2 February 2011

Foliage vs. Geology: Plants on volcanoes

Posted by Jessica Ball

In my office we have a collection of various plants – mostly ones that aren’t easily killable, like succulents. I recently added a Selaginella lepidophylla (more commonly known as a “Dinosaur plant”), which can be de- and re-hydrated without significant harm. But examining its growing “kit” got me thinking, since they packaged it with “real volcano rock from New Mexico” – otherwise known as scoria from some cinder cone, possibly a New Mexican one. Are those rocks really the plant’s ideal substrate? What plants really do like to live on volcanoes – and what kinds have I come across?

Plant types on volcanoes could be divided into two rough categories: rock and soil dwellers. A rock dweller might thrive on a young or an old volcano, but plants that need soil are only going to do well on an older volcano (which has had time to form soil), or a tropical one (where soil formation is accelerated because of the climate). Volcanic rocks are often (but not always) full of vesicles, which makes them porous and ideal for retaining water. Volcanic soils are generally referred to as andisols, which are soils that form on volcanic ash and contain volcanic glass and compounds of elements such as Fe, Al and Si. (Ions of those elements, which are released by leaching and weathering of volcanic rock, can form complexes with organic matter; in addition to an andisol’s ability to retain water, this can make for very fertile growing conditions.) Andisols in the US are, not surprisingly, concentrated in the Cascades and northern California – where we have large stratovolcanoes and an excellent supply of volcanic ash.

It turns out, according to one gardening website, that my Selaginella likes “dry places on rocky soil or on limestone talus” and thrives at “900–2000 m; N.Mex., Tex.; Mexico”. So it’s a good candidate for a plant that will hang out on volcanic rocks. I can’t say that I’ve ever encountered it before, but here are a few examples of plants I have seen on volcanoes:

Coffee plants with the Santiaguito lava dome (Santa Maria volcano, Guatemala)

Coffee (Coffea arabica): Coffee grows extraordinarily well in my field area – in fact, if there weren’t a lava dome at Santa Maria, coffee cultivation would probably go all the way to the base of the volcano. Coffee plants like wet conditions, which means volcanic soils (which retain water) are ideal; they do best at somewhat high altitudes (1000-2000 meters) and not-too-hot temperatures (20 C or so). I’ve also seen a few mentions of the plants preferring lower pH soils, which is also common in volcanic environments. This makes the highlands of Central America ideal, and during my time visiting the Santiaguito Volcano Observatory, I saw quite a bit (in fact, the Observatory is located in a coffee plantation, or finca).

Wine amphorae in the Roman town of Herculaneum, near Mount Vesuvius, Italy.

Grapevines (Vitis): Anyone who’s ever had a bottle of Lacryma Christi del Vesuvio would be hard put to miss the connection between volcanoes and wine grapes. The Geological Society of America’s meeting in Portland in 2009 was called “From Volcanoes to Vineyards” with good reason – the volcanic soil of the Cascades is a major factor in the thriving vineyards there. Italian wines are also often grown on volcanic soils, like those surrounding Mount Vesuvius; one depiction of the Roman god of wine (Bacchus) has him standing next to a stylized volcano. Grapevines need soil which retains water (to an extent) but doesn’t get too soggy; calcium, iron, magnesium, potassium, and nitrates/phosphates for plant health and photosynthesis; and slightly acidic soil pH (but not too acidic, or you get sour grapes).

Mauna Kea silversword

Silversword (Argyroxiphium): This is a very rare plant, found only in Hawaii, and it’s pretty impressive when you do get to see it. There are several varieties, which are found on separate volcanoes; the one on the left is a Mauna Kea silversword (Argyroxiphium sandwicense ssp. sandwicense). Silverswords are found at high (> 2000 m) elevations in either boggy soil or volcanic scoria; they do well in very low-nutrient soils, which are common on lava flows that haven’t had the time (or the correct climate) to weather into richer soil. This makes the high “alpine desert” slopes of Mauna Loa, Mauna Kea and Haleakala ideal, although the plants are in danger of extinction (from being eaten by animals, having their roots crushed, or just from lack of other plants for pollination). And, as you can see in the photo, they are indeed very silvery.

The author sheltering under a "poor man's umbrella" on Volcan Poas, Costa Rica

“Poor man’s umbrella” (Gunnera insignis): This is a pretty impressive plant that I’ve seen in Central America (Costa Rica and Guatemala). It’s commonly called “giant rhubarb” (not the edible kind) or “poor man’s umbrella” (for obvious reasons). It grows well in cool cloud forests, which are common on stratovolcanoes (and domes) in Central and South America. Orographic lift causes cool, moist air to collect around the high-elevation volcanoes, producing the humid but coolish conditions that this plant prefers. As you can see, it makes an excellent emergency umbrella (or sun parasol, as the situation demands).

Moss on the slopes of the Santiaguito lava dome complex

Moss (division Bryophyta): Okay, it’s not as exotic or tasty or pretty as the other plants I’ve mentioned, but because it doesn’t require soil, moss is one of the first colonizers (“pioneer species”) in many volcanic settings. Mosses prefer moist, slightly acid conditions, so vesicular, porous volcanic rocks are ideal substrates; on volcanoes that are constantly surrounded by clouds and don’t get a lot of sunlight (that orographic lifting again), moss is extremely common. (They’re also really common in Iceland, which is completely volcanic; in fact, mosses are sometimes the only plants on Icelandic tundra.) The moss that I’ve seen in my field area is also very common on lava dome rock which has been coated in volcanic ash; the ash seems to retain enough water to keep the moss happy, and the moss helps stabilize the ash and start forming soil by breaking it down. (It actually makes the domes much easier to navigate, since it forms a solid surface over the jumbled lava flows – otherwise I would have spent much more time scrambling over rocks than I did.)

Plant life in volcanic settings can vary from a few species to a few thousand; and although I (and other geologists) like to claim that plants get in the way of the rocks, they’re still worthy of attention. Looking at the plants in a geologic setting can give you basic information about soil conditions, rock types, and climatic conditions, all of which can be significant in a geologic study. (Those plants can also be handy before and after the study as well; starting the day with some coffee and ending it with a glass of wine is my idea of good field methods!)