Arboriculture & Urban Forestry 44(2): March 2018 ‘Weeds’ Versus Urban Spontaneous Vegetation The classification 'weed' is a cultural term, not a botanical one (Del Tredici 2014). A plant’s status can change over time; valued as an ornamental at one point in time but downgraded to a ‘weed’ at a later time. A classic example is Ailanthus altissi- ma or Tree-of-Heaven. Introduced to the U.S. by William Hamilton in 1784, the tree “supplanted” classic trees, such as horsechestnuts (Aesculus hip- pocastanum) and lindens (Tilia spp.), to become a “metropolitan favorite,” only to be “denounced and scorned as a vile, malodorous foreigner, per- haps even poisonous, sickening the vulnerable with its loathsome emanations” (Jonnes 2016). One of the primary limitations of the term 'weed' is that it is shorthand to mean an undesirable or unwanted plant; the term, however, obscures the particularities of place and time, as the evolution in the perception of Ailanthus altissima makes clear. Classifying a plant as a ‘weed’ does not provide any context-specific information. The public talks about weeding many different types of spaces, but not only are the actual plants different, but so are their effects. As Del Tredici (2014) notes, context matters in another way: it informs one’s aesthetic preference such that a ‘weed’ in an urban vacant lot is a wildflower in a rural meadow in Europe. Fur- thermore, dismissing plants as ‘weeds’ obfuscates the fact that human forms of land management cre- ate the conditions under which such plants grow. The spontaneous occurrence of vegetation on sites disturbed by natural-occurring phenomena, such as fires and windstorms, has been studied extensively in forest science. Spontaneous vegeta- tion on the types of disturbed landscapes associ- ated with urbanization began receiving serious scholarly scrutiny in German cities aſter World War II. In particular, studies of the vegetation colonizing rubble sites in post-war German cities were among the earliest strands of the field of urban ecology, and the publications produced by academics legitimized this urban flora (Lach- mund 2003). However, the elevated status of this vegetation type in the post-war period in Ger- many did not occur simultaneously in the U.S. or elsewhere. The perception of urban spontaneous vegetation is changing in response to both non-institutional studies (collegial citizen sci- 117 ence) and professionalized science (Lachmund 2003; Crimmins and Crimmins 2008; Burk- holder 2012; Del Tredici 2014; Rega-Brodsky and Nilon 2016; Rega-Brodsky and Nilon 2017). There have been academic studies of the lev- els of acceptance of floristic messiness in conven- tional landscapes (Nassauer 1995). However, the landscape types that are the focus of this study are not commonly understood as conventional land- scapes. De-industrialized lots, abandoned resi- dential parcels, un-stewarded street tree pits, and unmaintained pavements are landscapes where intentional, direct human management is absent (Del Tredici 2010a; Del Tredici 2014). The socio- ecological characteristics of these landscapes make the spontaneous vegetation growing there distinct from other forms of plant communities in cities and have catalyzed a different classification to indicate context-specificity of this urban flora (Lachmund 2003; Burkholder 2012; Del Tredici 2014). The new terminology exists in two forms: 1) spontaneous urban vegetation, and 2) urban spontaneous veg- etation. (Spontaneous vegetation is a variant of the latter and is assumed to be synonymous with non-native plants that spontaneously occur in cit- ies.) Urban spontaneous vegetation will be used in this paper. Urban refers to the fact that this palette of plants grows in urbanized environments. They establish and thrive in actualized niches created by human manipulation of the land. The spontaneous vegetation segment of the new terminology per- forms double work. It refers to the fact that these landscapes are not explicitly tended by humans in the same way that gardens and parks are, but also refers to typical plant behavior. Like their desirable counterparts, the establishment of 'weeds' is part of a natural process of succession in the same way that pioneer species regenerate aſter a forest fire, or when black birch (Betula lenta) takes advan- tage of sun access in a hemlock (Conium spp.) canopy (Karel et al. 2001; Swanson et al. 2010). Models of Public Participation in Scientific Research Public participation in scientific research (PPSR) is an umbrella term for scientific projects in which the public participates to varying degrees. That is, models of PPSR are defined by the degree of pub- lic participation in the research process (Shirk et ©2018 International Society of Arboriculture
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