Category Archives: limestone karst

Exploring Brazilian karst in Tocantins

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Karst in Aurora do Tocantins, central Brazil comprising highly dissected outcrops with open vegetation, and collapsed forested margins. Credits, Alex Monro.

I have been interested in karst since a child. My Grandparents lived at the foot of a large outcrop in France where I spent much of my holidays exploring for insects, rocks and plants. Through my work on nettles, a group of plants very diverse on karst, I have become to think of karst as a forgotten domain for plants. One that has received little focus from botanists. Karst is weathered limestone, a rock produced from the shells of organisms that lived hundreds of millions of years ago. It is soluble in water and the action of rain in warm climates results can in a dramatic landscape of razor-sharp rocks, deep chasms and caves. Its porous nature, slow accumulation of soil and chemical composition makes it a challenging habitat for plants that can require significant adaptations that frequently results in a very specialised flora. Because of its porous nature karst is also a major source of underground water, on which ca 25% of the world’s population depend.

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Forested on collapsed karst in Tocantins with several outcrops visible in the distance. Credits, Alex Monro

Having explored karst in China for several years, where it is associated with a high diversity of rare species, I was very curious to discover how this compared with South America, whose karst has been little studied. To do so, we sourced plant collections data for Brazil, the country in South America whose karst has been best mapped and collected, and the we compared the number of species and their rarity to Brazil as a whole. The result was a publication [https://doi.org/10.1038/s41598-019-53104-6] which shows that, whilst an important source of plant diversity and rare species, in Brazil it is less so than the surrounding vegetation in which it is located. This suggests that there are some fundamental differences between the karst floras of South America and Southeast Asia and prompts several hypotheses to be tested. Before doing so, however, I thought that it would make sense to spend some time collecting in one of the least known but most dramatic karst landscapes in Brazil, in Tocantins state. These are formed from 600 million year old deposits exposed ca 120 million years ago, early in the evolution of flowering plants.

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Some of the plants collected in Tocantins. L-R, T-B: An unidentified and possibly new species of Apodanthera in the cucumber family; an unusual Aristolochia species in the birthwort family; a possible new species of Acanthostachys bromeliad. Credits, Alex Monro

Together with Pablo Hendrigo, Gabriel Marcuso and Julio Lombardi of the State University of Sao Paulo we spent ten days in the field. The fieldwork was great fun, with lots of climbing and regular botanical surprises. It was, however, very hot on the exposed outcrops and the rock itself weathered into very thin serrated leaves, blocks balanced on each other, with narrow chasms between them which made clambering across a nerve-wracking and exhilarating experience. We observed that the core of an outcrop often comprised an open cactus and bromeliad dominated desert-like vegetation, whilst the collapsed edges were forested. These deciduous forests were botanically the more interesting in that they were more diverse, and each had understory species unique to it. For example, the Acanthostachys and Aristolochia species above. The open vegetation of the outcrops shared most species with each other, most of which showed adaptations to the very dry and exposed conditions. Small piles of animal droppings indicated the presence of a large rodent, that we regularly glimpsed clambering over rocks. After a rainfall many fast-moving black and yellow millipedes and snails appeared. The snails seemingly grazing the lichen covered rock. In the time that we spent there we managed to explore a small handful of outcrops and I was left feeling that we had only scratched the surface.

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L-R, Gabriel Marcuso, Pablo Hendrigo and Prof. Julio Lombardi. Credits, Alex Monro

Lost world discovered in twilight of Chinese caves

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Elatostema obscurinervium growing in stalagmite drip zones, one of 31 species known only from caves in China and Vietnam.

I never expected to be working in caves, that is, until I started to study nettles in Southwest China with Guangxi botanist Wei Yi-Gang. When Wei suggested that we collect in a local cave I was not hopeful. However, that first visit transported me to another world, an eerie moonscape in which plants thrived in powdery ‘soil’ and perpetual twilight. I was immediately gripped and determined to explore as many caves as possible. Over the next few years we visited over 60 caves, travelling on underground rivers, hiking across rice fields or sneaking into big tourist caves. Each time I got the same thrill from entering these strange and silent places. This has culminated in a paper just published in the scientific journal PLOS ONE.

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Yangtse cave, Fengshan County, Guangxi. Eight species new to science have been described from here.

There has been very little research on plants in caves. Having come across such a widespread and diverse flora we wanted to make sure that both botanists and cave biologists were aware of it. This is because plants growing in caves could tell us a lot about how plants adapt to extreme conditions. Also, as to how connected caves are to the surrounding landscape and each other. To do so we set out to answer a series of questions. Probably the most obvious was, are plants growing in light levels distinct to those outside? Did the species evolve in caves? Or are they survivors of a previous vegetation that at some time connected caves? And, are these plants important for conservation in the region? For example, in the restoration of the karst forests lost during the 20thC.

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Cave converted to a tourist attraction.

There are thousands of caves in Southwest China and the landscape that they helped form has long been famous in Chinese culture and art. A landscape covered in dense forest until the 20thC at which time the ‘Great leap forward’ and later the ‘Cultural revolution’ resulted in massive deforestation. Once cut, the forest has not returned but instead been replaced dry scrub. The 21stC has brought new threats from cement and tourism. The rock that forms these caves is used to make cement, now in high demand, that combined with a rapid growth in tourism has meant once untouched ‘lost worlds’ are being mined or filled with walkways, bright lights and litter.

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Collaborators from the Guangxi Institute of Botany, Fu Longfei and Chen Xiaoqin taking photosynthesis measurements.

We found that some of the plants growing in caves are surviving in very low levels of light. Levels much lower than previously known. From other cave studies we also know that daily changes in temperature and humidity are very small. There is also some suggestion from other studies that the cave atmosphere may be richer in CO2. It could be that low levels of light are being offset by more stable conditions and higher CO2 – an interesting question for future research.

We also found that whilst most of the plants we document are known from forest habitats elsewhere, 31 species are only known from caves. For reasons given in our paper we do not believe that this means that they evolved in caves but rather that they are species which were restricted to the forests lost in the 20thC.

So, it looks as if the plants that we are finding in caves are relics of forests that were lost to deforestation in the 1950s and 1970s. With a current focus on restoring these lost forests in China, these surviving populations could now become a valuable source of local plant material for restoration.

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Yin Jia cave close to Gu Lin village in Yunnan. At 1600 m above sea-level this is also one of highest elevations that we have collected in caves.

Other posts by me on this topic

2016: Studying cave-plants in SW China

2014: Caves explored last month