Hydraulic trait variation in Protea repens with change in climate in space and time

Master Thesis


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University of Cape Town

Global climate change and atmospheric CO₂ concentrations are affecting all levels of biodiversity in a number of ways. For example, the unique vegetation of the Cape Floristic Region (CFR) is expected to experience increased temperatures while rainfall becomes more seasonal, resulting in stronger summer drought with greater hydraulic stress in plants. Increased CO₂ concentrations, on the other hand, are expected to relieve hydraulic stress in plants that utilize the C3 pathway for photosynthesis (most fynbos species), by reducing the amount of time they have to keep their stomata open to take up the CO₂ they require. Observed weather data suggest that rainfall has remained relatively stable over the last 21 years, while temperatures for the region have increased marginally. Here I explore variation in the hydraulic traits (leaf and xylem anatomy) of Protea repens (L.) across a spatial climatic gradient in the CFR relative to a common garden experiment. I then compare the contemporary trait-climate relationships with a 21 year old xylem anatomy dataset. In the common garden experiment I explore xylem and leaf trait variation in P. repens from thirteen populations representing a gradient in temperature and mean annual precipitation. Because trait-gradient relationships can be confounded by genetic differences between populations along the gradient, I used a common garden experiment to test the degree to which trait variation was genetically constrained among populations. My results show that xylem vessel diameters and an estimate of hydraulic conductance increased with increases in maximum temperature and soil moisture days across the spatial gradient. My results for the common garden experiment does however show genetically constrained intra-specific differences in xylem vessel morphology between populations. Despite this, differences in xylem vessel and leaf morphology between plants in the common garden and their source locality demonstrate that P. repens has some ability to respond to changes in the environment through phenotypic plasticity. To determine the response of P. repens to changes in climate over the past 21 years, I compared contemporary xylem anatomy to an existing dataset collected from the same sites in 1994. My results show no significant change in vessel diameters since 1994 even though temperatures and atmospheric CO₂ have increased, with no change in rainfall amount. These results suggest that either P. repens is not experiencing increased drought stress under current climate conditions, or that xylem vessel anatomy is not a good proxy for small changes in drought stress in this species. The effect of increased drought stress due to higher temperatures and associated evaporative demand may be alleviated by increased atmospheric CO₂ reducing the amount of time the plants have to keep their stomata open to take up the CO₂ they require. Similarly, it is possible that drought stress has not changed substantially over this period, because a reduction in wind run across the CFR may have balanced the increase in evaporative demand created by higher temperatures. At a plant level, P. repens may not respond to small increases in drought stress by utilising deep water. In addition, P. repens is potentially able to reduce stomatal conductance thereby alleviating xylem anatomical responses to the small change in temperature since 1994.