Comparing phenology of Chihuahuan desert perennial grasses and shrubs between normal and below-average precipitation years

Background/Question/Methods

Arid and semi-arid regions of the southwestern USA are especially sensitive to changes in temperature, drought frequency and intensity. Observations of plant phenology in these water-limited environments serve as salient indicators of plant responses to changing climate. Repeated consistent phenology observations in conjunction with local climatological (e.g., precipitation, temperature) and soil (e.g., moisture, temperature) measurements are key to elucidating the role of abiotic drivers in the timing of lifecycle events.

Weekly observations of plant phenology were initiated in Mar 2010 at a Soil Climate Analysis Network (SCAN) site on the Jornada Experimental Range in southern New Mexico. Weekly observations for 20 individuals across three functional groups - perennial grasses (Aristida purpurea, Sporobolus spp., and Pleuraphis mutica), the deciduous shrub honey mesquite (Prosopis glandulosa) and the evergreen shrub creosote (Larrea tridentata) – were conducted using National Phenology Network (NPN) protocols. Phenology patterns by functional group were compared and examined in the context of precipitation. We determined dates of first production of leaves (initial growth), open flowers and fruits; in addition, we calculated the time from initial growth to full canopy and from flower to fruit production for 2010 and 2011 growing seasons.

Results/Conclusions

Rainfall in the 2010 rain year (213.4-mm) was near the long-term average (245-mm). Aristida and Sporobolus grasses initiated growth prior to 22 Mar 2010. Mesquite initiated growth later (19 Apr 2010), but achieved full canopy within an average of 4.5 weeks. In contrast, canopy development for grasses in 2010 occurred over 22 weeks. Time from first flower to fruit for grasses in 2010 ranged from 2 (for Sporobolus and Pleuraphis) to 2.7 weeks (for Aristida). Rainfall in the 2011 rain year was far below average (89.4-mm) and encapsulated 242 consecutive dry days. Initial growth for all grasses was delayed until 11 Jul 2011, presumably due to deficit rainfall conditions. In contrast, mesquite and creosote shrubs initiated growth prior to the first rain event after 08 Oct 2010 (06 Jun 2011). We hypothesize that changes in soil temperature at depth trigger initial growth for deep-rooted shrubs. No grasses produced flowers or fruit in 2011. Next steps are to evaluate the soil temperature hypothesis with soil measurements at the SCAN site and examine relationships between environmental variables and phenophase transitions using a proportional hazards model.