The Growing Season Index (GSI)

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 suboptimal 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.
Table 1. Upper and lower limits of the indicator functions used to calculate the Growing Season.

The product of the individual daily indicators for minimum temperature, vapor pressure deficit and photoperiod forms a single metric which can be monitored for canopy greenness, hereafter referred to as the Growing Season Index (GSI). The GSI is a daily indicator of the relative constraints to foliar canopy development or maintenance due to climatic limits. It is continuous but bounded between zero (inactive) and one (unconstrained). The daily metric is calculated from the simple equation:

iGSI = iTmin  * iVPD  * iPhoto

where GSI is the daily Growing Season Index, iTmin is the minimum temperature indicator, iVPD is the vapor pressure deficit indicator and iPhoto is the photoperiod indicator. The daily GSI is then calculated as the 21-day moving average of daily indicator, iGSI, for all sites. The moving average serves to buffer single extreme events from prematurely triggering canopy changes.

The Growing Season Index and Live Fuel Moisture in NFDRS 2016

In NFDRS 2016, the GSI is 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.

Table 2. Example values of the GSI, their interpretation and affect on NFDRS Live Fuel Moistures.

Figure 1. Example 2014 seasonal values of the GSI and Live Fuel Moisture. Watford North Dakota.

References

Jolly, W. M., R. R. Nemani, and S. W. Running. 2005. A generalized, bioclimatic index to predict foliar phenology in response to climate. Global Change Biology 11:619-632.