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Use of Underplanting to Enhance the Health and Sustainability of Oak Dominated Ecosystems in Kentucky and the Central Hardwood Region
J. Lhotka, J. Stringer
Department of Forestry
Oak dominated forests typify the Central Hardwood Region and have both economic and biologic significance. Oak species dominate 72% of Kentucky's forests and account for nearly 50% of all hardwood timber cut in the Commonwealth. Hard mast provided by oak dominated forests is also critical to the diet of important game and non-game wildlife species. Because of market and non-market benefits oak forests provide, the perpetuation of these forests is important to Kentucky and the larger Central Hardwood Region.
Threats such as invasive forest pests, altered historical disturbance regimes, and forest decline have the potential to affect the long-term health of oak forests. To reduce the vulnerability of these forests to threats and to enhance their resiliency following disturbance, foresters must implement sustainable management strategies. Important components of these approaches would be to promote vigor in young forests through density control (thinning and release) and to enhance desirable seedling recruitment in maturing oak stands or those facing imminent attack from insects or pathogens. The proactive recruitment of oak prior to a harvest or an impending natural disturbance requires that seedlings be established under existing forest canopies. Because the presence of an existing canopy has a mitigating influence on the forest microclimate, it affects the development of oak and its competitors.
Since invasive species can alter the forest environment, land managers must also understand how these species impact forest regeneration in order to develop management solutions that perpetuate oak in the presence of diverse forest health threats. A large-scale assessment completed by Woodall, et al. (2008) suggests that current seedling and sapling densities in many mature stands are insufficient to successfully regenerate oak following disturbance.
Underplanting of nursery grown seedlings can be an effective tool for increasing advance reproduction when natural seedling densities are insufficient and is an important technique for reintroducing high value species into degraded stands. The proposed study will evaluate underplanting of tree seedlings as a method for enhancing reproduction in stands whose current conditions are not conducive to oak seedling establishment. Because understory, midstory, and overstory vegetation can alter the forest microclimate, the study will evaluate how each of these structural components affects the response of underplanted seedlings. The study will quantify the competitive effect of invasive understory vegetation on planted seedlings (Objective 1), evaluate midstory removal as a method to increase growth and survival of underplanted seedlings (Objective 2), and assess the effect of canopy gaps and residual overstory cover on seedling development (Objective 3).
Results are expected to identify components of stand structure that have the largest impact on the success of an underplanting operation. Relationships documented by this study will aid in the development of regeneration strategies to maintain oak dominated forests in Kentucky and the Central Hardwood Region.
2010 Project Description
Activities during 2010 included the continuation of previous research as well as the initiation of new work. Activities were completed with cooperation and support of the USDA Forest Service, Southern Research Station, Upland Hardwood Ecology and Management research work unit.
Continuing work was associated with the monitoring and evaluation of two established midstory removal and underplanting studies through seven growing seasons. One study focuses on upland oak stands while the other focuses on riparian stands. With regard to the upland study, analysis of the response of underplanted white oak and black oak seedlings on four intermediate quality sites in the Cumberland Plateau region was completed and was presented to researchers and forest managers at the 17th Central Hardwood Forest Conference. Field sites were also utilized in experienced based learning exercises within an undergraduate course on silviculture. During the reporting period, data collection efforts were also expanded on the upland site. Installation of sampling equipment and initial data collection for the following microclimatic variables were completed: soil moisture, soil temperature, air temperature, and relative humidity. Data were collected simultaneously from experimental plots (midstory removal treatment and control) from May 10 to September 12. These microclimate data will be used in conjunction with the seedling growth data to further understand interactions between stand structure and the response of underplanted oak.
Project activities also included the establishment of a new study that will quantify the effects of spatial light patterns on the growth and competitive position of three oak species. Data collection for this study was completed during 2010; analysis and manuscript development will be completed in the upcoming year.
Other project activities during the reporting period included the evaluation of research sites for project objectives related to the response of underplanted seedlings following two gap-based regeneration systems and to the effect of invasive species on underplanted oaks. The establishment and initial data collection for one or more of these study sites is planned for the upcoming year.
Underplanting combined with silvicultural treatments can be an effective method for establishing competitive oak seedlings when natural seedling densities are insufficient in both upland and bottomland oak forests. Unlike reliance on natural regeneration, underplanting allows managers greater control over seedling densities and species composition. Underplanting can also facilitate the introduction of high value species into degraded stands.
The project will provide information regarding the response of underplanted oak seedlings to a suite of silvicultural treatments including understory competition control, midstory removal, and gap-based regeneration systems. To date, the project has yielded data on the seven-year response of underplanted white and black oak seedling following midstory removal on intermediate quality sites and cherrybark oak on high quality sites. Treatments on intermediate quality sites increased diameter growth of the underplanted seedlings, but had no effect on height growth. Height growth on intermediate quality sites was considerably lower than on higher quality sites. Much of the existing literature has focused on productive sites and red oak species.
This project suggests that the timeline for release of white oak seedlings may be considerably longer than has been prescribed for red oaks on productive sites. These data are important to managers in the region as they help in the refinement of silvicultural treatments that enhance the development of seedlings established by an underplanting operation.
The initiation of the new study that will quantify spatial light patterns following midstory removal in upland oak stands is important because the majority of existing research on oak advance regeneration has focused on observing seedling response over the entirety of a treatment and has ignored the influence of within treatment variability. The new study aims to look more closely at the spatial variability of light transmittance before and after midstory removal and how specific light levels relate to seedling success or failure. Because this work is assessing the spatial characteristics of light through the stand, more exact light levels needed for optimal growth may be determined. The possibility also exists for being able to predict the most effective harvest practices to create the necessary conditions for best growth.
Besides adding to the current literature on oak regeneration, the information obtained from this study will be essential to practicing foresters wanting to regenerate oak stands in Kentucky and the surrounding Central Hardwood Forest Region. Planned project activities will help further document the long-term response of underplanted oak seedlings and help to tailor underplanting strategies for oak-dominated stands on intermediate quality sites. This work will provide information regarding the impact of species and planting stock quality on seedling growth, the efficacy of understory and midstory control, and the timing and intensity of silvicultural release treatments that follow underplanting.