Summary1 Although drought frequency and severity are predicted to increase across numerous continental interiors, the consequences of these changes for dominant plants are largely unknown. Over the last decade, the south-western US has experienced six drought years, including the extreme droughts of 1996 and 2002, which led to widespread tree mortality across northern Arizona. 2 We examined the impact of these droughts on the co-dominant tree species of the pinyon-juniper woodland ( Pinus edulis and Juniperus monosperma ), a major vegetation type in the US. 3 Pinyon mortality following both droughts was 6.5-fold higher than juniper mortality. In addition, large pinyons suffered 2-6-fold greater mortality than small pinyons, a pattern associated with higher mortality of reproductively mature trees and survival of smaller pinyons resulting from facilitation by established vegetation. Differential mortality of large pinyons resulted in a vegetation shift such that the pinyon-juniper woodlands are becoming dominated by juniper, a species that is typical of lower elevations and more arid conditions. 4 Sites that experienced high pinyon mortality during the first drought suffered additional mortality during the second drought, so that reductions in tree densities and the resulting release from below-ground competition did not buffer surviving pinyons against additional mortality during the second drought. Such repeated mortality events also suggest that these stands may suffer chronic stress. 5 Reductions in biotic associations (e.g. avian seed dispersers, ectomycorrhizas and nurse plants) that will probably result from extreme mortality of large pinyons ensure that the observed vegetation shifts will be persistent. Because approximately 1000 species are associated with pinyon pine, the shift in the structure of these woodlands has large-scale community consequences.

SUMMARY Elasmobranchs, particularly sharks, function at speed and size extremes,exerting large forces on their cartilaginous skeletons while swimming. This casts doubt on the generalization that cartilaginous skeletons are mechanically inferior to bony skeletons, a proposition that has never been experimentally verified. We tested mineralized vertebral centra from seven species of elasmobranch fishes: six sharks and one axially undulating electric ray. Species were chosen to represent a variety of morphologies, inferred swimming speeds and ecological niches. We found vertebral cartilage to be as stiff and strong as mammalian trabecular bone. Inferred swimming speed was a good, but not infallible, predictor of stiffness and strength. Collagen content was also a good predictor of material stiffness and strength, although proteoglycan was not. The mineral fraction in vertebral cartilage was similar to that in mammalian trabecular bone and was a significant predictor of material properties.

Inaccurate age estimates can have severe consequences in the management of elasmobranchs. Numerous studies in shark age validation have demonstrated a disconnect between band pair counts and age, resulting in age underestimation, particularly in older individuals. To investigate the relationship between band pairs, vertebral shape and growth, we quantified intracolumn differences in centrum morphology (size and structure) and band pair counts in seven shark species: Squatina dumeril, Carcharodon carcharias, Lamna nasus, Isurus oxyrinchus, Alopias vulpinus, Prionace glauca and Carcharhinus obscurus. In all species examined, band pair deposition was closely related to body girth and the structural properties of the cartilaginous skeleton, relative to maximum size, and body type. These results have strong implications for accurately assessing age for fisheries management of these species.

SUMMARY Elasmobranch vertebral cartilage has a substantial mineral fraction(39–55%) and the arrangement of mineral varies among species. We examined vertebrae from one shark species, Mustelus californicus, to determine mineral content, the effect of mineral on material properties and the viscoelastic response of vertebral cartilage. We serially demineralized vertebrae and compressively tested them to failure at varying strain rates. Mineral in vertebral cartilage varies within individuals, intraspecifically and interspecifically; this is in contrast to bone, in which significant variation in mineral content is pathological or an interspecific effect. Within Mustelus, vertebrae with larger mineral fractions were significantly stiffer and stronger; however when variation is assessed across species, the structure has a larger effect. Shark vertebral cartilage did not show a substantial viscoelastic response at biologically relevant strain rates, validating the use of quasistatic testing for this material.

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