The impact of climate-change induced temperature increases on the suitability of street tree species in California (USA) cities

Climate change, resulting in increased temperatures, has the potential to alter the species composition of urban tree populations. We examined the likely changes in composition of common street tree species in California (USA), using a space-for-time substitution approach which paired each of the 16 cities covering the climatic range of the state (“representative city”) 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 82 species may be unsuitable for the future warmer climate. 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 ocean coast. In contrast, assessing climate suitability of tree species from published sources (professional judgement, resulting in implied climate envelopes), reduces the number of unsuitable species to as few as 14, although a hybrid approach that accounts for the substantial gaps in the climate envelope information results in an estimated 55 unsuitable tree species. The difference between the observed and estimated results (82 unsuitable species vs. 55) 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. Our results demonstrate the viability of the space-for-time substitution approach 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.