Abstract
A random sample of more than 500 streetside plantings of London plane, Plantanus x acerifolia and Norway maple, Acer plantanoides, were removed over a ten year period 1975-1985. London plane trees, attaining an average site of 39.03 years, were removed due to sidewalk damage, while dead Norway maples were removed after an average of 48.13 years (P<0.05).
It is well documented that in an urban setting, stress can weaken a tree to the point where normally ‘minor urban factors’ such as pests, diseases, or other environmental factors may become extremely deadly (5). It is for these reasons that studies on urban tree success and longevity are necessary.
The city of Jersey City is located just west of Manhattan in Hudson County, New Jersey. Jersey City has employed a full time forester and staff since the 1950’s. As early as 1960, policy was set that plantings would exceed removals of street and park trees by a margin of 2:1.
Tree removals in Jersey City have never been fully examined. London plane, Plantanus x acerifolia, and Norway maples, Acer platanoides, comprise nearly 55% of Jersey City’s urban forest and are the main focus of this study. Data were available as to planting and removal dates (longevity), and reasons for removal. This allows for comparisons to be made, thereby examining the effects of urban forestry decisions generations old.
Materials and Methods
Data were collected from three sources. The city has records of Shade Tree activity dating back to the turn of the century, but complete records only date back to 1960. It was determined that the ten year period 1975-1985 would be closely examined, as outside removal contracts and recorded citizen complaints would augment the historical data. These three sources, in combination, make the comparisions possible.
Once a removal location (address) was identified from the various contracts, complaint records were examined to verify species and reason for removal. These reasons were grouped into four categories: dead, sidewalk upheaval, request, and other. The last two categories contain such removals as those politically requested, invasive roots, traffic accidents, etc. A random sample of 270 trees/species was selected from the original removal data collected to study removal categories, while a randomly selected subsample of 90 trees/species was taken for longevity comparisons. Site age (longevity) of the trees involved was determined through available planting and removal dates previously recorded for each location.
Results
As seen in Table 1, significant removals of London plane occurred due to sidewalk upheaval (P<0.05). These sidewalk conditions had a major impact on the site age data for this species, which ranged from a low of 35.33 years to a high of 42.00 years. Norway maples, however, were removed significantly more often due to death of the tree (P<0.05). Norway maples “outlived” London planes by nearly ten years, ranging from 46.63 years to 49.57 years in site age.
Discussion
An informal 1973 inventory of Jersey City’s urban forest revealed that London plane far outnumbered Norway maple and other species not included in this study. At the time of this study, London plane continued to be the dominant street tree species in Jersey City. London plane has been heralded as a species which could grow anywhere and withstand rather adverse conditions
(1). Being much more tolerant of anthracnose Gnomoniaiplatani) than Americansycamore (4), London plane more often falls victimto mechanical damage (3) and rooting problems around curbs and sidewalks, especially in older communities (2,4).
Historically, tee removals in Jersey City were confined to dead or dangerous trees as strong neighborhood preservation groups attempted to stow the spread of urban blight. As tthe liability insurance crisis broadened, however, more citizens and their local representatives requested if not demanded the removal on the basis of side-walk upheaval. As shown in this study, ttwo thirds of those requests centered about a 35 to 40 year old London plane. These removals were mot due solely to insurance concerns, but also to the perceived usefulness of the tree, whefter by the city, the local politician, or the homeowner (6).
Compounding this ‘root problem’ is the test that Jersey City is known for an abundance of narrow, one-way streets affording little room for streetside parking, let alone streetside planting. It its not uncommon for the total distance between curb and house front to be fifteen feet or less. Therefore, it was extremely difficult to use the term “tree lawn’ to describe the two to three foot square openings cut through four inch thick concrete sidewalks that support the majority of streetside plantings. Recent species selection for new plantings and stricter planting site approval regulations reflect this severe space restriction.
This study also revealed that Norway maple tends to remain ‘on-site’ until it succumbs to the rigors of the urban environment, attaining a site age ten years greater than that of London plane. Despite the recent bad press Norway maple has been receiving, these findings indicate that, even with overly restrictive growing conditions, potential girdling root problems, and heavy pressure from pedestrian and vehicular traffic, this species remains a viable streetside selection for highly urbanized areas. Although no historical data or statistical evidence are available through this research, recent removals in Jersey city have shown that (dead Norway maples average 18 inch dbh, while London planes average 24 inch dbh. This size difference alone can be translated into a direct savings for the contracting authority. Future studies should be done to verify these observations, seeking establish acorrelation between species, diameter, storage, and removal costs in a highly urbanized environment.
Conclusion
London planes outgrew their available space and perceived usefulness necessitating their removal on the basis of sidewalk upheavals approximately 40 years after they were planted along the streets of Jersey City. In contrast, the majority of Norway maples remained on site until they succumbed to urban stress and died after a span of nearly 50 years. This difference alone provides information vital to the proper selection of species suitable for urban plantings, pertinent to problems which arise when certain considerations are ignored during the selection process.
- © 1991, International Society of Arboriculture. All rights reserved.