COS 32-9
Cross-scale perspectives on patterns and environmental cues driving plant phenology in an arid upland grassland
Projected changes in rainfall for the western United States are uncertain with respect to seasonality and direction. In many spatially-extensive arid and semi-arid environments in the western U.S., plant community responses to rainfall are dramatic and marked by high interannual variability. Plant phenological patterns (e.g., initiation of growth and the production of flowers and fruit) are discrete plant responses to changing climate and indicators for ecosystem services such as net carbon exchange and pollination. To elucidate the role of environmental drivers in the timing of phenological events, repeated consistent phenology observations are required along with coincident climatological and soil moisture measurements. We coupled daily phenocam estimates of canopy greenness with weekly field observations of plant phenology to evaluate the effectiveness of greenness metrics as a proxy for field estimates of percent green and examine the role of precipitation on canopy development. Daily phenocam greenness estimates and weekly field observations of phenology including ocular estimates of percent canopy greenness were made for deciduous C3 shrubs honey mesquite (Prosopis glandulosa) and C4 perennial black grama grasses (Bouteloua eriopoda) at an arid upland grassland site in southern New Mexico through two growing seasons (Feb 2012 - Dec 2013).
Results/Conclusions
Field estimates of canopy greenness closely corresponded with greenness index values providing confidence in interpretations of the greenness responses derived from phenocam images. Mesquite transitioned from minimum to maximum greenness over 15 days between 20 Apr and 5 May in both years. This pattern coincides with long-term phenology patterns in which mesquite leaf out has occurred in April in 75% of 90 observations prior to summer rainfall. Black grama green up quickly follows summer rain events over six days in 2012 (8-13 Sep) and 2013 (11-16 Aug). The synchronous decline in greenness for mesquite and black grama suggests that minimum air temperature or changes in day length may trigger senescence. Daily depictions of greenness demonstrate that canopy development in this water-limited system occurs rapidly and that phenocams can provide data needed to characterize greenness; however field sampling once to twice weekly is required to monitor flower and fruit or seed production. Next steps are to evaluate the soil temperature hypothesis and examine relationships between environmental variables and phenophase transitions using a proportional hazards model.