Structure Condition Services Stressors
Precipitation with high acidity (a lower pH)—also known as acid rain—was first noted as a threat to forests in the mid-20th century1 and is harmful to trees as it damages leaves and leaches important nutrients from soils. Some species such as red spruce (Picea rubens) and sugar maple (Acer saccharum) are particularly sensitive to acid rain, causing observed long-term decline. These observations led to the passage of the Clean Air Act in 1970 and subsequent amendments. Since then, there has been a steady decrease in the acidity of rain. Here, acid deposition is assessed as the mean annual pH of precipitation in Underhill, Vermont. The pH scale runs from zero (acidic) to 14 (basic), 7 being neutral. Unpolluted rainfall has a pH of around 5.6. A high score means that measured precipitation pH is at or near this number.
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 between minimum and maximum (scaled 1-5)
|Directionality of scores||
Higher values in the data are better.
|Minimum value used in scoring||
Data minimum - 10% of range
|Maximum value used in scoring||
Data on the pH of precipitation were accessed from the National Atmospheric Deposition Program (NADP) 1. Data were collected from the Underhill, Vermont, station. We set a target for precipitation acidity of 5.6, based on a previously established value 2. The annual score was computed as the difference between the lower scoring bounds (minimum value in the data minus 10% of the range) and the target value (5.6). This difference was then scaled between 1 and 5. Values above the target receive a 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.