Acorn Dispersal and Oak Regeneration in a Managed Landscape
Oaks (Quercus) are a dominant tree genus and foundational component of many eastern North American forests. In these forests, they shape the structure, wildlife habitat, and understory light conditions; produce acorns that are a high value food source for wildlife; and are a high-value timber resource. Across the region oaks are regenerating poorly in many forests, in part, because of widespread fire suppression and decades of management practices that did not align with oak disturbance ecology. This regeneration failure has spurred substantial research on historic disturbance regimes and led to an increased use of silvicultural methods that produce intermediate light levels, including shelterwood harvests, and prescribed fire. While these management methods hold promise for oak regeneration, they involve substantial modifications to forest structure and may affect crucial trophic interactions within these forests. One key interaction is the conditional mutualism between oaks and granivorous rodents that scatterhoard acorns, which shifts along a continuum from antagonistic to mutualistic depending on environmental factors. While these rodents consume a substantial number of acorns, scatterhoarded but un-retrieved acorns have a much higher chance of germination and establishment and may also be protected from fire. I conducted concurrent studies in a landscape-scale experiment in southern Indiana to assess the effects of expanding group shelterwood (Femelschlag ) harvests and prescribed fire on oak regeneration patterns and acorn-rodent interactions. First, I surveyed the natural regeneration at these sites to investigate the spatial patterns of woody regeneration within and outside of burned and unburned shelterwood harvest gaps. The first replicate of this study had significant competitive oak regeneration just outside of the harvest gaps on the northern, eastern, and western sides, but the more mesic second replicate did not. At a subset of these sites, I assessed northern red oak (Quercus rubra) acorn dispersal, survival, and cache pilferage rates in different positions relative to shelterwood gaps in burned and unburned stands. For both dispersal from aboveground presentations and cache pilferage, acorn survival was generally higher in the treatment stands than paired controls, and higher in burned than unburned stands. Acorn survival from aboveground presentations was higher in the forest matrix than harvest gap centers; however, there was no effect of position within the stand on cache pilferage. Our results suggest that prescribed fire and small shelterwood gaps may increase perceived habitat riskiness for rodents resulting in higher acorn survival up to 2 years post-treatment. Finally I assessed the effect of fall and spring prescribed fires on cached and un-cached northern red oak and white oak (Quercus alba ) seedling emergence. The odds of cached acorn emergence were 2.3-10.9 times higher following fall and spring prescribed fires than the unburned control. Red oak emergence was 1.9 times higher following fall burns than spring burns, whereas white oak acorns were unaffected by burn season. This study represents a link between the proposed oak-granivore conditional mutualism and the oak-fire hypothesis, suggesting that prescribed fire might produce environmental conditions that push oak-granivore interactions towards a more mutualistic relationship.
Swihart, Purdue University.
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