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Growing Season Index
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US48 | Alaska |
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The Growing Season or Live Fuel Index (GSI/LFI)
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 / Live Fuel Index 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 or Live Fuel Index.
| Input Variable | Unconstrained | Completely limiting |
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| Minimum Temperature | 5°C | -2°C |
| Vapor Pressure Deficit (Pascals) | 900 Pascals | 4100 Pascals |
| Photoperiod (Daylength) | 10 hours | 11 hours |
The Growing Season Index (GSI)
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 as shown in Equation 4, where iGSI 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 Live Fuel Index is simply a rescaled version of the Growing Season Index to provide a metric on non-decimal value of GSI and is derived by simply multiplying GSI by 100. GSI has a value of zero to one while LFI has a value of zero to 100.
Table 2 – Example values of the Live Fuel Index and their interpretation.
| LFI Value | Classification / Interpretation |
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| LFI Increasing |
| 0 to 50 | Pre-greenup; dormancy |
| > 50 | Green-up |
| 75 to 100 | Closed green plant canopies |
| LFI decreasing |
| 100 to 75 | Curing herbaceous vegetation |
| < 50 | Leaf senescence |
| Below 50 | Entering complete curing or dormancy |
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.
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