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Growing Season Index (GSI)

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.

nfdrs-gsi_us.png 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 Variable Unconstrained (1) Completely Limiting (0)
Minimum Temperature 5ºC /41º F -2ºC /28º F
Vapor Pressure Deficit (Pascals) 900 Pascals 4100 Pascals
Photoperiod (Daylength) 11 hours 10 hours

Learn more about the Growing Season Index (GSI)

 

NWCG Latest Announcements

Call for Nominations: Paul Gleason Lead by Example Award

Date: December 11, 2025
Questions? Please contact:
 Leadership Committee 

Do you know someone working in wildland fire who strives to make positive change and is undeterred by obstacles or setbacks? Now is your chance to give that person the acknowledgment they deserve by nominating them for a Paul Gleason Lead by Example Award.

This award, is presented by the NWCG Leadership Committee to remember Paul Gleason's contributions to the wildland fire community and to recognize individuals or groups that exhibit the same spirit and dedication to leadership – those who lead by example.

Nominations can be submitted via email with an attached Lead by Example Form to BLM_FA_Leadership_Feedback@blm.gov or through the online form. 

Tips for successful nominations and more information can be found on the Lead by Example webpage. All nominations must be submitted by December 31, 2025.

References:

Paul Gleason Lead By Example Award 

Paul Gleason Lead by Example Award Nomination Form 2025 

Leadership Committee 

Coming Soon! Ransom Road WFSTAR Module

Date: December 9, 2025
Questions about RT-130? 
Please contact: NWCG Audiovisual Specialist

On June 8, 1981, the Ransom Road Fire swept across Merritt Island National Wildlife Refuge in Florida, marking a defining moment in wildland fire management. This event led directly to the creation of the U.S. Fish and Wildlife Service's professional fire program.

Later this month, NWCG will release a compelling video that brings the Ransom Road Fire to life through firsthand accounts, official reports, and animated maps. Rob Lee, second on site during the deployment, shares his powerful recollections of that day. 

This video will be going live later this month in NWCG's RT-130, Wildland Fire Safety Training Annual Refresher (WFSTAR) Catalogue.

References:

WFSTAR Catalogue 

Wildland Fire Lessons Learned Center

2025-2026 Wildland Fire Leadership Campaign: "Leadership Levels" Now Available

Date: October 15, 2025
Questions? Please contact:
Leadership Committee

NWCG is excited to announce the release of the Leadership Committee’s 2025–2026 Leadership Campaign, themed “Leadership Levels.” 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.

The campaign is structured into four quarters and may be completed at any time between October 1, 2025 -December 31, 2026. Leadership levels will be released quarterly.

Quarter One materials are now available on the Wildland Fire Leadership Development Program website and focus on Leadership Level 1, Follower (Self-Leadership).

References:

2025/2026 NWCG Leadership Campaign - Leadership Levels

Wildland Fire Leadership Development Program

Leadership Committee

Updated! L-280, Followership to Leadership (Instructor-led) Course

Date: October 3, 2024
Questions? Please contact:
Leadership Committee

NWCG is pleased to announce the updated L-280, Followership to Leadership (Instructor-led) course is now available. L-280 is intended for operational personnel at the Firefighter Type 1 level, and for individuals pursuing Leadership Level 2, New Leader. 

This instructor-led course is a primer on leadership and the second formal course in the Wildland Fire Leadership Development Program. It helps emerging leaders build the confidence, skills, and mindset needed to lead effectively in high-risk environments. 

For more information about leadership development and available courses, visit the Leadership Committee Web Portal.

References:

L-280, Followership to Leadership (Instructor-led)

Wildland Fire Leadership Development Program

Wildland Fire Learning Portal