Redbud Woods in Context: The Urban Forest and Ecosystem Services

It doesn’t require close observation or astute ecological insight to determine that green space is a scarce resource in many cities. Ithaca is fortunate because it has a diverse and healthy array of green spaces that includes formal parks, street trees, remnants of primary forest in the gorges, athletic fields, residential landscapes, and successional communities occurring spontaneously on unmanaged fallow land. In cities, this last category includes vegetation on “vacant” lots. Characteristics common to all these urban vegetation types is that they are patchy and fragmented, have species compositions different from the pre-development condition and are embedded in a landscape matrix in which hard, impervious surfaces predominate. Because many ecosystem functions are “distributive”, that is, dependent on the spatial extent of and arrangement of the patches comprising the ecosystem, this combination of attributes has profound implications for how the biophysical features of urban ecosystems operate. Ecologists speak in terms of currencies that can be used to quantify fluxes of mass, energy and genetic information (arguably the most fundamental and inclusive term for organisms) through the systems they study. One of the most easily grasped currencies is water. We ignore the physics of water movement in human dominated landscapes at our peril, as is demonstrated repeatedly by flooding disasters and loss of topsoil.

It is obvious to anyone living in the Finger Lakes region for more than a few weeks that we typically receive abundant precipitation, much of which ends up as runoff due to our steep local topography and shallow soil. Observe the color of Cayuga Lake 24 hours after a storm if you need evidence of erosion and sediment loading resulting from runoff. An emerging, quite robust generality from many observational studies is that once 10% of a watershed is rendered impervious, the downstream hydrology is irreversibly affected: floods become more frequent and severe, base flows of streams are reduced, stream channels become destabilized and widen, and floodplain elevation rises. Although Ithaca is a small city with apparently abundant green spaces, its streams show many of these characteristics. Not all of these result from human construction; agriculture and logging play roles, too, which means that in our particular landscape setting, we need to be especially conscious of the amount of pavement and roof top we construct. I don’t know how close we are in Tompkins County to this 10% imperviousness threshold (probably quite far below it), but within the Ithaca City limits and on the Cornell campus, I would not be surprised if we had already crossed it. Destabilization of downtown stream channels is arguably moot because they have been straightened and armored, but storm runoff is responsible for sediment deposition in these channels (requiring periodic removal), basement flooding in the Fall Creek neighborhood and increased sediment loading in Cayuga Lake.

This brings us to Redbud Woods, which is part of a de facto greenbelt separating the Cornell campus “upstream” from the City of Ithaca “downstream”. Think for a moment: much of the water falling on the Arts and Ag quads and their adjacent academic buildings is “processed” by Redbud Woods, the cemetery and the old Treman estate. Much of it is “unprocessed” because it enters the sewer system and subsequently overloads the downtown treatment plant. There can be no denying that the amount of impervious surface on campus has increased steadily over the years and with the planned new construction, this trend is certain to continue. As skilled and thoughtful stewards of the campus and surrounding community, we should consider ways to buffer the hydrological effect that the campus is having on both the natural and social communities it borders. The greenbelt paralleling University Ave. provides some semblance of buffering because it permits water to infiltrate and be released gradually or be transpired by vegetation instead of running off, but in order to maintain some sort of baseline output, it would have to accommodate not only its share of runoff attenuation proportional to its area, but also compensate for the loss of infiltration capacity lost uphill ecosystem function cannot be regained by merely replacing one acre of, say, wetland with another located elsewhere, hence many ordinances call for a 2:1 or 3:1 mitigation. While this most often applies to wetlands, the concept pertains to ecosystem functionality in general. Given the strategic location of Redbud Woods, it makes most sense at least maintain it as pervious green space, if not making it hyperfunctional from a hydrological standpoint by adding retention ponds to capture accelerated runoff from campus. A quantitative hydrologic budget could be developed and used to design the way we wanted the campus to perform in a sustainable fashion.

I have roughly outlined only the hydrological services provided by the Woods because they are most readily apparent. Similar exercises could be performed for nutrient retention, carbon sequestration and atmospheric cooling. And these are just the physical attributes of the site, which operate somewhat independently of the floristic composition of the vegetation as long as the structure is the same. From this perspective, it may matter less that redbuds, not sugar maples, grow in Redbud Woods as long as it remains a wood. But the biotic features of the woods are interesting in their own right, as are the cascade of functional implications of changing floristic composition. Consider that a recent compilation by Peter Marks and others reveals that 40% of the Tompkins County flora is non-native. Similar findings for Baltimore, MD have come out of the Baltimore Ecosystem Study, one of NSF’s two Long Term Ecosystem Research sites specifically targeting cities. What are the effects of exotic species on total and native species diversity? How does shifting species composition affect biogeochemical cycling?

Questions like these are only now being are only now being resolved research topics by the scientific community.

The heuristic and pedagogical opportunities to study a university campus as an ecological unit in the landscape, with nearby witness sites representing communities affected by human activity to varying degrees, deserve special consideration given Cornell’s prominence in ecology. Isn’t it ironic that corporate headquarters for Ford, Nike, and 3M are at the forefront of sustainable building technologies? These are bottom line driven, zero sums corporations who find it to their benefit to apply the methods of industrial ecology to their physical plant as well as their manufacturing processes. Perhaps it is time for a non-zero sums enterprise like a university, with essentially unlimited intellectual capital, to follow this lead and learn something in the process.