The upper elevational limit of forests (or treeline) comes in two main forms: gradual and abrupt treelines. Most natural treelines around the world are a gradual transition from forest to alpine vegetation, where the height of trees gradually declines with elevation until there are only shrubs and grasses left. These are sometimes also called diffuse treelines. These differing growth forms are an adaptation to the stresses imposed by increasing elevation, including low temperature, wind and snow accumulation. In other parts of the world (in some places in the Southern Hemisphere and in the tropics), there are also abrupt treelines, where tall forests abruptly give way to alpine vegetation.
In New Zealand, we have both types of vegetation transitions (also called ecotones): abrupt treelines formed by mountain beech dominate in the eastern rain-shadow districts, and in the western, wetter regions, gradual ecotones are often formed by a diverse set of species. Even the early explorers recognised that, at similar latitudes, abrupt treelines form at higher elevations than gradual treelines. But nobody has investigated if there is also a difference in the temperature conditions experienced at the contrasting treeline ecotones.
In a newly published study, we measured soil and air temperatures across four gradual and two abrupt treelines ecotones in New Zealand for 2 years, and compared the climatic conditions between the treeline forms.
Cieraad E, McGlone MS 2014. Thermal environment of New Zealand’s gradual and abrupt treeline ecotones. New Zealand Journal of Ecology 38(1):12-25.
We found that air and soil temperatures mirror the change in tree stature in the ecotone. With increasing elevation through the gradual treeline ecotone, temperature decreased gradually. This has been shown elsewhere in the world to: short vegetation is generally warmer than tall vegetation on sunny days with little wind, but in areas with high winds, high humidity and/ or where cloud cover reduces solar radiation, the temperature difference between tall and short vegetation is not so clear. At the abrupt treeline–grassland interface of the abrupt treelines, temperature changed also abruptly: the soil beneath the mountain beech forest was 1° to 2.5°C cooler than soils of sites without trees less than 10 m away. The forest canopy creates its own microclimate by shading the soil beneath, and prevents the soil from warming up.
Trees at the gradual treelines experience similar summer temperatures as those at the abrupt treelines in the east. But temperatures in the shoulder seasons and during winter differed. At the gradual treeline sites, soil hardly ever froze and air temperature did not fall below −6°C. At the abrupt treeline sites freezing soils and snow were much more common. Compared to temperatures experienced at treeline sites in the Northern Hemisphere (for example the European Alps and North America), it was still not that cold – the coldest air frost recorded was −9°C. Trees growing at the New Zealand treeline can easily withstand those sorts of frosts (see a previous study).