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EU Horizon 2020 grant No. 776681 (PHUSICOS)..., Cunliffe, A.M., Anderson, K., Boschetti, F., Brazier, R.E., Graham, H.A., Myers-Smith, I.H., Astor, T., Boer, M.M., Calvo, L.G., Clark, P.E., Cramer, M.D., Encinas-Lara, M.S., Escarzaga, S.M., Fernández-Guisuraga, J.M., Fisher, A.G., Gdulová, K., Gillespie, B.M., Griebel, A., Hanan, N.P., Hanggito, M.S., Haselberger, S., Havrilla, C.A., Heilman, P., Ji, W., Karl, J.W., Kirchhoff, M., Kraushaar, S., Lyons, M.B., Marzolff, I., Mauritz, M.E., McIntire, C.D., Metzen, D., Méndez-Barroso, L.A., Power, S.C., Prošek, J., Sanz-Ablanedo, E., Sauer, K.J., Schulze-Brüninghoff, D., Šímová, P., Sitch, S., Smit, J.L., Steele, C.M., Suárez-Seoane, S., Vargas, S.A., Villarreal, M., Visser, F., Wachendorf, M., Wirnsberger, H., Wojcikiewicz, R.

Open- (e.g. grassland, savanna, shrubland) and closed-canopy (e.g. forest) biomes frequently coexist in the same landscape, where open environments tend to be fire-prone with higher light, but lower nutrient and water availability than closed environments. Environmental heterogeneity could select for divergent floristic assemblages and adaptive traits, from which emergent differences in resource availability and fire incidence contribute to excluding species from the alternate habitat. In this thesis, I investigated whether the coexistence of open–closed canopy biomes, such as forest and fynbos in the Cape Floristic Region, is contingent on environmental heterogeneity coupled with contrasting species traits. Given the heterogeneity in multiple environmental properties between open- and closed-canopy biomes, I hypothesized that boundaries between open- and closed-canopy biomes will display greater floristic turnover compared to boundaries between structurally similar biomes (e.g. open- and opencanopy biomes). To explore this, genus- and family-level turnover were correlated with climate, fire, leaf area index (LAI: proxy for understorey light) and soil properties across biome boundaries in South Africa. Both genus- and family-level turnovers were highest across open–closed boundaries and most strongly predicted by increased differences in LAI, suggesting that contrasting light regimes provide significant adaptive challenges for plants. The potential effect of contrasting light regimes is highlighted by the absence of open-canopy species from forest understoreys, where low, dynamic light could limit the ability of plants to acquire sufficient carbon. This apparent shade intolerance led to the hypothesis that open-canopy species lack the traits to maintain a positive carbon balance under low and dynamic light. To test this, leaf traits and photosynthetic response to continuous or dynamic light were compared between forest and fynbos species grown under three light treatments. Fynbos species experienced high mortality under shade treatments, produced leaves that were thicker, up to 1000 times smaller, had lower photosynthetic rates (0.8 versus 3.4mol m-2 s -1 ) under continuous low light (400 mol m-2 s -1 ) and lower light-use efficiency during dynamic light sequences than forest species. These differences imply that shade intolerance in fynbos species is associated with traits that are inefficient at harvesting light and require relatively continuous high intensity light for carbon assimilation. Moreover, these inefficiencies would make it difficult to support the carbon intensive traits (e.g. cluster roots, lignotubers, sclerophyllous leaves) that facilitate fire survival and nutrient acquisition/conservation in open habitats. In contrast, forest species are able to colonize open habitats during the long-term absence of fire, implying that they are able to tolerate high light and low nutrient conditions. Given that plants frequently cope with contrasting conditions through the expression of phenotypic plasticity, it was hypothesized that closed-canopy species possess greater plasticity than open-canopy species. To assess this, the response of leaf traits and foliar nutrition to changes in LAI and soil nutrition were compared between forest and fynbos species in the field. Leaf size and specific leaf area in forest species correlated positively with LAI and soil nutrition, whereas fynbos species response was weak, suggesting that forest species are more plastic. This plasticity may be realised by the variable light conditions forest species experience through their canopy and the occupation of higher nutrient soils, which alleviate belowground constraints. By comparison, the occupation of low nutrient soils by fynbos may inhibit plasticity given the selection of inflexible, conservative leaves. Consequently, I propose that the coexistence of open- and closed-canopy biomes arises from the steep turnover in selective regimes, which together with the contrasting adaptive traits and degrees of phenotypic plasticity they require, act together to competitively exclude species from the alternate habitat.

Blackwater rivers and lakelets are a common feature of the landscape in the South Western Cape. Contrastingly, white rivers can also be found in the region. Key to the colour of blackwaters is the increased presence of dissolved organic carbon. The vegetation of the regions is known to posses' large amounts of polyphenols and potentially low microbial decomposition. Therefore, are the plant-soil dynamics a possible answer to variation in river colour? To answer this, the chemical correlates of rivers in the region, along with the in vitro leaching of the fynbos vegetation compared to other types and the effects of nutrient fertilisation on microbial activity in soil were determined. The results revealed that organic carbon, Fe and pH are significantly correlated with blackwaters. The Fe in the water appears to be bound to humic compounds. Fynbos vegetation is able to produce greater concentrations of polyphenol leachates (315 mg/l humic acid) compared to savanna species (246 mg/ humic acid) over a four-day period. P fertilisations increased the concentrations of humic acids from 30 mg/1 to 200 mg/l in some soils through their affinity to bind with humic compounds. The N fertilisations moderately increased the humic acid concentrations and in some cases lowered the concentration by 10 mg/l, indicating that it provided a nutrient source to the microbes for carbon breakdown. The interactions between humic compounds, nutrients and Fe are key to the formation of blackwaters in the region. Attached to this is the limited decomposition that takes place due to nutrient limitations. Therefore, the interactions between the carbon leached from the vegetation and below ground activities are determinants of water colour in the South Western Cape.

The boundary between forest and fynbos vegetation in the Southern Cape of South Africa present a dynamic ecotone in which forest has previously been noted to have expanded into fynbos territory. Forest colonization of fynbos has been shown to be primarily a function of nutrients and light environments in the understory which may or may not be conducive for the growth of forest species. Additionally, it is understood that fire regime is the primary agent in determining forest/fynbos boundaries. Virgilia oroboides is a fast-growing leguminous tree confined to the margin between forest and fynbos. It has long been thought to facilitate the growth of forest species through the heavy shading and nutrient enrichment of soils; both of which facilitate the growth of forest seedlings. Furthermore, V. oroboides is predicted to reduce fuel loads and as a result prevent fire penetration of forests. These possible functions were tested by sampling soils, nutrient content of leaves and the shade cast by plants 18 months after a burn on a forest margin near Swellendam, Southern Cape. In addition, the effects of varying density of Virgilia on surrogates for fuel biomass, and forest seedling growth, were observed at Silvermine on the Cape Peninsula. Shaded environments produced by V. oroboides are shown to have detrimental effects on the growth of fynbos, while facilitating the growth of forests. V. oroboides had the highest soil nutrient enrichment potential of all species investigated and is therefore predicted to produce nutrient cycling processes conducive to the growth and regeneration of forests. Estimates of fuel in the understory of V. oroboides are predicted to reduce fynbos fire severity upon reaching the margin. These results suggest that Virgilia does indeed facilitate forest tree species more than light-demanding fynbos species so that its presence should result in greater stability or slow expansion of forests into fynbos. If supported by further work, this facilitative role of Virgilia has management implications for the conservation of fynbos and forests in the Southern Cape of South Africa.