Lightning, biodiversity, global change and Smarties in the mountains – say what?

By Paul Gordijn and Professor Tim O’Connor, SAEON

With intrigue, researchers have noted the co-occurrence of grasslands and areas with frequent lightning storms.

Lightning has played a key role in the evolution of these grassland ecosystems specifically, by influencing how often and when fires move through landscape.

Fire limits tree growth; without it, grasslands in tropical and subtropical regions would transform into shrublands and forest.

Lightning has played a key role in the evolution of our grassland ecosystems by influencing how often, when and how fires move through landscape (Photo: Sue Janse van Rensburg)

Before humans became dominant drivers in the grassland landscape, fire regimes were driven by patterns in lightning ignition. Although we may predict that a lightning-driven fire regime may be more variable than a rigorous anthropogenic or human-driven management regime, the particulars of historic lightning-driven fire regimes remain elusive.

What’s with the fascination in historic fire regimes anyway?

Well, the reasoning goes that since grassland flora and fauna have evolved under these ‘natural’ fire regimes, if we were able to reproduce a ‘natural’ regime, ecosystems would flourish.

Perhaps we may expect optimal functioning of ecosystems and maximal diversity in these grasslands? Maybe under these regimes, grasslands would be best suited to the extremes of climate change associated with global change? These are important questions to ask under current global change pressures and also realising that grasslands are one of the most transformed biomes on earth.

Drakensberg grasslands

Of southern African grasslands, Drakensberg grasslands stand out for their high levels of endemism and for being a biodiversity hotspot (that is, the area hosts numerous species and many of them are found nowhere else), plus the important ecosystem services they provide. Also, these grasslands are known as the ‘water tower’ of southern Africa, for from these mountains our great rivers such as the Orange and uThugela find their source.

In these grasslands, charcoal deposits and palynological (i.e. ‘pollen archaeology’) evidence from cores suggest that with the arrival of farmers and pastoralists, fire has intensified over the last thousand years or so. In particular, fire use may be expected to have become more finely managed or rigorous over the last century as settlement in the region increased.

The contrast of the presumed ‘natural’ fire regime versus the recent rigorous fire regimes begs a question of the effect of different fire regimes on these grasslands. If we were able to answer this question we are on track to promoting ecosystem functioning and biodiversity in the face of global change.

Cathedral Peak sentinel site

The SAEON Cathedral Peak research platform provides a unique opportunity to assess grassland vegetation change over time in response to different fire regimes or treatments. The platform is a world-class research site with a long history of research.

Importantly, the fire treatments at the Cathedral Peak research catchments have been running for over half a century. The fire treatments include both frequent (burnt every one to three years) and infrequent burn regimes (burnt every five to eight years).

We sampled an historic grass species monitoring project in the research catchments. We were confident that after at least two decades of fire treatments, an impact on grass communities would be clear (Gordijn et al., 2018). To our surprise, the influence of the fire regime on vegetation was only evident after more than half a century of fire treatments. This is a very interesting find, specifically in the face of global change.

Comparison with other nearby savanna ecosystems highlights these grasslands’ unique resistance. In nearby savannas, within a decade, vegetation composition has changed dramatically, with bush encroachment.

What about changes in trends in species abundance across all the fire treatments?

If global change was driving some of these, we may expect to find a few grass species to win and others to lose across the fire treatments?

Well, over more than three decades, the largest change observed across the fire treatments was less than six percent. This is a really small change and illustrates the ‘resistance’ of these grasslands to global climate change (at least so far).

So, the grasses may be ‘resistant’ to change, but what about the other flowering plants in the grass layer that make up 78 percent of the species richness of the area?

The answer to this question is not clear-cut - some comparisons with the work of other ecologists on the isolated indigenous forest patches in the Drakensberg are useful (Adie et al., 2017). These forest patches are restricted to the cooler slopes or aspects of this montane environment and to areas protected from fire (for example, rivers, cliffs and rocky outcrops).

‘Life is like a box of Smarties’

Since we’re thinking about forests, we’ll take a lead from the movie Forrest Gump, ‘Life is like a box of Smarties’. Imagine a box of Smarties being shared out in a family where each different colour Smartie represents a different species. Now, Dad shares out a strict three Smarties to each family member paying no attention to who gets what colour, being distracted by the Soccer finals.

Grassland fauna and flora have evolved under ‘natural’ fire regimes (Photo: Paul Gordijn)

In this round, everyone’s small handful only represents a small subset of all the different colours in the Smartie box. Next round, little Jordan the sweet-tooth gets hold of the Smartie box. With eagerness Jordan hands out the Smarties until the box is empty and everyone’s hands are filled with differently coloured Smarties. Jordan’s preference for different colours (and her bias for who should receive these) has affected who has what colours. So now even though everyone can say, ‘what a lot I got’ - everyone has a different set of coloured Smarties.

The forest patches are like the first round where everyone only had a few differently coloured Smarties due to Dad’s diligence, also in this process no one ended up with a unique set of Smartie colours. In contrast, grassland species assemblages are like Jordan’s eager dishing out of Smarties – everyone had many different colours (or species), plus, Jordan’s colour bias led to everyone having unique sets or assemblages of species.

In our grassland study, fire acted similarly to Jordan’s preferences for colours. That is, after more than half a century of different fire treatment applications, we found that the legacy of these treatments was important in shaping unique species assemblages. So, although these grasslands species assemblages were initially resistant to fire, over a long time period variation in the fire regime influenced the uniqueness of species assemblages over the landscape.

So how does this relate to interest in historic, variable lightning-driven fire regimes?

Well, this relation between unique species assemblages and variation in the fire regime is something that one would expect if species were adapted to a variable fire regime. This fits well with the hypothesis that the historic fire regime was variable, as one may expect a lightning-driven fire regime to be.

What’s more is that the highest number of species was found in areas with intermediate fire regimes. This provides further evidence that grassland flora is well adapted to a variable fire regime.

Does this mean that we should stop burning our grasslands and let lightning do the job?

The short answer is no, because we cannot impose a ‘natural’ fire regime on a human-dominated landscape - this has been tried before.

What are the take homes then for managing our grasslands in the face of global change? Well, yes, these grasslands appear to be resistant to rapid changes under global change and variable fire regimes. However, in the long term we would need to promote a variable fire regime over the landscape to maintain diversity.

Apart from influencing fire regimes, humans have also altered grazing regimes and are transforming the ‘natural’ functioning of these grasslands that are important reservoirs of biodiversity which provide our dams with clean water. So, what about the impacts of different grazing practices and other land uses (associated with different types of land tenure or ownership) around Cathedral Peak?

That’s for another time and another box of chocolates. For now, we appreciate the sheer biodiversity and associated clean water provided from the uKhahlamba Drakensberg mountains and, importantly, our collective responsibility in retaining the observed resistance, for our own good.

SAEON’s responsibility in the future is to continue observation programmes in these grasslands and evaluate response strategies to projected change.

Acknowledgements

SAEON's Grasslands-Forests-Wetlands Node would like to thank its host organisation, Ezemvelo KwaZulu-Natal Wildlife, for allowing node scientists to do this research in the Drakensberg and for providing excellent support.

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