Geology - Plant Relationships: Atlin Tufa

As a geologist, I am fascinated by geological influences on plant diversity and boundaries between species.

In this note, I describe some plants that grow on a calcareous substrate called tufa. This is not a comprehensive synthesis of plants in the area. The identity of all plants discussed has been confirmed by credible sources on iNaturalist (inaturalist.ca), where I posted all my observed plant species.

The tufa deposits are located in the Atlin area, northern British Columbia, Canada. Despite being located close to the 60th parallel, you may recognize some of the plants, because several also grow on calcareous substrates located much farther to the south.

You may be familiar with tufa because it is available at many garden centers and is commonly used in domestic rock gardens and as a specialized planter due to its porous and lime-rich character.

Atlin Context:

In 2019, we drove twelve days from Ottawa, Ontario, across Canada, through Yukon, to Atlin, located in northern British Columbia, to look at some examples where geology influences plant species distribution. Driving 12 days to Atlin may seem like a long way just to look at some geology and a few plants; however, the geology and plant diversity of Atlin area is fascinating. Atlin area has several types of groundwater springs, unusual rock types, and unusual habitats, including: a) serpentinite barrens and pine - lichen woodland (Photo 1); b) limestone barrens; c) dry lake basins, known as playa, that are filled with the mineral hydromagnesite [Mg5(CO3)4(OH)2·4H2O], which formed when lake evaporation exceeded recharge (Photo 2); d) tufa, a variety of limestone formed when carbonate minerals precipitate out of cool to warm spring water (Photo 3); and e) calcareous fens (Photo 4). The different habitats support different plant communities. Atlin’s spectacular, scenic landscapes, along with its unusual geological habitats and flowering plant communities, occur within the homeland of the Taku River Tlingit First Nation.

Once in Atlin, I faced a decision. Do I wade into a natural, warm water, spring-fed, calcareous pool (Photo 5) to get a closer look at bubbles breaking the water surface (Photo 6)? Or, do I listen to the voice inside my head yelling “Are you crazy? Those bubbles are warning that the bottom of the pool will open up and swallow you into the boiling bowls of the Earth!” Because I am sharing some observations, the Earth did not swallow me up and I conclude that my cautious voice overreacted yet again.

 Substrate - Plant Relationships:

Climate strongly influences distribution patterns of most, if not all, plants and other biota. Within a climatic region, plant distributions often reflect differences in the local geological history, topography, and substrate, which together create different habitats. That said, trying to identify specific substrate-plant relationships is challenging because geology is only one determinant.

In the Atlin tufa deposits, the composition of the tufa is one important factor that influences the types and distributions of plant species.

What Is Tufa?:  

Tufa is a terrestrial deposit of calcium carbonate that precipitated from calcium-enriched, spring waters, which flowed through limestone bedrock areas. Tufa is similar in composition to limestone rock; however, there is a big difference between the way tufa and limestone form. Limestone is a layered rock, which formed millions of years ago, from lime-rich muds and shells of dead marine animals that accumulated on the bottom a subtropical to tropical ocean. Conversely, tufa forms when calcium carbonate (CaCO3), generally in the form of the mineral calcite, precipitates from cold to warm, calcium-rich groundwater springs. Tufa (Photo 7): a) is generally not well layered; b) may contain ghostly casts of plants that grew where the spring discharged; and c) is generally very porous because many cast voids formed when trapped plants decayed. Globally, tufa forms in a variety of habitats, including: a) pools, creeks or rivers; b) lake bottoms or edges; and c) terraces and gentle slopes near spring discharge sites.

 Atlin Tufa:

The Atlin tufa is actively forming in an area where cool and warm calcium-rich springs discharge onto, and flow over, the land surface. In the rest of this note, I discuss three habitats and their associated plant species: a) pool-creek tufa habitat (Photo 8); b) sparsely vegetated, low-slope tufa habitat (Photo 3); and c) calcareous fen habitat (Photo 4).

Herbaceous vegetation related to pool-creek tufa deposits is best seen where a bubbling, warm pool (Photo 5, 6, and 8) became established at a warm spring discharge site and related drainage creeks. Conversely, sparsely vegetated areas are low-slope, open, and consist mostly of wet, unconsolidated calcium carbonate muds and crusts (Photo 3). Calcareous fens (Photo 4) are developed in water-saturated, low relief areas adjacent to calcium-rich creeks that flow away from spring discharge sites. 

Older tufa deposits occur in all three habitats where groundwater springs no longer discharge. This tufa consists of crumbly, lime-rich, surface crust and consolidated rock (Photo 9).

Pop Bottle Degassing:

Several chemical processes cause calcium carbonate precipitation from a groundwater spring, including: a) degassing of carbon dioxide (CO2) gas from the groundwater, much like a carbonated beverage effervesces violently when you quickly remove the bottle top; b) evaporation of the spring water on the land surface; and c) aquatic plant and algae photosynthesis in pools, creeks, and on the wet land surface. All of these processes remove CO2 gas from the spring water, which causes calcite to precipitate. Evidence of active CO2 gas degassing is illustrated by bubbles breaking the surface of the warm-spring pool (Photo 6).

Substrate Chemistry and Plants:

The Atlin tufa substrate has some important physical and chemical properties that influence the types of plant species present. The tufa substrate: a) is mesic to water saturated; b) contains a lot of calcium in the form of calcium carbonate (CaCO3), making it lime-rich; and c) the pore water is likely alkaline.

Scientists use the pH scale of 0 to 14 to describe how acidic or alkaline a substance is. By definition, a substance is alkaline if it has a pH greater than 7. The pH of typical tufa pore water ranges between 6 to 8, and is, therefore, alkaline. For comparison, bog water is acidic with a pH of 3.3 to 5.5, rainwater is slightly acidic with a pH of 5.0 to 5.5, and distilled water is neutral with a pH of 7.0. Several different types of fens exist in nature, each with pore waters having different pH values: a) poor fens have the lowest pH values of 4.5 to 5.5; b) moderately rich fens are slightly acid to neutral with a pH of 5.5 to 6.9; and c) rich fens have a pH above 6.9. Many plant species grow in slightly acidic to slightly alkaline conditions; however, some plant species, like some rhododendron species, grow in acidic conditions. Conversely, other specialized plant species called calcicoles, like Marsh Grass-of-Parnassus (Parnassia palustris), thrive in calcareous, alkaline conditions. Because the Atlin tufa is alkaline and wet in places, we would predict the presence of moisture- and lime-loving calcicolous plants. So, what plant species did I see on the tufa?

Pool-Creek Plants:

Seep Monkeyflower (Erythranthe guttata; Photo 10 and 11) and Watercress (Nasturtium officinale; Photo 12) both thrive in, and adjacent to, the warm pool and adjacent drainage creek.

Although Seep Monkeyflower (Photo 10 and 11) is common along the edges of the pool and drainage creek, I read that it can grow on a range of substrates, including harsh serpentinite rock and soils, where high concentrations of metals and the high magnesium to calcium ratio of the soil is poisonous to many plants.

Watercress floats (Photo 12 and 13) on the warm pool and grows in, and along, the edges of calcium-rich creeks that drain the warm pool and issue from limestone bedrock rock elsewhere in the area. Watercress is relatively rare in northern British Columbia leading some to suggest it was introduced to the tufa area by European settlers who used the plant as a culinary herb. Note also that Atlin is located in Canada Plant Hardiness Zone 1b, where you would not expect Watercress to survive the long, cold winters. Its year-round presence is a testament to geological processes that created the warm groundwater spring, which dramatically moderates the micro-climate and enables Watercress to survive year round. Another line of evidence supporting the micro-climate moderation by geological processes is the unfortunate presence of introduced Red Cherry Shrimp (Neocaridina davidi) in the Atlin warm pool. This introduced shrimp species requires a habitat equivalent to Canada Plant Hardiness Zone 12! Yet, due to the warm geothermal waters, this shrimp species survives year round in the Atlin warm pool.

Unconsolidated to Consolidated Tufa Plants:

The open, gently sloped, wet, unconsolidated tufa (Photo 3) ranges in texture from boot-sucking, unconsolidated muck, to semi-consolidated calcareous surface crust, to rare occurrences of rocky tufa (Photo 7). Despite being poorly vegetated, this habitat supports a number of herbaceous plants. Kalm's Lobelia (Lobelia kalmii; Photo 14) and Marsh Grass-of-Parnassus (Parnassia palustris; Photo 15) are two common calcicolous specialist species growing on this calcareous habitat.

Where the tufa is less moist, Mountain Death Camas (Anticlea elegans; formerly known as Zigadenus elegans), is present (Photo 16). As its ominous name suggests, this species contains toxic steroidal alkaloids that are poisonous to humans and domestic animals when consumed. Interestingly, moose frequent the Atlin tufa area to consume mineral-rich water and plants; however, I saw no evidence that moose, or any other herbivore, ate the Mountain Death Camas plants.

Lake Huron Green Orchid (also known as Green Bog Orchid; Platanthera huronensis; Photo 17 and 18), Hooded Ladies' Tresses (Spiranthes romanzoffiana; Photo 19), Sticky Tofieldia (Triantha glutinosa; Photo 20 and 21) and Western Butterwort (also known as Horned Butterwort; Pinguicula macroceras; Photo 22), also thrive on the moist, unconsolidated and less common consolidated tufa. While I cannot find an authoritative source attesting to their calcicolous nature, these plant species are known to grow on other calcareous substrates, such as near calcareous seeps, calcareous fens, and on alvars.

On the well-drained, drier unconsolidated crusty tufa areas, the calcicolous Entire-leaved Mountain Avens (Dryas integrifolia) grows with its characteristic mound-shape (Photo 23). Entire-leaved Mountain Avens is considered to be a pioneer species of calcareous gravelly and rocky barrens. Given Atlin’s northern location and the calcareous tufa substrate, it was not surprising to see this Arctic - alpine species.

 Other flowering plant species seen on the drier unconsolidated crusty tufa include Multi-rayed Goldenrod (Solidago multiradiata; Photo 24), Shrubby Cinquefoil (Dasiphora fruticosa; Photo 25), and Star-flowered False Solomon's Seal (Maianthemum stellatum; Photo 26), all of which grow on, and have a high tolerance for, the alkaline calcium carbonate-rich tufa substrate.

Although I visited the area late in the flower season, I also saw Balsam Groundsel (Packera paupercula; Photo 27 and 28) in flower. Balsam Groundsel is another plant species diagnostic of calcareous, alkaline substrates.

Calcareous Fen:

While I did not spend a lot of time in the calcareous fen (Photo 4), I did note the presence of Watercress (Photo 12) in the creeks and Seep Monkeyflower (Photo 10) along the edges of the fen creeks. Little Yellow Rattle (possibly Rhinanthus minor; Photo 29), and Hooded Ladies’ Tresses (Photo 19) were some of the flowering plant species in the vegetated calcareous fen. The striking American Cow Parsnip (Heracleum maximum; Photo 30) dots the transition zone between the calcareous fen and the boreal forest.

I saw several other vascular plants that grow on the calcareous Atlin tufa. If interested, I direct you to iNaturalist, where I have posted images of all the plants that I saw in this area.

Summary:

I have touched on a few of the native flowering plant species growing in the calcareous Atlin tufa habitats. Some of these species may be familiar to those of you who live father south. Some plant species, like Marsh Grass-of-Parnassus, are calcicoles adapted to thrive on an alkaline, calcareous geological substrate, which also grow much farther to the south. Others, such as Shrubby Cinquefoil, are generalists adapted to grow on a range of substrate types, including the calcareous tufa. Still other species, like the introduced Watercress and the Red Cherry Shrimp survive in the harsh Canada Plant Hardiness Zone 1b because geological processes created the warm geothermal springs that moderate the micro-climate.

These brief insights are an illustration not just the geological substrate - plant relationships seen on the Atlin tufa, but also the role played by geology in moderating the local micro-climate, resulting in a warm oasis in an otherwise harsh, cold northern area.

If you ever get an opportunity to travel to Atlin, often described as the Swiss Alps of Canada, you will not be disappointed by the scenic mountains, the varied geology, and the variety of plant species that occupy unusual geological habitats. But, recall this is a special place and visit with respect and a light footprint.

Acknowledgement:

I appreciate and acknowledge the reviews of an early version of this note by Sue Meades (adjunct professor, Algoma University) and Bruce Bennett (Yukon Conservation Data Centre Coordinator, Environment Department, Environmental Sustainability Division, Fish and Wildlife Branch, Government of Yukon). I note the neither Sue nor Bruce read the final draft.

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Jan. 08/22