Mortality patterns following a hickory decline event – Is density reduction key to maintaining bitternut hickory?

Abstract

Bitternut hickory (Carya cordiformis (Wangenh.) K. Koch) is an important component of many hardwood forest systems in the northern hardwood forests of the Lake States. Extensive mortality of the species was observed in a long-term field experimental site in a second growth northern hardwood forest of Wisconsin between 2010 and 2016. We quantified and documented the characteristics of the mortality, investigated the relationship between bitternut hickory density and mortality patterns, and assessed the persistence of hickory regeneration. The presence of a pathogenic fungus (Ceratocystis smalleyi Johnson and Harrington) and hickory bark beetle (Scolytus quadrispinosus Say) were identified as the main cause of mortality in other instances of hickory decline during a similar time period in the Midwest and Northeast. We investigated the role of these agents through post hoc analysis. Tree surveys of mapped stems ≥10 cm dbh conducted periodically from 2005 to 2016 indicated mortality of hickory rose sharply to 35% year−1 from 2010 to 2016. This is a substantial increase in mortality relative to the dominant species at the site (Acer saccharum Marshall, Tilia americana L., and Fraxinus spp.) which was 1.11% year−1 in the same time period. Mortality rates were higher in larger stems (≥20 cm dbh) than smaller stems (10 < 20 cm dbh). Additional analyses evaluated whether mortality rates were affected by localized species composition, using the proportion of hickory stems among neighboring trees. Proportion of hickory stems, diameter, and crown class proved to be statistically important in predicting mortality in individual hickory stems. The density of hickory regeneration in the <30 cm tall height class decreased from 2010 to 2016. Overall the bitternut hickory population significantly declined, especially the larger sized trees, and this decline will have lasting consequences on the persistence of the species in the area. Potential management actions for mitigating impacts of such rapid hickory decline events, mainly selective thinning, are discussed. Methods for maintaining this co-occurring species is increasingly important as several other codominant species in northern hardwood forests are also decreasing.