FightingTheFreeze: A biome level comparison of freezing resistance from a study site in the GCFR year-round rainfall regime

These analyses are aimed at exploring data on freezing tolerance collected during 2012. Part of this data has been published in Duker et al. (2012; Journal of Biogeography), but here we are exploring the freezing tolerance of species from across the fynbos, grassland, thicket and nama-karoo biomes.

Below is a per species summary of the control (blue line/points) and exposed (red line/points) photosynthetic efficiency values for each species across the three treatments (-4, -7 and -10 degrees C). The order is sorted by biome (AT: Thicket, FY: Fynbos, GS: Grassland, NK: Nama-karoo), and then ordered from highest susceptibility to lowest within each biome.

So, how does this translate into species-averages per biome? The three boxplots below shows the decline in photosynthetic efficiency (Exposed samples - Control Samples) averaged for each species and presented per biome per treatment. Each subplot represents a treatment: -4, -7 and then -10. Significance testing used non-parametric statistics.

---

Ok, so Fynbos and Grassland seems to have species much lower freezing tolerance than Nama-Karoo (that makes sense), but also Thicket (that’s a surprise!). What is going on at a species level? (The first figures are quite difficult to visually parse across species). The -4 and -7 degree C treatments don’t seem to have much influence at a biome level (although NK is sig. higher than FY for -7), so below is just the per species declines in photosynthetic efficiency at the -10 degree C treatment.

---

Summary

My original thinking was that the nanophyllous and sclerophyllous leaves of the fynbos and grassland would provide an inherent protection from freezing. This is, most clearly, not the case. Some of the smallest-leaved species are the most susceptible!!! I would not have predicted Metalasia denudata to have been the most affected by freezing!

Some questions…

• Do we have any idea of minimum temperatures of high elevation sites in the Cape? I realise that many of the mountains experience snow, but I think there is a distinct difference between the effects of snow and frost on plant physiology.
• Are our fynbos and grassland species an adequate representative sample? (Based on genera, it certainly looks like it, but it would have been nice to have had a member of the Restionaceae).
• My interpretation of these results is that Fynbos lineages may have come from arid-adapted rather than cold-adapted ancestors, if phylogenetic niche conservatism is still at play. And this makes complete sense if we take Tony Verboom’s short-term post-fire Fynbos invasion by what we today consider Renosterveld lineages.