Regional variation in xylem anatomy of a wide-ranging desert shrub, Larrea tridentata

Tyler, Anna

Brief Discussion: The author studied reginal variations in xylem anatomy of Larrea tridentata populations found in the Chihuahuan, Mojave, and Sonoran Desert. Full Discussion: Climate is a strong determinant of plant species’ distributions and community structure. Xylem structure is critical to maintaining optimal water transport to the leaves, and is an important determinant of plant tolerance to climate extremes such as drought and freezing. However, neither inter-population variation in xylem anatomy nor its relationship t climate is well understood. To evaluate the variability in xylem anatomy across climatic gradients, I sampled stems of the wide-ranging, drought-tolerant shrub, Larrea tridentata, across its southwestern USA, range in the Chihuahuan, Sonoran and Mojave deserts. Sampling across the species range incorporated variation in freezing frequency, long-term minimum temperature and amount and timing of precipitation. In addition, L. tridentata ploidy ranges from the diploid populations in the Chihuahuan desert to hexaploid populations in the Mojave desert, and with the Sonoran desert dominated by tetraploid populations but with regions of mixed ploidy. Across the three regions, colder long-term minimum temperature and higher mean annual precipitation were significantly correlated with smaller diameter vessels. The level of ploidy, however, explained more variation in L. tridentata xylem conduit diameters across the three regions than climate. Plants from the Chihuahuan desert diploid region, with the highest average freeze frequence, the coldest minimum temperatures, and the least amount of winter precipitaion, had the smallest mean diameter vessels and the most narrow range of diameters vesus plants from the polyploid Mojave and sonoran retions. Although increased freezing frequency has been shown t be directly related to smaller xylem diameters across speicies, it did not strongly explain variaiton in conduit diamters across L. tridentata populations in this study, and mmy data provide only mixed support for this trend in L. tridentata within each individual ploidy region. The percent of standing dead sood (dieback) on L. tridentat shrubs from the Chihuahuan diploiid region was also significantly greater than shrubs from the Mo[h]ave hexaploid or the Sonoram tetraploid regions.