Structure Condition Services Stressors
Stand age diversity represents the presence of a large range of age classes in the forest. The age of the trees in a forest provides us a sense of the amount of time since a stand-replacing disturbance event has occurred, such as a hurricane or a timber harvest. Across our forests, we want a diversity of stand ages, with some stands being young and new and others being old and in place for a long time. This provides a diversity of habitat for a range of wildlife, and resilience to change. Here, we use stand ages (in 40 year grouping) extracted from Forest Inventory and Analysis data1 to quantify the proportion of our forest in three age groups: very young (0-40 years), young (41-80 years), and mature (81+ years). A higher score means that the relative amount of forest by age class is remaining stable over time.
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||
No change from the long-term mean is better
|Minimum value used in scoring||
Data minimum -10% of the rang
|Maximum value used in scoring||
Data maximum + 10% of the range
Using the USFS Forest Inventory and Analysis Program data accessed via the FIA EVALIDator1 , we extracted the acreage of forest for every 20 year age class. We re-categorized stand ages into 40 year buckets (0-40, 41-80, and 81-140 years) and computed age class diversity using a Shannon-Weiner Diversity Index calculation (see equation below). Per FIA protocol, plots were reassessed every 5 years until 2014 (i.e. data reported for 2007 used 2003, 2004, 2005, 2006 and 2007), and then every 7 years after that2. The target for this dataset was set to the long-term mean, and the score was then computed as the deviation from this target, scaled to be between 1 and 5.
Shannon-Weiner Index (H’)
Where pi is the proportion of trees in the ith size class
Forest cover is the percent of the state of Vermont with tree cover.
Regeneration of sugar maple seedlings provides information about the future of Vermont's hardwood forests.
Regeneration of red spruce seedlings provides information about the future of Vermont's softwood forests.
Forests with greater stand complexity have trees in a range of sizes and as a result, may be more productive and resilient to stress.
Forest patch sizes provides information on the average size of contiguous forest blocks.
Forest connectivity is a measure of the linkages among Vermont's forests.
With greater diversity in tree species, forests can support more biodiversity, exhibit higher resilience to stress, and store more carbon.
Across the landscape, having a range of forest stand ages provides diversity, varied habitat conditions, and resilience to stressors.