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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.  


Back to the Suppression BMU data requirements


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Last updated on October 19, 2005
The contact for this web page is: tim.ebata@gov.bc.ca

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