Uncovering underground Botany

Hidden in the shadows, plants have survived catastrophic deforestation by taking refuge in caves.

Prehistoric – and modern-day – humans and human-like groups have exploited caves (“natural underground chambers in a hillside or cliff”) for millennia, whether for protection, as a space for creating art, or, simply because they offer a degree of shelter from the everyday, and are somewhere to call ‘home’. Some plants seemingly also ‘choose’ to live in these out-of-the-way geological excavations. And, since caves are often in places where access is difficult, plant communities in such places can remain relatively unknown and undisturbed for millennia, and even longer – until inquisitive botanists decide to investigate.

Yangzi cave, Guangxi, China.
Yangzi cave, Guangxi, China. Image Monro et al. 2018.

Although defined by their “saturated humidity, buffered temperature and … absolute darkness deep within caves”, there is “often a unique ecosystem that develops in the very limited zone in and around the entrance”. In some respects this near-entrance, light-permissive – albeit light-restricted – region is akin to the so-called Goldilock’s Zone (the region in which conditions are ‘just right’ for life to exist) of those who seek out Earth-like planets beyond our own orb. However, unlike the astronomical equivalent, that ‘photosynthetic plant-permissive’ part of a cave is zoned, as Alexandre Monro et al. have discovered in the limestone caves of SW China, with three growing zones of decreasing light levels: entrance, twilight and dark.

Importantly, they find that the light environment in those caves is distinct from that of terrestrial habitats, and that some of the 418 species of plants they found there probably grow in the lowest light levels documented for vascular plants*. Whilst 93% of the species documented were known karst forest species, 7% were endemic (“native and restricted to a certain place”) to caves, and 81% of the species were angiosperms (flowering plants).

OK, but apart from cataloguing the flora of a slightly unusual habitat, is there real relevance to this study? Yes. Monro et al. propose that the cave flora is an extension of the karst forest understorey that was present in the area prior to catastrophic deforestation in the 20th century. They therefore suggest that such caves serve as both refuges and a valuable source of germplasm (“living tissue from which new plants can be grown”) for restoration of the karst forest.

Furthermore, they also suggest that the caves represent a distinct habitat for plants, which, although similar to that of the forest understorey, is distinguished therefrom “with respect to the absence of trees, leaf litter, root mats, higher levels of atmospheric CO2, and lower diurnal and annual variation in temperature and humidity”. Somewhat disappointingly, they also highlight tourism (which – and somewhat ironically – might increase now that this flora has been recognised, documented and made known to the world..?), agriculture and the absence of legislated protection of caves as the main current threats to this flora. But, maybe, the cave flora is sufficiently out of sight that it’s also ‘out of mind’, and can survive the ‘attentions’ of mankind for a long time to come.

By way of a distraction from that potentially pessimistic prediction, we have a challenge for you. Knowing how much we like names and new words at Cuttings HQ, we have a question: If cave-dwelling animals are termed troglodytes, should the equivalent plants be called troglophytes? [Ed. – Hmmm, maybe. However, it should be pointed out that there appears to be a perfectly acceptable existing name for such plants: Sciophytes]. Finally, and in the time-honoured tradition of suggesting appropriate item(s) based on the reader’s browsing habits, our unpatented ‘Cuttingsbot’ suggests you might also be interested in the item entitled “New plants for a dimmer future?”.

* Which surely makes at least some of these plants worthy of the designation of ‘phytophotoextremophiles’.

Reference List

Shipton, C., Roberts, P., Archer, W., Armitage, S. J., Bita, C., Blinkhorn, J., … Boivin, N. (2018). 78,000-year-old record of Middle and Later Stone Age innovation in an East African tropical forest. Nature Communications, 9(1). https://doi.org/10.1038/s41467-018-04057-3

Tena, G. (2018). Alone in the dark. Nature Plants, 4(3), 132–132. https://doi.org/10.1038/s41477-018-0122-0

Monro, A. K., Bystriakova, N., Fu, L., Wen, F., & Wei, Y. (2018). Discovery of a diverse cave flora in China. PLOS ONE, 13(2), e0190801. https://doi.org/10.1371/journal.pone.0190801