Adaptations to fire and other natural disturbances in American chestnut and red oak regeneration
The American chestnut (Castanea dentata) has been functionally extirpated as a canopy species since the mid-20th century, due to the importation of multiple pathogens in the 19th and early 20th centuries. Modern breeding programs are seeking to produce disease-resistant genotypes, but the ecological and silvicultural knowledge necessary for successful restoration is lacking. In Chapter 2, we compare the growth response of hybrid chestnut seedlings to those of frequent competitors, northern red oak (Quercus rubra), sugar maple (Acer saccharum), and red maple (A. rubrum), planted in the understory of mixed-oak forests. These plantings were designed to evaluate the effects of various sources of competition on seedling growth and survival. Height and diameter growth, as well as seedlings mortality was tracked for 4 years after planting. We found that chestnut had out-competed the other species in the most open environments. Chestnut responded positively to the removal of competition, regardless of the source, and displayed a notable ability to capture newly opened growing space. We concluded that American chestnut restoration would be most successful where early stand management is used to provide chestnut a period of reduced competition. We suggest that midstory removal immediately preceding underplanting would increase growth of chestnut seedlings without drastically increasing competition, and may help minimize mortality. An establishment strategy which minimizes competition could benefit chestnut seedlings planted at high quality sites; if combined with multi-stage shelterwood removals and some amount of competition control, midstory removal may be a beneficial tool during chestnut restoration in eastern oak-hickory forests.^ American chestnut is closely related to northern red oak and the two were common overstory associates in the eastern hardwood forests of North America throughout the last several thousand years. In Chapter 3, we sought to elucidate those relationships by collecting a variety of crown architecture and leaf morphology measurements from 28 chestnut and 46 red oak saplings. This study was intended to evaluate the potential differences in the seedling establishment strategies of the two species, and how those strategies might be adaptive in relation to local disturbance regimes. We also investigated species' nonstructural carbohydrate (NSC) reserve strategies and compared whole-tree biomass partitioning patterns. Overall, chestnut invested less than oak in structural roots, but relatively more in branches and foliage. Chestnut also had higher NSC concentrations than oak indicating some adaptation to withstand aboveground disturbance (e.g., fire or animal browsing). Oak had smaller crown projection area and lower specific leaf area than chestnut, both maladaptive traits in shaded understories and small gaps. We concluded that chestnut's morphology may have historically helped it to function as an opportunistic and plastic species, and that well-established saplings may have benefitted from a variety of gap sizes to facilitate release.^ In Chapter 4, we provide an initial evaluation of chestnut seedling's response to fire-induced topkill. We topkilled 94 American chestnut and 143 red oak seedlings across a range of ground-line diameters. Seedlings were topkilled using a burn chamber and growth and survival of resulting seedling sprouts were monitored for a full year. We observed that chestnut sprouts tended to emerge sooner after topkill than oak sprouts, and noted strong differences between species in point of attachment, with oak storing dormant buds further belowground than chestnut. Oak sprouts outpaced chestnut sprouts, and by one year post-treatment, had greater total volume and taller individual stems than chestnut. We also saw some evidence that oak sprouts expended more resources on re-filling NSC reserves compared to chestnut, highlighting their adaptation to repeated surface fires. We found that sprout growth of both species was more closely linked to initial seedling size than to canopy openness, though canopy openness did have a significant effect on both height and volume of sprouts. Our results indicate that chestnut seedlings can be expected to respond positively to a single surface fire, but are likely more vulnerable than red oak when exposed to more frequent prescribed burning or when they are below 3 cm ground line diameter. These fires will damage seedlings, and may lead to blight infection. However, due to the prevalence of chestnut blight within its historic range, we suggest that a single, low-intensity fire may still be beneficial as a release treatment in restoration plantings, while not unduly endangering planting stock. Fire will be most effectively applied once seedlings are greater than 3 cm ground line diameter, and have sufficient NSC reserves to fund resprouting. (Abstract shortened by UMI.)^
Michael R. Saunders, Purdue University.
Biology, Ecology|Agriculture, Forestry and Wildlife|Environmental Sciences