Documenting effects of urbanization on flora using herbarium records

As human populations increasingly live in cities, urban floras and the ecosystem services they provide are under increasing threat. Understanding the effects of urbanization on plants can help to predict future changes and identify ways to preserve biological diversity. Relatively few studies document changes through time in the flora of a focal region and those that do primarily address European floras. They often rely on contemporary spatial gradient studies as surrogates for changes with time. We compare historical species records (prior to 1940) with the current flora for Marion County, Indiana, USA, home to Indianapolis, the 13th largest city in the United States. Specimens from the Friesner Herbarium of Butler University and other vouchered records for the county provided the basis for historical records. Current records are derived from inventories of 16 sites conducted by Herbarium staff and other botanists over the past 15 years. Physiognomic group, wetland classification and nativity (native vs. non-native) were determined for each species. Fidelity to high-quality habitat was quantified using coefficients of conservatism (C-values). The last 70 years have seen a significant turnover in species presence, most notably a decrease in native plant species number (2.4 per year) and quality, with an accompanying increase in non-native plants of 1.4 per year. Loss of species has been non-random, with a disproportionate number of high-quality wetland plants lost. The signature of past land use can be seen in physiognomic changes in the composition of the flora that reflect the shift from agriculture to urban/suburban land use. Many invasive non-native shrubs now present have escaped from cultivation, highlighting the combined threats of habitat conversion and human plant preference to native flora in cities. These invasives likely present the greatest threat to remaining biodiversity. Synthesis. This study demonstrates the value well-documented historical records, such as those housed in herbaria, can have in addressing current ecological issues