Recent literature describes the increase in downed coarse woody debris (DCWD) following the Emerald Ash Borer (EAB - Agrilus planipennis) invasion and ash (Fraxinus spp.) tree deaths. We previously documented the arrival of EAB (2001) and peak ash mortality (2009) in an 8 ha forest preserve. We have data on woody plant densities and basal area (BA) for 33 permanent plots, sampled in 1974, 1986, 1998, & 2015, which allow us to compare forest community changes pre- and post-EAB. Data show a 10% density increase from 1974-1986 followed by 6% and 5.6% decreases between 1986-1998 and 1998-2015, respectively. BA increased in all three intervals (22%, 3.4%, & 4.2%). Ash tree densities increased 25% between ‘74-86, but decreased by 49% between ’86-98 (prior to EAB), likely due to three severe droughts. By 2015, only ash saplings remained. Photographs taken of each plot in 1986, 2016, and 2020 allow us to compare DCWD, with the hypothesis that peak ash death will produce as much DCWD in the 4-year interval as in the 30-year period. We compared DCWD patterns using t-tests for paired samples, and chi-square tests to compare DCWD for different ash-loss densities based on the census data.
Results/Conclusions
Our prediction is supported by a two-fold increase in DCWD in 2016 (p = 3.8*10-6), with an average of 3.2/year, with comparable increases of 3.0/year in 2020 (n.s.). Between 1986-2016, 23 of the 33 plots had increases, with a significant contribution from medium-to-high ash-loss plots (p = 0.02). In 2020, only 18 of the plots showed increases, but those averaged 11.5/year, which included the four highest ash-loss plots. The relationship with the EAB ash decline recorded in 2015 (p = 2.0*10-4) is confounded by the drought-induced ash declines pre-EAB between 1986-1998 (p = 1.6*10-4). Ash trees accounted for 93% of the decline in tree density pre-EAB and 48% of the decline 1998-2015. Ash BA decline in both the pre- and post-EAB periods was offset by the increased growth in other tree species, notably sugar maple (Acer saccharum), tulip poplar (Liriodendron tulipifera), and red oak (Quercus rubra). We found significant increases in DCWD in upland (p = 0.003) and slope (p = 0.002) habitats, but not in floodplain plots (p = 0.08). Our research shows the value and complexity in disentangling the consequences of an invasive species such as EAB from changes due to climate-driven weather patterns.