Climate case study: Invasive plants in FDR Park

Context: FDR Park is a sprawling park located at the southernmost point of South Philly, not far from where the Schuylkill River joins the Delaware River. I visited it for the first time on my first pandemic birthday, and have since become a member and a volunteer. As I’ve pulled weeds and picked trash, I’ve become closely acquainted with the impacts of climate change on this vital public space. At the same time, I’ve become aware of how much I don’t know about the causes and consequences of the impacts I’ve observed. This post is more about questions than answers, but there may be a follow-up post as I learn more.

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It was a bright, sunny, but soggy morning in the Meadows when we tackled the multiflora roses. This opportunistic plant–some would say invasive plant–has long, creeping vines covered in fine prickles which snagged my sleeves as I ducked beneath the tree they were choking. The tree was some kind of elm, judging by the yellowing ovoid leaves–although it was hard to distinguish tree branches from the rose vines that twined up and curtained down from them. The ground was drenched after heavy autumn rains, so it was easy to uproot slender new rose vines as I pushed my way toward the elm’s trunk. Above me, more nimble volunteers clipped multiflora roses from the elm’s lower branches and sawed at the places where the scrambling vines had twisted and hardened into obstinate cords. I traced the path of a thick, brittle braid of vine and found that its ascent had caused one northbound elm branch to grow south, strangling its own trunk.

It took our volunteer team all morning to release the elm from the climbing multiflora roses. When we finished, we stood back and blinked at the blue sky suddenly visible through the tree branches. The discarded vines stacked waist-high around us were destined for compost.

This tree was just one trouble spot in the sprawling Meadows, park acreage that had once been a tidal marsh surrounding a river island, then filled in and landscaped by the same firm that created Central Park, then converted to a golf course, then abandoned. Annexed by FDR Park, the Meadows have been rewilding for two years: the water traps have grown over with algae, lilies, cattails, and phragmite, and much of the manicured green has gone shaggy with nettles and tall grasses. It is not a completely untended area: the park staff cuts trails through the dense flowering shrubs and mows grass under the maples so visitors can picnic on the former putting green. But in this once over-managed land, it’s easy for opportunistic plants to exploit the recovering ecosystem. Hence the volunteer team of Weed Warriors. Hence the occasional bright patches of poison beneath the trees.

The terms “native” and “invasive” are current terms of art in conservation policy and law, due in part to a 1999 executive order intended to curb the spread and negative impacts of invasive species. But identifying and removing invasive plants is not a straightforward matter. Even in my brief introduction to the work, I’ve encountered some points of debate.

What even is an invasive species? What defines a native species?

It depends on who you ask. According to the U.S. Fish and Wildlife Service, native plants are those that historically occur in a particular habitat. In the United States, some agricultural guides specify that “historically” means “prior to European colonization.” When I sat in on an Master Watershed Steward infosession recently, the coordinator described native plants as those that grew without human intervention.

The definition of invasive species also varies, but generally the term is not applied to all nonnative plants–just those that cause harm to the new ecosystem they find themselves in. Harm may look like the fast-growing multiflora roses strangling a massive elm as they climb it. Harm may also look like the non-native porcelain berry and mile-a-minute weed blanketing fields and even trees in the Meadows, the way kudzu does in the south, outcompeting everything and preventing other species from getting a foothold. Harm may also entail unbalancing the ecosystem in some way–eroding the soil, draining the soil of nutrients, displacing plants that attract pollinators. In an indigenous ecosystem, plants and animals have evolved together, adapted to the climate and terrain, and developed checks and balances that maintain a viable, sustainable food web; one way or another, invasive species disrupt that web.

As a Tree Tender–a volunteer trained by Pennsylvania Horticultural Society to plant and care for street trees in Philadelphia’s urban forest–I was taught a taxonomy of native, exotic, and invasive trees. The distinction can be very fine! Red maples are native, naturally occurring in the Philadelphia area. Norway maples are considered invasive, meaning that they are not naturally occurring and that they are likely to outcompete native maples due to their shade tolerance. Japanese maples, a very desirable sidewalk tree due to its small size and beautiful foliage, is considered exotic. Exotic is a term for trees which aren’t native to the habitat, but which can grow and reproduce in the environment without overtaking native species or causing harm.

A diverse urban forest can be a healthy urban forest. Native trees, in theory, are the best suited for the ecosystem and the most attractive to local wildlife. Exotic trees that become naturalized (i.e. able to thrive and reproduce on their own) may improve the biodiversity and overall health of an ecosystem, since they may be resistant to the impact of species-specific pests (such as the emerald ash borer) and disease (such as needle rust). Philadelphia’s urban forest is 46% nonnative trees; the nonnative tree population includes our friendly ornamental cherry and lilac trees as well as disruptive species like tree-of-heaven (favorite snack of the loathsome spotted lanternfly) and the shallow-rooted paper mulberry that sprouts everywhere, all over the city.

I find this taxonomy serviceable enough when choosing trees for a city sidewalk or backyard. In a green space–especially one like FDR Park, where the native marsh ecosystem has been remade more than once over the centuries–the relationships among plants might be more complex.

How do you decide if a plant is invasive or exotic?

I look forward to discussing this further with experts–but to me, it seems that the categorization is inextricably bound up with other human values: what we want to eat, what we can use, what we need.

The rose-laden elm I described above shares a hillock with leggy blackberry plants that would probably also have liked to climb the tree, if only the multiflora roses hadn’t gotten there first. The blackberry plants–at least, this particular species–are not native to the ecosystem, and we volunteers took no great care to preserve the blackberry vines. But we didn’t uproot them either: the vines didn’t seem to be in danger of overtaking or killing their neighbors, and perhaps their fruit would prove attractive to human and animal visitors.

On that same hillock, there were also two honeysuckle trees that the park manager would have liked to keep, except that they were slowly being throttled by a Virginia creeper vine whose broad palmate leaves had turned a cheerful autumn red. Virginia creeper is native to the region; honeysuckle tree is not, but trees arguably have more to offer a city park ecosystem than a climbing vine. Trees sequester carbon, provide shade, and counterbalance urban heat islands. Trees attract birds, and FDR Park is a designated Important Bird Area. Trees are picturesque and we like to look at them. And vines kill trees: when I cut away the native creeper vines and pruned the honeysuckle tree, its strangled branches were as dry and brittle as cardboard.

There are words we can use instead of invasive when the harm is clearly defined: plants like mile-a-minute weed, multiflora roses, and tree-of-heaven can be called noxious or nuisance species because they kill other plants and/or attract other nuisance species such as spotted lanternflies. Other plants don’t seem to warrant such strong terms–like mugwort, a tender green plant that blankets the ground beneath some trees in the park and lines a few of the overgrown trails. Mugwort is not native and is likely taking the place of some regional ground-dwelling herb; volunteers have been taught to harvest the leaves for tea and incense, snipping the top of the plant before it can flower and seed. If mugwort has the capacity for greater harm, I don’t know it.

As far as I can tell, there’s no way to predict whether a nonnative species will naturalize and become part of a flourishing ecosystem or if they will overtake native plants. Norway maples were planted by John Bartram in the 18th century, in part to replace shade trees lost to Dutch Elm disease; if he had known they would outcompete local sugar maples and red maples, would he still have planted them?

What about climate change?

That is the big question! As the global average temperature rises and causes a cascade of other environmental impacts to ecosystems, like too much or not enough water in different regions, some species find their native habitats too changed to thrive. As the wonderful book Hurricane Lizards and Plastic Squid: The Fraught and Fascinating Biology of Climate Change by Thor Hanson explains, species threatened by climate change must move, adapt, or die.

Last fall I read an article at Vox that recommended moving away from the term invasive, given that we are going to see more and more species migrating to find new habitats. We’re used to nonnative species being introduced to ecosystems through direct human intervention–like Bartram and his horticultural mission, or the accidental import of the spotted lanternfly via global trade. Is it fair to call a butterfly shrimp invasive when it wanders out of its historic habitat to follow its migrating food?

Before the pandemic, I experienced as much plant blindness as the average city dweller probably does–I enjoyed looking at plants but rarely knew what I was looking at, especially since the species that surround me in Philadelphia differ from the ones I learned growing up in Memphis. As I learn to identify plant species and understand how they thrive or fail to flourish together in my local ecosystem, I am in that stage of education where I mostly know what I don’t know. This post is a my way of putting a pin in these questions–to return to when I’ve learned more.

Sources

Bolotnikova, Marina. “It’s time to stop demonizing ‘invasive’ species.” Vox. November 28, 2021. https://www.vox.com/down-to-earth/22796160/invasive-species-climate-change-range-shifting

“Frequently Asked Question About Invasive Species.” U.S. Fish and Wildlife Service. November 20, 2012. 
https://www.fws.gov/invasives/faq.html

Pearce, Reagan. “How Relevant is the ‘Native vs Invasive’ Argument in a Warming World?” Earth.org. August 18, 2021. https://earth.org/how-relevant-is-the-native-vs-invasive-argument-in-warming-world/

“The Urban Forest of Philadelphia.” United States Department of Agriculture. November 2016. https://www.itreetools.org/documents/337/PhiladelphiaUrbanForest.pdf

“Norway Maple : History in North America.” Center for Teaching and Learning, University of Vermont.
http://libraryexhibits.uvm.edu/omeka/exhibits/show/uvmtrees/norway-maple-introduction/norway-maple-history