Anthony S. Aiello and Peter J. Zale seek native trees that can adapt to the shifting climate.
While plant collecting is often romanticized as occurring in pristine natural habitats, much of the most successful seed collecting is done in more prosaic locations. This is especially true when searching for tree species, where roadsides or power line rights-of-way provide light (for tree growth) and ease of access to fruiting branches (for collecting convenience). This certainly was the case in September 2020, as we searched for Quercus falcata (Spanish oak, or southern red oak) in southern Chester County (PA). After a fruitless morning at the Goat Hill Serpentine Barrens Preserve, we found much better success along local county roads.
Why Quercus falcata? Based on biological and climatic threats to tree species traditionally grown at Longwood Gardens, we have focused recent efforts on native species that combine ornamental traits with disease resistance and greater heat tolerance. For example, red oak (Quercus rubra), one of the more prominent shade trees at Longwood Gardens, in recent years has been among the most susceptible to bacterial leaf scorch (Xylella fastidiosa), which can weaken and eventually kill mature trees. As possible substitutes for this and other susceptible oaks, we targeted populations of Quercus phellos, Quercus michauxii, and Quercus falcata native to southeastern Pennsylvania. Maturing at approximately the same size as red oak, all three are potential substitutes as high-canopy, overstory shade trees. Each of these three species, which are widespread further south, reach the northern limits of their native ranges in southeastern Pennsylvania, southern New Jersey, and Long Island. Southern red oak barely extends its range into southeastern Pennsylvania, southern New Jersey, and possibly Staten Island and Long Island. In fact, due to its rarity in Pennsylvania (fewer than fifteen populations are known in the state), the Pennsylvania Natural Heritage Program lists it as a species of special concern (an S1, for those familiar with the coding).
Faced with travel restrictions due to the COVID-19 pandemic, starting in the late summer and fall of 2020, we decided to look for local opportunities for collecting seed, pursuing a range of regional collecting objectives that we had previously not achieved. This included targeting the southern ranges of northern species (such as Larix laricina) and, vice-versa, the northern limits of southern species (for example Quercus virginiana), with the goal of growing plants suitable for changing climatic conditions in the Delaware Valley. For those southern species that reach into the mid-Atlantic, the extreme populations might possess traits, including cold tolerance, that provide opportunities to grow these beyond their traditional horticultural ranges. Conversely, for the northern species, the southernmost populations could possess greater heat tolerance, allowing us to continue to grow these as their native populations retreat northwards in the face of warming climates.
In September 2020, we focused our collecting on nearby populations of Quercus falcata, which occur on serpentine soils and their associated barrens. (Serpentine barrens, with thin, nutrient-poor soils, support high levels of unusual, rare, or endangered species, in contrast to adjacent areas.) We had targeted Goat Hill based on recent herbarium records from this location, but we did not find any southern red oaks there. Collecting was easy along the county roads, however, where we made three separate collections from trees whose acorns were within reach of our pole pruners. These three populations, within two miles of each other, were made up of large mature trees that were at least 50 feet tall. Quercus falcata stand out among other oaks, having coarsely lobed leaves with sickle-shaped (falcate) terminal lobes, and dense grey down (pubescence) beneath. For two of these collections, the southern red oaks were mixed among other native trees species in a dense forest; the third location was a grove of separate mature trees growing in a heavily grazed cow pasture, all within sight of the Herr’s Snack Factory, a local landmark.
Seeking additional Pennsylvanian locations of Quercus falcata, we pored through herbarium collections shared through the Mid-Atlantic Herbaria Consortium. Historical records from the late 1800s through the mid-1900s show a distribution in southeast Pennsylvania along much of the Piedmont-Coastal Plain boundaries, including southwest Philadelphia. But due to urbanization of much of this historic range (which includes Philadelphia International Airport), herbarium collections since the late 1980s center on three areas: southern Chester County near the Maryland border, southern Montgomery County along Militia Hill in Fort Washington State Park, and southern Bucks County along the Delaware River.
While collecting southern red oak within a few square miles of West Nottingham Township (Chester County), we came across a remarkable diversity of eight oak species—in addition to Quercus falcata, we also encountered Q. alba, Q. ilicifolia, Q. marilandica, Q. prinoides, Q. rubra, Q. stellata, and Q. velutina. Though familiar with the local diversity of oaks in southeast Pennsylvania, we rarely see this number of species in a single day’s outing. Together, these represent a significant portion of the 11 oaks reported by Hugh Stone in his two-volume 1945 Flora of Chester County Pennsylvania, and nearly half of the approximately 20 oak species found in Pennsylvania. This wealth of oak species serves testament to the richness of the Chester County flora, historically the most botanically diverse in the state, though heavily impacted by human activities since the publication of Stone’s Flora.
We returned in 2021 to make duplicate collections from the easily accessible roadside trees found in 2020. Oaks are famous for having years of heavy acorn production (mast years) followed by years of lower production. In 2020, we experienced a postmast year when looking for Quercus phellos in southern Bucks County. Despite seeing a few dozen trees during a day in the field, we did not see a single acorn on any of these. Our luck was better with Quercus falcata: in 2020 we collected a total of approximately 250 acorns, and in 2021, just under 200 acorns.
As with seed collecting, patience is the main ingredient needed to grow oak seedlings. To germinate, acorns usually need a few months of cold treatment, followed by warm conditions and the increasing day lengths of spring. Ultimately the seedlings derived from these collections will be evaluated in our Research Nursery for growth rate and form, disease resistance, and fall color, before being introduced into public areas of Longwood Gardens.
Anthony S. Aiello and Peter J. Zale are Associate Directors for Conservation, Plant Breeding, and Collections at Longwood Gardens in Chester County, Pennsylvania.
Citation: Aiello, A. S. and Zale, P. J. Quest for southern red oak—north of the Mason-Dixon Line. Arnoldia 79 (2): 11–13.
From “free” to “friend”…
Established in 1911 as the Bulletin of Popular Information, Arnoldia has long been a definitive forum for conversations about temperate woody plants and their landscapes. In 2022, we rolled out a new vision for the magazine as a vigorous forum for tales of plant exploration, behind-the-scenes glimpses of botanical research, and deep dives into the history of gardens, landscapes, and science. The new Arnoldia includes poetry, visual art, and literary essays, following the human imagination wherever it entangles with trees.
It takes resources to gather and nurture these new voices, and we depend on the support of our member-subscribers to make it possible. But membership means more: by becoming a member of the Arnold Arboretum, you help to keep our collection vibrant and our research and educational mission active. Through the pages of Arnoldia, you can take part in the life of this free-to-all landscape whether you live next door or an ocean away.