Applying a classification approach to categorizing urbanized landscapes in California and their invasion by the Maltese starthistle Centaurea melitensis

There is increasing evidence that the traits of organisms can differ in urban environments, but defining what makes an environment urban is difficult. There are many variables associated with increasing human impacts that might be associated with urbanization, and this makes it challenging to identify both how traits vary across diverse urbanized landscapes and the variables that might drive that variation. To better define the mosaic of urban environmental heterogeneity and identify similar types of environments that are comparable across its complexity, here we develop a multivariate landscape classification framework and apply it to classification of land area in the state of California, USA. We used a hierarchical cluster analysis to group 7,829 government census tracts into Environmental Zones based on a set of 19 independent environmental characteristics, including climate, land cover, pollution, and socio-economic variables. This analysis identified nine major Environmental Zones, which were differentiated based upon complex combinations of variables that did not align with conventional urban vs. natural dichotomies or gradients. Environmental Zones also occurred as mosaics of many zones within cities and differed in their relative abundance between cities, reflecting complex urban landscapes unique to each area. We then asked if these Environmental Zones were better able to explain trait variation than conventional urban vs. non-urban classification using a case study of the invasive, annual plant Centaurea melitensis, commonly found throughout much of California. Seeds from seventeen populations of C. melitensis were collected from six Environmental Zones, including two heavily urbanized, and four more natural/agricultural. Seeds were grown in greenhouse conditions, and eight vegetative traits were measured. No trait differed significantly between urban and non-urban sites, but four traits differed according to Environmental Types (length of longest leaf, SLA, root diameter, and the number of flowerheads). Traits differed between the heavily urbanized zones, as well as among the relatively more natural zones. Our results reveal that more complex multivariate classifications of the urban mosaic can identify similar, comparable environments across complex landscapes and better explain trait variation in organisms navigating urbanized environments.