Abstract
Campbell Island, which is 600 km south of New Zealand, has the southernmost tree line in this ocean sector. Directly under the maximum of the westerlies, the island is sensitive to changes in wind strength and direction. Pollen records from three peat cores spanning the tree line ecotone provide a 17,000-year history of vegetation change, temperature, and site moisture. With postglacial warming, tundra was replaced by tussock grassland 12,500 years ago. A subsequent increase of shrubland was reversed at 10,500 years ago and wetland-grassland communities became dominant. Around 9000 years ago, trees spread, with maximum tree line elevation reached around 6500 to 3000 years ago. This sequence is out of step with Southern Ocean sea surface temperatures, which were warmer than 12,500 to 9000 years ago, and, subsequently, cooled. Campbell Island tree lines were decoupled from temperature trends in the adjacent ocean by weaker westerlies from 12,500 to 9000 years ago, which leads to the intrusion of warmer, cloudier northern airmasses. This reduced solar radiation and evapotranspiration while increasing atmospheric humidity and substrate wetness, which suppressed tree growth. Cooler, stronger westerlies in the Holocene brought clearer skies, drier air, increased evapotranspiration, and rising tree lines. Future global warming will not necessarily lead to rising tree lines in oceanic regions.
Highlights
Tree lines are confidently predicted to rise as a result of global climate change in the near future, which will lead to a profound impact on alpine ecosystems [1]
Despite the issue of downward intrusion of roots, which is a problem with peat bog dates, there are
While the postglacial spread of the forest on sub-Antarctic Campbell Island broadly follows the warming of the surrounding ocean, wind direction, wind strength, and cloudiness modified the timing and created gradients within the island, according to the degree of wind exposure
Summary
Tree lines are confidently predicted to rise as a result of global climate change in the near future, which will lead to a profound impact on alpine ecosystems [1]. A global survey of tree line response to climatic warming over the last 100 years showed that, while some were rapidly advancing, many had either responded sluggishly or not at all [3]. Oceanic tree lines, such as those that characterize New Zealand and southern. South America, are among those that have failed to respond to recent warming. Why this may be so, and under what circumstances these tree lines may change, is, of great interest [4]. The sub-Antarctic islands lying several hundred kilometers to the south of New Zealand offer a unique opportunity to explore these questions
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