Effects of increased heat and drought stress approximating climate warming on the reproduction, photosynthesis and growth of Proteaceae species in a southern African Mediterranean climate ecosystem

Abstract

In this research project, the effects of warming and drought on three Proteaceae species (Protea repens, Protea laurifolia, Leucadendron laureolum) were investigated through various monitoring and experimental studies on an altitudinal moisture gradient. This study tested the hypothesis that reproduction, photosynthetic capacity and growth of selected Proteaceae species from warm, arid sites at low elevations along the gradient will be more resilient to heat and drought stress accompanying climate warming than those from cool humid sites at high elevations along the same gradient. Experimental approaches were applied to obtain numerical data on diurnal stem diameter variations, sap flow rates, vapour pressure and photosynthetic capacity of Protea repens growing at five different climate stations along an altitudinal moisture gradient. Enforced seed dormancy was examined by calculating seed germination in 11 Proteaceae species in experimental mesocosms. Germination responses (% germination) of three Proteaceae species growing along a moisture gradient were tested and also in a greenhouse to increase the understanding of the impact of changing environmental conditions on germination. Drought resilience was tested on one-year-old seedlings of 16 Proteaceae species in a greenhouse. The results of this study have indicated that the lowest elevation exhibited lower dewfall over the entire experimental period compared to the dewfall at the highest elevation. Protea repens indicated significantly negatively correlations in total daily amplitudes in sap flow vs station maximum diurnal temperature during four seasons. Proteaceae germination indicated that measured reductions in seedling recruitment were closely associated with increases in diurnal soil temperature minima and maxima. Measured transpiration rates declined linearly with increasing duration of drought and decreasing soil volumetric content in all 16 Proteaceae species. The test species proved to be highly tolerant of water stress/drought and are well adapted to Mediterranean climatic conditions. This study emphasizes the importance of the non-rainfall precipitation as an insignificant, continuous supply of water to dry and semi dry areas, and also it provides an important supply to the areas water equilibrium, particularly during low rainfall episodes. Findings indicate that Proteaceae seedlings are tolerant of summer dry periods over a large part of the South African Cape Floristic Region, and that enforced seed dormancy, is induced by elevated night-time temperatures. However, the post-fire stage among Proteaceae is most germination of Proteaceae is highly dependent on temperature. It was concluded that Proteaceae reproductive stages, growth and survival are sensitive to drought, higher temperatures and water availability. Long term monitoring, more test species and more climate change experiments is needed to get more results to fully explain the effects of temperature and drought on Proteaceae. This study has contributed to our understanding and knowledge of climate change effects on Proteaceae.