Ecology and management of oak regeneration in bottomland hardwood forests in Missouri
Authors:   , Hayford Isaac 
Published: October 2023

Managing oak regeneration is a challenge in bottomland hardwood forests due to variations in acorn production, acorn predation, competition with other species, and flooding. This study was designed to investigate bottomland oak ecology and management approaches that can guide forest managers to successfully regenerate oaks in bottomland hardwood forests. We investigated limitations on seed establishment through acorn production, predation, and germination (Chapter 2). Chapter 3 examined the effects of initial overstory harvests and subsequent midstory release on survival and growth of underplanted oak advance reproduction. Chapter 4 explored the photosynthetic light responses of artificial oak advance reproduction growing in two light levels and a hydrological gradient. Chapter 5 examined the effects of initial midstory and understory removal and subsequent overstory harvests on underplanted and naturally regenerated oak advance reproduction. We found that low acorn production and high rates of acorn predation were prevalent throughout the stands. Thus, in the absence of oak advance reproduction, mast years or underplanting may be needed to establish new seedlings. We also found that germination improved when acorns were buried, but germination was unaffected by light availability. Our findings suggest that midstory release will not increase germination and therefore should not be expected to increase the initial establishment of new seedlings. However, midstory release could be used to enhance development of seedlings following establishment as our study findings indicated in Chapter 3 of this dissertation. Our results showed the importance of establishing oak advance reproduction prior to overstory harvest and demonstrated the difficulty of getting successful enrichment planting after overstory harvest. Midstory release enhanced the survival and development of oak advance reproduction. However, our results suggested no benefit of understory release on survival or growth of oak advance reproduction. Overstory harvest treatments (selection cut and shelterwood) applied eight years following midstory release enhanced the development of oak reproduction. Our findings reiterate the importance of developing competitive oak advance reproduction using an initial midstory release prior to canopy disturbance to improve oak regeneration success in bottomland forests. While the shelterwood harvest in our study increased the growth of planted oaks, its likelihood for oak regeneration success depended on sufficient size and numbers of oak advance reproduction prior to overstory harvest. Although the shelterwood treatment enhanced growth of oak advance reproduction, it also released a lot of oak competitors. However, the selection harvest increased growth of oak advance reproduction with less competitor release. Thus, a lighter overstory harvest such as that in our selection cut may provide better balance between increasing the growth of oak advance reproduction while also suppressing the development of competitors. The results from this study indicated that survival of bareroot pin oak seedlings increased with increasing initial basal diameter. We observed that to achieve more than 60 percent survival fifteen years after planting, the initial basal diameter must be > 1.4 cm. However, the RPM [registered trademark symbol] pin oak seedlings did not show any clear pattern of dependence on size at the time of planting. Considering RPM [registered trademark symbol] seedlings are more expensive to purchase, we recommend planting big bareroot oak seedlings to ensure better survival in the long term. We also found that while browsing at any stage of seedling development could negatively affect survival, browsing in the first year was the most impactful and a determinant of later survival. We found that light availability strongly influenced photosynthetic rates of oak seedlings, with greatest photosynthetic rates observed in high light levels. The more light-demanding pin oak exhibited lower rate of photosynthesis in response to decreased light availability compared to swamp white oak. The different light requirements of the oak species could directly influence the length of time seedlings can remain in the understory prior to their release. Our results also suggest that changes in light availability can alleviate the impacts of soil moisture stress on foliar nitrogen and photosynthesis of oak seedlings. Specifically, the effect of soil moisture stress on foliar nitrogen and photosynthesis was more pronounced in low light environments. We recommend light levels [greater than or equal to] 20 percent of full sunlight to lessen moisture stress and enhance the development of oak advance reproduction in bottomland hardwood forests. Further, foliar nitrogen was directly related with light availability. We also found that photosynthetic rate was positively related with foliar nitrogen. Thus, any decrease in foliar nitrogen resulting from low light levels and soil inundation could adversely influence photosynthetic capacity of oak seedlings and their subsequent development in bottomland hardwood forests.