Frost-driven lower treelines in Angola and their implications for tropical forest–grassland mosaics

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Prepared by Manfred Finckh, Janika Wendefeuer & Paulina Meller

The transition of ecotone grasslands to grasslands at the confluence of the Río Cusseque and its western tributary, Río Sovi. Our network of temperature data loggers yielded the lowest temperatures in this area, where the nocturnal flows of cold air from both valley systems converged. The fresh-green ground layer in the centre of the image is dominated by the geoxyle Brachystegia russelliae. The eastern treeline of the Cusseque valley and the southern treeline of the Sovi valley can be seen in the background of the photo. (Photo credit: Paulina Meller, date 11/10/2018).

Travellers in the dry tropics will often encounter landscapes where forest stands and open land meet with clear boundaries. The central Angolan highlands are such a landscape, with forests on the hills and open grassland in the valleys. Generally, such forest-grassland mosaics are interpreted as the result of disturbances by fire, waterlogging and/or large herbivores. However, anyone who has spent a night during the dry season there will quickly wonder whether frost might not also play a role in these vegetation patterns.

We therefore wanted to systematically investigate whether there is a frost-dominated lower treeline on the Angolan highlands and, if so, what role valley systems play in this and what this could mean for our understanding of tropical forest-grassland systems worldwide.

For our study, we chose two different approaches. Firstly, we used the supervised classification of a Landsat scene covering large areas of the still very natural eastern Angolan plateau to systematically investigate the forest-grassland distribution. We then placed a 1000 m grid over this classification and, using a digital elevation model placed underneath, queried the vegetation types and their elevation at each grid point. In addition, we used a “valley depths” analysis for each point to determine how deep it was below the crest line of the nearest hill range.

Secondly, we measured microclimatic data across topographic gradients at two sites along the Plateau. We were particularly interested in when and how often frost occurs during the year and how high the daily temperature fluctuations are.

Our results show that lower treelines separate forests from open habitats in all major valley systems. These treelines are located 150–200 m below the headwater divides. This shows that topography controls the frost patterns, with minimum temperatures of -7.5°C and diurnal temperature amplitudes up to 40°C. The physical mechanism causing the frosts is nocturnal outgoing thermal radiation facilitated by the clear sky during the dry season and resulting cold-air pooling in the valleys.

In the dry season, hoarfrost often appears early in the morning on the low-lying grasslands in the Sovi Valley (Photo credit: M. Finckh, June 2017).

Our results show that natural landscapes and vegetation in tropical mid-altitudes can be strongly shaped by frost. The physical processes causing frost depend particularly on air humidity. This has relevance for our understanding of the vegetation history of tropical mid altitudes. The considerably drier atmosphere and lower global temperatures, particularly during the cold phases of the Pleistocene about 12,000 to 2.6 million years ago, may have brought a longer frost season for many tropical highlands, with still lower minimum temperature and frost extending to lower elevations and latitudes than today. Thus, Pleistocene forest fragmentation in south-central Africa and the rise of endemism-rich grasslands in comparable tropical highlands like the Brazilian Cerrado and the Indian Ghats may have been a consequence of it becoming too cold for tropical tree species.

You can find more information about these fascinating landscapes at the Virtual Excursion to Cusseque, presented at the 63rd IAVS Annual Symposium (this year virtual), and on Twitter under #geoxyleoftheweek from @Undercovertrees.

This is a plain language summary for the paper of Manfred Finckh, Janika Wendefeuer and Paulina Meller published in the Journal of Vegetation Science (https://doi.org/10.1111/JVS.13084).