Estimating Snag Densities Using Aerial Survey Data
Keith Sprengel1, Julie Johnson2, Bruce Hostetler3 Kim Mellen4, Beth Willhite5
USDA Forest Service
Introduction
Forest land managers interested in maintaining healthy ecosystems must
evaluate effects of existing or proposed management activities on organisms
that use snags, down wood and other wood decay elements. Recent
development of the “DecAID Advisor” has provided an important tool for
helping estimate dead wood requirements in analysis areas. DecAID is an
internet-based synthesis of published scientific literature, research data,
wildlife databases, forest inventory databases and expert judgment and
experience. It addresses current vegetative conditions (unharvested and
managed); provides relevant summaries of snags and down wood; and
presents information on wildlife use of snags and down wood data. It also
provides information on insects and pathogens and their role in creating and
retaining dead wood. Information in DecAID should be applied at minimum
scales of 5th field watersheds.
Pacific Northwest Region
Snag density (#/acre); snags > 25 cm dbh
as a percent of Forest cover
60.0%
Upper Sycan River
Watershed (USRW)
showing aerial survey
reported mortality recorded
in the > 25 cm dbh class
between 1995 and 2004 for
the PPDF wildlife habitat
type.
Aerial Survey Data 1995-2004
Unharvested Inventory
Plots
50.0%
40.0%
30.0%
USRW
20.0%
>0 <4 tpa
10.0%
>4 <8 tpa
>8 <12 tpa
>16
>12<16
>8<12
>4<8
>0<4
0.0%
0
One important aspect of dead wood analyses is ascertaining the current
status of snags and down wood within and in the vicinity of a project area.
This project examines how aerial survey data can aid managers in
developing a picture of the ‘current situation’ for the incidence, distribution
and condition of snags (which are potential down wood) across a landscape.
>12 <16 tpa
>16 tpa
Distribution of the unharvested area of the PPDF_S Vegetation Condition
among snag density classes (#/acre) for snags > 25 cm dbh, based on 302
unharvested inventory plots compared to Aerial Insect and Disease Survey
data converted to snags > 25 cm dbh using conversion factors generated
from inventory plot data.
Photo by Alan Dyck
Upper Sycan River
Watershed (USRW)
showing aerial survey
reported mortality
recorded in the > 25 cm
dbh class between 1995
and 2004 for the
EMC_ECB wildlife habitat
type.
Snag Density (#/acre); > 25 cm dbh
as a percent of Forest Cover
60.0%
Aerial Survey Data 1995-2004
Unharvested Inventory
Plots
50.0%
40.0%
30.0%
>6 <12 tpa
10.0%
>12 <18 tpa
>30
>24<30
>18<24
>12<18
>6<12
>0<6
Methods
Key Findings
•Aerial survey data consistently provided a conservative estimate of snag densities in the study watershed in both the PPDF and EMC_ECB wildlife
habitat types for all of the examined diameter classifications.
•Mean snag density predicted from analysis of 10 years of aerial survey data = 13.34 trees per acre (TPA). Inventory plot data based conversion factor
applied to aerial survey data for summarizing snags > 25 cm dbh = 5.17 TPA. Snag density estimate derived from field study for snags > 25 cm dbh =
16.32 TPA.
•Inventory plot generated conversion factors underestimated the percent of snags in all diameter classes and for all species measured, providing
conservative estimates of snag densities in diameter classes of interest (> 25 cm dbh and >50 cm dbh).
•Analysis of aerial survey data provides a cost effective method for estimating snag densities and distributions at 5th field watershed scales.
•Results of aerial survey data analysis may eliminate the need to conduct expensive ground surveys in some strata and/or for some size classes of
snags.
Upper Sycan River
Watershed
>24 <30 tpa
Distribution of the unharvested area of the EMC_ECB_S Vegetation
Condition among snag density classes (#/acre) for snags > 25 cm dbh,
based on 754 unharvested inventory plots compared to Aerial Insect and
Disease Survey data converted to snags > 25 cm dbh using conversion
factors generated from inventory plot data.
•The study area, located in south-central Oregon, consists of forested federal lands within the Upper Sycan River Watershed. Two wildlife habitat
types (WHT) were sampled, the Ponderosa Pine/Douglas-fir (PPDF) and Eastside Mixed Conifer/Eastside Cascades and Blue Mountains (EMC_ECB).
In order to use aerial survey data to estimate snag densities, we developed aerial survey code conversion factors based on inventory plot (CVS, FIA
and BLM) summaries of snag data from each WHT. Ten years of cumulative mortality data, as recorded by aerial detection surveys from 1995 through
2004 (see http://www.fs.fed.us/r6/nr/fid/data.shtml for more information on Pacific Northwest Region Aerial Surveys) were then compared to ground
data collected from stratified systematic random samples within two wildlife habitat types.
•Areas which were harvested or burned by wildfire from 1995 through 2004 were excluded from the study. Only cover types classified as either
Evergreen or Mixed Forest as defined in the National Land Cover data (NLCD) developed by USGS were used to represent “forested” conditions for
inclusion in the study.
•As a result of a pilot survey, the study area was stratified into three strata: EMC_ECB (small/medium trees); PPDF Open (large trees); and PPDF
Closed (small/medium trees). WHT strata were used as defined by the Northwest Habitat Institute and Northwest Power and Conservation Council,
2000. Open/Closed strata were developed using Landsat7 imagery.
•Using the ArcView Sample Point Generator (SPGen), developed by Vern Thomas and Marla Downing (USDA Forest Service, Forest Health
Technology Enterprise Team) and Robin Reich (Colorado State University), a grid of 200 X 200 m cell polygons with centered points was placed over
the entire study area and 5 cells wre randomly selected from each stratum. At each selected point, azimuths were randomly selected using the random
number function (RAND) in Excel. Starting at each sample point, four 50 m samples were taken along the continuous 200 m transect. Snags within 20
meters either side of the transect center line were measured for perpendicular distance to the line, species, diameter, height and structural decay class.
Measured snags had to be > 1.8m tall and: > 20.3 cm dbh (ponderosa pine or white fir); or > 15.2 cm dbh (lodgepole pine) or > 7.6 cm dbh (whitebark
pine). Diameter breakpoints were established relative to insect biology and what sizes of trees an aerial observer could be expected to see on an
overview survey relative to various wildlife habitat types (Sprengel, Hostetler and Bridgwater, pers. com.).
•Pilot survey data were analyzed using protocols developed by Bate et. al. (1999) to determine numbers of samples and optimal plot sizes needed for
each strata to provide a stratified mean density estimate with a 90% confidence interval for qualifying snags in the study area.
PPDF wildlife
habitat area
>18 24 tpa
>30 tpa
DBH
Range
cm
Habitat
Type
1
EMC_ECB wildlife
habitat area
>0 <6 tpa
0
Aerial view of ponderosa and lodgepole pine mortality attributed to mountain pine beetle.
Random sample
points
USRW
20.0%
0.0%
Upper Sycan River
Watershed Study area
showing wildlife habitat areas
and sample points.
Total
Plots w/
snags
Sum of
Wtd.
Plots w/
snags
Sum of
Wtd.
snags/
acre
Ave.
snags/
acre
1.
Set-up rules for each
insect species host &
habitat type
combination that is
recorded during aerial
surveys
2.
Using these rules we
looked at the proportion
of the inventory snag
data by wildlife habitat
type that could have
been recorded by aerial
surveys
Percent
by size
class
Ponderosa pine
EMC_ECB
1080.5
4.3
20-25
>20
950
219.1
0.9
20.3
25-50
251.8
573.9
2.3
53.1
287.5
1.1
26.6
>50
Lodgepole pine
EMC_ECB
964.2
12.0
15-25
649.4
8.1
67.4
25-50
>15
311
80.1
306.4
3.8
31.8
8.4
0.1
0.9
>50
Inventory plot data summary example resulting in the derivation of the aerial survey
conversion factors. Example note: actual percent lodgepole > 25 cm in the
EMC_ECB habitat type was 64.5% whereas only 34.7% of the aerial survey data was
converted to represent lodgepole greater than 25 cm.
Aerial Survey Recorded mortality
Fir Engraver
Mountain pine beetle in whitebark
Mountain pine beetle in lodgepole
Mountain pine beetle in ponderosa
W estern pine beetle in large diameter ponderosa
W estern pine beetle in pole-sized ponderosa
Conversion factors developed
from inventory plot snag data
(CVS, FIA and BLM) and expert
knowledge of insect/host biology.
Conversion factors were
summarized by wildlife habitat
types. Diameter and trees per
acre breakpoints identified as
significant to wildlife in DecAID.
3.
These proportions were
applied to the aerial
survey data to estimate
the number of trees in
each size class
4.
This information was
compared to the data
gathered in the field
study.
EMC_ECB
PPDF
DBH Breakpoints (cm) DBH Breakpoints (cm)
>25
>25<50 >50
>25 >25<50 >50
TPA breakpoint code
TPA breakpoint code
3
1
1
2
1
1
ASD Conversion factor ASD Conversion factor
0.716
0.551 0.165 0.687 0.505 0.182
0.372
0.34 0.032 0.372
0.34 0.032
0.327
0.318 0.009 0.402 0.392
0.01
0.797
0.531 0.266 0.711 0.543 0.168
1.000
0.000 1.000 1.000 0.000 1.000
0.797
0.531 0.266 0.711 0.543 0.168
Xst (acres)=
Varst=
16.321
2.125
Se =
1.458
Bound=
2.435
>2<6
>4<8
>6<12
>6<10
>8<12 >12<18
>10<14
>12<16 >18<24
>14<18
>18
>16 >24<30
>30
ksprengel@fs.fed.us phone 503-668-1476 Aerial Survey Program Coordinator; 2jjohnson@fs.fed.us phone 503-808-2998 GIS Analyst; 3bhostetler@fs.fed.us phone 503-6681475 Entomologist; 4kmellen@fs.fed.us phone 503-808-2677 Wildlife Ecologist; 5bwillhite@fs.fed.us phone 503-668-1477
Entomologist
Acknowledgements
Funding for this project was provided by Forest Health Monitoring Evaluation Monitoring
Program and Forest Health Protection, Pacific Northwest Region. Excellent field work
was provided by Paul Handy, Jennifer Deshong, and Ben Smith. Special thanks go to
Sue Puddy, Rich Stubbs and Mike McNeil of the Fremont National Forest for rounding up
spatial data on harvest and fire histories. Graphic design by Shelly York, Visual
Information Specialist, Mt. Hood National Forest.
References Cited
Bate, Lisa J.; Garton, Edward O.; Wisdom, Michael J. 1999. Estimating Snag and
Large Tree Densities and Distributions on a Landscape for Wildlife Management. Gen.
Tech. Rep. PNW-GTR-425. Portland, OR: U.S. Department of Agriculture, Forest
Service, Pacific Northwest Research Station. 76 p.
http://www.fs.fed.us/pnw/pubs/gtr_425.pdf
Northwest Habitat Institute and Northwest Power and Conservation Council, 2000.
http://www.nwhi.org/index/gisdata.
Upper Limit= 18.755
Lower Limit= 13.886
TPA breakpoint Codes
1
2
3
>0<2
>0<4
>0<6
Future Research and Applications
•Assess the impact of bark beetle caused mortality on standing dead and down fuels.
•Assess the ability of aerial surveys to consistently make conservative estimations of
snag and large tree densities and distributions at watershed scales in other wildlife
habitat types, and to estimate the magnitude of the under estimation.
•Investigate possible correlations between fuel loading and cumulative mortality as
reported by aerial surveys.
•Evaluate efficacy of plot (FIA/CVS/BLM) derived conversion factors for aerial survey
relative to wildlife habitat types and significant trees per acre and diameter breakpoints
identified in the Decayed Wood Advisor (DecAID).
•Develop interactive aerial survey-derived cumulative mortality database for integration
with DecAID.
Level of Precision
(%)=
15
Snag density estimates for non-aspen
snags > 25 cm dbh and > 1.8 m tall:
Results from data collected from
stratified systematic random sample
transects.
United States Geological Survey National Land Cover data
http://landcover.usgs.gov/prodescription.asp.
Information on the Decayed Wood Advisor (DecAID) can be found at:
http://wwwnotes.fs.fed.us:81/pnw/DecAID/DecAID.nsf
ArcView Sample Point Generator
http://permanent.access.gpo.gov/websites/fsfedus/www.fs.fed.us/
foresthealth/technology/spatialstatistics/sample.php.htm