II. What about tree health?

(All data summarized from Forest Health Monitoring plots in the North Central Region.)

Crown dieback

One measure of tree health is crown dieback. This is one of the variables measured in the Forest Health Monitoring plots. Dieback is measured as the percent of branch tips in the crown that are dead. However, in making this measurement branch tips that are dead due to shading or competition are not included because they are dead due to the normal processes of tree growth. Therefore, the measurement only includes dead branch tips in the upper portion and outer edges of the live crown--these branch tips would generally be alive if the tree were not experiencing stress. Dieback amounts differ among tree species. Some species will normally show a small amount of dieback. While such small amounts of dieback are not considered to be a problem, larger amounts of dieback indicate that a tree is under extra stress. Trees with high dieback values have a reduced amount of leaf area for producing food. Dieback has varied among years and between hardwoods (e.g., maple, oak) and conifers (e.g., pine, fir):

{dieback}

Hardwood trees tend to have higher amounts of dieback rates than conifer trees. This is partly due to physiological differences between these two groups. The energy needed to grow a completely new set of leaves each year and the additional effect that drought has on hardwood trees is partly responsible for their higher dieback rates.

Dieback also can be described as none (0-5%), light (6-20%), moderate (21 -50%), or severe (51 -100%). In a detailed look at the data for 1996, we can see the percent of the trees that were in each of these classes:

{percent of trees}

Over 97% of all trees in 1996 were classified as having either no crown dieback or light crown dieback. More than 6% of the American elm trees in the plots were in the moderate and severe dieback classes. This is due in many cases to the effects of Dutch elm disease.


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