Emerald ash borer (EAB), Agrilus planipennis, may cause extensive changes to forest composition in North America due to rapid ash (Fraxinus spp.) mortality. Ashes succumb to EAB in 1-4 years, potentially increasing resource availability. Extent of ash decline and initial vegetation layer dimensions may shape canopy and understory woody plant responses. Furthermore, differences in initial volume and cover coupled with differences in light level adaptation may alter seedling abundance, recruitment and survival influencing long-term forest structure and composition.
We sampled 24 sites (three plots per site), representing a chronosequence of ash mortality throughout western Ohio. Nested within plots (400m2), were a sub-plot (200m2), and four (4m2) micro-plots. Within plots we assessed ash condition and measured all trees ≥10cm diameter at breast height (DBH). In sub-plots we measured all sub-canopy trees (3-9.9cm DBH), measured basal area (BA) of the eight largest shrubs for each species, and measured shrub cover. Within micro-plots, canopy openness was measured and all seedlings 20-100cm were identified and measured. We tested whether an index of stand ash decline affected canopy openness and non-ash canopy growth. We also investigated effects of ash decline, canopy BA, and sub-canopy BA, and shrub cover on seedling abundance, recruitment, and survival using linear mixed models and evaluated using Akaike’s Information Criterion. In a sub-set of 16 sites containing the invasive shrub Amur honeysuckle (Lonicera maackii) we tested the effect of ash decline on L. maacki growth. We also tested the effect of ash decline and L. maackii abundance on L. maackii fecundity.
Results/Conclusions
Canopy openness in 2012 was similar across all sites suggesting that EAB-caused ash decline did not affect forest floor light availability. Non-ash canopy tree growth was not dependent on ash-decline. When all woody seedlings were pooled there was no difference among sites in species richness, abundance, recruitment, or seedling survival. There was a significant relationship between L. maackii growth and ash decline. Where ash quality was poorer L. maackii growth was greater. Sites with the greatest L. maackii percent cover had the greatest fruit production.
Additional analyses will be conducted using hierarchical linear models to investigate if ash decline and initial vegetation layer volume affects shrub or sub-canopy growth, which in turn impact seedlings. Additionally, seedling parameters will be investigated separately for: invasives, shade tolerant natives, and shade intolerant natives. Further investigations should assist in disentangling forest vegetation responses to EAB and how best to manage forests following invasion.