Understory trees are important drivers of forest composition and succession, yet the effects of canopy loss and climate change on understory tree growth are underrepresented in the literature. We used permanent plot data and dendrochronology to study four understory species: Aesculus glabra, Cornus florida, Hamamelis virginiana, and Ostrya virginiana in NE Ohio. Between 2001-2012, the site lost all of its adult ash trees (Fraxinus spp.) to the invasive emerald ash borer (Agrilus planipennis - EAB), which caused major changes in forest canopy structure and composition, including altered light and soil conditions. The study site also experienced three significant droughts (1988, 1991, & 1994). We predicted that temperature, precipitation, and ash-loss density will be reflected in growth patterns of understory trees. We used survey data from 1974, 1986, 1998, and 2015 to analyze changes in understory species density and basal area. We recorded GPS coordinates for all individuals of the four target species, and took increment cores from representative samples. We analyzed tree rings with a Velmex system. We dated samples by comparing them to an established Quercus alba dendrochronology using COFECHA and estimated climate correlations using dplR, ARSTAN, and Climate Explorer, as well as ash-loss density correlations, determined through ArcGIS.
Results/Conclusions
The survey data, based on thirty-three 20x20m plots, represent 16% of the study area, which reveal trends of declines for three of the four species (C. florida, H. virginiana, and O. virginiana). We believe that our data represent the first such dendroclimatology analyses for Ostrya virginiana and one of the first for Aesculus glabra. Not surprisingly, we found varied tree-ring patterns among the four understory species. Growth patterns reflected differences when comparing trees of these four understory species with Quercus alba for their responses to temperature and precipitation. Where white oak correlates (p<0.001) positively with annual precipitation, Ostrya and Cornus showed significant growth with only 2004 precipitation, which we believe reflects the importance of microclimate and site differences. We were also able to categorize ash loss into high, medium, and low-density regions. Aesculus had significantly lower basal area growth (p<0.05) in areas of high ash-loss, which is complicated by similar growth patterns due to habitat differences. We observed that ring growth patterns of Ostrya trees were affected in only the high ash-loss zones. Further investigations on site variability and physiological responses are necessary to better understand growth patterns in understory tree species.