The introduced riparian tree, Russian-olive (Elaeagnus angustifolia L.), is abundant in the interior western United States, except in the extreme south.  We carried out an intensive survey of the occurrence of Russian-olive at the southern limit of its range in order to explore the factors controlling southward expansion.  We used a vehicle-mounted global positioning system to record occurrences of Russian-olive visible from roads in southern California, Nevada, Arizona, New Mexico and Texas.  We overlaid the track log from this expedition on the PRISM climate data set to locate pixels (2 km x 2 km) containing planted Russian-olive (n = 200), naturally occurring Russian-olive (n = 123), or no Russian-olive (n = 5688).  We then used logistic regression to relate occurrence of Russian-olive to mean annual extreme minimum temperature, mean annual chilling hours (number of hours below 7 °C), mean annual maximum temperature, mean annual number of growing degree days, mean annual precipitation, and distance to nearest mapped water.  To account for variation in search effort among pixels, observations were weighted by the length of road searched within the pixel divided by pixel area. 

Results/Conclusions

There is a sharp southern boundary to the Russian-olive distribution, which is strongly related to winter low temperatures, but only weakly related to summer high temperatures and precipitation.  For example, pseudo r² values for single-variable quadratic regressions predicting occurrence of naturally established Russian-olive were 0.15 for mean annual chilling hours, 0.08 for mean annual maximum temperature, and 0.03 for mean annual precipitation. Occurrence of naturally established, but not of planted, Russian-olive is strongly negatively related to distance from water (pseudo r² =  0.10, 0.01), demonstrating that this species is planted in uplands, but escapes to wetlands and riparian zones. These results are consistent with the hypothesis that southward spread of Russian-olive is limited by insufficient chilling hours to break bud dormancy.  Because of high correlations among temperature variables, however, we cannot rule out the possibility that some other facet of winter cold temperatures is controlling Russian-olive occurrence in this region.