Treelines and tree-rings

During our weekend in Aviemore, we discussed treeline dynamics in Scotland. Being an Arctic researcher, I am used to hillslopes where trees grow up to a certain limit, above which the low temperatures and harsh conditions simply prevent any establishment or survival of seedlings.

In the Cairngorms, most slopes we saw were bare, or were home to only a few scattered patches of trees. Granted, Scotland is not famed for its pleasing weather, but it seems hard to believe that climate is what prevents trees from colonising the Scottish heather uplands. What, then, could prevent tree regeneration? The students provided many answers, and we concluded that the browsing of young saplings by deer and repeated burning of the heather (muirburn) to maintain habitats suitable for grouse are probably the main constraints. Some natural treelines still occur in less managed and more remote places. Creag Fhiaclach in the Cairngorms (which was right across Loch an Eilean where we had lunch!) is considered to be one of the best examples of a natural treeline in the UK.

Our lunch spot with a natural treeline in the background

Our lunch spot with a natural treeline in the background

We also discussed age distributions, which can tell us a lot about the dynamics and regenerative state of a forest. There was no better place to discuss this than in a beautiful Scots pine forest where young saplings grew next to massive, century-old trees! We aged a 5-year-old sapling using shoot elongation (the trick is to count the sections between each node of lateral stems) and came up with a study design to sample older trees and count their annual growth rings.

Talking forest regeneration

Talking forest regeneration

Finally, the word of the day: krummholz! This is how we call trees growing in a stunted, shrubby form (the word means “twisted wood” in German), generally above the tall tree line. This growth form is caused by the abrasion of apical buds by wind and ice particles, causing stem dieback and triggering a response from lateral buds. Whether to include krummholz or not in the definition of treeline is a hot debate, because they can respond quickly to change. If climate warming promotes their growth to the point that they become “proper” trees (like the spruce in the picture below), should we say there has been a treeline advance even if no new trees have become established? I will leave you to ponder that one… Feel free to come and see me with your thoughts!

A spruce krummholz in the Canadian Arctic

A spruce krummholz in the Canadian Arctic

Links of interest:
Definitions of altitudinal zones by the Scottish Natural Heritage
The Science of tree-rings: all you need to know about dendrochronology!

By Sandra, devoted course demonstrator  : )

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