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Growing Season Index (GSI)

ARCHIVED

This committee has been archived July 1, 2026.  Please reach out to your NWCG Coordinator if any content needs to be moved to the new committee structure.


 

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The Growing Season Index (GSI) In NFDRS 2016, controls live fuel moistures, replacing the 1000-hour and X1000 controls in the 1978/88 versions. The moisture bounds  (30-250 for herbaceous and 50-200 for woody shrubs) and behavior of annuals versus perennials are maintained.

 The Growing Season Index is a simple metric of plant physiological limits to photosynthesis. It is highly correlated to the seasonal changes in both the amount and activity of plant canopies. It predicts the green-up and senescence of live fuels and the influence of water stress events on vegetation. Increasing values of GSI indicate periods of improving conditions for live fuels and decreasing values indicate periods of detrimental weather conditions. GSI is calculated as a function of the three indicators of important weather factors that regulate plant functions. These indicators are combined into a single indicator that integrates the limiting effects of temperature, water and light deficiencies. The importance of each of the three indicators is explained below and a summary of the Growing Season is given at the conclusion.

Minimum Temperature

Many of the biochemical processes of plants are sensitive to low temperatures. Although ambient air temperatures certainly influence growth, constraints on phenology appear to be more closely related to restrictions on water uptake by roots when soil temperatures are sub optimal and many field studies show variable ecosystem responses over a range of minimum temperatures.

Vapor Pressure Deficit (VPD)

Water stress causes partial to complete stomatal closure, reduces leaf development rate, induces the shedding of leaves, and slows or halts cell division. Although models are available to calculate a soil water balance, they require knowledge of rooting depth, soil texture, latent heat losses, and precipitation. As a surrogate, we selected an index of the evaporative demand, the vapor pressure deficit (VPD) of the atmosphere.

Photoperiod or Daylength

Photoperiod provides a plant with a reliable annual climatic cue because it does not vary from year to year at a given location. We assume that photoperiod provides the outer envelope within which other climatic controls may dictate foliar development. Studies have shown that photoperiod is important to both leaf flush and leaf senescence throughout the world.

Upper and lower limits of the indicator functions used to calculate the Growing Season

Input VariableUnconstrained (1)Completely Limiting (0)
Minimum Temperature5ºC /41º F-2ºC /28º F
Vapor Pressure Deficit (Pascals)900 Pascals4100 Pascals
Photoperiod (Daylength)11 hours10 hours

Learn more about the Growing Season Index (GSI)

 

NWCG Latest Announcements

2025 Paul Gleason Award Winners Announced

Date:  August 13, 2026
Questions? 
Please contact: Leadership Development Subcommittee

The NWCG Qualifications Management Committee (QMC) and Leadership Development Subcommittee (LDSC) are proud to announce the recipients of the 2025 Paul Gleason "Lead By Example" awards. Award categories include Initiative and Innovation, Mentoring and Teamwork, and Motivation and Vision.

Congratulations to the awardees:

  • Hunter Bell, U.S. Forest Service — Initiative and Innovation
  • Shannon Deane, U.S. Wildland Fire Service— Initiative and Innovation
  • Chris Delaney, U.S. Wildland Fire Service — Motivation and Vision
  • Sean Peterson, U.S. Forest Service — Mentoring and Teamwork

References:

Paul Gleason Lead By Example Award

Wildland Fire Leadership Development Program (WFLDP)

Leadership Development Subcommittee

LMC Equipment Bulletin 26-001: Aquatic Invasive Species – Seasonal Reminder for Mussels

Date:  July 30, 2026
Questions? 
Please contact: Equipment and Technology Subcommittee

The Logistics and Equipment Management Committee (LMC) issued Equipment Bulletin 26-001: Aquatic Invasive Species - Seasonal Reminder for Mussels, reminding the wildland fire community that aquatic mussels continue to be a problem. Mussels rapidly colonize hard and soft surfaces, including aquatic plants and fire equipment, clogging pipes, fouling motors, disrupting water systems, and impacting native species.

To help prevent their spread, fire personnel must follow the decontamination procedures outlined in the NWCG Guide to Preventing Aquatic Invasive Species Transport by Wildland Fire Operations, PMS 444. Additionally, report any sightings immediately to your Lead Resource Advisor and include clear, close-up photos (with a pen or your hand for size reference) and location details.”

Read the complete LMC Equipment Bulletin 26-001, to learn more. 

References:

NWCG Alerts

LMC Equipment Bulletin 26-001: Aquatic Invasive Species - Seasonal Reminder for Mussels

NWCG Guide to Preventing Aquatic Invasive Species Transport by Wildland Fire Operations, PMS 444

Intermediate Faller (FAL2) Position Task Book Update

Date:  July 24, 2026
Questions? 
Please contact: Tree Felling Subcommittee

NWCG Operations Management Committee (OMC) has issued an update to the NWCG Position Task Book for Intermediate Faller (FAL2), PMS 311-91. To ensure clarity for both trainees and evaluators, the updated FAL2 Position Task Book (PTB) explicitly states who is authorized to sign off on your tasks:

“Evaluators must be qualified as a FAL2, or an Advanced Faller (FAL1).”

If you are currently working out of a previously published version of the FAL2 PTB, you do not need to restart. However, OMC highly encourages you to take the following step to prevent any confusion in the field:

  • Print and replace the updated evaluator information page (page 2) in your current physical PTB.

References:

Intermediate Faller (FAL2) Position Page

RMC Memo 26-01: Transition Plan for Implementing Updates to Intermediate Faller (FAL2) Position Qualifications

NWCG Position Task Book for Intermediate Faller (FAL2), PMS 311-91

Equipment Advisory 26-001: Potential Front Driveline Joint Failure on 2018 and Earlier Ram Pickups

Date:  July 10, 2026
Questions? 
Please contact: Equipment and Technology Subcommittee

The Logistics/Equipment Management Committee (LMC) has released Equipment Advisory 26-001: Potential Front Driveline Joint Failure on 2018 and Earlier Ram Pickups. This advisory highlights multiple U.S. Forest Service regions reporting front driveline joint failure incidents.

This advisory recommends firefighters and fire managers to: 

  • Follow all Ram-issued inspection, maintenance, and service guidance for front driveline joints. 
  • Consider installing a driveline loop. 
  • Coordinate with fleet managers and service providers to determine the most appropriate driveline loop kit for each pickup.

For further details, please refer to the complete advisory.

References:

Equipment Advisory 26-001: Potential Front Driveline Joint Failure on 2018 and Earlier Ram Pickups

Technical Service Bulletin: 03-002-21