Fish, forests, and snow

You’re standing on a stream bank in your local mountain watershed. The sun is rising, warming the back of your head, and you see a fish drifting lazily in a still pool just off the rock you’re standing on. You peer closer, but with a flick of the tail it’s gone. You wonder if it was a rainbow trout, or perhaps even an at-risk westslope cutthroat trout. But does it even matter what species it was?

Scientists are showing that it does matter, as the westslope cutthroat trout and other native salmonids like it are at risk of extinction through a combination of hybridization with and out-competition by fierce rainbow trout, introduced into these waters by decades of conservation approaches focused on sport fishing. Research has shown that these pure populations are declining, and invasive fish are rushing in to take their place.

Few would suspect, however, that the future of an entire fish species is rooted in the forest that surrounds its aquatic habitat, largely due to the impacts that forest has on snow accumulation and melt. The inter-connectedness of environmental processes is one reason they’re both fascinating – and frustrating – to study, and the link between fish and trees is no exception.

Fish are sensitive to a range of factors throughout their lifecycle – being in constant contact with water means that water flow timing and amount, as well as water temperature (and the associated dissolved oxygen content), are critical for fish population success. These properties originate in the headwaters of a catchment, and are affected by processes happening all along the length of a stream as it hurls itself down the mountainside to the foothills and lowlands. The stream habitat is dynamic, responding to the seasons and to changes in land cover. Most of the time the fish don’t even know what’s hit them.

Our story starts in the headwaters of the Eastern Slopes of the Rocky Mountains, where snow accumulates over the long, cold winter and waits for the touch of spring warmth to start melting. In high alpine areas, where bare rock is the main landscape feature, snow accumulates directly on the ground – at least where the slopes aren’t too steep to just shake it off. But as you creep down the windswept scree slopes and into the shelter of the forest that covers the lower end of the alpine zone like a bushy green skirt, everything changes.

Mt Coulthard, Crowsnest Pass, AB (Photo: S. Boon)

Snow gets caught in the tree branches before reaching the ground, and some is sublimated directly back to the atmosphere. The snow that collects on the ground is therefore less than what falls directly from the sky, and forms a pattern of thinner snow under trees and thicker snow between them (anyone who’s ever fallen in a tree well while skiing of snowshoeing knows what I’m talking about).

Tree well in an undisturbed forest (Photo: S. Boon)

Spring brings warmer temperatures and a higher sun angle, driving snow melt especially on south facing slopes. While it might seem as though the melting snow flows directly into the local streams, its trip there is delayed by several stops along the way.

First the soil under the snowpack itself has to be rehydrated, and depending how dry the previous fall was, can take a significant amount of water. Meltwater also percolates down to the groundwater table, which is one of the more fascinating aspects of hydrology (maybe because we always think topics outside our specialty are cooler than topics we know well). Groundwater that has contributed to keeping streams running over winter is now topped up by snow melt coming in from the surface.

All of this means that there’s a bit of a delay between the start of spring snowmelt and the increase in water levels in the stream. But when the water levels do increase they can be phenomenal. If you go from subzero to well above zero temperatures for a long enough time period, the snow can melt quickly and synchronously across an entire watershed. This fills the soil and groundwater reservoirs in no time and generates high spring flood peaks.

If the weather is cooler, melt can be stretched out over a much longer period – unless you get rain on snow (or ROS, because we like our acronyms). This is what happened during the Alberta floods of 2013 – a rainy system parked itself over the Rocky Mountain snowpack and added up to 200 mm of extra water to what was already there. The soil and groundwater reservoirs were filled to capacity, snow melt was sped up, and a whole lot of water came down out of the mountains in a short period of time.

Now imagine the temperature of that water. With so much snowmelt, plus shading from the trees as it flows through the forest, plus the relatively low sun angle during the spring period – it’s pretty chilly! I wouldn’t want to take a bath in it, but think of our fish swimming somewhere along the stream system. They’re being bombarded with cold water, which some species like, but others? Not so much. Those that like it are the ones that thrive: native salmonids.

Unfortunately, changes in climate have caused a few scenarios that alter this well-functioning system, affecting water temperature and the timing of spring runoff.

• A reduction in snowpack at low- and mid-elevations has been observed across the North American west. When there’s less snow, it melts earlier in the year and doesn’t contribute as much to either groundwater replenishment or directly to streamflow. This leads to lower stream flows and higher water temperatures, as a lower proportion of streamflow is coming from snowmelt.

• Warmer summer air temperatures and reduced snowpack increase the likelihood of wildfires, and of insect infestation in drought-stressed trees. This changes the forest canopy from a carpet of green throughout all seasons to a patchwork quilt of burned, defoliated due to insects, and a few remaining green, patches. More snow reaches the ground in the burned and defoliated forests, and it melts earlier and faster. Streamflow is high (and cold) early, but then drops quickly and warms up. And does it ever warm up. Without the green forest to shade mountain streams, they can warm up significantly and affect aquatic organisms (like our fish) that depend on a specific water temperature range.

Snow in a burned forest (Photo: S. Boon)

Now let’s bring back our fish. Subject them to changes in stream flow and water temperature at critical stages in their life cycle – while they’re also battling it out with invasive species like rainbow trout that like these new flow and temperature conditions better – and what do you get? Hybridization and loss of pure populations of at-risk species like westslope cutthroat trout, and loss of some populations altogether.

Westslope cutthroat trout (Photo: W. Warnock).

Right back where we started – and all because of trees, and snow, and the lingering effects of climate change…

Note: Maybe you’ve noticed one potential loophole out of this mess. In watersheds that are largely groundwater-fed, stream temperatures are less affected by what happens to the forest canopy and rely instead on groundwater, with a temperature fluctuating around annual average air temperature. In watersheds that are seeing high elevation increases in snow accumulation, this could mean continued groundwater replenishment in spring and colder stream temperatures. So high elevation groundwater-dominated systems may be able to ride out the changes a bit longer than surface-water dominated ones, though they’ll likely see those changes eventually.

Further reading:

1. Arismendi I, Johnson S, Dunham J, Haggerty R, Hockman-Wert D. 2012. The paradox of cooling streams in a warming world: Regional climate trends do not parallel variable local trends in stream temperature in the Pacific continental United States. Geophysical Research Letters, doi:10.1029/2012GL051448.
2. MacDonald RJ, Boon S, Byrne JM, Robinson MD, Rasmussen JB. 2014. Potential future climate effects on mountain hydrology, stream temperature, and native salmonid life-history. Canadian Journal of Fisheries & Aquatic Sciences 71(2): 189-202.
3. Muhlfeld CC. Kovach RP, Jones LA, Al-Chokhachy R, Boyer MC, Leary RF, Lowe WH, Luikart G, Allendorf FW. 2014. Invasive hybridization in a threatened species is accelerated by climate change. Nature Climate Change, doi:10.1038/nclimate2252.
4. Wagner MJ, Bladon KD, Silins U, Williams CHS, Martens A, Boon S, MacDonald RJ, Stone M, Emelko MB. 2014. Catchment-scale stream temperature response to land disturbance by wildfire governed by surface-subsurface energy exchange and atmospheric controls. Journal of Hydrology 517: 328-338.