The ForestWerks™
Model:
A watershed processes and forest management simulation
Outline
Frank
Heinzelmann, Ph.D.
www.ForestWerks.ca
The objectives
of this research are to identify potential long-term trade-offs of forest
management options on aquatic habitat of small and intermediate streams
in coastal British Columbia, Canada. Next to many biotic processes,
the production of salmonids and other aquatic organisms is dependent
on the structural complexity of aquatic habitat within stream systems.
The approximation of timber harvest-induced changes in pattern of runoff
generation, coarse sediment input, and large wood recruitment, and their
likely impact on the channel network and hence aquatic habitat, is essential
for evaluating relative impacts of forest management options.
As no
field experimentation on a large scale is possible, I have built a spatially-explicit,
pixel-based, and stochastic management model using Visual Basic and
Access. I use a 3,200 ha catchment in the Tsitika valley on northeastern
Vancouver Island as a management platform. The spatial resolution is
1 ha per pixel. I obtained process information and data from the literature.
Semi-hypothetical landscapes are generated by running the model (without
harvesting) for, say, 200 years. Forest management activities are then
simulated for these landscapes. The rule-based model represents the
major hydrology, coarse sedimentology, forest stand dynamics, and large
wood recruitment processes. It approximates the cumulative, downstream-propagating
impacts on the stream system and aquatic habitat. I evaluate the effects
of management actions in a qualitative way. The focus is on trends in
frequency distributions of processes and in relative answers, rather
than on accurate and absolute predictions. The model is based on the
assumption that major precipitation events and the ensuing storm runoff
are 'driving' the system. These major events initiate debris slides,
mobilize sediments, entrain large wood in stream channels, and result
in changes in stream habitat characteristics.
I want
to gain an understanding of the implications of road network configuration,
and areal extent and disposition of cutblocks for cumulative impacts
on the stream system, and hence aquatic habitat. I contrast the following
management options: 1) rate of harvest (no logging versus logging at
a long-term yield rate); 2) alternative spatial harvest patterns (small
versus large cutblock sizes arranged in dispersed and concentrated patterns);
3) alternative temporal harvest patterns (continuous versus intermittent
harvest); and 4) road density of alternative harvesting systems (grapple
versus skyline yarding).
50 years
of daily temperature and precipitation data from the closest weather
station are input to a Thornthwaite water balance to track soil moisture
over time based on precipitation, evapotranspiration and soil data.
Using the meteorological data, the effective precipitation is calculated
based on antecedent soil moisture and infiltrability. For threshold
precipitation events, the model generates design storms from the historical
data and calculates daily runoff and peakflows in stream channels using
a modified Clark unit hydrograph where antecedent precipitation is accounted
for. A high and a low density road network for the short- and long-yarding
systems, respectively, were designed in conjunction with large (~ 80
ha) and small (~ 25 ha) cutblocks. An extension of the channel network
by forest roads can be approximated by a change in the time of concentration
of the unit hydrograph. After each critical discharge event, the model
calculates erosional events on hillslopes and in stream channels. Probabilities
for debris slide initiation are based on hydraulic forcing due to major
precipitation events, time since last disturbance, land conditions,
hillslope gradient, and road construction (if a road is present). The
deposition of a debris slide is deduced from its relative LWD content
and the hillslope gradient or stream power. Forest stand dynamics is
simulated based on nine dominant stand types and alternative successional
trajectories. Stand attribute data like height, density, stem diameter
and volume are based on the stand level growth and yield model TIPSY.
Forest stand dynamics and forest management (harvesting and road construction)
are simulated yearly. Given the downslope path of a debris slide, the
quantity and quality of large wood reaching streams is tracked. The
large wood input into channels from riparian stands is approximated
by an average annual rate. The formation of log jams is approximated
based on the relative large wood load in the channel. Log jam dynamics
with regard to size, height and dominant tree species are tracked and
sediment storage capacity is computed. Channel bedload movement is initiated
based on stream power. Bedload transport is computed by the Bagnold
equation. Using the B.C. Channel Assessment Procedure classification
of stream channel morphologies, the channel type and state is calculated
based on hydraulic equations for bankfull width and depth. The state
of stream channels can change from 'highly aggraded' via 'stable' to
'highly degraded' based on sediment and large wood input, and flow.
Finally, the channel network accessible to coho salmon (Oncorhynchus
kisutch) is rated for its spawning and incubation, summer rearing
and over-wintering habitat capability based on fish expert ratings of
stream channel morphologies.
Results
of sensitivity analysis for key parameters and Monte Carlo runs for
management scenarios are presented in my doctoral dissertation. In the
future, this modelling approach may also be applied to other fish species,
and other aquatic and riparian-dependent wildlife species.
forest planning, simulation, decision
support tool, habitat supply model, land use practice, harvest scheduling,
riparian buffer, aquatic resources, fish habitat, channel morphology,
stream morphology, coho salmon, hydrology, debris flow, large woody
debris, LWD, large wood dynamics, ForestWerks Model, ForestWerksModel,
Forest Werks Model, ForestWorks Model, ForestWorksModel, Forest Works
Model, fish-forestry interactions, forestry-fish interactions, fish
forestry interactions, forestry fish interactions