Climate change will alter the species composition of urban tree populations through a combination of modified precipitation regimes and increased temperatures. Likely changes in the composition of common street tree species in California were examined using the ‘space-for- time’ substitution method. Sixteen cities covering the climatic range of the state (“representative city”) were paired with a “warm city” counterpart, where the climate of today approximates the climate of the representative city in 2099. Of the 140 tree species found, as many as 83 species may be unsuitable for the future warmer climate in specific cities. This change is geographically non-uniform, with greater losses (up to 100% of common species unsuitable for future climate) found in cities away from the coast. In contrast,applying the “climate envelope” approach, i.e., assessing climate suitability of tree species from published sources based on the geographic distribution of a species, reduces the number of unsuitable species to as few as 14, although a more rigorous “professional judgment” approach that accounts for the substantial gaps in the “climate envelope” information results in 56 unsuitable tree species. The difference between the observed and estimated results (83 unsuitable species vs. 56) is likely caused by the climate envelope information insufficiently accounting for the irrigation needs of newly-planted street trees in the Mediterranean-type climates like California. The results of this study demonstrate the viability of the ‘space-for- time’ substitution method for evaluating possible climate change effects on urban trees, and suggest both an immediate need to re-evaluate the planting palette of street trees, and a long-term imperative to trial new tree species.

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