What a tree looks like can tell us about the conditions it grows in and about its past. By looking at growth form, we can get clues to what the climate has been like for the life of the tree.

The sections below explain three concepts in the following order:
1) Different Tree Growth Forms-- why are trees shaped differently?
2) What did we do? What did we measure?
3) What can we learn from what a tree is shaped like? What have we learned so far?

New branches coming up from an old twisted base

Click here to listen to Dr. Andrea Lloyd describe growth form. Click here for text.
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1) Different Tree Growth Forms

Some trees grow straight and tall. Each year, the tree produces buds that develop into new branches and leaves. A stem called the "leader" grows up above the rest and, therefore, the tree keeps its upright form. How does this happen? Something called "apical dominance" happens. Click here to learn more.
Other trees are not straight and tall; some grow as twisted "krummholz" forms. Krummholz trees often have more than one leader, like in the picture above (the red arrows).Why is there more than one leader? The "apical dominance" fails when the apical buds are killed. Often times, blowing ice crystals kill apical buds... Click here to learn more about apical dominance.
...And these buds tend to die at a certain point in the tree's vertical growth. Notice how this tree is "flagged"-- it has a section of stem that has little or no branches. This branchless region (red arrow) is where the snowpack is: everything underneath it hides under the snow during harsh winter winds, while just above the snow, the harshest winds blow into the apical bud.
So once a tree gets up above this snowpack region (during a few years that are not as harsh), it can usually grow straight again. Notice how these leaders grow straight, yet come from a base that is twisted because of many apical deaths.

2) What did we do?

So what did we do? We classified and documented each tree's growth form (or morphology) in order to learn about conditions at and above treeline.
Since we also want to know what a tree has grown like in the past, we look for signs of that, too. This tree, for instance, shows a "skirt"-- an area that used to be twisted krummholz (see red arrow). Even though the tree appears straight and is growing well now, the skirt suggests that growing conditions were once challenging for this tree. It probably had a hard time getting above the snowpack. We record these clues of the past when we classify each tree's growth form.

3) What did we find?

In summary, we record what the tree looks like so that we can reconstruct what the growing conditions may have been like for the past several hundred years. Is there more than one leader? Is the tree a twisted krummholz or straight? What did it look like 50 years ago? We are particularly interested in how these growth forms have changed over time. Take the tree above, for instance: new leaders have quickly grown up from an old, twisted krummholz base. (How do we know it's old? Click here to find out.) This suggests that conditions have ameliorated-- that they are more favorable for growth. So what the trees look like may be clues to what the climate has been like in the past.
Graphs like this one compare growth form over time. Click here to see a bigger graph and learn more. From the the growth forms we have looked at so far and the graphs we have made with that information, it appears that growth form might be changing over time. In recent times (around 1960), trees have been growing more upright than in the past (from 1700 to 1960). This is interesting because the climate in the area has also gotten warmer during this time. Click here to see climate data.

A word of caution:

Growth form is determined by what conditions a tree grows in (wind, cold temperatures, etc.). Because these conditions are usually different between treeline and above treeline, there are often more krummholz trees above treeline-- conditions tend to be harsher. But even though there are more krummholz trees above treeline, there are fewer trees total because the harsher conditions above prevent as many trees from establishing.

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