These included 10m buffer, 30m buffer, 50% thinned and unharvested Controls, each replicated 3 times. We divided the stream into reaches, and characterized channel and valley attributes. Each downed tree that was greater than 7.5cm in diameter at mid creek and that had fallen since the 1998 harvesting dates was tagged and measured. Each piece of LWD was classified by species, decay classes and end characteristics (e.g. uprooted, broken or intact). Diameter at breast height, diameter at mid creek, length of log, span length, height above creek were measured and recorded for each piece.

IntroductionLarge woody debris (LWD) is an important part of forest ecosystem. It helps to structure fish habitat (Bisson et al. 1987), shape channels (Swanson et al. 1976), trap sediments (Swanson and Lienkaemper 1978). In this project, we are focusing on post harvest windthrow as a pulse source of LWD inputs in the riparian buffer. For designing effective riparian prescriptions we need to estimate the probability of windthrow and its probable impacts. By sampling a range of riparian buffer conditions, we will be able to establish empirical relationships for predicting LWD recruitment as a function of log dimensions, species, spanning width, valley/gully and channel conditions. This knowledge will be useful to forest managers, habitat ecologists and terrain stability specialists who are designing prescriptions for riparian management areas.

Figure 1. Number of spanning logs by treatment, all species, all decay classes.

Preliminary ResultsWe found that the 30m buffer treatment has the maximum number of spanning logs followed by the Control, 10m buffer and 50% thinned treatments (Fig 1). Logs are in decay classes 1-3 (see classification of decay class in Table 1). Western hemlock is the dominant species, followed by western redcedar. Fig 2 shows the number of spanning logs by treatment and by species.

Fig 2. Number of spanning logs by treatment, by species. Cw stands for western redcedar, Hw for western hemlock, OC for other conifers like sitka spruce, douglas –fir, Dr for red alder, Ep for paper birch and OD for other deciduous like big leaf maple, bitter cherry.

Aerial View of Malcolm Knapp (study site) showing buffer strips

Tagged log across the creek

30m buffer10m buffer

Logs lying across the creek after windthrow event.

Fig 3. Average number of logs by species among decay classes 1-3. Cw stands for western redcedar, Hw for western hemlock, OC for other conifers like sitka spruce, douglas-fir, Dr for red alder, Ep for paper birch and OD for other deciduous like big leaf maple, bitter cherry.

Fig 4. Height of logs above bank-full height at mid creek vs log diameter for the major species.

-50

0

50

100

150

200

250

300

350

0 10 20 30 40 50

Mid creek Diameter(cm)

He

igh

t o

f lo

gs

ab

ov

e b

an

k-f

ull

cre

ek

(c

m) Cw Ep Hw

0

1

2

3

4

5

6

7

8

9

10

Cw Hw OC Ep Dr OD

decay class 1 decay class 2 decay class 3

0

5

10

15

20

25

30

35

10m buffer 30m buffer 50% thinned Control

Cw Hw OC Dr Ep OD

0

5

10

15

20

25

30

35

10m buffer 30m buffer 50% thinned Control

Treatment

No

. o

f lo

gs

> 7

.5 c

m d

ia.

at m

id c

reek

Table 1. Classification of decay class (Bartels et al. 1985).

Class 1 Class 2 Class 3 Class 4 Class 5

Bark Intact Intact Trace Absent Absent

Twigs Present Absent Absent Absent Absent

Texture Intact Intact to partly soft

Hard large pieces Small, soft blocky pieces

Soft and powdery

Sharp Round Round Round Round to oval Oval

Color of Wood Original color Original color Original to faded Light brown to reddish brown

Red brown to dark brown

Portion of tree on ground

Tree elevated on support points

Tree elevated on support points but sagging slightly

Tree sagging near ground

All of tree on ground

All of tree on ground

Invading roots None None In sapwood In heartwood In heartwood ReferencesBartels, R., J.D. Dell, R.L. Knight and G. Schaefer. 1985. Dead and down woody material. In: Management of wildlife and fish habitats in forests in western Oregon and Washington. E.R. Brown (ed.). USDA For. Serv. Pub. No. R6-FoWL-192. Portland, Oregon. 171-86.

Bisson, P.A., Bilby, R.E., Bryant, M.D., Dolloff, C.A., Grette, G.B., House, R.A., Murphy, M.L., Koski, K.V., and Sedell, J.R. 1987. Large woody debris in forested streams in the Pacific Northwest: past, present, and future. In Stream-side management: forestry and fishery interaction. Edited by E.O. Salo and T.W. Cundy. Institute of Forest Resources, University of Washington, Seattle. Contrib. No. 57. pp. 143-190.

Swanson, F.J., and Lienkaemper, G.W. 1978. Physical consequences of large organic debris in Pacific Northwest streams. USDA For. Serv. Gen. Tech. Rep. PNW-69.

Swanson, F.J., and Lienkaemper, G.W., and Sedell, J.R. 1976. History, physical effects, and management implications of large organic debris in western Oregon streams. USDA For. Serv. Gen. Tech. Rep. PNW-56.

A large proportion of conifer logs are in decay classes 1 and 2 whereas logs from deciduous trees are in decay classes 2 and 3, which can be seen in Fig 3. Interestingly, 10% of uprooted western redcedar are still alive and spanning the creek whereas all of the uprooted western hemlocks are dead. The majority of logs are still suspended over the creek with very few logs (from 10-19cm diameter at mid creek) in the creek. The latter are mostly western hemlock. Fig 4 shows the height above bank-full at mid creek vs log diameter.

ConclusionDeciduous species appear to decay faster than conifers. A large proportion of the LWD is from small trees 8-25 cm diameter at mid creek, but these logs are still suspended well above the creek 8 years after harvesting. It will be interesting to see how much time it takes a log to drop into the creek.

AcknowledgementsMalcolm Knapp Research Forest