The ghost of fire regimes past (and future)

By Jasper Slingsby and Glenn Moncrieff, SAEON Fynbos Node Tweet

Fire is a key disturbance affecting the structure, composition and function of many ecosystems around the world, and is essential for the healthy maintenance of the Grassland, Savanna and Fynbos Biomes of South Africa.

Alteration of fire regimes can have profound effects on ecosystems, driving shifts in the dominance of species and growth forms, or in extreme cases, causing transitions between fire-tolerant (or dependent) and fire-sensitive biomes, such as between Fynbos or Savanna and Forest.

While humans have influenced fire regimes in South Africa for tens of millennia or more, population growth and the spread of settlements and agriculture has seen this influence grow exponentially over the past few centuries and decades. Humans have become the most common source of ignitions, our farms, roads and houses limit the spread of fire, and we invest great effort in suppressing fires when and where they threaten our lives or property.

Unfortunately, detecting, understanding and managing our influence on fire regimes and the subsequent impacts on our ecosystems is incredibly challenging. While modern satellites allow us to detect and map fires in near real time, most records of fire activity extend back only a few decades, limiting our ability to detect change.

A map of the count of fires detected by the MODIS mission satellites over South Africa for the period 1 November 2000 to 1 July 2019. Fire is common across all but the most arid or densely forested areas.

Even where we do have good records, by the time we can detect change in the fire regime the impacts on the ecosystem may have already occurred and may be irreversible, leaving few or no management options.

Using ignition catchments to predict change in the fire regime

Managing the impact our influence on fire has on ecosystems requires the tools not only to determine where we have altered fire in the past, but to predict where our ongoing activities are altering them currently and into the future. In short, long-term observation alone is not enough. We need conceptual, mathematical and/or statistical models that allow us to make defensible projections into the past and future. These are the issues that we begin to address with our collaborators in a study published this month.

We sought to identify factors that determine or alter the vulnerability of a site to fire a priori and use them to predict where we’d expect to see changes in the fire regime over time. To do this, we expanded on a concept proposed by van Wilgen et al (2014) to explain why fire is less frequent in areas adjacent to major rivers in the Kruger National Park. Namely, because they have ‘a smaller ignition catchment, as fires that originate on one side of a river rarely cross to the other side’.

While van Wilgen et al’s concept was purely spatial, we expanded this heuristic to include both the spatial extent and temporal range where an ignition is likely to result in a site burning. In other words, sites are more likely to burn where ignitions are more readily available and where any ignition across a large area would result in the site burning under the dominant or a broader range of weather conditions.

Conversely, sites are less likely to burn when they are surrounded by barriers to fire such as rivers and other non-flammable boundaries, or if they experience few ignitions or only experience ignitions at times when fire is unlikely to spread.

Now consider how natural ignition catchments may be altered by human activities that change the frequency, timing and location of ignition sources or that affect fire spread through changes in land use and land cover. If one compared a landscape with and without these human influences, you’d rapidly be able to infer where the fire regime may be affected.

Take this one step further and compare the results of fire spread models run for the natural and human-altered landscapes, and you should be able to predict the change in fire regime with a high degree of accuracy. This is exactly what we did, using the Cape Peninsula as a case study.

Most Fynbos species are adapted to tolerate or even depend on fire. Here a recently burnt Protea species has released its seeds from the old inflorescences or “cones” in which they hold their seeds to protect them from fire. Without fire, recruitment is rare, and adult plants begin to senesce, causing a crash in the population and allowing forest or alien species to invade.

Anthropogenic fire shadows and the spread of forest on the Cape Peninsula

The Peninsula provided the perfect location to test our ideas, because there is a 60-odd year record of fire activity and urban expansion. There’s also a record of the spread of fire-sensitive forest into fire-dependent Fynbos over the same period, providing us with a benchmark to validate our predictions.

We parameterised our fire spread models with the record of weather conditions (temperature, humidity, wind strength and direction) experienced during observed fires, topography (slope, aspect) and an existing Fynbos “fuel model” that describes the various parameters of vegetation that determine flammability and fire spread.

Running the models for many iterations allowed us to generate a map of the probability of fire occurrence (or burn probability) for each scenario. Comparing the two revealed the change in burn probability due to human influence. Finally, we used this map of the change in burn probability to predict the observed fire record and where forest has expanded into fynbos. It worked!!!

While our models weren’t perfect, and could do with many refinements, that we were able to predict the fire record and change in forest extent with some confidence shows that the ignition catchment concept is a useful heuristic for predicting human influence on fire regimes past and future. With improved models this approach could be used to inform fire and vegetation management in the face of the many global drivers of change, from direct human influence to invasive species (changes on fuel properties) or even climate change (e.g. changes in fire weather).

It is imperative that we improve our understanding of how and where we are altering fire regimes so that this insidious threat to biodiversity can be assessed and the potential impacts managed. Ideally, the ignition catchment framework would be used proactively, informing spatial land use decisions a priori to avert or minimise the impact of human activities on fire regimes and ecosystems.

Much of the forest behind Kirstenbosch would once have been Fynbos, but the spread of agriculture and later suburbia has altered the ignition catchment and excluded fire. One can still find ancient individuals of fire-dependent Fynbos species like kreupelhout (Leucospermum conocarpodendron) clinging on in the newly forested areas.

Literature cited

Slingsby, Jasper A., Glenn R. Moncrieff, Annabelle J. Rogers and Edmund C. February. 2020. Altered Ignition Catchments Threaten a Hyperdiverse Fire-Dependent Ecosystem. Global Change Biology 26 (2): 616–28. https://doi.org/10.1111/gcb.14861.    Request article 

Wilgen, Brian W. van, Navashni Govender, Izak P. J. Smit, and Sandra MacFadyen. 2014. The Ongoing Development of a Pragmatic and Adaptive Fire Management Policy in a Large African Savanna Protected Area. Journal of Environmental Management 132 (January): 358–68. https://doi.org/10.1016/j.jenvm an.2013.11.003