Tradeoffs between hydraulic efficiency and mechanical strength in the stems of four co-occurring species of chaparral shrubs

Possible tradeoffs between efficiency of water transport and mechanical str ength were examined in stems of two congeneric pairs of co-occurring chapar ral shrubs. First, since previously published results indicated that Adenos toma sparsifolium (Rosaceae) had greater specific conductivity (k(s) or hyd raulic conductivity per xylem transverse area) than A. fasciculatum, it was hypothesized that A. sparsifolium would have greater vessel lumen area per square millimeter of xylem area, and less mechanical strength, than A. fas ciculatum. Secondly, since Ceanothus megacarpus (Rhamnaceae) is a non-sprou ter (unable to sprout from the root crown following fire or other major dis turbance) whereas C. spinosus is a sprouter and thus able to form new stems following disturbance, it was hypothesized that C. megacarpus would have g reater mechanical strength, but lower k(s), than C. spinosus. Both hypothes es were supported. Based upon computer-aided image analyses, A. sparsifoliu m had significantly higher mean and maximum vessel diameters (16.4, 40.5 vs . 14.6, 35.7 mu m), a 34% greater percent vessel lumen area, and a two-fold greater measured and theoretical k(s) than A. fasciculatum. This correspon ded to 14% lower stem density (wet weight/volume) and less mechanical stren gth, with a 37% lower modulus of elasticity (MOE) and a 30% lower modulus o f rupture (MOR) than A. fasciculatum. Similarly, C. spinosus had a signific antly higher maximum vessel diameter (52.7 vs. 41.8 mu m) and a 92% higher theoretical k(s) (and 43% higher measured k(s)) than C. megacarpus. This co rresponded to a 9% lower stem density and 20% lower MOR than for C. megacar pus. Thus, at least within these two congeneric pairs of chaparral shrubs g rowing together in the same habitat, there may be tradeoffs between mechani cal strength and conductive efficiency of the stem xylem which correspond t o differences in transport physiology and life history traits of sprouter v ersus non-sprouter species.