The Mojave Global Change Facility (MGCF) examines the impact of increased summer precipitation, nitrogen deposition and crust disturbance on the Mojave Desert. Given the large number of plots (96) at this facility and the difficulty in making time-intensive field measurements of primary productivity, we were interested in assessing the correlation between plant spectra, synoptic plot images and field measurements. Our hypothesis was that spectra and images would provide an accurate assessment of treatment responses that would correlate with other field measurements and perhaps indicate trends that were not significant in the field data. Field spectra (Unispec field spectrometer with leaf clip) were acquired simultaneous with gas exchange measurements throughout several growing seasons and very low altitude multispectral images were acquired during one growing season. Several spectral indices were calculated including the normalized difference vegetation index (NDV), photochemical reflectance index (PRI), normalized difference pigment index, etc.
The best relationship between net photosynthesis and field spectra was the result of morning data collection and the PRI (r-square 0.65). The NDVI and other red to near infrared indices corresponded well with shoot growth measurements acquired within a similar time frame and revealed significant differences between the control and summer irrigation treatment. While significant differences were not found for the disturbance and nitrogen treatments for all plant species, trends were apparent and particularly on a seasonal basis. For example, disturbance appeared to have a negative impact on individual plant species that were not receiving the summer irrigation treatment regardless of nitrogen treatment. Both the multispectral images and field spectra revealed a significant positive summer irrigation treatment effect even at the start of a new spring growing season. Not surprisingly, the multispectral images revealed a significant increase in whole plot plant cover from the summer irrigation treatment. Subtle negative differences in whole plot cover were also noted for the disturbance treatment but not the two nitrogen treatments. These results show that the summer irrigation treatment had a relatively immediate impact on this Mojave Desert system at both the leaf and whole plot scale during the first five years of treatment. Trends in the impact of disturbance at both plant and whole plot level were apparent within this initial five year period, and we anticipate that this effect will become significant over time.