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Forest Practices > Forest Health > Forest Health Information Management
Forest Health Information Management
How to Estimate Green:Red
July 2001
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Problem:
- Green:Red are commonly reported to
express the rate of increase of beetle populations based on
information gathered from ground surveys conducted for other
operational purposes
- This statistic is vital to estimate the
size of the beetle population
- The numbers vary considerably within and
between districts and regions.
- Very large numbers (20 - 100 greens per
red tree) are reported which indicate that the ratio was not
accurately estimating the average growth rate but was influenced by an
outside source of beetles. (see Maximum
Biological expansion rate discussion)
- A standardized method is required to
provide more meaningful estimates
Proposed Methodology:
OPTION 1 - Unbiased Sampling (preferred)
- Stratify the district's susceptible
types by Susceptibility Class (high, moderate, low)
- Within each strata, randomly select
starting points within 10 - 15 stands and use a 50 m x 10 m strip survey to
tally the number of red and green attacked trees along the
line.
- Calculate an estimate of the number of
trees/ha (red/ha and green/ha).
- Average the red/ha and green/ha for the
entire district and then use the average ratio
(green/ha) / (red/ha) =
green/red = district green:red
OPTION 2 - Use of Existing Ground Data (operationally
feasible for 2001/01)
-
Alternative method:
Using operational beetle probe data, select only survey data from small
(<50 red tree) spot infestations not adjacent to large infestations
(i.e., 10 ha red polygons). Delete sites that have unusually high
(>5:1 green:red) or sites that have only green attacks (see Maximum
Green:Red discussion). Randomly select 20 infestation sites
from this short-list and insure that they were collected across the
district. Sum the total number of green attacks
detected and divide by the sum of the total number of red attacks.
These spot infestations are usually on the leading edge and are targeted for treatment by fall and
burn or small scale salvage.
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Last updated on October 19, 2005
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