COS 251-1 - Using a tree species stand-level evenness index to track temporal shifts in tree species evenness of forest stands at the landscape scale

Abstract: Background/Questions/Methods

Diversity measures are commonly used to assess the overall health of forest ecosystems. Two common metrics at the taxon level are species richness and species evenness, the number of species and the abundance of species, respectively. Focusing on the evenness metric, healthier forest ecosystems should exhibit a higher level of evenness among tree species than those systems with strong single species dominance. Shifts in the degree of tree-species evenness 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 tree-species evenness over a 15-year period. These data came from four sample re-measurements made in 2005, 2010, 2015, and 2020 across the state of Arkansas, a typical southern state that had a high level of tree cutting during this period. I used the McIntosh Evenness Index (MEI) to measure stand evenness on each plot with values ranging between 0.0 and 1.0 (where 1.0 represented full evenness in the species space and 0.0 represented no evenness).

Results/Conclusions

Approximately 4000 FIA plot conditions were re-measured during each of the four survey periods. The MEI decreased slightly in each survey between 2005 and 2020, going from 0.6160 in 2005, to 0.6053, 0.5990, and 0.5830 in 2010, 2015, and 2020, respectively. All trees >2.54 cm dbh were included in the MEI. A continued increase in plantation forestry over this period may have contributed to the decline. Between 2005 and 2020, forest plantation area increased from 1190100 ha to 1534700 ha. Typical monocultures in plantations would lower the overall MEI for the State. Other reasons for a decline in the MEI may be attributed to a high number of acres in younger early successional stands, much brought on by substantial forest cutting activity. Monitoring the MEI of a state’s forest is helpful in establishing forest policy. However, no guidelines exist concerning how low an index, such as the MEI, should go, i.e., guidelines need to be developed to determine what is a healthy MEI for any particular region or state and when to sound the alarm. Monitoring efforts, such as used in this study, are an important first step in ecosystem risk assessment.