larger structural classes, and also by a decrease - fs.fed.us · PDF file1941, when the Rio Grande experienced a major flood (Funk 1993). Regulation of flooding events and reduction

larger structural classes, and also by a decreasein the number of coyote willow polygons.

In numerous polygons that contained Russianolive in 1984, vegetation structure has changed asindividual plants have matured. Numerous polygonshave changed from structural class 5 to 3 (see fig. 1for general depiction of vegetation structural types)as a result of increased height and density of Russianolive in the middle story of the canopy. Further, anumber of polygons that did not support Russianolive in 1984 now have the species in the understory.

Salt cedar has been a vigorous invader of south-western riparian habitats, and continues to advancesteadily in the Rio Grande bosque. Numerous vegeta-tion polygons that did not have salt cedar in 1984now contain the species. Further, coyote willow hasbeen replaced by salt cedar in a number of polygons.The addition of salt cedar as a polygon componentis perhaps the most widespread and obvious changethat has occurred since the 1984 maps were produced.

Our field assessment identified 31 fires withinthe study area since 1984, making fire a significantcontributor to vegetation change. On burnedpolygons, the first woody species that appeared toreturn was coyote willow, followed by salt cedar,then Russian olive, and finally, cottonwood.

A number of changes in vegetation structureand types within the mapped polygons indicatesthat the Rio Grande bosque is maturing. Numer-ous polygons have changed from structure class 5to 3 as previously mentioned, and a few stage 3polygons have reached structure stage 1. Further-more, most of the stage 1 polygons have additionalspecies (e.g., salt cedar, Russian olive) in theirstory, and the number of sites containing coyotewillow is decreasing pointing to successional,maturing vegetation. Interestingly, field assessmentshowed that while the bosque vegetation is aging,cottonwood recruitment appeared low. The major-ity of cottonwood trees in the bosque date back to1941, when the Rio Grande experienced a majorflood (Funk 1993). Regulation of flooding eventsand reduction of water quantity in the Rio Grandemay contribute to low cottonwood regeneration.

CONCLUSION

The 1995 middle Rio Grande vegetation mapsindicate that vegetation classes and species compo-sition have changed extensively since 1984. Over-

all, the bosque is aging as detected by vegetationgrowth and maturation using structural stageassessment. Non-endemic woody species arebecoming more prominant, often appearing asunderstory species in cottonwood woodland, andalso as shrub communities without cottonwoods(see fig. 1 on next page). Because wildlife speciesare associated (sometimes strongly) with vegeta-tion type and structure, it is clear that as bosquevegetation changes and matures, species compositionand population sizes of various species of bosquefauna will change. Such shifts may have alreadytaken place, but long-term monitoring data areneeded to detect changes in animal populations. Werecommend long-term monitoring programs beestablished to detect further changes in bosque floraand fauna. Monitoring data are necessary to ensure ascientific basis for establishing goals and priorities forbosque conservation and restoration.

The middle Rio Grande vegetation maps werecreated to provide baseline vegetation data to helpaddress this and many other topics critical to thesuccessful management of the Grande.

ACKNOWLEDGMENTS

We thank Ed for technical help and theAlbuquerque Army Corps of Engineers for fundingthis project. We are grateful to WilliamMark Sifuentes, and Jeff Whitney for their guidance.

LITERATURE CITED

Crawford, C. S., A. C. Cully, R. Leutheuser, M. S.Sifuentes, L. H. White, J. Wilber. 1993. MiddleRio Grande ecosystem: Bosque biological man-agement plan. U.S. Fish and Wildlife Service,Albuquerque, NM. 291 p.

Funk, R. 1993. Rio Grande Valley State Park. Pp.159-161 B. J.J. Cortner, M. G.Wallace, L. F. R.H. Hamre, technicalcoordinators. Riparian management: commonthreads and shared interests. USDA ForestService. Rocky Mountain Forest and RangeExperiment Station, Fort Collins, CO. GeneralTechnical Report RM-226.

Hink, V. C., and R. D. 1984. Middle RioGrande Biological Survey. Final Report. ArmyCorps of Engineers Contract No. 0015. Tempe, AZ: Center for EnvironmentalStudies, Arizona State Univ. 160 p.

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