Patterns in reproductive phenology for dryland grasses and shrubs from 1993 to 2010 in the Chihuahuan Desert

Background/Question/Methods

Arid and semi-arid regions of the southwestern USA are especially sensitive to changes in temperature as well as drought frequency and intensity. Timing of periodic life cycle events (i.e., phenology) is an integrated and salient indicator of plant responses to climate change. Spatially extensive phenology observations in these environments are uncommon and with few exceptions, there is a paucity of long-term records collected using standardized protocols. We examine an 18-year dataset of monthly observations of reproductive phenology for two species of perennial grasses and a deciduous shrub distributed across three upland grassland sites and three mesquite-dominated sites on the Jornada Basin LTER in southern New Mexico, USA. Our objective was to compare and contrast timing and duration of flower and fruit production for two perennial grass species (Bouteloua eriopoda [black grama],  Sporobolus flexuosus [mesa dropseed]) with one co-existing shrub that has historically displaced grasses in this system (Prosopis glandulosa [honey mesquite]). Field observations are made monthly along two 75-m x 1-m belt transects at each site and constitute a count of individual plants in each of five phenophases: (1) dormant, (2) leaf-out with no reproductive structures, and producing (3) flowers, (4) seeds, or (5) fruits. The protocol simultaneously yields estimates of status and abundance.  

Results/Conclusions

The appearance of first fruit for grasses (i.e., grains) occurred consistently in August or September, although the number of plants producing fruit was highly variable from year to year. Mesquite demonstrated remarkable synchrony in the production of first leaves (in April), flowers (in May), and fruit (in June) across four sites. The appearance of first fruit and peak fruit production was most often in the same month for mesquite and mesa dropseed. The time from first to peak fruit production in black grama was one month at two sites and in the same month at a third, suggesting local variability in rainfall or soil moisture dynamics may play a role in peak fruit production. Future analyses will examine relationships between the amount and seasonal distribution of rainfall with timing and abundance of fruit production. Understanding the role of abiotic factors in influencing the timing and success of reproductive events is especially relevant to the conservation and restoration of species of concern as well as in predicting plant responses to changes in climate.