Crown density

Another way to measure tree health is crown density. Density tells us the amount of plant material, such as leaves, branches, and fruit, that block skylight from shining through the tree crown. It is measured as the percent of total light that is blocked by tree material. Unlike transparency, this measurement takes into account both the live and dead parts of the crown rather than just the live portion. Because density measurements consider all parts of the crown, density is not a perfect mirror image of transparency. Like transparency, density differs among tree species and is affected by the tree's shape and growing conditions. For transparency, high values indicate poorer tree condition. For density, however, high values indicate that the tree has a large amount of leaf material available for photosynthesis and has growing conditions that enable full and symmetrical growth. Low density values indicate poor amounts of foliage, a thin crown, or a missing section of crown and may result from stresses such as insect and disease damage or other environmental factors such as drought, wind, competition, or soil compaction. Density fluctuates from year to year as shown in the graph below:

This variation in density is natural and is influenced by a variety of factors. One of the most important factors influencing the variation is drought. Pest outbreaks also can lead to density change. The minor fluctuations seen in the graph above appear to be associated with these types of stress events. Density would have to change by more than 20% for the difference to be significant.

Density also can be described as being good (51 -100%), moderate (21-50%), or poor (0-20%). In a detailed look at the data for 1996, we can see the percent of the trees that were in each of these classes:

Most trees are doing very well in terms of foliage density. Over 98% of all trees were classified as having good or moderate foliage density in 1996.

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