Disturbances, such as insect outbreaks, are natural occurrences in forests. These processes change forest structure (the number of trees of different sizes in a given area). In 2014, an article was published in the journal Forests which outlined a quantitative methodology for measure forest structural sustainability. The paper defined structural sustainability as the stability of baseline and observed tree mortality. Forest Service scientists, in collaboration with the index developers, tested this new quantitative forest structural sustainability index on lodgepole pine forests in northern Colorado that had been heavily impacted by mountain pine beetles. The data used for the evaluation included tree mortality, including pre-outbreak conditions and annual mortality attributed to the mountain pine beetle during the outbreak. The purpose was to see if the index could be used to assess the change in forest structure as a result of beetle-caused tree mortality and to characterize whether the changes in structure could be characterized as sustainable in terms of maintaining tree density across tree size classes. A successful outcome of using the index would offer forest managers with a tool to help characterize forest health and disturbance impacts.
1. The structural sustainability index was sensitive to the effects of mountain pine beetle as identified by significant differences in structural sustainability across years during the outbreak. The index indicated impending changes to current forest conditions by identifying areas at landscape and watershed scales where lodgepole pine was over-crowded and thus poised for a mountain pine beetle caused stand-replacing disturbance and characterized changes in tree density across tree sizes as unsustainable.
2. Through the collection of easily measured forest census data, structural sustainability can be an effective indicator of ecosystem change and can provide critical quantitative data as part of a forest health monitoring program. This is relevant because managers need easy ways to assess the impact of disturbance on forests.
In conclusion, this study found that structural sustainability is an effective indicator of ecosystem change; the structural sustainability index reflected forest structure changes after the mountain pine beetle outbreak; and these patterns varied spatially across watersheds. The study provides quantitative data as part of a forest health monitoring program.