Representing Timberline

When I taught map reading in the U.S., there was a piece of folklore that we used to tell our students: that the green areas on the USGS topographic map indicated there was a sufficiently dense forest there that you could hide a platoon of soldiers (about 40 people) per acre. It was a good way to explain why small clumps of trees (or “tree islands”) didn’t appear on the map.

This story lives on on the Internet, but there’s no evidence that it’s really true. I like the implied subtext — cartographers producing the maps for military officers involved in some kind of domestic war, and needing to know where they could hide their men from aircraft –- but it doesn’t take much reflection to realize that the U.S. Geological Survey never could have visited all those places, looked at the tree cover, and decided where you could or couldn’t hide 40 guys. Or even how to divide it up into suitable one acre blocks.

In Canada, the green area on the topographic maps has a specific definition. According to Natural Resources Canada it’s “An area at least 35 per cent covered by perennial vegetation of a minimum height of 2 m.” And they probably estimate that 35% coverage from air photos.

NRCan topo_sm

So, when you’re ascending a mountain and the green ends on the map, is that treeline, or is it timberline? I’d always thought these were just variant terms for the same thing, but then I read Jim Pojar’s book Alpine Plants of British Columbia. In the Introduction to this photo-rich handbook of all the plants you’re likely to see up there, I learned that treeline and timberline are different:

The term treeline designates the upper limit of the occurrence of tree species, regardless of their stature, whereas timberline refers to the upper limit of forest, of continuous cover of upright trees 3 m or more in height.

So timberline, being where the solid forest ends, is the end of green on our classic NRCan topos. Treeline is the last little, twisted, stunted tree.

Neither, of course, is really a line. As map scale decreases and you zoom in, the timberline becomes impossibly complex, and has to be generalized somehow. And no map, I think it’s fair to say, tries to represent treeline, since this would somehow be defined by many isolated clumps of krummholtz (“twisted-wood”, the bonzai-like tree clumps also known affectionately as shintangle) that you see after ascending past timberline. And just to make things a bit more complicated, as the climate changes it has become easy (in northern BC at least) to find areas above treeline where dozens of tiny seedlings are coming up and now surviving. How big would they have to be before one moves treeline?

Timberline however is a very important landmark for hikers and skiers, and how to represent it is a question that comes up frequently in topographic mapping. Of course using using generalized green and white is not the only option. I first learned this when I was bushwhacking across a 1:50,000 scale “provisional” series, black-and-white topographic map in northern BC. On these there is actually a black line that snakes across the elevation contours. It has “W” on one side of it (for wooded) and “C” on the other (for clear). Little pairs of tick marks pointed into the forested side. It was hard enough to read that I took a pencil crayon and shaded in some green on the W side.

NRCan BW topo_sm

There’s also the solution that National Geographic used years ago in mapping northern Canada; the Northern Limit of Wooded Country is represented by a line of tiny tree symbols.

NatGeo_sm

Sometimes using green is just out of the question. If you want to use a range of colours to represent elevation (hyposgraphic tinting), having green forest is going to be quite confusing. In these cases I have I tried a technique of having a dashed green line at timberline, bordered by some fill on the downhill side, fill that quickly fades out. You can do this in QGIS by using shapeburst fill, shading to a set distance of a few millimetres.

McDonell_Lake_detail_sm

I have also played around with not marking timberline at all, and just putting a note beside the trail at the point when one would clear the trees.

OpalRidge_sm

Another option available to you if you have access to landcover data, is to give map-readers more information about how the forest makes that complicated transition to grassy tundra. In this case I used the Land Cover, circa 2000-Vector data available at Geogratis, and assigned progressively lighter colours to “coniferous dense” (which captures the main forest of spruce and fir), “coniferous open” and “broadleaf open” (which captures willow).

Seaton_sm

If timberline is something you just want to suggest, but don’t really need to accurately show, a method that can still look good is to style the digital elevation model in a series of fading greens. Have it become completely white at the elevation where timberline is typically encountered in the area. You get the effect of the “naked” mountains rising above forested slopes without introducing the complexity of avalanche tracks and the differences in where trees grow between north- and south-facing slopes.

RedRose_sm

Timberline in this area tends to be at 1500m

Sometimes the highest elevations on the map just touch timberline, in which case there can be a large zone where small meadows alternate with clumps of diminutive trees, an ecology often called parkland. I’ve tried representing this by scattering a host of little tree shapes, sort of trying to show how trees are still there, but not connected to the “mainland” of the forest.

QuickHills_sm

And this is just scratching the surface. The more you study timberline, the more you realize the folly of trying to show it accurately, yet the importance of indicating where we see it!

 

 

 

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