Across eastern North America, many forests that were historically dominated by oaks (Quercus spp.) and hickories (Carya spp.) are undergoing a shift in species composition. While mature oak and hickory trees are still an important component of the overstory, the smaller size classes are dominated by the more shade-tolerant maples (Acer spp.) and American beech (Fagus grandifolia). Oak regeneration failure in particular, has been the subject of extensive research and management efforts. Several hypotheses have been proposed to explain the change in oak forest dynamics over the last century, including 20th century fire suppression, increased deer browsing, land use change, dense understory shade, and a general pattern of decreasing multi-year droughts over the last few hundred years, all of which may contribute to declining oak regeneration success.
However, there has been no such examination of hickory regeneration over this same time period. Are hickories also experiencing declining regeneration success? To explore the long-term trends in hickory regeneration, we used data from permanent plots located in southeast Ohio. The changing contribution of hickory to total basal area, basal area of the regeneration layer and stem numbers were examined over the past 30 – 50 years.
Results/Conclusions
Five species of oak still dominate the canopy – white oak (Q. alba), black oak (Q. velutina), red oak (Q. rubra), scarlet oak (Q. coccinea) and chestnut oak (Q. montana) – and together comprise 71–94% of the total basal area in plots. Hickories account for 1 – 10% of the total basal area in these same plots. Oak recruitment into the sapling strata (defined as trees that have reached a minimum DBH of 6.5 – 10 cm) has declined over time. Oaks represented 4–9% of all new recruits before 1984, but 0–3% of new recruits after 2005. Hickory recruitment is variable among sites, accounting for 4–5% of new recruits before 1984, and 0–9% of new recruits after 2005. The greatest difference between hickories and oaks appears in the mid-sized trees (10 – 25 cm DBH). Since the 1970’s, the percentage of oaks in this size class has steadily declined while the percentage of hickories has steadily increased. This could represent increased hickory recruitment and (or) an alternative shade tolerance strategy in hickory (i.e., persisting as smaller individuals for extended periods of time). Understanding the differences between hickory species is the next step, for improving our predictions for future forest composition.