Mortality and recruitment of desert perennials as related to extreme drought: The possible elimination of drought deciduous shrubs from low elevations

Background/Question/Methods The deserts of western North America have been subjected to drought for most of the last decade.  The drought has been particularly extreme in the northwestern Sonoran Desert where low rainfall has been accompanied by increasing average yearly high temperatures, conditions which are believed to be result of climate change.  Due to the drought, a high percentage of perennials have died and recruitment for perennials is extremely rare.  Because temperature and rainfall are influenced by topography, especially in arid environments, the mortality and recruitment of perennials was studied with respect to altitude at the Boyd Deep Canyon Research Center in Palm Desert, California.  It was hypothesized that because temperature decreases and rainfall increases with increases in elevation, desert perennials should experience decreased mortality and increased recruitment with increases in elevation under the current climate conditions.  Furthermore, shallow-rooted drought deciduous shrubs should experience the greatest mortality among perennials because they depend on pulsed rainfall events more than deeply-rooted evergreens and phraetophytes and they do not have enough stored water to persevere long dry periods without rainfall like stem and leaf succulents.  Results/Conclusions At 250 m elevation, 59% of the perennials had died and 92% of the drought deciduous shrubs, which made up 50% of the perennials, were dead.  Mortality of perennials was higher at a 500 m elevation site where 65% of the perennials had perished and 70% of the drought deciduous shrubs, which represented over 90% of the perennials at this elevation, had died.  At 820 m elevation, 31% of perennials were dead and 42% percent of drought deciduous shrubs, which made up 56% of the perennials, were dead.  Recruitment of drought-deciduous shrubs, particularly Encelia farinosa, which represented over 97% of all perennial recruits, was prominent at the two higher elevation sites but was essentially non-existent at 230 m over the last few years.  Thus, the combination of high mortality and low recruitment is decimating the perennials, primarily the drought deciduous shrubs, at the low elevation site.  It is believed that if the drought in this region of the Sonoran Desert continues as has been predicted in some climate change models, that drought deciduous shrubs may be virtually eliminated from the lower elevations in the northwestern Sonoran Desert for the near future.