Declining fynbos diversity

Increasingly prolonged periods of severe hot and dry weather during the first summer after wild fires is inhibiting vegetation recovery and causing loss of plant diversity in the fynbos, according to a new study published in the Proceedings of the National Academy of Sciences this week.

The importance of fire in sustaining and enhancing biodiversity in fynbos and many other ecosystems has long been known, but the cycle of regeneration is being negatively impacted by climate change.

The 44-year study of permanently marked vegetation plots in the Cape of Good Hope section of Table Mountain National Park — one of the most botanically diverse regions of the world — was led by the South African Environmental Observation Network (SAEON) in collaboration with researchers from three South African universities and four US institutions.

They found that species diversity has declined due to increasingly prolonged spells of hot, dry weather in the first year after fire, impacting particularly those species that sprout after fire. Furthermore, the species unique to each survey have shown a 0.5°C increase in maximum temperature tolerance. Species that have low tolerance of high maximum temperatures have been disappearing, while those that have been colonising plots have a higher maximum temperature tolerance – suggesting that species are tracking shifting climate.

The importance of fire in sustaining and enhancing biodiversity in fynbos and many other ecosystems has long been known. This protea shows the effects of fire damage.

The observed increase in temperatures and extreme weather patterns over the past half-century suggests that opportunities for successful regeneration of vegetation will become increasingly rare.

While attributing biotic responses to climate change is typically very difficult, there is high confidence in this study’s findings, because it explored multiple potential drivers of change. Over and above the impact of climate, the study found a legacy impact of a history of invasion by alien plants, which were common in the reserve until the mid 1980s.

Climatic Russian roulette

“The increasing bottleneck that extreme weather places on the post-fire regeneration phase may compromise the stability of plant populations,” noted Jasper Slingsby, biodiversity scientist at SAEON and research associate at UCT’s Statistics in Ecology, Environment and Conservation group and Department of Biological Sciences.

“Climatic variability may continue to provide years sufficiently benign to allow successful recruitment, but many species in ecosystems that regenerate in the first year after a fire event – most species in our study – are subject to a form of climatic Russian roulette. Unfortunately, as climate change intensifies, there are fewer empty chambers in the gun.”

An Orange-breasted Sunbird perches on a blooming Staavia radiata.

The exacerbation of post-fire mortality by increasingly severe weather extremes is likely to drive major shifts in the composition, and hence, structure and function of fire-prone ecosystems, according to the researchers. This is a concern for dry-summer fire-prone ecosystems around the world, including much of the Mediterranean Basin, the fynbos and renosterveld of South Africa, South Western Australia, California, and forests along the Western United States. Most of these ecosystems are global biodiversity hotspots, and all have experienced large numbers of fires and extreme drought and heat in the past few years.

The study represents one of the few examples of climate-drive diversity loss in natural communities and demonstrates an important interaction between climate change and disturbance by fire that suggests flammable ecosystems may be particularly sensitive to climate change.

The only other empirical evidence of the impact of climate change on fynbos species was published by UCT authors recently.

Watch the video:

The impact of climate change on the fynbos in the Cape of Good Hope section of Table Mountain National Park.