Temporal trends in tree species richness in harvested stands over a landscape scale
Understanding biodiversity dynamics across managed forest landscapes is important for the establishment and implementation of policy that addresses the improvement in and conservation of species diversity. A temporal study identifying the impacts of tree harvesting requires a long-term data set with consistent 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 sample units over time on natural and plantation stands. Forest stands that were harvested (>75 percent of basal area removed) between 1967 and 1977 across Mississippi were used as baseline data. These harvested sample units were measured again in 1987 and 1994 (the last year of this sample design) and those that experienced no more cutting were used in the analysis. Tree species richness and stand evenness dynamics were the metrics used to determine diversity changes over time where tree species richness was the count of species (on each sample unit) and stand evenness was the McIntosh Evenness Index (MEI), ranging between 0.0 and 1.0 (where 1.0 was perfect evenness of all species).
Between 1967 and 1977, approximately 97,902 ha of natural forests and 100,233 ha of plantation forests were cut. In natural stands, overstory richness was 3.4 species per sample unit (SU-1) before the cut, declined to 1.7 species SU-1 by 1977 (after the cut), then increased to 3.4 species SU-1 by 1987 and to 4.1 species SU-1 at 1994. For plantations, overstory richness was 3.2 species SU-1 before the cut, decreased to 0.4 by 1977, then increased slightly to 1.7 and 2.8 species SU-1 in 1987 and 1994, respectively. The MEI differences between natural and plantation stands were more substantial; natural stands averaged 0.54 and plantations 0.46 before cutting. By 1994, natural stands had returned to the precut level of evenness (0.53). Plantations, however, were at 0.33 MEI. This is because of the tendency toward extreme monoculture management in plantations. Understory richness was more similar between natural stands and plantations, 6.4 and 5.3 species SU-1, respectively. This is a result of the degree of understory control measures on plantations. Studies such as this are valuable tools in quantifying the dynamics of species diversity under various types of anthropogenic regimes. It is important that sample designs are stable for reliable long-term monitoring.