Permanent plots are valuable for examining ecological change, such as succession and the effects of disturbance or invasions on community structure. In 2015, we resampled thirty-three 0.04 ha forest plots in northeast Ohio that were previously surveyed in 1974, 1986, and 1998. Our null hypothesis predicted that pre-EAB (emerald ash borer) trends would continue through 2015, against which we examined changes in species diversity associated with the widespread death of ash (Fraxinus sp.) trees following the 2002 invasion of EAB. As in the previous studies, we identified species, counted individuals, and recorded DBH for all trees ≥ 2.54 cm, and counted saplings (DBH < 2.54 cm and > 45 cm height) and seedlings in sub-plots. We analyzed variation in species diversity among plots and among years by comparing changes in the actual (S) and effective (e H’- from the Shannon-Weiner index H’) numbers of species per plot. We compared changes in percent similarity (PS) among plots and years for both count and DBH (basal area) data. Because patterns within years were not normally distributed, we analyzed variation among years, species, and plots with Kruskal-Wallis tests, and we analyzed changes between sample years with Wilcoxon Signed-Rank tests for paired data.
Results/Conclusions
Classical late-forest successional trends marked the pre-EAB invasion period (1974-1998) with increased dominance of Acer saccharum and Liriodendron tulipifera. The S (mean = 8.5 in 1974, 8.0 in 1998; n.s.) and e H’ (mean = 4.8 in 1974, 4.0 in 1998; p = 0.01) decreased, while PS between plots increased (mean = 30.3 in 1974, 33.3 in 1998; p = 0.009). Saplings and seedlings experienced similarly significant declines in species diversity and evenness. During the pre-EAB period, adult ash species decreased in both numbers from 122 to 77 trees, and total basal area from 56,034 cm2 to 41,737 cm2, while average basal area/tree increased from 459 to 542 cm2. Ash saplings and seedlings also experienced dramatic declines (-58% and -71% respectively). The post-EAB sampling revealed continuing declines in actual (mean = 7.1; p = 0.005) and effective numbers (mean = 3.2; p = 0.00001) of species per plot, with percent similarity of plot densities increasing to mean = 35.4 (p = 0.00002). Ash was reduced to just 34 total trees with an average basal area of 29 cm2. However, ash saplings and seedlings both increased (495% and 236% respectively), indicating the potential for recovery and continued community structure changes.