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|Large woody debris (LWD) recruitment plays
an integral role in the production and maintenance of riparian
and aquatic habitat (Beechie and Sibley 1997; Bisson et al.
1987; Bryant 1983; Harmon et al. 1988; Swanson et al. 1977;
Swanson and Lienkaemper 1978; Triska and Cromack 1979). Recruitment
processes can be classified in one of two categories: biological
(e.g. natural mortality, insect/disease - induced mortality)
and physical (e.g. windthrow, streambank failure) (Keller and
Swanson, 1979). Although recruitment events are stochastic in
time and space, the potential of a given riparian forest to
recruit LWD can be measured at some snapshot in time. Thus,
the impact of management changes on LWD recruitment potential
can be modeled to support the development of better forest management
Riparian vegetation is the primary source of large woody debris
inputs into adjacent streams. Recruitment of large woody debris
is a binomial event, there are only two possible outcomes: success
and failure. When a biological or physical agent causes a tree to
fall, either it will hit the stream (success) or it will not (failure).
||Even the most conducive set of biological,
physical, spatial, and temporal characteristics will recruit
from only a subset of the total forest inventory. The probability
of recruitment success is a function of a tree's height and
distance from the stream (Robison and Beschta 1990). The probability
space for a tree falling is a disk centered on the tree with
radius equal to the tree's height.
Figure 1 illustrates how only those trees whose height is
greater than the distance from the stream (such as tree N
in the figure) will have a positive probability of recruitment
success, which is the proportion of the total probability
space that overlaps the stream. All other trees (such as tree
M) will have zero probability of success. The set of all trees
with positive recruitment probabilities (i.e. the set of all
trees similar to N) is the set of LWD "candidates."
If the riparian forest inventory information is spatially explicit
then the set of candidates and their associated probabilities of
success can be measured directly. However, by making an assumption
regarding the distribution of tree heights across the forest, the
set of candidates and their probabilities can be deduced even if
the inventory data is not spatially explicit.
By measuring the recruitment capacity of riparian forests under
various conditions, the effects of (active or passive) management
can be analyzed and compared. By employing growth models, temporal
comparisons can be made, and the enduring effects of different management
scenarios can be analyzed.
||Let the inventory visualized in Figure
2 represent an unmanaged scenario, the chart below illustrates
the marginal and cumulative recruitment by distance class along
750 feet of an adjacent stream. The chart illustrates that more
than 90% of all recruitment potential is achieved within the
first 100 feet from the stream, and there is no effectiveness
beyond 140 feet.
We can compare this to the inventory visualized and charted in
Figure 3, representing an alternative management scenario. While
both marginal and cumulative recruitments are lower across the forest
under this scenario, the effective width of the buffer is largely
unchanged at 130 feet. Most importantly, 90% of the effectiveness
is still achieved in the first 100 feet.
||If minimum recruitment targets are set, then managers
can create and analyze a matrix of silvicultural strategies
that provide required levels of recruitment and achieve other
management objectives, such as shade production. If analyses
reveal that active management does not change the effective
width of a buffer, then measures can be taken to recoup the
loss in cumulative potential from a passive management scenarios.
For stands that are significantly overstocked, active management
may indeed increase the recruitment capacity of the stand over
time. The silvicultural implication of managing for maximum
recruitment potential is creating conditions with the maximum
number of tall trees, since tall trees increase the effective
width of a buffer.
There are systematic temporal and physical influences that affect
recruitment potential. Research indicates that recruitment potential
varies between stand development stages (Triska and Cromack 1979;
Spies et al. 1988; Van Sickle and Gregory 1990). Physical characteristics
such as species composition, soil composition, soil stability, valley
form, aspect, and management history also affect recruitment potential
(Bisson et al. 1987).
Recruitment potential does not address the issue of recruitment
effectiveness. More pieces may not be as desirable as larger pieces.
If fewer but larger pieces are more effective, then this creates
even more flexibility in creating silvicultural pathways that produce
and maintain aquatic habitat.
Beechie, T.J., and Sibley, T.H. 1997. Relationships between channel
characteristics, woody debris, and fish habitat in northwestern
Washington streams. Transactions of the American fisheries society.
Bisson, P.A. and 8 others. 1987. Large woody debris in forested
streams in the Pacific Northwest: past present, and future. In:
Salo, E.O, and Cundy, T.W. (eds.). Streamside management: forestry
and fisheries interactions. University of Washington, Institute
of Forest Resources, Contribution No. 57. Seattle, Washington.
Bryant, M.D. 1983. The role and management of woody debris in west
coast salmonid nursery streams. North American Journal of fisheries
Harmon, M.E. and 12 others (1988). Ecology of coarse woody debris
in temperature ecosystems. Advances in ecological research. 13:133-276.
Keller, E.A and Swanson, F.J. 1979. Effects of large organic material
on channel form and fluvial processes. Earth Surface Processes.
Robison, G.E., and Beschta, R.L. 1990. Identifying trees in riparian
areas that can provide coarse woody debris to streams. Forest Science.
Spies, T.A., Franklin, J.F., and Thomas, T.B. 1988. Coarse woody
debris in Douglas-fir forests of western Oregon and Washington.
Swanson, F.J., Lienkaemper, W.L., and Sedell, J.R. 1976. History,
physical effects, and management implications of large organic debris
in western Oregon streams. General Techical Report PNW-56. USDA
Forest Service, Pacific Northwest forest and range experiment station,
Swanson, F.J., and Lienkaemper, G.W. 1978. Physical consequences
of large organic debris in pacific northwest streams. General Technical
Report PNW-69. USDA Forest Service, Pacific Northwest forest and
range experiment station, Portland, Oregon.
Triska, F.J., and Cromack, K.J. 1979. The role of wood debris in
forests and streams. In: Forests: Foresh perspective from ecosystem
analysis. Proceedings of the 40th annual biology colloquiam, Oregon
State University Press, Corvallis, Oregon.
Van Sickle, J., and Gregory, S.V. 1990. Modeling inputs of large
woody debris to streams from falling trees. Canadian Journal of
Forest Research. 20:1593-1601.