Restoration of Hardwoods through Conversion of Non-native Conifer Plantations
Forest tree species such as American chestnut (Castanea dentata (Marsh.) Borkh) and oaks (Quercus spp.) have been heavily impacted by forest changes in the eastern US during the past century. American chestnut, a once-important eastern US forest tree, was decimated by chestnut blight fungus. Blight-resistant chestnut hybrids have been developed, however, and so restoration is now conceivable. Regeneration of oak is poor in eastern forests due to disturbance regime changes including lack of fire, deer overabundance, and competition with faster-growing species. Concurrently, large areas of mature introduced pine (Pinus spp.) plantations exist in the Midwest Central Hardwood Forest following their wide-scale establishment 50-60 years ago. These monoculture plantations are in decline due to storm damage and other threats. They also have little economic and ecological value due to lack of a viable proximate market and limited biodiversity. Pine, oak, and chestnut are associates with similar site preferences suggesting that these pine plantations may represent ideal sites for planting of threatened hardwood species before less desirable species naturally regenerate and replace pine. We conducted two experiments to determine optimal pre- and post-planting management strategies for converting pine plantations to valuable hardwood species in the Central Hardwood Forest. For the first experiment (Chapter 2), we manipulated the canopy of pine plantations with clearcut and shelterwood treatments, and a fully intact canopy was retained as a control. We underplanted hybrid and pure American chestnut and northern red oak (Quercus rubra L.) seedlings in the three canopy treatments. Herbaceous and woody competition was either controlled or not through two growing seasons. Microenvironmental measurements were taken throughout the course of the study. Soil results indicated an acidic pH and low levels of some nutrients. After three growing seasons, oak performed best in the clearcut and chestnut in the shelterwood treatment. This reflects variation in shade tolerance of the two species; chestnut exhibits intermediate to high shade tolerance while northern red oak is less shade tolerant. Chestnut height and RCD were double that of oak, and chestnut leaf N content increased with light availability while oak did not. These results can be explained in large part by different adaptive strategies in biomass allocation between the two species; oak tends to allocate resources proportionally more toward root growth and chestnut toward aboveground growth. Both species had highest photosynthesis in the clearcut. Chestnut seedlings had significantly higher RCD (root collar diameter) in weeded clearcut and shelterwood plots, and oak had significantly higher RCD in weeded clearcut plots. Weeding in the uncut control plots was ineffective because shade limited the degree of herbaceous competition. For the second experiment (Chapter 3), we manipulated pine canopy to create the same three silvicultural treatments and underplanted hybrid American chestnut and northern red oak. Additionally, a fourth ‘open field’ treatment was added. Conifers often affect soil chemical characteristics, and we sought to compare pine (three original silvicultural treatments) and non-pine (open field) soils to determine whether pines did reduced soil fertility and thereby impacted seedling growth. For two growing seasons, two levels of controlled release fertilizer (CRF) were applied directly to seedling root systems along with an unfertilized control. Chestnut and oak seedlings both had significantly greater height and RCD after two growing seasons in the clear cut and shelterwood than uncut control and open field. Chestnut height and RCD were triple that of oak, likely due to contrasting growth strategies of the two species and disparate initial planting stock size. Fertilization increased seedling growth more in the clear cut and open field than shelterwood and uncut control, probably due to higher light and water availability. Open field seedling survival and leaf nutrient concentration were less than the other treatments, likely associated with occurrence of flooding and incidence of ambrosia beetle, rather than soil effects. Soils were not very different between open field and pine-forested treatments, except for significantly higher pH, potassium, and sulfur in the open field likely due to high buffering capacity of soils conferred by soil characteristics. Though the soils of current or former conifer plantation sites in this study have low pH and base saturation, oaks and chestnuts are adapted to these conditions. Results from this study suggest that pine plantations are ideal locations for northern red oak and American chestnut restoration and provide insight regarding the two species’ ecological growth strategies and habitat preferences. Shelterwoods and clearcuts are each viable silvicultural conversion options for oak and chestnut provided that herbaceous competition is controlled. Addition of CRF augments further growth increase, especially in open environments.
Jacobs, Purdue University.
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