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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This committee has been archived July 1, 2026.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.


 

NFDRS header graphic. Decorative.

 

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

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

NEW! NWCG Standard Operating Procedures, PMS 900

Date:  July 9, 2026
Questions? 
Please contact: NWCG Staff

NWCG is excited to announce the publication of the NWCG Standard Operating Procedures, PMS 900.

The new publication establishes general, consistent, and uniform standards for conducting business with the National Wildfire Coordinating Group.

References:

NWCG Standard Operating Procedures, PMS 900

Quarter Four Materials for the 2025-2026 Wildland Fire Leadership Campaign Now Available

Date:  July 8, 2026
Questions? 
Please contact: Leadership Development Subcommittee

NWCG is excited to announce that Fourth Quarter materials for the 2025/2026 Wildland Fire Leadership Campaign are now available. This annual initiative offers all students of leadership and wildland fire management an opportunity to engage with essential leadership skills and knowledge needed to lead effectively in dynamic environments.

Quarter Four materials focus on Leadership Level 4: Leader of Leaders (Provide Direction). Leadership Level 4 is where you are setting the conditions for others to do things well, even when you are not present. This quarter focuses on equipping experienced leaders with skills to provide direction, build trust across teams, and foster strong decision making. 

References:

2025/2026 NWCG Leadership Campaign - Leadership Levels

Wildland Fire Leadership Development Program

Leadership Development Subcommittee