Relationship between Photosystem Physiology, Phenology, Reflectance and Ecosystem CO2 Flux in a Chihuahuan Desert Shrubland

Background/Question/Methods

By mid century, arid ecosystems will likely comprise the largest terrestrial biome on the planet – largely as a result of anthropogenic disturbance and climate change. The size, changing extent, increased prevalence of shrubs, and large pool of soil carbon are just some of the underlying reasons why it is important to advance our understanding of biogeochemical cycling and energy balance in these landscapes and how change may alter feedbacks with other components of the Earth System. Although substantial progress has been made over the past decade, few studies have simultaneously examined how plant stress can constrain larger scale phenomenon (e.g. plant and landscape phenology and land-atmosphere trace gas and energy flux), and how large scale phenomenon (e.g. climatic extremes and variability) can impact relatively small scale processes such as plant photosynthetic stress. This study, conducted in a creosote shrubland on the USDA Jornada Experimental Range in southern New Mexico during 2012-13, documents seasonal changes in the chlorophyll fluorescence of five common plant species, and compare these with time series for plant and landscape phenology (NPN protocols and phenocams), land-atmopshere carbon, water and energy exchange (eddy covariance); and plant to landscape reflectance (hyperspectral reflectance collected from a robotic tram system). 

Results/Conclusions

Results span August 2012 - January 2013. Electron Transport Rate (ETR) was higher and peaked earlier (late August – mid September) for deciduous shrub species (Prosopis glandulosa, Flourensia cernua), followed by the evergreen shrub Larrea tridentata, which peaked mid-late September. ETR was lowest for the graminoid Muhlenbergia porteri, which peaked early September. ETR trends reflect relative lag responses to rainfall events in August and early September. ETR declined with air temperature for all species but the response was slowest for Larrea tridentata. Declines in ETR followed the onset of leaf senescence for all species except Larrea tridentata. Peak landscape greenness detected with a network of phenocams and the robotic tram system (2GRBI and NDVI respectively), occurred mid - late September and followed the peak in ETR for all species by approximately 1-3 weeks. The same lag between ETR and landscape level Gross Ecosystem Exchange (eddy covariance) was also noted. Ongoing analyses will explore this time series data through July 2013. So far, it appears that plant photosystem responses lag climatic events, but precede processes measurable at landscape scales. Thus, photosystem responses may serve as ‘early indicators’ of future landscape level ecosystem responses to climate variability.