I wrote my previous post whilst on an EasyJet flight to Malta and, yes, I did feel pangs of guilt writing about climate change at the same time that I was adding significantly to my carbon footprint. That’s one of the great conundrums of my life: on the one hand, I need a global (or, at the very least, a European) perspective in order to do my job. Yet, in gaining that perspective I become part of the problem.
A pond at Argotti Botanic Gardens, Valletta, Malta, April 2015.
So here I am in Valletta, capital of Malta, a country whose properties as summarised by Luke in the Acts of the Apostles still remain true (ch. 28 v. 2: “… the natives showed us unusual kindness …”). And, Lo and behold, I find myself at the University of Malta’s Argotti Botanic gardens, (which may well be coming to a blog near you before too long) where there is a pond into which I stare. And in that pond I see a mass of vegetation that, from a distance, resembles duckweed in appearance. However, in close-up they resolve into a series of tiny overlapping leaves with, hanging beneath them, a bunch of roots, a couple of centimetres long. Most of the leaves are green but several are reddish in colour. This is Azolla, which is, despite appearances, an aquatic fern, very common in the warmer parts of the world. It contains a symbiotic nitrogen-fixing Cyanobacterium (blue-green alga) which means that, like terrestrial legumes, it can grow in situations where most other aquatic plants cannot thrive. There has been a lot of research over the years into the potential for encouraging Azolla to grow as a “green manure” in paddy fields.
Azolla sp. in a pond at Argotti botanic gardens. There are a few leaves of Lemna in the foreground to give an indication of scale.
A little further on, I saw another pond whose surface was covered by another aquatic fern, Salvinia. This is larger than Azolla and lacks a nitrogen-fixing partner, but it is also very common in warmer parts of the world. Both grow by extending and branching their underwater stems and adding new leaves in such a way that they can rapidly cover large areas of a pond’s surface if conditions are favourable.
Salvinia sp in a pond at Argotti botanic gardens.
And there is evidence that conditions are getting increasingly favourable for both Azolla and Salvinia. Azolla filiculoides is not native to the UK but was introduced during the 19th century and, for a long time, the scattered records were confined to the south and midlands. The latest distribution maps, however, suggest that it is now widespread in these areas and has also been recorded from the north of England and Scotland.
Salvinia has not been recorded from the UK but there is an interesting study from Poland where this species, too, has been introduced and is on the increase. There is good evidence from northern Poland of warming associated with the North Atlantic Oscillation. This allows spores of Salvinia to germinate earlier in the year and, crucially, fewer frosts in the Spring means that the delicate young stages are more likely to survive. The fronds are also able to continue to grow – and spread – later in year too. All these factors combine to help this alien species extend it’s distribution not just in Poland but elsewhere in northern and Central Europe where it has not previously been seen.
There lies the irony. Plants such as Azolla and Salvinia are opportunists, adapted to spreading fast when conditions are favourable. You can find evidence for many other plants and animals whose distribution has changed in recent years. And, yes, I can’t avoid the comparison with the opportunism of humans, offered a cheap flight for a brief escape from the chilly British spring. At times such as these it is better that I accept that I am not a dispassionate observer of nature and, in fact, a small part of the systems that I study.
Gałka, A. & Smezja, J. (2012). Phenology of the aquatic fern Salvinia natans (L.) All. in the Vistula delta in the context of climate warming. Limnologica 43: 100-105.