PS 30-167 - A multi-species dendroecology approach to understanding pre- and post-EAB forest changes

Abstract: We have permanent plot surveys showing ash declines prior to the 2001 Emerald Ash Borer (Agrilus planipennis - EAB) invasion in our 8ha NE Ohio forest preserve. The four surveys (1974, 1986, 1998, & 2015) also reveal pre-EAB declines in tulip poplar (Liriodendron tulipifera) and sassafras (Sassafras albidum). Tree-ring data show synchronous basal area increment (BAI) declines in these three species and in white oak (Quercus alba) following arrival of EAB. We used GPS to map tree locations for all four species. We first focused on tulip poplar, whose BAI patterns showed steady post-EAB decline of the master chronology trees. We examined growth patterns against survey intervals, habitats, and climate using Wilcoxon and Chi-square tests. We then used BAI measures for all the tree cores from our surveys – not just cores from master chronologies, which we believe hold ecological clues to forest changes. We measured growth rings to the nearest 0.001mm using a Velmex system, developed chronologies with COFECHA, and used dplR for calculating BAI values. We used SEA (superposed epoch analysis) to compare tulip poplar average BAI values. We used a hierarchical mixed-effects model to test for effects of years on BAI values all four species.

Our initial focus on tulip poplar showed a significant (p=0.037) growth decline in 2002, the year after EAB arrival, with significance (p=0.0221) due to floodplain trees. Tulip trees in the master chronology, which showed post-EAB declines, were significantly (p=0.0025) older than trees that did not cross-date well, and that were not in decline, indicating the value of using all cores in a dendroecology study. We found non-significant relationships between BAI patterns with survey period or drought patterns. The multi-species analysis revealed a highly significant (p< 2x10^-16) relationship for the effect of year on BAI patterns across all species, which combines small year-to-year changes in tree growth that might be non-significant and overlooked when studying growth patterns of individual species. The synchronized pattern for four species over time indicates forest-level responses to environmental influences in our tree-ring chronologies, which date to the late 19^th century. Our permanent plot surveys date from 1974-2015, dovetailing with significant increases in average monthly precipitation (p< 0.001) and temperature (p< 0.01) at our location since 1950, and with major atmospheric changes resulting from the 1970 and 1990 Clean Air Acts. We are now focusing on methods to interpret the synchronized shifts in tree growth with environmental signals.