American Chestnut

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The American Chestnut, Castanea dentata (Marsh.) Borkh., was once one of the most common trees in the eastern United States.  An important ecological and economic component of American forestry until the early 20th century, the American chestnut population began to rapidly decline due primarily to an introduced fungal pathogen, the Chestnut blight [Cryphonectria parasitica (Murrill) Barr].  It is believed that the fungal blight was introduced by the importation of Old World Chestnut varieties, such as the four Castanea species found in Asia: C. mollissima Bl., C. henryi (Skan) Rehder & Wilson, C. seguinii Dode. in China and C. crenata Sieb. & Zucc. in Japan.  Today, the American Chestnut survives in low densities and poor condition, mainly due to the repeated regrowth of sprouts from surviving root masses.  The sprouts are eventually re-infected and die back.  Restoration efforts are underway and consist of traditional approaches, such as inoculating infected individuals with fungicides, as well as an extensive genetic hybridization program lead by the US Forest Service aimed at producing a blight resistant American-Chinese chestnut species for reintroduction.  These breeding programs are dependent upon the collection of a diverse sample of American Chestnut gametes in order to maintain as much genetic diversity as possible in the restored population.  Therefore, determining the location of mature, breeding C. dentada individuals is necessary to the success of the genetic hybridization restoration efforts.  Additionally, more accurate information on the current distribution of the species would provide the opportunity for further analyses focused on plant-pathogen ecology, climate change adaptations of threatened tree species, and biological conservation.

In 2013 I hiked the Appalachian Trail and volunteered for the American Chestnut Foundation by counting individuals along the way. As it turns out, the foundation compiles these counts for the MEGA Transect Project, and they allowed me the data. Below are some preliminary results from my analysis, with the ultimate intention being to provide information on the most probable location of large flowering individuals for the breeding programming.

ChestnutDensityMap

American Chestnut densities along the Appalachian Trail, representing 81,500 individual counts (assuming many repeated trees). Some of the greatest densities lie in Georgia along the southern limit of the species’ range, but this is also one the most counted portions of the trail. Another densely populated region is the trail through northern Virginia, near the Shenandoah. Big mature trees, however, are most dense in Pennsylvania, Maryland, and North Carolina, each with around 0.2 trees per kilometer of trail.

PoissonLogRegressionsGrid_lowres

Poisson distributions sometimes show good fits with count data like the MEGA project. In this case, elevation is one of the strongest predictors of chestnut density, followed by some soil properties, such as the organic matter content (OM), or the flooding frequency.