Where Did All 

the Deer Go?

Investigating the

Disappearance of

Habitat from

Historical Photos

in a GIS System

by Christopher Ross, PhD.

Physical Scientist/Reclamation

BLM, Nevada State Office 920

Background

Over the last 50 years the

Washoe-Lassen Interstate deer

herd population, which straddles

Longitude 120 degrees West

between western Nevada and eastern California, has dropped from

50,000 to less than 3,000 animals.

Loss of winter forage and thermal

cover has been suspected as a major

factor, but has not been accurately

tracked. Two long-lived members of

the Rose family, Curlleaf Mountain

Mahogany (Cercocarpus ledifolius

L.) - a small tree - and the shrub

bitterbrush (Purshia tridentata)

have historically provided the critical deer habitat, but fires and other

changes have greatly reduced their

populations. I have documented

this change and analyzed the causes

from aerial photos. While there is

much overlooked information

available in historical aerial photography, there are many obstacles to

quantitative analysis of the data.

Discussion

I assembled a series of aerial photos

from 1954 to 1996 from BLM and

other archives. These were scanned

into a Map Image Processing

System (MIPS) as raster images,

where they were orthocorrected and

georeferenced (placed in known

map coordinates). Orthocorrection

is necessary in steep, high-relief

terrain to warp or rubber-sheet the

images over the topography so that

accurate area measurements can be

made. These photos were then

placed in mosaic in MIPS, and

imported into ArcInfo and

ArcView as TIFF files, where the

polygons representing old growth

mountain mahogany were digitized

on the photos. In ArcView Spatial

Analyst, Digital Elevation Models

(DEM) of the terrain were clipped

to the study area to derive large

data sets which include the slope,

elevation, and aspect of each of the

42,160 thirty-meter pixels. The

digitized mountain mahogany coverage was then overlain on these

sets, resulting in five data sets for

1954, 1974, 1993 (two sets - one

of all trees young and old, and one

of only old trees) and 1996, each

with four variables (slope, elevation, aspect, and whether each pixel

had mountain mahogany or not).

Aspects (from 0 to 360 degrees)

were transformed into a continuous

variable with a range of 0 to 2,

with 2 being the optimal value for

a chosen aspect, to allow statistical

analysis by regression. Multiple logistic regression in the SAS system was

then used to examine relationships

between the geographic variables

and the presence of mahogany. A

50-year history of fire perimeters

was digitized from hand maps in

ARC/Info and added as a layer

between the various photo mosaics. 

Conclusion

The resulting GIS project shows

dramatically the decrease in the

mountain mahogany stands after

fires, and the resulting drop in deer

populations. The results of this

project also demonstrate that there

is little regeneration of the very

long-lived trees. It further shows that

they have been increasingly restricted

to higher elevations, gentler slopes,

and northeast to northwest aspects

(more common now than they were

50 years ago). Of special interest was

the result of plotting probability

curves (the probability of finding

mountain mahogany at a given

combination of aspect, slope, and

elevation) for each of the five years

of data sets. Data sets representing

years soon after fire (1974, 1993,

and 1996) had nearly identical

curves. Data sets not representing

the effects of recent fires, where

new trees had opportunity to

recruit, also had similar curves.

However, curves in non-fire years

were different from curves in fire

years. The similarity within these

two groups of curves suggests very

strongly that the trees regenerate in

the same areas after fires, and that

fires tend to destroy trees in similar

geographic locations over time.

However, the distribution of trees

killed by fires also shows the stochastic spread of fires. Field work

has established that there are very

few young and middle-aged trees

present now to sustain the

population - most of the surviving

specimens are hundreds to perhaps

over a thousand years old. Statistical

analyses show the potential effect on

mahogany survival of decreases or

increases in slope and elevation and

changes in aspect. 

While the project represents an

application of state-of-the-art computer technology and statistical

analysis to data collected decades

before such technologies existed,

they also provide much direction

for habitat rehabilitation techniques. Areas most suitable for

replanting and reseeding have been

identified and are being treated

RESOURCE NOTES

NO. 37 DATE 08/10/00

Range

Wildlife Biology
S l o p e 6 0 - E l eva t i o n 7 0 0 0 , va r y t ra n s fo r m e d a s p e c t

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

probability

0.1 0.25 0.5 0.75 1 1.25 1.5 1.75 2

aspect

1954

1974

1993 (all)

1993 (old)

1997

S l o p e 6 0 - E l eva t i o n 8 0 0 0 , va r y t ra n s fo r m e d a s p e c t

1

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

0

probability

0.1 0.25 0.5 0.75 1 1.25 1.5 1.75 2

aspect

1954

1974

1993 (all)

1993 (old)

1997

with aerial seeding and seedling

transplants. Those existing habitats

most vulnerable to future fire are

also being identified, and may be

given priority for suppression or

protected with vegetative strips

where topography permits.

Contact

Christopher Ross, PhD.

Physical Scientist - Reclamation

Bureau of Land Management 

Nevada State Office 920

P.O. Box 12000

Reno, Nevada 89520-0006

phone (775) 861-6571

fax (775) 861-6711

email clross@nv.blm.gov

Range

Resource Notes are intended to be early

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Bureau of Land Management personnel and some of their customers.

Information in this Resource Note is based on the opinion and

experience of the author and has not been peer-reviewed. Conclusions

and opinions expressed herein do not

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Figure 1. Two sets of probability curves.