There is a magical moment that occurs after slogging your way up mountains through tangled and dense forest and bush. First, you notice more light in front, a few steps later, a noticeable reduction in tree height and then, often between one step and the next, you are out of the bush and into the open with nothing but tussock grasslands ahead. And a view. Your world goes from watching for errant branches and roots a metre or two away from you, to vistas that take in many kilometers of scenery. All in a couple of steps. The treeline is a special place with the abrupt transition from one habitat to the next, especially in New Zealand. I have crossed this edge in many places, from the Routeburn and Queenstown areas in the south, to Arthur’s Pass, Lewis Pass and the Landward Kaikouras further north, and even in the Tararua Range of the North Island. Every time it was a marvelous moment, an opening up of the world, a chance to see how far you had come and to see where you were going. A good excuse to stop for a well-earned break, fresh air to suck in, a breeze to cool your sweaty body. Once I have had my fill of the mountain scenery, my curiosity is always drawn back to the treeline. Why is there such an edge? If a tree or shrub can grow at the current edge then why not 30 cm uphill? Or a metre? What makes these treelines so sharp, so definitive?
What is clear is that there are probably all sorts of factors that contribute to the setting of treeline elevation. Ultimately, growing season temperature sets an upper limit for where a tree can grow. However, most treelines are often several hundred metres lower than this maximum, so there must be more to it. Plenty of research has shown that factors, like frosts, aspect, latitude, slope angle, dryness and so on, can contribute to producing the tree boundary. While these studies have been useful for identifying what might be happening at one site, there has been little work that has tried to look at explaining treeline throughout an area, like the whole of New Zealand. This may be because the treeline elevation is determined by both large scale factors, like latitude and climate, and small scale factors, like the aspect and shape of the slope, and that it is difficult to sort all of these out in one study. Until now. Brad Case (GIS expert) and Hannah Buckley (community ecologist) from Lincoln University have developed an approach to investigating many of the factors that could be playing a role. They obtained data from 28 alpine areas from throughout the Southern Alps and into the mountain ranges of the southern North Island. Within these areas they collected data from around 2100 sites (each about 15 metres in width) that were at the treeline for southern beech forest. For each site they collected elevation data (and the highest elevation recorded from each area was set as the potential treeline that all local trees could get to), slope steepness, slope curvature, seasonal temperature, mountain size, rainfall, and previous earthquake intensity. In addition they were able to re-purpose a model developed for estimating air pollution levels (especially within cities) called The Air Pollution Model (TAPM) to generate weather data for every 200 x 200m cell within each area for mid-summer and mid-winter. Using the TAPM data Case and Buckley could calculate climate factors for each location that might be affected by the local landscape, such as dryness, frost nights, and how much light reaches the location. With this data they hoped to be able to answer the following questions. Are treelines around New Zealand the result of distinctive topology and climate conditions and how much of the variation in treeline elevation is the result of local or regional processes?
The results of this complex analysis have been published in the open access journal PeerJ. Treeline elevation ranged from 763 to 1486 m through the 28 areas. Treelines were usually within 100-250m of the maximum potential treeline for an area. Interestingly, the results were not obviously driven by latitude. Rather, elevation differences seemed to be largely driven by regional differences in moisture, solar radiation, warmth and frosts. Digging further, the factors that most determined these differences were slope curvature, with highly concave slopes decreasing treeline elevation, as well as rainfall, length of growing season, and other climate-related measures. Individual trees that grow even a little further upslope than the treeline are hit by whole bunch of factors that make their survival unlikely. What was also apparent from the study was that site traits were very important and this pointed to the fact that treeline altitude is often locally variable and determined by the landscape and its affect on these local areas. Treelines in dryer areas with warm growing season temperatures were much more likely to be close to their maximum potential elevation. Ultimately, what Case and Buckley show by collecting data at both the large and small scales and using tools like TAPM, is that knowledge of both scales is required to answer the question of treelines. Next time I break through a treeline and throw myself down to catch my breath, I will be able to gaze at the surrounding landscape and better understand why the edge is where it is.