Large woody debris inputs after harvesting in small stream buffers

1
Large woody debris inputs after harvesting in
small stream buffers at Malcolm Knapp Research
Forest
Devesh Bahuguna, MSc. Candidate and Dr. Stephen
J. Mitchell, Associate Professor Department of
Forest Sciences, University of British Columbia,
Vancouver, BC, V6T 1Z4, deveshbahuguna_at_gmail.com
Abstract A 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.
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.
10m buffer
30m buffer
Aerial View of Malcolm Knapp (study site) showing
buffer strips
Tagged log across the creek
Preliminary Results We 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.
Introduction Large 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.
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.
Table 1. Classification of decay class (Bartels
et al. 1985).
Fig 4. Height of logs above bank-full height at
mid creek vs log diameter for the major species.
Conclusion Deciduous 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.
References Bartels, 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.
Logs lying across the creek after windthrow event.
Methods We 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.
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.
Acknowledgements
Malcolm Knapp Research Forest