>> What do we mean by 'tree line'?   Tree line refers to the point on a hill or mountain where trees (in the Rockies most often pine, spruce, and fir) cease to grow, so in the image to the right it's the stark line between green and grey. Usually the line is less clearly defined, but a reasonable estimate of its position can still be made. Location of tree line is determined by at least three kinds of factors. One is the local physical environment—soil depth, availability of moisture, etc. Since these conditions can vary dramatically from one place to another, tree line is generally not uniform even across a fairly small area. In some places, forest fires can play an important role as well. In the Rockies, for instance, fire suppression is fairly common, with the result that tree line is 'reset' less frequently than in the past and trees are free to slowly expand beyond their historical range. A third major factor is temperature. Locally, one can often observe a difference in tree line between shady and sunny hill faces; more broadly, since temperature decreases with altitude, it establishes a threshold beyond which it is too cold for trees to grow even if soil and moisture requirements are met. The precise altitude at which trees can no longer grow varies with the local climate but, all else being equal, the higher the minimum temperature in an area the higher trees can grow. Not surprisingly, the global trend of rising temperatures leads, among other things, to tree growth at higher altitudes than ever before.

Meadow with forest below... Alpine meadows and rising tree line

Alpine meadows are high-altitude grassland communities home to a diverse and unique group of plants, animals and insects. Like forests, alpine meadows can only persist where certain basic conditions, including temperature, sunlight, moisture retention, and soil characteristics, are met. Alpine meadows are thus relatively uncommon in the Rockies where terrain is highly variable, but often follow a pattern where they do occur: a moderate altitude of ~2000m, a relatively flat incline more common to ridge-tops than mountainsides and, because they nearly always occur just above tree line, complete encirclement by trees. Many alpine meadows occur as 'caps' on hills and lower mountains, rather than as 'bands' on taller peaks—like islands in a sea of trees. So if altered conditions do allow tree growth beyond its historical maximum elevations, alpine meadows have literally nowhere to go. What begins as meadow shrinkage and fragmentation ultimately results in full-scale 'treeing over' of the meadow. Few meadow species can survive in a tree-dominated environment. >> View an example of meadow reduction HERE.

Case in point: the Rocky Mountain Parnassian (Apollo) butterfly

Rocky Mountain Parnassians (Parnassius smintheus) are a relatively common butterfly in alpine meadows throughout the Rocky Mountains. Although they are not currently endangered, they are very similar to other Parnassian species that are threatened or endangered—so research on the Rocky Mountain Parnassian has much wider relevance. Some of the most interesting research on these butterflies has examined their movement across the landscape. As we've seen, alpine meadows are usually separated from each other, often by distances of several kilometers. While some meadows are independently large enough to sustain a healthy butterfly population, others require occasional or even constant 'reinforcements' from surrounding meadows in order for their butterfly populations to remain healthy in the long run. And regardless of meadow size, the arrival of individuals from other populations mitigates the potentially negative effects of inbreeding and keeps the overall population viable. It follows that movement is vital to the Rocky Mountain Parnassian.

Research has already shown that rising tree line affects butterfly movement in a couple of ways. First, if a meadow is reduced to such an extent that it can no longer sustain butterflies, it also ceases to function as a 'stepping stone' for butterflies en route to other meadows. Although smaller, less significant meadows are usually the first go, their disappearance may effectively isolate other, larger meadows from each other, cutting off the flow of individuals between them. This process would lead to local extinction in some areas, but would most likely take considerable time before it significantly threatened Rocky Mountain Parnassians across their entire range. Of more immediate concern is the fragmentation of meadows, even where total meadow area remains more or less the same. For example, it takes only a few trees—comparatively speaking—to change a formerly expansive meadow into a collection of much smaller meadow patches, each separated from the other by a thin band of trees. This is actually quite common, especially where natural features of the meadow like depressions or narrowings facilitate tree-growth. Long-term research in the Alberta Rockies indicates that trees present a very significant barrier to butterfly movement, and that Rocky Mountain Parnassians will actively avoid flying into forested areas. The implication is that a large meadow can be reduced with relative ease to a collection of small, all-but isolated meadows, with potentially damaging consequences for Parnassian populations.

What about more subtle effects of rising tree line?

As adults, Rocky Mountain Parnassians drink nectar from a variety of flowering alpine plants. As larvae (caterpillars), however, they eat the leaves of a single plant—Lance-leaved Stonecrop (Sedum lanceolatum)—almost exclusively. When I first saw the alpine meadows of Kananaskis Country, I noticed that Stonecrop grows much more densely near tree line, whereas Parnassian larvae generally stay away from tree line in the main body of the meadow. The reason for this pattern is still unknown, but I have some guesses: perhaps tree line conditions (more shade, say) are less attractive to larvae, and less grazing means more Stonecrop plants survive. Or, perhaps proximity to tree line makes Stonecrop less palatable, driving larvae away from these areas to where the Stonecrop is 'tastier' (there is circumstantial evidence that this may in fact be the case, but more research is needed to say for sure). Whatever the reason, it's clear that tree line does affect the relationship between Parnassian larvae and their primary food source, and it seems reasonable that tree line rise should have further implications. My PhD research examined exactly this.

On the face of it, these issues may seem rather less important than the more obvious effects of trees on adult butterfly movement. But their significance is that they represent a mechanism whereby tree line may negatively influence Rocky Mountain Parnassians even before the obvious effects of tree encroachment and meadow fragmentation are visible. For example, consider a scenario where the 'unattractive' Stonecrop grows in a 10-m band along tree line, and then imagine that tree line as a whole moves 5 m into the meadow over a certain period of time. The overall meadow area will be only slightly reduced, as will the total number of Stonecrop plants in the meadow. But assuming the new tree line also retains a 10-m band of 'unattractive' Stonecrop, the relative numbers of less palatable plants will have increased compared to tasty plants. The result is less or lower quality food available for Parnassian larvae, even while the meadow as a whole appears healthy. The consequences of this could be large.

My own research sought to determine whether such a scenario is realistic and, if it was, to assess potential implications for the Rocky Mountain Parnassian and similar species. My results suggest that tree line does indeed influence larval feeding behaviour in precisely the manner discussed above. For the technically-minded, my full Ph.D thesis is available HERE via the U of A Libraries, and a full list of my academic publications is available HERE via Google Scholar; alternately, check out my project methodology HERE, or take an aerial tour of my study site on Google Maps HERE.


>> Read more about...

...the Rocky Mountain Parnassian at the Canadian Biodiversity Information Facility HERE.

...Lance-leaved Stonecrop at Montana plant life HERE.

...tree line around the world at Wikipedia HERE.

...the features and ecology of Kananaskis Country at Alberta Parks and Recreation HERE.

...my project in more technical terms at my departmental website HERE.

...other work taking place in my lab at the University of Alberta HERE.