Diverse functional responses to drought in a Mediterranean-type shrubland in South Africa

Mediterranean-type ecosystems contain 20% of all vascular plant diversity on Earth and have been identified as being particularly threatened by future increases in drought. Of particular concern is the Cape Floral Region of South Africa, a global biodiversity hotspot, yet there are limited experimental data to validate predicted impacts on the flora. In a field rainout experiment, we tested whether rooting depth and degree of isohydry or anisohydry could aid in the functional classification of drought responses across diverse growth forms. We imposed a 6-month summer drought, for 2 yr, in a mountain fynbos shrubland. We monitored a suite of parameters, from physiological traits to morphological outcomes, in seven species comprising the three dominant growth forms (deep-rooted proteoid shrubs, shallow-rooted ericoid shrubs and graminoid restioids). There was considerable variation in drought response both between and within the growth forms. The shallow-rooted, anisohydric ericoid shrubs all suffered considerable reductions in growth and flowering and increased mortality. By contrast, the shallow-rooted, isohydric restioids and deep-rooted, isohydric proteoid shrubs were largely unaffected by the drought. Rooting depth and degree of iso/anisohydry allow a first-order functional classification of drought response pathways in this flora. Consideration of additional traits would further refine this approach.