This is documented in a paper by Jennifer Fitchett and Kestrel Raik, of the School of Geography, Archaeology and Environmental Studies at the University of the Witwatersrand.
In the 1920s and 1930s, jacaranda trees only started to bloom in mid-November, wrote Fitchett in a recent column in The Conversation. But gradually, over the decades, the date of bloom has shifted to October and then the early weeks of September.
This is referred to as a phenological shift and is being observed across a range of species globally. It is closely linked to changes in climate, with individual species responding to specific combinations of increases in temperature, changes in the timing and amount of rainfall, changes in frost occurrence, humidity and sunshine hours, the researchers note in their paper, which was published in the journal, Urban Forestry & Urban Greening.
The researchers describe how the cities in Gauteng host one of the world’s largest and most densely vegetated urban forests, with the City of Johannesburg alone recording more than 10-million trees.
Pretoria is colloquially called the Jacaranda City, with an estimated 33 630 jacarandas. The trees are classified as an alien invasive species because of their water consumption in the semi-arid region. Deliberate replanting is forbidden but existing trees are not removed.
“The spring blossoms transform the city-region into a purple vista, and remain a popular attraction for locals and tourists alike,” according to the paper. “The trees and their blossoms therefore fall into an unusual category of being very popular and a flagship species of a region due to their aesthetics, but because of conservation laws cannot be replanted. This heightens the concerns among the residents and visitors regarding the health and longevity of the trees and their flowers, as once the existing trees have died, they will not be replaced.”
Over the past century, the flowering seasons for jacarandas have been reported in local newspapers, and more recently documented by the public through social media platforms Instagram, Flickr, Facebook and Twitter. The researchers trawled these records, comparing the flowering dates to meteorological data from across the province.
“Analysing these reports of Jacaranda mimosifolia flowering, an advance of 2.1 days per decade is calculated for the period 1927–2019,” the paper says.
This advance unfolded against a backdrop of warming temperatures, ranging from 0.1°C to 0.2°C a decade for daily maximum temperatures and 0.2°C to 0.4°C a decade for daily minimum temperatures. This was driven by winter climatic conditions, of which maximum temperatures in June are the most marked.
“This advance reflects the response of the tree to regional climate warming, which poses threats to the species and the urban forest in the long term when thresholds for adaptation are surpassed,” according to the paper.
The advance in the flowering timing is consistent with global phenological trends, yet provides a “concerning outlook” for this species, the researchers warn.
Jacarandas comprise a key component of the extensive urban forest, which contributes to carbon sequestration and storage, and are part of the “aesthetic identity” of the cities, which has both intrinsic and economic value as a tourist attraction.
Although phenological shifts represent an adaptation in plants and animals, these advances in flowering dates cannot continue indefinitely, the researchers say, adding: “At a critical threshold, the flowering season will become unsuccessful.”
The phenological changes may be indicative of a gradual strain on this species, and replanting with indigenous trees is advised, they say.