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Seeing The Forest Through The Trees
Sampling secondary structure in pine stands with novel
aerial photography
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Beetle-attacked lodgepole pine stands in the B.C. Interior are being
harvested in order to try to utilize the wood before it loses its economic
value. These forests will be managed for rapid reforestation. However, the
epidemic is so extensive that thousands of hectares of attacked pine stands will
remain on the landscape. What will these forests look like in the future?
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How a beetle-attacked pine stand might change over time |
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The way that these stands develop over time is the result of the combined effect
of deterioration of dead trees and the growth of new trees. Deterioration rates
depend on factors such as snow loading, and the chance occurrences of windstorms
and fire. Growth depends on what’s there already and the locations of seed
sources. Many pine stands are even-aged because they regenerated after a
stand-destroying fire. However, other stands have more complex histories which
have resulted in gaps, uneven tree ages, and a mix of tree species. Also, this beetle epidemic has not killed every pine tree. Mortality has been very
high in many stands but other pine stands will have varying numbers and sizes of
conifers that survive this epidemic. |
| When the overstory dies, the physical environment on
the forest floor changes, thereby affecting the growth of understory
trees. Changes include more water and light being available. Light
levels in deteriorating and growing pine stands are being documented as
part of this project. Effects of canopy mortality
on snow accumulation and ablation |
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Conifers that survive the beetle epidemic will tend to be the smaller pine trees
and the non-pine species. However, some mature lodgepole pine will also survive.
These are collectively termed “secondary structure”, (Coates et al., 2006). As
those authors noted, the beetle epidemic has increased the value of secondary
structure in pine stands because when the pine overstory dies, any remaining
live trees will benefit wildlife habitat, accelerate hydrologic recovery, and
could provide mid-term timber supply. Unfortunately, there is a dearth of
inventory information on understory conifers, partly because understory
trees are almost impossible to see on regular aerial photographs. Forest
inventory information that cannot be mapped on standard aerial photos or
satellite imagery is often obtained by sampling. The most accurate sampling
method is ground-based surveys but this is slow and expensive. This project
is investigating a new sampling method that would operate at a scale between
ground-based and standard remote methods. |
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Vertical aerial photos taken when snow covers the forest floor and when the
sky is overcast provide a unique view of forest structure. This combination of
lighting conditions provides a diffusely backlit silhouette of all vegetation
and woody debris that is not covered by snow. Aerial photographs of the same
research plots taken under different snow and lighting conditions show the
clarity with which this method can reveal both overstory and understory trees.
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Complete snow cover
and clear sky. |
Complete snow cover
and overcast sky. |
Partial snow cover
and overcast sky. |
Here are aerial photos of a 50 by 50 metre plot in a 100% pine pine stand.
According to the provincial inventory, this stand is 135 years old, 22 metres
tall, and has 650 stems/ha (B.C. Vegetation Resource Inventory). These images
show that lighting has a radical effect on the visibility and appearance of
trees. Under clear skies, large scale aerial photos of trees are dominated by
patterns of light and dark due to uneven illumination (first photo). This is not
a problem if one is interested mainly in the canopy-forming trees. Indeed,
traditional aerial photography normally requires cloud-free conditions due to
flight altitudes. When the sky is overcast, the distraction of shadows is
removed but these photos reveal another large difference. The appearance of the
trees is very different depending on whether they are seen as silhouettes
against a white background (second image), or illuminated from the front against
a dark background (third image). On the image taken when there was complete snow
cover and overcast skies, understory spruce trees are clearly visible under the
pine overstory. This is enhanced by the fact that understory spruce tend to have
crowns that are much larger in diameter and denser than those of pine trees –
especially dead pine trees, and by the fact that pine stands in the Interior
tend to have relatively low crown closures.
The effects of different lighting conditions on the ability to detect
different scene features are well-known in the fields of microscopy and machine
vision. In contrast, unusual scene illumination has not been well-utilized in
photographic remote sensing of forests because the lighting cannot be
controlled. Aerial photography under these conditions must be done
opportunistically.
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Aerial photos taken in late winter and early spring of 2006 for
another project (
Effects of
canopy mortality on snow accumulation and ablation ) provided the
initial sample of images. In March and April 2007, new aerial photos
were acquired with funding from the Ministry of Environment’s Ecological
Restoration Program. In early 2008, VRI Interpreters were contracted to
create stem maps and measure crown diameters of live understory conifers
on a subset of these photos in order to test the usability of the
imagery, the new interpretation process, and two stem mapping platforms
(Ozi Explorer and Google Earth).
| Example of georeferenced
aerial photo and stem map of live understory conifers.
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| 2007 aerial
photo overlaid in Google Earth |
One-quarter
hectare plot superimposed on aerial photo. |
Stem map of live
understory based on photo interpretation. |
This Google Earth image overlay can be downloaded here
695 full res.kmz |
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Project participants include the University of Victoria, West
Fraser Mills Ltd., Williams Lake Woodlands, and B.C. Timber
Sales, Williams Lake.
In 2007, new research plots were selected where secondary
structure will be sampled by two different methods for
comparison. Aerial photos will be acquired using the new method
and will be used to map and measure secondary structure as in
the above example. Secondary structure will be mapped and
measured in the same plots in the field and the two maps and
sets of measurements will be compared.
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| This
MoFR website has links to documents pertaining to stewardship in the
context of the Mountain Pine Beetle epidemic, including guidance from
the Chief Forester and information on secondary structure. |
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| Coates, K. David, Craig DeLong, Philip J. Burton, and Donald L.
Sachs. 2006. Abundance of Secondary Structure in Lodgepole Pine Stands
Affected by the Mountain Pine Beetle. Report prepared for the Chief
Forester of British Columbia. Available at
MoFR website |
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Contact:
Pat Teti
Research Hydrologist
Southern Interior Region
200 – 640 Borland Street
Williams Lake, B.C., V2G 4T1
Phone: 250 398-4752, eMail:
pat.teti@gov.bc.ca
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