Treeline ecotone patterns matter

Abrupt (and discrete) treeline composed of Lophozonia menziesii (Nothofagaceae), southern Alps, New Zealand. Photo by M. Y. Bader.

By Johanna Toivonen and Maaike Bader

As treeline researchers, we have been fascinated by treeline spatial patterns and the processes behind these for quite some time. The idea for this paper was born during a workshop in the Pyrenees, in Jaca (Spain), September 2017. Since then the authors have been brainstorming ideas around and beyond this paper, especially in three face-to-face meeting (pre-COVID time obviously): twice at iDiv, Leipzig (Germany), and once in New Zealand. Thus, one could say that a substantial amount of man and women power has been allocated for this paper – and for a good reason.

Anyone who has ever walked through a treeline must have noticed that it is not actually a line at all, but a transition zone, i.e. an ecotone. But this transition between forest and alpine vegetation is not the same at every treeline: the spatial configuration of the trees and the change in tree size and shape can differ considerably between treelines in different mountain ranges, or even within them. Tree cover may stop suddenly to form a discrete boundary between closed forest and alpine vegetation, or the transition may be more diffuse, dominated by single trees or by tree islands. Simultaneously, tree stature may change from tall to small abruptly or gradually, while trees might also get crippled into krummholz (German for “crooked wood”). Accordingly, different types of treelines can be recognized: abrupt, diffuse, gradual, abrupt-diffuse, abrupt krummholz, krummholz-island, etc., with of course a continuum of intermediate forms in between. Additionally, species diversity may differ strongly: most treelines consist of one or two tree species only, but some, especially in the tropics, can consist of multiple species (up to 30!). But why do these patterns matter? Why try to characterize them? What can they tell us?

The answer, and the core idea in our paper, is that with the treeline ecotone patterns, defined by tree spatial distribution and tree stature, we can predict the underlying fundamental ecological processes that shape these patterns and that are repeated at treelines around the world. And once the hypotheses on the pattern-process relationships are properly tested, we can also predict treeline shifts in a changing climate, shifts that are observed to differ strongly at treelines around the world.

Gradual diffuse krummholz treeline composed of Polylepis tarapacana, Sajama, Bolivia. Photo by M. Y. Bader.

To classify the underlying processes, we followed the previous hypothesis by Harsch and Bader (2011) classifying treeline-forming processes at three hierarchical levels: 1) first-level processes including growth limitation, dieback and mortality, 2) second-level processes causing the gradients in these first-level processes, including reduced cell formation (due to a lack of warmth or nutrients), loss of foliage (due to e.g. frost, wind), snow-mould damage, fire damage, dispersal limitation etc., and 3) third-level processes, referring to species interactions modifying the second-level processes, for example through sheltering or shading.

We expect that the framework for describing treeline ecotone patterns that we present in our paper will be very useful by 1) clarifying the common treeline terminology, 2) encouraging treeline researchers to explore treeline ecotones in more detail, and finally 3) allowing to analyze treeline ecotone patterns globally and to link them to underlying processes. With these aims and expectations, we welcome all treeline researchers to visit our alpine-treeline webpage (https://alpine-treelines.de/) and to contribute to the joint effort to create a global treeline ecotone database allowing the future global analysis of treeline ecotone patterns. This webpage can also work as a networking environment for treeline researchers with shared interests. Treeline pattern detection, analysis and classification tools will be available on the website as soon as they are developed.

Gradual diffuse krummholz treeline - showing how along the treeline there are ecotone sections that are more and that are less diffuse and with more or less krummholz formation - composed of Pinus uncinata, Ordesa, Pyrenees, Spain. Photo by M. Y. Bader.

Last but not least, and to be more precise, the paper on the framework of treeline ecotone pattern-process relationship, as well as this blog post, was actually written not by treeline researchers but treeline ecotone researchers, aiming to grasp the global rules for pattern formation in alpine treeline ecotones to help understand their behavior in space and time. 

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