The vegetation of the Jalisco-to-Gila migratory corridor is structured in part by regional physiography - the spine of the Sierra Madre Occidental on the east, the Pacific and Gulf of California on the west, Basin and Range topography characterized by rocky uplands and broad valleys in the north. Regional phenology - the temporal pattern of leaf, flower, and fruit production - is largely determined here by highly seasonal rainfall.

This time series shows the monthly vegetation greenness for the Jalisco-to-Gila migratory corridor based on AVHRR satellite imagery from April 1992 to March 1993. The AVHRR sensor orbits Earth on a satellite and acquires images of almost every square kilometer on the Earth on a daily basis. The apparent greenness of the vegetation in the eye of the satellite is converted to numerical values.

These vegetation index values are displayed using a color gradient ranging from brown to yellow to green. The images below are monthly composites of vegetation greenness. The color for each square kilometer pixel in the image represents the greenest stage of that pixel during that month.

VEGETATION LEGEND BROWN AREAS - bare ground or sparse vegetation (the lowest parts of the Sonoran Desert around the northern Gulf of California, and the Mojave Desert and Great Basin in California and Nevada) - sometimes clouds and snow (but sometimes these appear white, along with bodies of water) YELLOW AREAS - grasslands (parts of Sonora and Southern Arizona) - desertscrub (drier shrubland and middle-elevation forest) GREEN AREAS - evergreen forests at higher elevations (the Sierra Madre and the Sky Islands) - deciduous forest during the monsoon - agricultural fields The image for October 1992 is shown at right. At this resolution, you can even see the riparian and agricultural vegetation along the Gila and Lower Colorado Rivers in Arizona, the Rio Grande in New Mexico, and the Arkansas River in southern Colorado.

April 1992	May 1992	June 1992	July 1992	August 1992	September 1992
October 1992	November 1992	December 1992	January 1993	February 1993	March 1993

The sequence of images follows the phenology of the vegetation in the region. Notice the change in greenness from May, the driest month along the entire coastal route, through October. Follow this link to compare MODIS images of west Mexico from May and October 2001, side by side.

The summer monsoon is particularly important in the south, and influences the productivity of the forest even during the ensuing winter months. The extent of snow cover (darkest brown) during the winter months in the northern portion of the area shown (Nevada, Utah, and Colorado) is also apparent in this time series, although snow rarely occurs in the desert and coastal plains.

Where does it begin and end?

Migratory creatures leave southern Mexico in January or so, and return in the Fall. All parts of their corridor are important to them, even when they are away -- the plant resources that pollinators need for their travels must grow and store energy year round to provide this important service.

Vegetation, especially in this arid region, is always vulnerable to variations in climate and human land uses, both of which can complicate the important job of making flowers and nectar!

News Item:

Ruby-throated Hummingbirds reach the U.S. (NASA Earth Observatory, 21 Mar 2007)
http://earthobservatory.nasa.gov/Newsroom/NewImages/images.php3?img_id=17589

References:

Global Land 1-KM AVHRR Project
http://edcdaac.usgs.gov/1KM/1kmhomepage.html

Holbrook, N. M., J. L. Whitbeck, & H. A. Mooney. 1995. Drought responses of dry tropical forest trees. In Seasonally Dry Tropical Forests, S. H. Bullock et al. (eds.), pp. 243-276. Cambridge University Press, New York.

Maass, J. M., J. M. Vose, W. T. Swank, & A. Martínez-Yrízar. 1995. Seasonal changes of leaf area index (LAI) in a tropical deciduous forest in west Mexico. Forest Ecology and Management 74: 171-180.

Martínez-Yrízar, A., J. M. Maass, L. A. Perez-Jimenez, & J. Sarukhan. 1996. Net primary productivity of a tropical deciduous forest ecosystem in western Mexico. Journal of Tropical Ecology 12: 169-175.

Mora, F. & L.R. Iverson. 1998. On the sources of vegetation activity variation, and their relation with water balance in Mexico. International Journal of Remote Sensing 19 (10): 1843-1871.