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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.

 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

2025/2026 Leadership Campaign Quarter 2 Now Available!

Date: February 13, 2026
Questions? 
Please contact: Leadership Committee

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

This quarter focuses on Leadership Level 2: New Leader (Conveying Intent). Conveying intent is a crucial leadership competency in the high-risk, dynamic environment of wildland firefighting. Months 4 - 6 will concentrate on how successfully conveying intent enhances situational awareness, promotes shared understanding, and empowers firefighters to exercise initiative and adapt to changing conditions.

References:

2025/2026 NWCG Leadership Campaign - Leadership Levels

Wildland Fire Leadership Development Program

Leadership Committee

Opportunity to Serve the Wildland Fire Community as an IPTM SME!

Date: February 10, 2026
Questions? 
Please contact: NWCG SME Specialist

NWCG is seeking experienced wildland firefighters and support personnel to serve as Subject Matter Experts (SMEs) for the Incident Performance and Training Modernization (IPTM) initiative. The intent of IPTM is to transition NWCG’s training and qualification system to a performance-based model through updated position descriptions, Incident Position Standards, Next Generation Position Task Books (Next Gen PTBs), and updates to training where needed.

NWCG is currently recruiting SMEs for the following positions:

  • Aircraft Timekeeper (ATIM)
  • Compensation/Claims Unit Leader (COMP)
  • Cost Unit Leader (COST)
  • Crew Representative (CREP)
  • Deck Coordinator (DECK)
  • Fire Effects Monitor (FEMO)
  • Field Observer (FOBS)
  • Geographic Information System Specialist (GISS)
  • Ordering Manager (ORDM)
  • Receiving/Distribution Manager (RCDM)
  • Staging Area Manager (STAM)

Projects begin April/May 2026 and require 15–20 hours per month for 6–8 months, including a five-day workshop in Boise, Idaho this May. Apply by February 27, 2026, at the link below and review the attached document for details.

References:

NWCG SME Interest Sign-up

IPTM Subject Matter Expert Duties and Expectations Document

Incident Performance and Training Modernization (IPTM)

NEW! M-280, Resource Advisor, Fireline (Instructor-Led) Course Available Now

Date: January 29, 2026
Questions? 
Please contact: Resource Advisor Subcommittee

NWCG is pleased to announce the new M-280, Resource Advisor, Fireline (Instructor-Led) course is now available. This training supports individuals working towards Resource Advisor, Fireline (REAF) incident qualifications.

M-280, Resource Advisor, Fireline (Instructor-Led) training prepares students to serve as a Resource Advisor, Fireline (REAF) as defined in NWCG Incident Position Standards for Resource Advisor, Fireline, PMS 350-112. This course presents scenarios that reflect the diverse challenges a REAF may encounter during an incident. Through these scenarios, participants gain hands-on experience and an understanding of REAF responsibilities in a controlled learning environment.

References:

Resource Advisor, Fireline Position Page

NWCG Position Standards for Resource Advisor, Fireline, PMS 350-112

M-280, Resource Advisor, Fireline (Instructor-Led)

Wildland Fire Learning Portal

2026 Updates to the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1

Date: January 23, 2026
Questions? 
Please contact: Incident and Position Standards Committee

The Incident and Position Standards Committee has updated the NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1.

These updates address changes to incident position names and requirements for qualification, training, and experience. Updates related to the implementation of Complex Incident Management (CIM) and Incident Performance Training Modernization (IPTM) are also included.

The NWCG Wildland Fire Position Qualification Flowchart, PMS 308 has also been updated to reflect the recent changes.

References:

NWCG Standards for Wildland Fire Position Qualifications, PMS 310-1

NWCG Wildland Fire Position Qualification Flowchart, PMS 308

IPSC Memo 26-01: January 2026 Updates to the NWCG Standards for Wildland Fire Positions Qualifications, PMS 310-1