Thermal environment of New Zealand’s gradual and abrupt treeline ecotones

In New Zealand, there are treelines of two main forms: abrupt southern beech treelines and gradual conifer–broadleaved treelines. At similar latitudes, abrupt treelines form at higher elevation than gradual treelines, but it is unclear whether this difference is also reflected in the climatic conditions experienced at the contrasting treeline ecotones. In this 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. Although gradual treelines form at lower elevations, they experience similar summer temperatures as the higher abrupt treelines. In contrast, temperatures in the shoulder season and during winter differed between sites of contrasting treeline forms. Soil scarcely froze and air temperature did not fall below −6°C at the gradual treeline sites, whereas freezing soils and snow were more common (extreme air frosts down to −9°C) at the abrupt treeline sites. Air and soil temperatures mirror the change in tree stature in the ecotone: with increasing altitude through the gradual treeline ecotone, temperature decreased gradually; whereas abrupt temperature changes were found at the abrupt treeline–grassland interface. These altitudinal patterns provide insights into potential mechanisms that drive treeline form and position, and their response to climatic change.