Average Site Index

The Indicator

The productive capacity of a forest is a good indicator of its overall health. One measure of the productive capacity of a forest is the site index of the dominant tree species in the area. On Marylandís Eastern Shore, the site index will likely be measured on loblolly pine; in Central Maryland, tulip poplar or northern red oak; and in Western Maryland on black cherry and red oak. This forest site quality measurement is an estimate of the capacity of particular sites to grow trees. It is similar to various measures of the productivity of land for growing agricultural crops. For Eastern forest species, the site index is defined as the average height of dominant trees at 50 years of age. Tree height growth has been found to be closely correlated with tree volume growth and therefore site productivity.

Data to develop this indicator were derived from the National Resources Conservation Service (NRCS) State Soil Geographic (STATSGO) Database and were mapped to the soil unit. For any given soil unit, several commercially important tree species for that soil type and geographic area may have been listed, each species having been assigned a site index value (average height of dominant trees at 50 years of age). The average of these site indices for any given soil unit was then calculated and mapped.

Indicator Use

The average site index helps to determine the influence of soil related growth conditions on tree productivity for a particular site. Sites with high average site indices might be selected for the most intensive management, if producing timber were the primary objective for maintaining and managing the forest. From an ecological perspective, high site index areas may also in some cases support large numbers and multiple types of flora and fauna, although high site index values are also found in some areas where monoculture is practiced, where biological diversity is relatively low.

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