A study on an altitudinal gradient investigating the potential effects of climate change on fynbos and the Fynbos-Succulent Karoo boundary

[摘要] Global circulation models predict that the Cape Floristic Region (CFR), a biodiversity hotspot, inthe near future will be subjected to rising temperatures and widespread droughts as a result ofrising atmospheric CO2 causing global climate change. It is predicted that climate change willlead to a southward shift of the Succulent Karoo, a neighbouring more drought tolerant biome,and a possible invasion of Fynbos, the main vegetation type of the CFR, by succulent species.In this research project, the effects of climate change on Fynbos, and the likelihood of SucculentKaroo invading Fynbos are assessed by means of various monitoring and experimental studieson an altitudinal gradient spanning a natural transition between fynbos and succulent karoovegetation. An analysis of plant species diversity and turnover on the gradient revealed highspecies turnover between succulent karoo and the rest of the gradient, associated with aboundary between two soil types: shale (associated with succulent karoo) and sandstone(associated with fynbos). Phenological monitoring of fynbos species across the gradient showedhow growth of fynbos species is affected negatively by high temperatures, and that low butregular rainfall is required to sustain growth during the dry Mediterranean summer.Retrospective growth analysis of Proteaceae species pairs with contrasting range sizes revealedthat small geographic ranges do not signify low tolerance of climate variation, but rather thatfaster growing species are more sensitive to interannual climate variation than slow growingspecies. Exposing fynbos species to experimental drought confirmed that faster growing specieswill be more severely affected by climate change than slow growing species with conservativewater use strategies. This experiment also confirmed the importance of rainfall reliability forgrowth in fynbos species when a naturally occurring prolonged dry period affected some speciesmore severely than the drought treatment of an average reduction in rainfall. A reciprocaltransplant experiment exposed fynbos seedlings to both warmer and drier conditions when theywere planted outside of their natural ranges in the succulent karoo. Soil type as a barrier toinvasion of fynbos by succulent karoo was also tested. Soil type was found to be not limiting tosucculent karoo species and competition and disturbance was revealed to be more important indetermining the fynbos-succulent karoo boundary than climate. It was concluded thatproductivity in fynbos will be adversely affected by rising temperatures and that differingresponses to climate change between slow and fast growing species will lead to shifts indominance among species, and consequently altered community structures and vegetationdynamics. Fires are likely to facilitate invasions of marginal habitats by succulent karoo becauseof sensitivity of fynbos regeneration stages to high temperatures and drought.

[发布机构] Stellenbosch University