The history of North American forests is dynamic. Alternating glacial and interglacial periods have reshaped forest communities over the past million years, causing species to migrate south during cooling periods and north during warming periods. Evidence of these migrations can be found in fossil pollen deposits, which provide a snapshot of the plant communities present in an area at a particular time. In Massachusetts, for instance, glaciers covered much of the land area twenty thousand years ago. As the climate warmed and the glaciers receded, boreal species arrived from the south and were eventually replaced by today’s deciduous forest.
Many of the North American tree species that comprise these forests span large geographic areas. Red maple (Acer rubrum) is common in Massachusetts, Minnesota, Florida, and even Texas. Other species with widespread distributions include white oak (Quercus alba), tulip tree (Liriodendron tulipifera), pignut hickory (Carya glabra), basswood (Tilia americana), black cherry (Prunus serotina), and flowering dogwood (Cornus florida). Within these species, there is often significant variation in traits like flowering time, leaf shape, or disease resistance. And often, this variation within species is underpinned by genetic variation.
As a Katharine H. Putnam Research Fellow at the Arboretum, I am studying genetic variation in blue ash (Fraxinus quadrangulata) and the North American Castanea, chestnuts and chinquapins. These are widespread, ecologically important tree species under attack from pests or pathogens brought to North America by human activities. Once a common tree in eastern North America, the American chestnut (Castanea dentata) was famously decimated by the chestnut blight (Cryphonectria parasitica)—first identified in 1904, the blight spread rapidly and by 1950 had killed nearly every adult tree across its range. The collapse of the beloved American chestnut is infamous as an ecological disaster that permanently transformed North American forests.
Meanwhile, the story of North American ash trees is still unfolding. In 2002, a metallic green beetle from eastern Asia called the emerald ash borer (Agrilus planipennis) was found in Michigan, and has since been observed throughout the eastern United States, including eastern Massachusetts (it was first detected at the Arboretum in 2014). Its larvae feed on the phloem and outer cambium of ashes, eventually girdling and killing them. Given the advance of these lethal invaders, it is increasingly urgent to learn everything we can about Fraxinus and Castanea.
My research at the Arboretum focuses on two types of genetic variation: genetic structure and genetic diversity. Genetic structure describes how populations are similar or different from one another and is often correlated with geography. For instance, populations in the western portion of a species’ range may be more similar to one another than they are to the populations in the eastern part of the range. This kind of pattern can arise through local adaptation or through historical processes like migration. Glacial cycles can generate population structure by forcing a species into isolated refugia, where the separated populations spend several generations evolving independently. Genetic diversity, on the other hand, is a more general metric of variation within species. It is based on counting how many different versions of a gene—think blood types in humans—occur within an individual or a population. In a population with very low genetic diversity individuals would be like clones, almost genetically identical. Most natural populations have significant genetic diversity, but the amount of diversity varies among populations and species. Higher genetic diversity generally means greater potential for adapting to changing environments.
These research projects began in the Arboretum’s living collections. I visited the Arboretum’s chestnut and ash trees last spring, checking the winter buds and waiting for leaves to emerge. Most of the chestnuts and ashes at the Arboretum have been collected from wild populations. Some werecollected close by, like an American chestnut from Petersham, Massachusetts (24-80*A); others originated on the distant edges of the species’ ranges, like a chinquapin (Castanea pumila) collected in Arkansas (21486*A). I sampled young leaves (often the best for DNA extraction) from all of these trees in early spring. To draw a broader picture for each species, I made several field excursions to collect leaves from populations throughout each species’ range, visiting sites in Arkansas, Mississippi, Florida, South Carolina, Georgia, North Carolina, and Virginia. After extracting DNA from all of the leaves I amassed, I should obtain the first genetic sequences within the next few months.
Essentially, I’m looking for insights into both the past and future of these species. Genetic variation in a species carries clues of past events that can be disentangled to reconstruct aspects of its history. In this way, genes can show us how natural forces have caused adaptation, changes in population size, migration, or even hybridization. This project will also provide important information for future conservation efforts by identifying populations that are particularly distinctive or genetically diverse. Often, conservation strategies that protect the broadest range of genetic variation within a species will give that species the best chance to adapt to current threats and future climatic changes. This kind of genetic information can also be useful for horticultural breeding programs that aim to produce individuals adapted to a specific environment or with particular characteristics, like disease resistance.