A Visual Obstruction-based Technique for Photo Monitoring
of Willow Clumps
Chad S. Boyd and Tony J. Svejcar
Introduction
Willow and associated riparian
shrubs fill a variety of important
ecological roles in many riparian
ecosystems. With this importance
comes the management need to
document changes in willow abundance over time. This is particularly
true in areas where willows are
being restored or where herbivory
may negatively impact willow
resources. However, measuring
changes in abundance of woody
plants has proven difficult in field
application. One potential solution
to this monitoring challenge is the
use of ground-based photographs to
estimate willow abundance. Photo
monitoring can reduce observer
bias, provide a permanent record
of vegetation status, and samples
(photographs) can be reanalyzed at
a later date if new technologies become available. Our objective was
to evaluate the use of a visual obstruction/photo-based approach for
monitoring point-in-time abundance
of willow clumps and changes in
abundance associated with defoliation by herbivores. We focused on
young willows (<6.5 ft in height)
because this size class represents
a critical life stage in the development of willow clumps and can be
impacted easily by herbivory.
technique focused on the relationship between visual obstruction of
a photoboard and the abundance
of obstructing willows, in this
case Booth’s willow. Rather than
moving the photoboard to a willow clump, clumps—or portions
of clumps—were cut, transported
to the photoboard location, and
clamped in a holding device located
immediately in front of the photoboard (Fig. 1). The photoboard (59
by 79 inches) was constructed from
plywood painted fluorescent orange
to increase color variation between
the board and willow clumps when
determining visual obstruction.
Twenty-five willow clumps were
sampled in each year of the study.
Willow clumps were defoliated by
hand in four to seven increments
and harvested material was dried
and weighed. Defoliation removed
leaves plus the tips of new stem
growth. For simplicity, we refer
to this material as current annual
growth or CAG. Photographs were
taken before and after each removal
(Fig. 1); these defoliations were
meant to simulate the effects of herbivory. The visible photoboard area
was determined using color recognition software. We determined
visual obstruction by comparing the
photoboard area in each photograph
to that visible for the same board
without obstructing willow.
Results and Management
Implications
Results suggest that the technique presented here can be used
to predict both qualitative (i.e.,
increasing or decreasing amount)
and quantitative (i.e., amount of
increase or decrease) changes in
Experimental Protocol
The study site was located in
the Big Creek drainage in Grant
County, Oregon. Sampling took
place during peak biomass (August) of 2000 and 2001. The
Figure 1. These images show a willow clump during (left) and after defoliation
(right). Clumps were defoliated in four to seven increments, and photographs were
taken before and after each defoliation.
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willow abundance within the size
class of clumps used in our work.
The technique accurately predicted
the weight of total clump CAG
(Fig. 2a), and the strong relationship
between disappearance of CAG and
change in visual obstruction
(Fig. 2b) suggests this technique
may be useful for monitoring
changes in willow abundance
associated with herbivory.
For field monitoring purposes, we
constructed a portable lightweight
version of the photoboard on an
aluminum frame fitted with a board
of 2-mm-thick fluorescent orange
(59 by 79 inches) that was elevated
16 inches from the ground surface
(Fig. 3). Stabilizing legs allowed
the board to be freestanding. The
board was painted fluorescent orange, and the frame was constructed
in two pieces so that the more portable half frame could be used for
very small willow clumps. Visual
obstruction values for the half board
may be expressed as a percentage of
either the half or whole board. For
clumps wider than our photoboard,
it was not possible to estimate total
clump CAG. However, with accurate repeat board placement it was
possible to estimate changes in the
amount of willow CAG influencing
the photoboard over time. These
data can be used as both a quantitative (i.e., index to weight of total
clump CAG) and qualitative (i.e.,
is visual obstruction, and thus CAG
weight, increasing, decreasing, or
staying the same over time?) monitoring tool.
In summary, our technique
provides managers with a method
for reliably and accurately estimating willow abundance, changes in
abundance over time, and degree of
utilization by herbivores. However,
users should keep in mind that the
relationships we report may differ
by species of willow, leaf structure,
and the ratio of woody to leaf plant
material. For these reasons, we
recommend that predictive equations be developed on a site-specific
basis. Additional work is needed
to test the relationships described
herein with other willow species
and larger clump sizes.
Figure 2. The relationship between percent visual obstruction of a 59- by 79-inch
photoboard and (A) total weight (g) of current annual growth (leaves and tips of
current annual stem growth, CAG) for harvested willow clumps and (B) remaining
weight of CAG for harvested willow clumps following sequential defoliation in
four to seven increments.
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Figure 3. For field monitoring, we designed a lightweight photoboard constructed
of an aluminum frame with a painted plastic board. For smaller willow clumps,
the apparatus may be disassembled (left); the full board is used for larger clumps
(right).
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