Climate drivers of tree fall on the streets of Sao Paulo, Brazil

Key message Street trees are vulnerable to extreme climate conditions such as precipitation, wind gust and high temperatures. But trees may also fail in the cities as a result of poor stewardship. Urban trees are renowned for improving the livability in the cities through their ecosystem services. Long-lasting services, however, are only possible if local responsible authorities adequately manage the trees. Wood decay, inappropriate pruning, presence of xylophagous organisms, trunk constriction by the sidewalks, altogether increase the probability of trees to fall under adverse climate conditions. Tree fall is closely related to disservices like damages to the infrastructure, risks of injury and death. Given the previously reported poor conditions of the street trees in Sao Paulo/Brazil, one of the world's largest cities, we assessed the main climate causes of tree fall. We evaluated a 3-year daily series of fallen trees in the city. The effects of precipitation, wind gust and temperature were evaluated using cross-wavelet analyses. Out of the 652,000 street trees of Sao Paulo, 7034 fell in the city during this period. Cross-wavelet analyses revealed that tree fall is seasonal and in phase with, and driven by, precipitation, wind gust and temperature during the wet season. While temperature directly influences tree fall, both precipitation and wind gust may show lagged effects. Such associations with climate were not observed during the dry season despite the records of fallen trees. Falling trees were observed in two-thirds of the days during the dry season. Although it could be partially caused by lagged effects of precipitation and wind gust, these records during the dry season confirm the poor stewardship and conditions of the street trees in the city. This study highlights how vulnerable street trees are to extreme climate conditions and that yet poorly managed trees fall on the streets daily without any apparent climate causes. (AU)