If epiphytes were people, we would call them clever strategists. I guess this holds true for any organism that manages to occupy a hostile corner, but I am fascinated by life forms that grow on toxic substrates. In this context, epiphytes (or air plants) are the pioneers that prepare the terrain for the arrival of other organisms.
Encyclopædia Britannica (2016) defines epiphytes as ‘any plant that grows upon another plant or object merely for physical support’. They harvest water and some nutrients from mist and rain and gain other nutrients from debris that falls upon them. Because of that, epiphytes are able to establish themselves in poor soils and on toxic ground, as their roots or rhizoids act mainly as anchors.
Pioneers that are interesting for mine sites are largely lichens and mosses (bryophytes). Lichens are quite ‘special’ and often misunderstood: They are not plants, but a symbiotic relationship between a fungus and something that provides food (a source of carbon) through photosynthesis, a type of algae or cyanobacteria, or even both. Fungi provide the outer appearance of lichens and host their ‘carbon factory’ within their structure. Lichens also gain some of their nutrients by feeding off the substrate they are attached to, through excreting acids that dissolve the minerals they need to take up.
Fascinating as definitions may be, let us explore what lichens and mosses do for mining waste. Both can recover from prolonged desiccation and use their entire surface area to take on moisture and nutrients. In addition, lichens ‘liberate’ essential minerals and trace elements that are locked up in rocks. In this way, lichens are among the first (with bacteria) to colonise bare rock and occupy this challenging habitat. This happens in all sorts of places, on freshly cooled lava, boulders by the sea, roof slates, concrete walls, bricks and the broken and milled rocks that make up mine waste.
As with all organisms, lichens have a finite life span and when they die, their remains, organic matter, will be broken down by micro-organisms, such as bacteria and fungi. You could call it ‘composting’, but that’s a little far fetched. But it is enough to provide a substrate that can be utilised by tough pioneer plants, such as mosses. In time, through several cycles of life and death, enough organic matter may accumulate on mine waste to provide some nutrients and hold enough moisture for higher plants, such as certain grasses, to get a foothold…and natural succession has started.
What makes this process special are the particular characteristics of many metal mine waste heaps:
- lose substrate, prone to erosion and into which it is difficult to anchor roots
- steep slope angles and exposure to the elements
- sandy or silty texture with poor water retention
- low nutrient concentrations and nutrient retention
- low content of organic matter
- high acidity
- high concentrations of potentially toxic elements, such as arsenic, cadmium and lead
I have written more about challenging ‘soils’ here.
Cocking, D (no date) Bryophytes. Biology Encyclopedia. Advameg, Inc. http://www.biologyreference.com/Bl-Ce/Bryophytes.html [accessed 26/03/2016]
Encyclopædia Britannica (2016) Epiphyte plant type. http://www.britannica.com/plant/epiphyte [accessed 26/03/2016]
Stewart RJ (2013) Devon Great Consols. A Mine of Mines. The Trevithick Society, Camborne. [link]
University of Sydney (2004) Lichen Biology. http://bugs.bio.usyd.edu.au/learning/resources/Mycology/Plant_Interactions/Lichen/lichenBiology.shtml [accessed 26/03/2016]