Effects of an invasive shrub and emerald ash borer-caused tree mortality on tree seedling survival and recruitment in Ohio deciduous forests
Emerald ash borer (EAB), Agrilus planipennis, may cause extensive changes to forest community composition in North America. Canopy ash (Fraxinus spp.) trees succumb to EAB in 1-4 years, with effects on understory light likely mediated by sub-canopy tree cover. We hypothesize that the effects of ash mortality on tree seedling recruitment and survival are influenced by sub-canopy cover and abundance of the invasive shrub Lonicera maackii.
We censused 24 sites (three plots per site) established throughout western Ohio by the U.S. Forest Service; representing a range of time since EAB infestation. Nested within each plot (400m2), was a sub-plot (200m2) and four (4m2) micro-plots. Within plots we assessed ash health, measured all trees >10cm diameter at breast height (DBH), and calculated weighted average ash condition. Within each sub-plot we measured all sub-canopy trees (3-10cm DBH) and the two largest L.maackii in each quadrant to calculate sub-canopy basal area (BA) and L. maackiiBA. At the micro-plot level, we censused tree seedlings 20-100cm to calculate survival and annual recruitment. Tree seedlings were categorized as shade tolerant, intermediate, or intolerant based on Niinemets and Valladares (2006).
We investigated effects of weighted ash condition, sub-canopy BA, and L. maackii BA on tree seedling recruitment and survival using linear mixed models and Akaike’s Information Criterion for model evaluation. Response variables were seedling survival 2012-2014 and annual (2013, 2014) recruitment of shade intolerant, intermediate, shade intolerant, and total seedlings. Eight of the 24 sites without L. maackiiwere dropped from these analyses.
Weighted ash condition and L. maackii BA were the best predictors for total recruits and the number of intolerant seedlings recruited (2013, 2014); fewer seedlings reached 20cm in plots with greater L. maackii basal area and healthier ash trees. The best predictor for shade tolerant seedling recruitment (2013, 2014) was L. maackii BA, with fewer recruits in plots with larger shrubs. Sub-canopy BA and L. maackii BA were the best predictors for recruitment of seedlings of intermediate shade tolerance; plots with less sub-canopy and larger shrubs had fewer recruits. Survival of seedlings from 2012 - 2014 was best predicted by L. maackiiBA; plots with larger shrubs had lower seedling survival.
All seedling responses were negatively impacted by L. maackii, while ash mortality was associated only with recruitment of shade intolerant and total seedlings, and sub-canopy had few effects. This suggests that L. maackii plays a larger role in seedling dynamics than EAB.