How geraniums are sending a climate change distress signal to the Cape

Geraniums in the Western Cape are blooming nearly 12 days earlier than they did a century ago due to climate change.

The plants — more correctly known as pelargoniums — are at the centre of a new data-gathering and number-crunching exercise which combed through 108 years of records from weather stations and herbariums.

The conclusions, just published in the journal Climate Change Ecology, are that a temperature increase more than three times the global average is the reason pelargoniums flowered 11.6 days earlier in 2009 than they did in 1901.

American ecologists responsible for the study, which detected an average temperature increase of 2.9°C since 1850, said it was a grim warning about the future impact of higher temperatures in the Cape Floristic Region.

“Climate change projections [for the region] predict a 1-3°C warmer climate by 2100,” said Tanisha Williams and colleagues from the department of ecology and evolutionary biology at the University of Connecticut.

“Projections focused on the fynbos biomes ... showed that if temperatures warm 2.3°C above pre-industrial levels by 2050, habitat loss could reach 51-65% [and] 23% of the species in this region are projected to go extinct.

“Not only is the faster rate of temperature increase a cause for alarm, the temperature levels needed to invoke such catastrophic impacts to this biodiversity hotspot are similar to the temperature changes found in this study.”

Williams' team set out to use phenology — the timing of seasonal events such as flowering, fruiting, insect emergence and seasonal migration in animals — to investigate the impact of climate change.

To do so, they combined records from more than 6,200 herbarium specimens of pelargoniums with historical temperature data from more than 460 weather stations in the 91,000km² Cape region.

Their findings — in one of the few studies of its type in the southern hemisphere — echoed similar studies in North America and Europe, and they said they had been particularly keen to investigate one of the world's five Mediterranean climates.

“All of the Mediterranean regions are classified as biodiversity hotspots, those with high levels of biodiversity, endemism and habitat destruction. These ecosystems may be among the most greatly affected due to anthropogenic climate change,” they said.

“Climate change projections in these regions paint a grim picture of temperature increases, rainfall reductions, and increased fire frequencies and intensities.

“These environmental changes coupled with other threats, such as urban development and invasive species, will exacerbate droughts, water availability, growing-season lengths, plant activity and ecosystem productivity, and impact food and other ecological systems.”

Williams said pelargoniums' earlier flowering could also pose a threat to the plants' pollinators, which are bees and long-proboscid hovering flies.

“Pollinator populations may be impacted by the advancement in flowering times if the pollinators’ phenology does not respond or respond in the same direction as pelargoniums,” said the study.

“Insect declines will have devastating impacts on plant-pollinator interactions, and could cause a 'cascade' of negative impacts ... ultimately destroying ecosystem function and services.”

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