Los Angeles Tops List for Most Tree Diversity

Los Angeles has the most tree diversity, according to tree survey data from 20 cities that was analyzed by a team of researchers led by a University of California, Riverside, landscape ecologist.

By combining datasets from 20 cities in the United States and Canada, the research addresses a little-studied area – how cities compare to one another in terms of plant species diversity and what drives variations in urban biodiversity across the United States.

Among the findings (the city name is followed by the number of species statistically adjusted to make up for variation in sample sizes):

  • Four California cities top the list: Los Angeles (104), Santa Barbara (85), Irvine (77) and San Francisco (77).
  • Position of larger cities in the United States varies greatly, from Washington D.C. (72) to Phoenix (52) to Chicago (51) to Boston (31).
  • Canadian cities appear on each extreme. Toronto, Ontario (67) and London, Ontario (57) appeared near the top. Edmonton (26) and Kelowna, British Columbia (20) were the lowest ranked.

The pattern the researchers found mirrors the general trend in natural ecosystems (with more species in the warm tropics) but the underlying mechanisms are quite different.

In warm cities, such as Los Angeles, tree diversity is dominated by species imported from other regions and continents. In cold cities, such as Minneapolis, the urban forest is dominated by native species.

Similarly, plant traits that are attractive to urban residents such as large, ornate flowers were also strongly driven by temperature, with such flowers of tropical origin dominating warm cities.

In the paper, the researchers propose a new hypotheses – the climate tolerance and trait choice hypothesis – that relates to how urban forests form.

The new hypothesis combines restrictions imposed by climate on what trees are planted (tropical plants can’t survive outdoors in Minneapolis) with the ability of local land management officials to engage in activities such as irrigation (regular watering of tropical plants in relatively dry cities such as Los Angeles).

“If you add water in LA, you can pretty much grow anything,” said Darrel Jenerette, an associate professor of landscape ecology at UC Riverside.

The hypothesis helps resolve inconsistencies in two urban ecology hypotheses. The first, “biome matching,” predicts a high concentration of species that are native or from a biome similar to that city’s geographical location. The second, “urban homogenization,” predicts that, because of globalization, urban communities include less variation in species than their surrounding native communities.

All the city tree surveys were carried out between 2000 and 2010. The 20 cities span a climate gradient from cold and wet (Minneapolis) to cold and dry (Kelowna, British Columbia) to hot and wet (Tampa) to hot and dry (Phoenix).

The number of trees sampled varied from 131 in Riverside, California, to 4,000 in Tampa, Florida. In total, nearly 26,000 trees from 416 species were identified.

The following is a full list of the cities analyzed and the number of tree species, adjusted for sample size: Los Angeles (103), Santa Barbara, California (85), Irvine, California (77), San Francisco (77), Washington, D.C. (72), Toronto, Ontario (67), London, Ontario (57), Philadelphia (56), Roanoke, Virginia (54), Baltimore (53), Syracuse, New York (52), Phoenix (52), Chicago (51), Minneapolis (51), Tampa, Florida (47), Riverside, California (46), Raleigh, North Carolina (44), Boston (31), Edmonton, Alberta (26) and Kelowna, British Columbia (20).

In the future, the researchers are planning to take a closer look at the Los Angeles tree data to get a better understanding of what makes the city the most diverse for trees. They also hope to examine other groups of plants, including annuals, to see how they compare to trees.

The paper, “Climate tolerances and trait choices shape continental patterns of urban tree biodiversity,” was published in the journal Global Ecology and Biogeography. In addition to Jenerette, the authors are Lorraine W. Clarke, Meghan L. Avolio, Diane E. Pataki, Thomas W. Gillespie, Stephanie Pincetl, Dave J. Nowak, Lucy R. Hutyra, Melissa McHale, Joseph P. McFadden and Michael Alonzo.

The work stems from a National Science Foundation grant that Jenerette, Pataki, Pincetl and Gillespie received to study the biogeography of the urban forest of Los Angeles.

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