Diversity and drought: Exploring hydraulic failure and carbon limitation of plant functional types in a South African shrubland

Background/Question/Methods

The global increase in drought-related tree mortality has sparked renewed interest in mortality mechanisms in plants. However, the impact of drought on highly diverse ecosystems is not well understood. Drought-related plant mortality is of particular concern in the Fynbos Biome of South Africa, a well-known global biodiversity hotspot. This winter-wet, summer-dry region is predicted to experience an increase in regionally acute drought, with unknown consequences for the endemic flora. We asked whether drought impacted key functional types in this diverse ecosystem in a predictable manner, and if these patterns fit existing frameworks of drought mortality mechanisms in plants. We conducted two independent, multi-year drought studies in Mountain Fynbos. The first was a 3-year rain-exclusion experiment on the Cape Peninsula. The second was a 2-year sapflow study conducted 100km inland from the former study site, but in a similar vegetation type. For both studies, a variety of responses to moisture stress were measured in the three key functional types: large proteoid shrubs, small, heath-like Erica shrubs and graminoid restioids. Species varied between sites allowing us to explore whether responses were species- or functional-type specific. Key traits measured were water potential, vulnerability to cavitation, gas exchange, sapflow and growth.

Results/Conclusions

Our two studies showed remarkably consistent results within the functional types. The shallow-rooted Erica shrubs experienced considerable water stress, with 80% loss of conductivity under normal summer conditions, and 100% during the more severe experimental drought. A model of leaf-level carbon gain indicated extended periods of carbon deficit during drought for Erica. Thus Erica shrubs experienced both high levels of hydraulic failure and carbon limitation during drought. In contrast, the proteoid shrubs showed little evidence of water stress during both the natural and experimental droughts. The combination of deep-rooting and stomatal regulation of water potential resulted in <25% loss of conductivity and few periods of carbon deficit during the summer drought. Surprisingly, the shallow-rooted restioids showed little signs of drought stress, maintaining high water potentials (>-2MPa), as well as positive gas exchange, through both natural and experimental droughts. High vulnerability to cavitation of the culms resulted in up to 90% loss of conductivity over the drought, yet with few periods of carbon deficit. Our studies provide increasing support for the use of 1) functional types to characterize drought responses in this diverse shrubland, and 2) a hydraulic framework for predicting drought-related mortality under future climate.