Structure Condition Services Stressors
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 -- 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 index of drought called the Standardized Precipitation Evapotranspiration Index (SPEI), which incorporates both rainfall, temperature, and plant water requirements1,2. 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. A high score means drought severity is not deviating from historical patterns.
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.
NOAA, Northeast Regional Climate Center at Cornell University (2020)
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.
Distance away from long-term mean (scaled 1-5)
|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 long-term mean from 1961-1990 based on the baseline set 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 plus 10% of the range) or the lower scoring bounds (minimum value in the dataset minus 10% of the range), scaled to be between 1 and 5.
Acid rain harms forests and other ecosystems by damaging leaves and leaching nutrients.
The length of the growing season varies from year to year, but large or persistent changes can be problematic to forests.
Ozone can cause many negative impacts to forests by reducing regeneration, productivity, and species diversity.
Mercury is a toxin that persists in the environment for long periods by cycling back and forth between the air, water, soil and organisms - resulting in long-term, negative effects to forest ecosystems.
Warmer winter minimum temperatures can allow for non-native species to proliferate, while at the same time stressing native forest trees.
Higher maximum summer temperatures can stress forests, reducing productivity and health.
Changes to precipitation can alter the water balance in Vermont’s forests, causing either drought or deluge.
Snow insulates the soil and tree roots from cold temperatures and provides water when it melts.
Climate change will continue to result in more extreme weather events, which can stress forests beyond what they are accustomed.
Lack of sufficient precipitation can cause both immediate and long-term stress to trees.
As native trees are not adapted to defending themselves from non-native, invasive insects and diseases, widespread damage and mortality can result.