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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.
2011 Project Description
Project activities included final measurement and analysis of two long-term oak underplanting studies. The study database now includes fifth, sixth, and seventh year growth responses of naturally regenerated oak and maple as well as underplanted white and black oak across seven research sites.
One analysis path explored the seven-year effects of midstory removal treatments on the understory microclimate and seedling response of underplanted and natural oak reproduction.
A second focus took a modeling approach to develop predictive survival and growth equations for oak seedlings based upon initial seedling characteristics and metrics of vertical and horizontal canopy structure.
A secondary study was also started during the reporting period to evaluate the effect of simulated fire top kill (i.e., top-clipping) to determine whether seedlings growing in the absence of midstory removal had a more pronounced sprouting response. Outcome of this follow-up study will come in a future reporting period.
Project field sites were also used for instruction in two undergraduate classes. These field labs included exercises to help students comprehend the effects of midstory removal on forest structure and understory light environments. These relationships were further linked with measurements of seedling response, so students could see developmental trends among treatments.
Data analysis during the reporting period yielded five important implications of the project to regeneration of upland oak stands. These findings are as follows:
1) Midstory removal as applied in the project can appreciably modify understory light availability. However, we could not detect an effect of midstory removal on understory temperature/relative humidity, soil temperature, or soil moisture.
2) Six and seven year responses following midstory removal strongly suggest that the treatment greatly enhances height and diameter growth of natural and underplanted oak reproduction. However, even after seven growing seasons natural and underplanted seedlings have not yet reach an acceptable release height (> 3 ft).
3) the presence or absence of tall intervening vegetation (e.g., midstory trees) was generally the best predictor of seedling growth,
4) data indicate no consistent trend regarding whether underplanted or natural regenerated seedlings respond more favorable to midstory removal.
5) Observed red maple growth suggests that while midstory removal can increase oak's growth it also greatly promotes the development of shade tolerant competitors. The number and size of red maple would indicate the likely need for competition control treatments before or after overstory removal to increase the competitive ability of oak seedlings.
Craig, J.M., J.M. Lhotka, and J.W. Stringer. 2012. Response of naturally regenerated and underplanted white oak (Quercus alba L.) seedlings six years following midstory removal. in Proceedings of the 18th Central Hardwood Forest Conference. USDA For. Serv. Gen Tech. Rep.