COS 142-7
Temporal shifts in tree species dominance of forest stands at a landscape scale
A certain degree of oligarchy is a common characteristic of temperate forests and is usually evident in tree-species abundance distributions or in stand-level tables produced from sample data. Shifts in the degree of oligarchy, or in the dominant species, may be indicative of disturbance or of successional stages in developing forest stands. Investigations into such shifts in dominance require long-term data sets with a consistent sample design over time, both difficult to apply and maintain. We used long-term landscape-level data from the USDA Forest Service, Forest Inventory and Analysis (FIA) program, to empirically track shifts in species dominance over a 27-year period. These data came from four sample measurements made in 1967, 1977, 1987, and 1994 across Mississippi, a typical southern state that had a high level of tree cutting during this period. We used two dominance metrics to test for changes over time in natural stands (no plantations): McNaughton’s dominance measure (MCN) and Misra’s dominance measure (MIS), both ranging between 0.0 and 1.0 (where 1.0 represented full dominance in the species space). McNaughton’s measure was the relative aggregate of the top two species and Misra’s was the relative aggregate of the top three species.
Results/Conclusions
Substantial forest cutting (a cut stand was defined as that where >5 percent of stand basal area was removed) occurred in Mississippi over the three re-measurement samples: approximately 2.5 million ha in 1977, 2.3 million ha in 1987, and 2.3 million ha in 1994. The MCN dropped from 0.67 (± 0.01 C.I.; n=2410) in 1967 to 0.62 (±0.01 C.I.; n=2472) in 1994. A similar drop occurred in the MIS, from 0.78 (±0.01 C.I.; n=2410) in 1967 to 0.71 (±0.01 C.I.; n=2472) in 1994. Some of this decrease in the degree of oligarchy can be attributed to cutting disturbance where many of the most dominant species are the most economically valuable, thus targeted for harvesting and reducing the abundance of dominants. This is especially true where cutting operations use selective cutting methods. The decrease in dominance shown in this study follows what Darwin had noted, that an increase in disturbance may lessen the degree of dominants in a community and subsequently increase diversity. However, this does not mean these stands have improved in quality. Though they may have a lessened oligarchic structure they may be absent important keystone species that provide important ecosystem functions necessary for community stability.