Xylem Transport Safety and Efficiency Differ among Fynbos Shrub Life History Types and between Two Sites Differing in Mean Rainfall

Abstract

Xylem safety and efficiency were analyzed for stems of evergreen shrubs that inhabit fynbos communities in the Mediterranean-type climate region of South Africa. We hypothesized that species with different life history types would differ in xylem function on account of their different regeneration niches. Comparisons were made among postfire nonsprouters, facultative sprouters, obligate sprouters, and opportunists. Measurements included xylem resistance to water stress–induced cavitation (xylem safety) and xylem-specific hydraulic conductivity (xylem efficiency) at a dry site and at a wetter site. Life history types differed in hydraulic traits: xylem safety was greater in life history types with disturbance-dependent recruitment. By contrast, water stress resistance was lowest in postfire obligate resprouters that recruit seedlings during fire-free intervals in the litter layer of shady microsites. Among life history types, greater xylem safety came at the cost of reduced hydraulic efficiency. This pattern was also observed between field sites, with most taxa from the drier site having greater levels of cavitation resistance and lower levels of xylem-specific hydraulic conductivity than taxa from the wetter site. We conclude that xylem traits are linked to differences in life history types in fynbos species.