Abstract
Restoration of subtropical thicket in South Africa using the plant Portulacaria afra (an ecosystem engineer) has been hampered, in part, by selecting sites that are frost prone—this species is intolerant of frost. Identifying parts of the landscape that are exposed to frost is often challenging. Our aim is to calibrate an existing cold-air pooling (CAP) model to predict where frost is likely to occur in the valleys along the sub-escarpment lowlands (of South Africa) where thicket is dominant. We calibrated this model using two valleys that have been monitored during frost events. To test the calibrated CAP model, model predictions of frost-occurrence for six additional valleys were assessed using a qualitative visual comparison of existing treelines in six valleys—we observe a strong visual match between the predicted frost and frost-free zones with the subtropical thicket (frost-intolerant) and Nama-Karoo shrubland (frost-tolerant) treelines. In addition, we tested the model output using previously established transplant experiments; ∼300 plots planted with P. afra (known as the Thicket-Wide Plots) were established across the landscape—without consideration of frost—to assess the potential factors influencing the survival and growth of P. afra. Here we use a filtered subset of these plots (n = 70), and find that net primary production of P. afra was significantly lower in plots that the model predicted to be within the frost zone. We suggest using this calibrated CAP model as part of the site selection process when restoring subtropical thicket in sites that lie within valleys—avoiding frost zones will greatly increase the likelihood of restoration success.
Highlights
Frost can hamper the restoration of plant communities (Snowcroft & Jeffrey, 1999; Snowcroft et al, 2000; Curran, Reid & Skorik, 2010; Rorato et al, 2018)
We calibrate a cold-air pooling (CAP) model to predict where frost may occur in the complex terrain of the sub-escarpment lowlands of South Africa where subtropical thicket is the dominant vegetation
The frost-prone, continentally dry, and high-elevation interior plateau of South Africa is dominated by frost-tolerant Karoo shrubland vegetation with small patches of thicket vegetation found in frost-free refugia such as steep slopes and rocky outcrops (Hoare et al, 2006; Mucina et al, 2006; Fig. 1C)
Summary
Frost can hamper the restoration of plant communities (Snowcroft & Jeffrey, 1999; Snowcroft et al, 2000; Curran, Reid & Skorik, 2010; Rorato et al, 2018). When wind speeds are low or wind is absent, atmospheric buoyancy forces can drive the formation of steep temperature gradients—this is caused by denser cooler air decoupling from the free atmosphere and sinking below relatively more buoyant and warmer air (Goulden, Miller & Da Rocha, 2006; Lundquist, Pepin & Rochford, 2008; Dobrowski et al, 2009; Smith et al, 2010; Dobrowski, 2011) Such CAP in flat and low-lying areas is especially common in regions with complex terrain, where changes in elevation and catchments result in intense frost on valley floors while slopes remain frost-free (Barr & Orgill, 1989; Neff & King, 1989; Lindkvist, Gustavsson & Bogren, 2000; Chung et al, 2006; Bigg et al, 2014). This has led to the exclusion of taller, dense thicket vegetation from valley floors where one would expect it to be dominant on the deeper bottomland soils (Becker et al, 2015); this includes P. afra
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