A warming planet and changing rainfall is a significant challenge for agriculture. A new paper Succulent plant diversity as natural capital, by Olwen Grace, looks some plants that you’d expect to thrive in hotter times. A closer look shows it’s not that simple.
I asked Dr. Grace some questions to find out more.
I started by asking what a succulent is. I think I’d know one if I saw it, but that’s not good enough when you’re running a research programme. But even people who work with succulents have struggled, as Dr. Grace explained: “Experts have found it difficult to pin down exactly what is or isn’t, succulent. A very wide approach includes plants that have some mechanism to store water, but there is a fine line between those that store water long-term, and those that are fleshy but not able to remain independent of water from the environment for a period, which is what really defines a succulent. My research focuses on xeric (arid-adapted) succulents like cacti, which are the iconic examples of desert succulence.”
This is an approach that means some plants that I wouldn’t have expected are classed as succulents, like orchids and baobab trees. As succulents are defined by what they do instead of their family, it means there are plenty of species to find. Dr Grace said: “We know there are around 12,000 species but this number could be much larger because we’re not certain how many orchids are succulent, and Orchidaceae are one of the largest flowering plant families on Earth.”
Another feature of succulents is they occupy a surprising range of habitats. I’d thought of them in deserts, but there’s a lot more to succulent plants than this. Dr. Grace showed how diverse the group is. “Succulent plants occur in a wide range of environments because succulence has evolved as an adaptation to drought-prone habitats, and these are not limited to deserts. For instance, small alpine species might experience water shortages when water is frozen in the soil and becomes unavailable to the plant. Epiphytic orchids experience drought when moisture from the forest canopy doesn’t reach them. Others are orchids adapted to semi-arid and arid terrestrial habitats.”
Given that succulents are plants adapted for dry conditions, you might expect that more droughts would give them a competitive advantage. Dr. Grace mentions the danger that some species may become invasive in the future – but she also warns of extinction: “Plants that have evolved to occupy very specialised niches can tolerate only limited fluctuations in the environment in which they occur. They are usually quite particular about the soils they grow in, and are not adapted to disperse pollen or seed long distances to reach similar habitats far away. Hence, any disturbance to the finely tuned niche can be disastrous. If the species is able to undergo a range shift, cities and roads can get in the way. These are all typical of succulent desert species which tend to be localised and therefore vulnerable to extinction.”
Extinction isn’t a good thing, but is it going to matter if we lose some species? Dr. Grace says yes: “The value of wild diversity as genetic resources for the future is increasingly acknowledged, as we rely on a limited pool of plant species or crops for our needs (see the UN’s State of the World’s Biodiversity for Food and Agriculture report just published). If species that are adapted to thrive in hot, dry conditions go extinct, we are losing the potential to grow and use those resources in the future.”
This isn’t just about losing varieties of tequila. Dr. Grace thinks some of the most exciting applications of succulence are ones that don’t exist yet. “I am really excited about the innovations that plants use to store water under drought conditions. We are only just starting to understand the process, since we can now combine sophisticated high-resolution microscopy with -omics technologies to look at form, function and genetic regulation. Perhaps we will harness a biomimetic approach to water storage in future, inspired by succulents.”
One of the lines that struck me from the paper was “Non‐commercial horticulture has a more pivotal role in the ex situ conservation of succulent plant diversity”. This is in the context of botanical gardens, but I have a terrarium with succulents in. And they’re not all dying. Is there something the home gardener can do to help? I asked if I’m looking for something for a terrarium, are there things I should look for, or avoid when buying succulents? “The main thing to worry about is whether you are purchasing plants are grown from seed in a nursery, or harvested from the wild. If you’re looking at a large, mature specimen of a rare plant then you should be asking questions about its origin. The trade in many succulent plant species is regulated by CITES because they are threatened with extinction due to harvesting pressure (for the ornamental trade) or habitat loss (being cleared for new roads, towns etc) and the CBD requires that appropriate permissions and equitable benefits are in place before plants are harvested from the wild.”
If you’re looking to build a career in plant science, then you should take a look at succulents. Dr. Grace concluded: “It’s an exciting time to become a plant scientist because we are working against the clock to document and conserve plant diversity before it might go extinct. There are new tools and big data yielding exciting insights into the evolution of traits such as succulence, and with this information we can understand which traits might be most valuable for, say, crop breeding or other innovations in the future. Plant diversity is pivotal to tackling challenges facing humanity today – hunger, the changing environment, health, etc.”
You can hear a recent podcast featuring Dr. Olwen Grace from Linnean Learning at Soundcloud.
Grace, O. M. (2019). Succulent plant diversity as natural capital. Plants, People, Planet. https://doi.org/10.1002/ppp3.25