Latest Score:

2.9/5

in 2019

score trend is flat over time
Weight: 10%

While mild, short-term droughts are not an uncommon event in Vermont, with higher temperatures resulting from continued climate change, we may see more severe or longer duration droughts. Lack of water, particularly during the growing season, can result in short-term changes in our forests, for example, a tree will halt photosynthesis and growth until there is sufficient water. But droughts can also result in more serious consequences to forests, like lack of a viable seed crop or large die offs of vulnerable species, locations, or age classes. Seedlings are particularly vulnerable to drought due to their shallow root systems. Further, droughts can increase the chance of forest fire -- a condition to which many of our tree species are not adapted. Insect pest outbreaks can also be more destructive when they occur with or following drought. Here, we used an pre-computed index of drought called the Standardized Precipitation Evapotranspiration Index (SPEI), which incorporates both rainfall, temperature, and plant water requirements12. We set the target for the dataset as the long-term mean with the understanding that droughts are a natural phenomenon, but changes in the frequency or duration of seasonal droughts could pose serious threats to forests as we know them. Annual scores were computed as the deviation from the data target, scaled from 1-5.

1Vicente-Serrano S.M., Beguería S., López-Moreno J.I., 2010: A Multi-scalar drought index sensitive to global warming: The Standardized Precipitation Evapotranspiration Index – SPEI. Journal of Climate 23(7), 1696-1718, DOI: 10.1175/2009JCLI2909.1.-- http://digital.csic.es/handle/10261/22405.
2Vicente-Serrano S.M., Beguería S., López-Moreno J.I., Angulo M., El Kenawy A. (2010) A global 0.5° gridded dataset (1901-2006) of a multiscalar drought index considering the joint effects of precipitation and temperature. Journal of Hydrometeorology 11(4), 1033-1043, DOI: 10.1175/2010JHM1224.1.-- http://digital.csic.es/handle/10261/23906.

Short-term summer and fall droughts are expected to increase. Lack of moisture during the growing season can harm the health of trees. Drought stress reduces trees’ ability to defend against boring insects and drought itself can lead to tree mortality, especially when combined with warmer temperatures. Drought conditions increase the risk of wildfires.

Interpretation provided by:

NOAA, Northeast Regional Climate Center at Cornell University (2020)

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The score is calculated using a target value and the historical range of the the entire long-term dataset. The higher the score, the closer this year's value is to the target.

Once the score is computed for each year, the trend in scores over time is calculated. If the trend is significantly positive or negative, the long-term trend is marked as increasing or decreasing respectively.

Component Description
Scored as

Distance away from long-term mean (scaled 1-5)

Target value

Long-term mean

Directionality of scores

No change from the long-term mean is better.

Minimum value used in scoring

Data minimum - 10% of range

Maximum value used in scoring

Data maximum + 10% of range

Drought was assessed through the Standardized Precipitation Evapotranspiration Index (SPEI) for Vermont1. The SPEI more fully captures the effect of drought on plants than the Palmer Drought Severity Index, as the former includes the loss of water through evapotranspiration. We selected a six month SPEI value which spans from April-September to capture drought in the functional growing season. We set the target for the dataset as the mean duration of snow cover from 1961-1990 which is used as the baseline normal for climate comparisons by the Intergovernmental Panel on Climate Change (IPCC). The current year is scored for where it falls between the target and the upper scoring bounds (maximum value in the dataset + 10% of the range) or the lower scoring bounds (minimum value in the dataset – 10% of the range), scaled to be between 1 and 5.

1 Vicente-Serrano S.M., Beguería S., López-Moreno J.I., 2010: A Multi-scalar drought index sensitive to global warming: The Standardized Precipitation Evapotranspiration Index – SPEI. Journal of Climate 23(7), 1696-1718, DOI: 10.1175/2009JCLI2909.1.-- Available at: http://digital.csic.es/handle/10261/22405

STRESSORS INDICATORS